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  • richardmitnick 11:14 am on November 12, 2016 Permalink | Reply
    Tags: , , , Black Holes, Using LISA to Learn How Pairs of Black Holes Formed   

    From AAS NOVA: “Using LISA to Learn How Pairs of Black Holes Formed” 

    AASNOVA

    American Astronomical Society

    11 November 2016
    Susanna Kohler

    2
    ESA/LISA

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    A still image from a simulation that shows a black-hole binary that formed inside a globular cluster. Future gravitational-wave detections may reveal how the binaries that we detect originally formed. [Northwestern Visualization/Carl Rodriguez]

    How are black-hole binaries built? Observations of gravitational waves from these systems — made using the upcoming Laser Interferometer Space Antenna (LISA) — may be able to reveal their origins.

    Formation Channels

    There are two primary places where stellar-mass black-hole binaries are thought to form:

    In isolation in the galactic field, as the components of a stellar binary independently evolve into black holes but remain bound to each other.
    In dense stellar environments like globular clusters, where the high density of already-formed black holes can cause a pair to dynamically interact and form a binary before being ejected from the cluster.

    Can we differentiate between these origins based on future detections of gravitational waves from black-hole binaries? A team of scientists led by Katelyn Breivik (CIERA, Northwestern University) thinks that we can!

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    The gravitational-wave spectrum and how we detect it (click for a closer look!). While ground-based interferometers like LIGO detect black-hole binaries in the final moments before merger, LISA’s lower frequency band will allow it to detect binaries earlier in their inspiral. [NASA Goddard SFC]

    Differentiation by Eccentricity

    Breivik and collaborators believe that the key clue is the binary’s eccentricity. Gravitational-wave emission will eventually circularize all black-hole binaries during their inspiral. But in the first formation scenario, binary evolution processes like tidal circularization and mass transfer will reduce the binary’s eccentricity early on — whereas in the second scenario, the binaries that form in globular clusters may retain eccentricity in their orbits long enough that we can detect it.

    Ground-based interferometers won’t be up to this task; by the time the binary orbits shrink enough to evolve into the LIGO frequency band, the orbits won’t have measurable eccentricity anymore. But the upcoming space-based LISA mission, which will operate in a lower frequency band, might be able to pick up this signature.

    To determine if LISA can pull it off, Breivik and collaborators simulate two populations of binary black holes: one evolved in isolation in galactic fields, and the other formed dynamically in globular clusters and then ejected. The authors then explore the evolution of these populations’ masses and eccentricities as their orbits narrow into the LISA-detectable frequency band.

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    Eccentricity evolution tracks as a function of gravitational-wave frequency for black-hole binaries formed in dynamical scenarios (black) and in isolation (blue for those with a common-envelope episode, green for those without). Eccentricities above 10-2 are measurable for all binaries; those above 10-3 are measurable for 90%. LISA’s frequency band is shown in grey. [Breivik et al. 2016]

    Separating Populations

    Breivik and collaborators find that LISA will be able to make several important distinctions. First, if LISA detects binary black holes with eccentricities of e > 0.01 at frequencies above 10-2 Hz, we can be fairly certain that these originated from dynamical processes in dense stellar environments.

    For binary black holes detected with eccentricities of e > 0.01 at lower frequencies, they could either have formed in dense stellar environments or they could have formed in isolation. Based on this study’s results, however, those with measurable eccentricities that formed in isolation most likely originated from a common-envelope formation. Measuring eccentricities of such systems in the future could provide constraints on the physics of how this formation mechanism works.

    Though the field of gravitational-wave astronomy is only just beginning, its future is promising! Theoretical studies like this one will help us to extract a greater understanding from the observations we can expect down the road.

    Citation

    Katelyn Breivik et al 2016 ApJL 830 L18. http://dx.doi.org/10.3847/2041-8205/830/1/L18

    See the full article here .

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  • richardmitnick 2:29 pm on October 28, 2016 Permalink | Reply
    Tags: Black Holes, , , Why Doesn't Dark Matter Form Black Holes?   

    From Ethan Siegel: “Why Doesn’t Dark Matter Form Black Holes?” 

    10.28.16
    Ethan Siegel

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    Black Holes Could Turn You Into a Hologram, and You Wouldn’t Even Notice

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    An illustration of a black hole. Despite how dark it is, all black holes are thought to have formed from normal matter, not dark matter. Image credit: NASA/JPL-Caltech.

    Dark matter is the most abundant form of mass in our Universe. If you were to add up all the stars, planets, lifeforms, gas, dust, plasma and more — all the known, “normal” matter in our Universe — it would only account for about 15-to-17% of the total gravitation that we see. The remaining mass, outclassing the normal matter by a 5:1 ratio, must be completely invisible, meaning it doesn’t absorb or emit light at all. Yet it must interact gravitationally, enabling it to form large-scale structure in the Universe and to hold galaxies together. So why, then, can’t it form black holes?

    Black holes aren’t the only thing dark matter can’t form; it also can’t create dark matter stars, planets or dark atoms. Imagine the Universe as it might have been back in the very, very early stages, before there were any black holes, stars, planets or atoms.

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    The early Universe was full of matter and radiation, and was so hot and dense that the quarks and gluons present didn’t form into individual protons and neutrons, but remained in a quark-gluon plasma. (Image credit: RHIC collaboration, Brookhaven, via http://www.bnl.gov/newsroom/news.php?a=11403)

    All we had was a hot, dense, expanding “sea” of matter and radiation of all the different types allowed. By time the Universe has aged to be a few minutes old, the atomic nuclei are there, all the electrons are there, all the neutrinos and photons are there, and all the dark matter is there, too.

    They’re all flying around at incredible speeds, sure, but they’re also all exerting forces on one another. It’s true that they all feel the gravitational force (even photons, thanks to Einstein’s energy-mass equivalence), but gravity isn’t the only thing that matters here.

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    In the hot, early Universe, prior to the formation of neutral atoms, photons scatter off of electrons (and to a lesser extent, protons) at a very high rate, transferring momentum when they do. (Images credit: Amanda Yoho)

    hotons and electrons have it the worst: they interact very frequently through the electromagnetic force, scattering and “bouncing” off of one another, exchanging energy, momentum and colliding at an alarming rate. Nuclei fare only a little better: they’re much more massive, so their interaction rate is lower, and they pick up (or lose) less momentum with each collision.

    Neutrinos are much luckier: they don’t have an electric charge, and so they don’t interact through the electromagnetic force at all. Instead, they can only interact (besides gravity) through the weak force, which means collisions are incredibly infrequent. But dark matter gets it the best in terms of freedom: as far as we can tell, it only interacts through gravity. There are no collisions at all, and so all dark matter can do is be attracted to the other sources of matter.


    Access mp4 video here .

    This might, you worry, make things worse! While normal matter has collisions and interactions preventing it from collapsing gravitationally, forming denser clumps, etc., the dark matter density begins to grow in the overdense regions. But this doesn’t happen the way you think of “collapse” happening. When a gas cloud collapses to form stars, what happens?

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    A massive, gaseous nebula is where new stars in the Universe are born. (Image credit: ESO/VPHAS+ team, via http://www.eso.org/public/images/eso1403a/)

    The gas interacts through the gravitational force, becoming denser, but the matter that makes up that gas sticks together, allowing it to reach a denser state. That “stickiness” only happens thanks to the electromagnetic force! This is why things can collapse down to produce bound objects like stars, planets and even atoms.

    Without that stickiness? You’d just end up with a diffuse, loosely held together, “fluffy” structure bound together only through gravity. That’s why you hear of dark matter halos on galaxy and cluster scales, of dark matter filaments on even larger scales, and of no other dark matter structures.

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    Now, these diffuse, fluffy halos are incredibly important: they represent the seeds of all the bound structure in the Universe today. This includes dwarf galaxies, normal galaxies, galaxy groups, galaxy clusters, superclusters and filaments, as well as all the substructure that makes these objects up. But without that extra force — without some “sticky” force to hold it together, to exchange energy and momentum — the dark matter is destined to remain in this fluffy, diffuse state. The normal matter can form the tightly-bound structures you’re used to, but the dark matter has no way to collide inelastically, to lose momentum or angular momentum, and hence, it has to remain loosely bound and “halo-like.”

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    While stars might cluster in the disk and the normal matter might be restricted to a nearby region around the stars, dark matter extends in a halo more than 10 times the extent of the luminous portion. (Image credit: ESO/L. Calçada)

    It’s a little disconcerting to think that it’s not the gravitational force that leads to planets, stars, black holes and more, but gravity is just part of the equation. To really drive this point home, imagine that you took a ball of some type and launched it, with the ball — as you know — made out of atoms. What’s the ball going to do?

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    A projectile under the influence of gravity will move in a parabola, until it strikes other matter (like the floor) that prevents it from moving further. (Image credit: Wikimedia Commons users MichaelMaggs Edit by Richard Bartz under c.c.a.-s.a.-3.0)

    Of course, it’ll move in a parabolic path (neglecting air resistance), rising up to a maximum height and falling down until it finally strikes the Earth. On a more fundamental scale, the ball moves in an elliptical orbit with the center-of-mass of the Earth as one focus of the ellipse, but the ground gets in the way of that ellipse, and so the portion we see looks like a parabola. But if you magically turned that ball into a clump of dark matter, what you’d get would surprise you greatly.

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    Normal matter is stopped by the Earth, but dark matter would pass right through, making a near-perfect ellipse. (Image credit: Dave Goldberg of Ask A Mathematician/Ask A Physicist, via http://www.askamathematician.com/2012/01/q-why-does-gravity-make-some-things-orbit-and-some-things-fall/)

    Without the electromagnetic force, a whole bunch of terrible things happen:

    There’s no interaction, other than gravity, between the particles making up the ball and the atoms of the Earth. Instead of making a parabola, the dark matter clump goes all the way through the layers of the Earth, swinging around the center in an (almost-perfect) ellipse (but not quite, due to the layers and non-uniform density of the Earth), coming out near where it entered, making a parabola again and continue to orbit like that interminably.
    There are also no interactions holding this clump together! So while atoms in a ball do have some random motions, they are held together by the electromagnetic force, keeping that ball-like structure to it. But if you remove that electromagnetic force, the random motions of the dark matter particles will work to unbind this from being a clump, since the gravitation of the clump itself is insufficient to keep it bound together.

    This means that over time (and many orbits), the dark matter gets stretched into a long ellipse, and that ellipse gets more and more diffuse, similar to the particles that make up the debris stream from a comet, only even more diffuse!

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    (Image credit: Gehrz, R. D., Reach, W. T., Woodward, C. E., and Kelley, M. S., 2006, of the trail of Comet Encke)

    Dark matter can’t form black holes or other tightly-bound structures because gravity alone isn’t enough to bind something tightly together. Because the force of gravity is so weak, it can only bind it loosely, which means huge, diffuse, very massive structures. If you want a “clump” of something — a star, a planet, or even an atom — you need a force that’s stronger than gravity to make it happen.

    There may yet be one! It is possible that dark matter self-interacts (or interacts with matter or radiation, at some level), but if it does, we only have constraints on how weak that interaction is. And it is very, very weak, if it’s even non-zero at all.

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    If dark matter does have a self-interaction, its cross-section is tremendously low, as direct detection experiments have shown. (Image credit: Mirabolfathi, Nader arXiv:1308.0044 [astro-ph.IM], via https://inspirehep.net/record/1245953/plots)

    So even though we think of gravity as the only force that matters on the largest scales, the truth is when we think about the structures that we see — the ones that give off light, that house atoms and molecules, that collapse into black holes — it’s the other forces, in concert with gravity, that allow them to exist at all. You need some type of inelastic, sticky collision, and dark matter doesn’t have the right interactions to make that possible. Because of that, dark matter can’t make a galaxy, a star, a planet or a black hole. It takes more than gravity alone to do the job.

    See the full article here .

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    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
  • richardmitnick 2:13 pm on October 5, 2016 Permalink | Reply
    Tags: , , Black Holes, , , XJ1417+52: X-ray Telescopes Find Evidence for Wandering Black Hole   

    From Chandra: “XJ1417+52: X-ray Telescopes Find Evidence for Wandering Black Hole” 

    NASA Chandra Banner

    NASA Chandra Telescope

    NASA Chandra

    October 5, 2016

    1
    Credit X-ray: NASA/CXC/UNH/D.Lin et al; Optical: NASA/STScI

    A “wandering” black hole has been found in the outer regions of a galaxy about 4.5 billion light years from Earth.

    Evidence suggests this newly discovered black hole has about 100,000 times the Sun’s mass, and was originally located in a smaller galaxy that merged with a larger one.

    Chandra data show this object gave off a tremendous amount of X-rays, which classifies it as a “hyperluminous X-ray source”.

    The burst of X-rays may have come from a star that was torn apart by the strong gravity of the black hole.

    Astronomers have used NASA’s Chandra X-ray Observatory and ESA’s XMM-Newton X-ray observatory to discover an extremely luminous, variable X-ray source located outside the center of its parent galaxy.

    ESA/XMM Newton
    ESA/XMM Newton

    This peculiar object could be a wandering black hole that came from a small galaxy falling into a larger one.

    Astronomers think that supermassive black holes, with some 100,000 to 10 billion times the Sun’s mass, are in the centers of most galaxies. There is also evidence for the existence of so-called intermediate mass black holes, which have lower masses ranging between about 100 and 100,000 times that of the Sun.

    Both of these types of objects may be found away from the center of a galaxy following a collision and merger with another galaxy containing a massive black hole. As the stars, gas and dust from the second galaxy move through the first one, its black hole would move with it.

    A new study reports the discovery of one of these “wandering” black holes toward the edge of the lenticular galaxy SDSS J141711.07+522540.8 (or, GJ1417+52 for short), which is located about 4.5 billion light years from Earth. This object, referred to as XJ1417+52, was discovered during long observations of a special region, the so-called Extended Groth Strip, with XMM-Newton and Chandra data obtained between 2000 and 2002. Its extreme brightness makes it likely that it is a black hole with a mass estimated to be about 100,000 times that of the Sun, assuming that the radiation force on surrounding matter equals the gravitational force.

    The main panel of this graphic has a wide-field, optical light image from the Hubble Space Telescope. The black hole and its host galaxy are located within the box in the upper left. The inset on the left contains Hubble’s close-up view of GJ1417+52. Within this inset the circle shows a point-like source on the northern outskirts of the galaxy that may be associated with XJ1417+52.

    The inset on the right is Chandra’s X-ray image of XJ1417+52 in purple, covering the same region as the Hubble close-up. This is a point source, with no evidence seen for extended X-ray emission.

    The Chandra and XMM-Newton observations show the X-ray output of XJ1417+52 is so high that astronomers classify this object as a “hyper-luminous X-ray source” (HLX).

    These are objects that are 10,000 to 100,000 times more luminous in X-rays than stellar black holes, and 10 to 100 times more powerful than ultraluminous X-ray sources, or ULXs.

    At its peak XJ1417+52 is about ten times more luminous than the brightest X-ray source ever seen for a wandering black hole. It is also about 10 times more distant than the previous record holder for a wandering black hole.

    The bright X-ray emission from this type of black hole comes from material falling toward it. The X-rays from XJ1417+52 reached peak brightness in X-rays between 2000 and 2002. The source was not detected in later Chandra and XMM observations obtained in 2005, 2014 and 2015. Overall, the X-ray brightness of the source has declined by at least a factor of 14 between 2000 and 2015.

    The authors theorize that the X-ray outburst seen in 2000 and 2002 occurred when a star passed too close to the black hole and was torn apart by tidal forces. Some of the gaseous debris would have been heated and become bright in X-rays as it fell towards the black hole, causing the spike in emission.

    The location and brightness of the optical source in the Hubble image that may be associated with XJ1417+52 suggest that the black hole could have originally belonged to a small galaxy that plowed into the larger GJ1417+52 galaxy, stripping away most of the galaxy’s stars but leaving behind the black hole and its surrounding stars at the center of the small galaxy. If this idea is correct the surrounding stars are what is seen in the Hubble image.

    A paper by Dacheng Lin (University of New Hampshire) and colleagues describing this result appears in The Astrophysical Journal and is available online.

    See the full article here .

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    NASA’s Marshall Space Flight Center in Huntsville, Ala., manages the Chandra program for NASA’s Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory controls Chandra’s science and flight operations from Cambridge, Mass.

     
  • richardmitnick 7:59 am on October 4, 2016 Permalink | Reply
    Tags: , , Black Holes, Do Black Holes Die?,   

    From SPACE.com: “Do Black Holes Die?” 

    space-dot-com logo

    SPACE.com

    October 3, 2016
    Paul Sutter

    EXPERT VOICES

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    Artist’s illustration of a supermassive black hole emitting a jet of energetic particles. Credit: NASA/JPL-Caltech

    Paul Sutter is an astrophysicist at The Ohio State University and the chief scientist at COSI Science Center. Sutter is also host of Ask a Spaceman, RealSpace, and COSI Science Now.

    There are some things in the universe that you simply can’t escape. Death. Taxes. Black holes. If you time it right, you can even experience all three at once.

    Black holes are made out to be uncompromising monsters, roaming the galaxies, voraciously consuming anything in their path. And their name is rightly deserved: once you fall in, once you cross the terminator line of the event horizon, you don’t come out. Not even light can escape their clutches.

    But in movies, the scary monster has a weakness, and if black holes are the galactic monsters, then surely they have a vulnerability. Right?

    Hawking to the rescue

    In the 1970s, theoretical physicist Stephen Hawking made a remarkable discovery buried under the complex mathematical intersection of gravity and quantum mechanics: Black holes glow, ever so slightly, and, given enough time, they eventually dissolve.

    Wow! Fantastic news! The monster can be slain! But how? How does this so-called Hawking Radiation work?

    Well, general relativity is a super-complicated mathematical theory. Quantum mechanics is just as complicated. It’s a little unsatisfying to respond to “How?” with “A bunch of math,” so here’s the standard explanation: the vacuum of space is filled with virtual particles, little effervescent pairs of particles that pop into and out of existence, stealing some energy from the vacuum to exist for the briefest of moments, only to collide with each other and return to nothingness.

    Every once in a while, a pair of these particles pops into existence near an event horizon, with one partner falling in and the other free to escape. Unable to collide and evaporate, the escapee goes on its merry way as a normal non-virtual particle.

    Voila: The black hole appears to glow, and in doing so — in doing the work to separate a virtual particle pair and promote one of them into normal status — the black hole gives up some of its own mass. Subtly, slowly, over the eons, black holes dissolve. Not so black anymore, huh?

    Here’s the thing: I don’t find that answer especially satisfying, either. For one, it has absolutely nothing to do with Hawking’s original 1974 paper, and for another, it’s just a bunch of jargon words that fill up a couple of paragraphs but don’t really go a long way to explaining this behavior. It’s not necessarily wrong, just…incomplete.

    Let’s dig into it. It’ll be fun.

    The way of the field

    First things first: “Virtual particles” are neither virtual nor particles. In quantum field theory — our modern conception of the way particles and forces work — every kind of particle is associated with a field that permeates all of space-time. These fields aren’t just simple bookkeeping devices. They are active and alive. In fact, they’re more important than particles themselves. You can think of particles as simply excitations — or “vibrations” or “pinched-off bits,” depending on your mood — of the underlying field.

    Sometimes the fields start wiggling, and those wiggles travel from one place to another. That’s what we call a “particle.” When the electron field wiggles, we get an electron. When the electromagnetic field wiggles, we get a photon. You get the idea.

    Sometimes, however, those wiggles don’t really go anywhere. They fizzle out before they get to do something interesting. Space-time is full of the constantly fizzling fields.

    What does this have to do with black holes? Well, when one forms, some of the fizzling quantum fields can get trapped — some permanently, appearing unfortunately within the newfound event horizon. Fields that fizzled near the event horizon end up surviving and escaping. But due to the intense gravitational time dilation near the black hole, thy appear to come out much, much later in the future.

    In their complex interaction and partial entrapment with the newly forming black hole, the temporary fizzling fields get “promoted” to become normal everyday ripples — in other words, particles.

    So, Hawking Radiation isn’t so much about particles opposing into existence near a present-day black hole, but the result of a complex interaction at the birth of a black hole that persists until today.

    Patience, child

    One way or the other, as far as we can tell, black holes do dissolve. I emphasize the “as far as we can tell” bit because, like I said at the beginning, generality is all sorts of hard, and quantum field theory is a beast. Put the two together and there’s bound to be some mathematical misunderstanding.

    But with that caveat, we can still look at the numbers, and those numbers tell us we don’t have to worry about black holes dying anytime soon. A black hole with the mass of the sun will last a wizened 10^67 years. Considering that the current age of our universe is a paltry 13.8 times 10^9 years, that’s a good amount of time. But if you happened to turn the Eiffel Tower into a black hole, it would evaporate in only about a day. I don’t know why you would, but there you go.

    See the full article here .

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  • richardmitnick 11:20 am on September 28, 2016 Permalink | Reply
    Tags: , , , Black Holes, Is There a Size Limit for Supermassive Black Holes?   

    From AAS NOVA: “Is There a Size Limit for Supermassive Black Holes?” 

    AASNOVA

    Amercan Astronomical Society

    28 September 2016
    Susanna Kohler

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    In this artist’s illustration, a supermassive black hole with billions of times the mass of our Sun accretes matter in the heart of a galaxy. A new study questions whether there is a maximum mass that these monsters can attain. [NASA/JPL-Caltech]

    Supermassive black holes (SMBHs) lurk in the centers of galaxies, and we’ve measured their masses to range from hundreds of thousands to ten billion solar masses. But is there a maximum mass that these monsters are limited to?

    Observed Maximum

    Since the era when the first SMBHs formed, enough time has passed for them to potentially grow to monstrous size, assuming a sufficient supply of fuel.

    Instead, however, we observe that SMBHs in the centers of the largest local-universe galaxies max out at a top mass of a few times 10^10 solar masses. Even more intriguingly, this limit appears to be redshift-independent: we see the same maximum mass of a few 10^10 solar masses for SMBHs fueling the brightest of quasars at redshifts up to z~7.

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    Accretion rate (solid) and star formation rate (dashed) vs. radius in a star-forming accretion disk, for several different values of black-hole mass. Though accretion rates start out very high at large radius, they drop to just a few solar masses per year at small radii, because much of the gas is lost to star formation in the disk. [Inayoshi & Haiman 2016]

    So why don’t we see any giants larger than around 10 billion solar masses, regardless of where we look? Kohei Inayoshi and Zoltán Haiman (Columbia University) suggest that there is a limiting mass for SMBHs that’s set by small-scale physical processes, rather than large processes like galaxy evolution, star formation history, or background cosmology.

    Challenges for Accretion

    Growing an SMBH that’s more massive than 10^10 solar masses requires gas to be quickly funneled from the outer regions of the galaxy (hundreds of light-years out), through the large accretion disk that surrounds the black hole, and into the nuclear region (light-year scales): the gas must be brought in at rates as high as 1,000 solar masses per year.

    Modeling this process, Inayoshi and Haiman demonstrate that at such high rates, the majority of the gas instead gets stuck in the disk, causing star formation at radii of tens to hundreds of light-years and never getting close enough to fuel the SMBH. The remaining trickle of gas that does accrete onto the SMBH is not enough to allow it to grow to more than 10^11 solar masses in the age of the universe.

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    Cygnus A provides a stunning example of the tremendous jets that can be launched from SMBHs at the center of galaxies. [NRAO]

    What’s more, for a large enough SMBH, this trickle of gas can become so small relative to the black hole mass that the physics of the accretion itself changes, causing the inner disk to puff up and launching strong outflows and jets. Once this transition occurs, the black-hole feeding is suppressed, preventing the SMBH from growing any larger.

    The authors show that the critical mass for this transition is 1–6 x 1010 solar masses — consistent with the maximum masses that we’ve observed for SMBHs in the wild. This consistency supports the idea that the small-scale physics around the SMBH may be setting its size limit, rather than the large-scale environment around the galaxy.

    Citation

    Kohei Inayoshi and Zoltán Haiman 2016 ApJ 828 110. doi:10.3847/0004-637X/828/2/110

    See the full article here .

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  • richardmitnick 7:17 pm on September 22, 2016 Permalink | Reply
    Tags: , , , Black Holes   

    From astrobites: “Black holes and populations” 

    Astrobites bloc

    Astrobites

    Sep 22, 2016
    Paddy Alton

    TITLE: Stellar populations across the black hole mass – velocity dispersion relation
    AUTHORS: Ignacio Martín-Navarro, Jean P. Brodie, Remco C. E. van den Bosch, Aaron J. Romanowsky, and Duncan J. Forbes
    FIRST AUTHOR INSTITUTION: University of California Observatories
    STATUS: Accepted for publication in the Astrophysical Journal Letters

    Introduction:

    A supermassive black hole is a kind of cosmic parasite that preys on galaxies. As the host galaxy grows larger, so too does the black hole, consuming gas that would otherwise be turned into stars. Worse, it guzzles gas so quickly that the gas surging down its gravity well gets superheated; the hot gas radiates strongly, heating up surrounding gas and driving it away from the black hole. This prevents it from being turned into stars, causing the galaxy to starve. This effect actually sets a limit on how fast a black hole can grow. To cap it off, it doesn’t always finish its meal, launching jets of material clear of the galaxy which expel yet more gas (there’s an obvious simile here, which I’m not going to employ for reasons of good taste).

    Naturally, all this has a profound effect on the host galaxy and its stellar population, ultimately shutting down star formation. The bigger the galaxy, the bigger the black hole – and the more aggressive it gets. A negative feedback loop is created which causes those galaxies that grow quickest to also fail quickest, a kind of cosmic ‘boom and bust‘. This leads to a tight correlation between the mass of a galaxy’s supermassive black hole and its total mass (more precisely with its velocity dispersion, which is just a stand-in for galaxy mass).

    In today’s paper, the authors search for clues as to how the presence of a supermassive black hole affected the formation of the stars that did make it before the gas supply was cut off.

    How it works:

    We know that just as there is a connection between galaxy mass and black hole mass, so too is there a connection between galaxy mass and the chemical makeup of stars. The most massive galaxies are ‘alpha-enhanced’ – a term that, like most technical language, packs in a lot of detail. Alpha elements are common elements created by sticking together a bunch of alpha particles (Helium nuclei). Their atoms therefore have atomic masses divisible by four: Oxygen, Neon, Magnesium, Silicon, Sulphur, Argon, Calcium, and Titanium, in case you haven’t got a periodic table to hand.

    (Carbon doesn’t count. It’s more like the seed to which you attach alpha particles in order to make the legitimate alpha elements. Making Carbon is hard, but once you’ve got some making alpha elements is easy. Sorry Carbon.)

    There’s another class of elements called ‘iron peak’ elements. Heavier elements tend to be rarer but there’s an exception for elements with atomic numbers similar to iron. When a galaxy is alpha-enhanced, it means that the alpha elements are more abundant relative to the iron peak elements than they are, for example, in the Sun. When this happens it’s actually telling us something interesting about the history of that galaxy. All these elements are formed in stars and released back into the wild via supernova explosions, which come in two types. Core-collapse supernovae are what you’re probably most familiar with: in these a massive star runs out of fuel, can no longer support itself against its own gravity and so collapses … before rebounding in a huge explosion. The massive stars that end in these explosions don’t last long in cosmic terms, sometimes only a few million years. By contrast, other supernovae occur when a white dwarf star (the kind of thing our Sun will eventually turn into) grows above a critical mass, probably due to wrenching material away from another star or merging with another white dwarf. These supernovae almost exclusively make iron peak elements but can’t possibly occur until you actually have some white dwarfs. That means there is a delay of several billion years while you wait for stars like the Sun to reach the end of their lives.

    This time delay is critical. If you grow a galaxy steadily, over billions of years, these late supernova start to go off. This seeds the galaxy with iron peak elements while it’s still making stars. If you grow your galaxy quickly, they still go off, but it’s too late: star formation is finished and those heavy elements sail off into the void. In either case the core-collapse supernovae go off quickly, so you make lots of alpha elements. In summary, alpha-enhancement means you formed your stars very quickly. It’s therefore significant that the most massive galaxies have massive black holes (which we think cut star formation off early on) and are also alpha-enhanced: the two effects are directly related.

    Today’s paper:

    In order to isolate the effect of the black hole, the authors look at the outliers – galaxies whose black holes are a bit more/less massive than expected given the size of the galaxy. This is shown in Figure 1. Their sample spans a wide range of masses and other galaxy properties, the idea being that the particular effect of the black hole can be isolated in this way.

    1
    The authors’ sample of galaxies. The usual relationship between black hole mass and galaxy mass (velocity dispersion, on the x-axis, is just an indirect measurement of this) is plotted as a thick black line. Galaxies with slightly overweight central black holes are in orange, whilst those with comparatively light black holes are in blue (galaxies represented by circles are more compact than those represented by stars, but that’s not very important here). Figure 1 from the paper.

    The authors find a clear correlation between underweight black holes and younger stellar populations, bearing out the idea that less massive central black holes are not so good at cutting off star formation. In the ‘blue’ galaxies in Fig. 1 black hole growth has lagged behind galaxy growth for some reason, which has meant those galaxies were able to form stars for longer.

    2
    Both central black hole mass and the production of alpha elements (see text) such as Magnesium, Mg, are related to galaxy mass. Here we see there is a more fundamental, direct connection between the two: those galaxies with slightly more massive central black holes than expected are also slightly more abundant in Mg than expected (and vice versa). The axes show the excess black hole mass and excess Mg enhancement respectively. Figure 3 from the paper.

    I said earlier that both central black hole mass and enhanced production of alpha elements are linked to total galaxy mass. What the authors are able to show is that there is a more fundamental direct link between the two (see Figure 2); this confirms that these correlations are no coincidence but really do arise from the theory I sketched out. The authors have shown us the direct effects of black hole feedback on the stellar populations of their host galaxies.

    See the full article here .

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    What do we do?

    Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
    Why read Astrobites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.
    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

     
  • richardmitnick 4:34 pm on September 13, 2016 Permalink | Reply
    Tags: , , Black Holes, , , Star arrangement that hid for a decade spotted at galaxy’s heart   

    From New Scientist: “Star arrangement that hid for a decade spotted at galaxy’s heart” 

    NewScientist

    New Scientist

    13 September 2016
    Adam Mann

    1
    Part of our galaxy’s centre, as seen in near-infrared wavelengths. ESO/S. Gillessen et al.

    There’s a party in the galactic centre. We may have found the first solid evidence of a dense conference of stars around the Milky Way’s heart, which may one day help us observe the supermassive black hole living there.

    Sag A*  NASA Chandra X-Ray Observatory 23 July 2014, the supermassive black hole at the center of the Milky Way
    Sag A* NASA Chandra X-Ray Observatory 23 July 2014, the supermassive black hole at the center of the Milky Way

    The structure is known as a stellar cusp, and it has played hide-and-seek with astronomers for more than a decade. It was first proposed in the 1970s, when models predicted that stars orbiting a supermassive black hole would jostle around every time one was devoured. Over the course of a galaxy’s lifetime, this should leave an arrangement with many stars near the black hole and exponentially fewer as you move farther away.

    But it has been hard to prove this happens. Other galaxies are too far away for us to see their centres as anything more than fuzzy blobs. Observations in the early 2000s seemed to support a cusp in the Milky Way, but better data showed that we had been tricked by obscuring dust.

    Now, Rainer Schödel at the Institute of Astrophysics of Andalusia in Granada, Spain, and his colleagues have combined images of the galactic centre to map faint old stars, which have been around long enough to settle into a cusp. They also studied the total light emitted by all stars at varying distances from our galaxy’s central black hole, and compared the results with simulations.

    Perfect probes

    These methods point to the same conclusion: the cusp exists. Around our galaxy’s central black hole, the density of stars is 10 million times that in our local area, says Schödel, who presented the work on 7 September at the LISA Symposium in Zurich, Switzerland.

    Many of those stars will eventually explode as supernovae, leaving behind black holes with masses comparable to that of our sun. If one of these merges with the black hole in the galactic centre, it will emit telltale gravitational waves that can be picked up by future observatories, like the proposed Laser Interferometer Space Antenna (LISA).

    ESA/eLISA
    ESA/eLISA

    Those waves will help figure out the mass, rotation rate and other properties of the black hole with extreme precision.

    “These stellar mass black holes would be absolutely perfect probes of spacetime around the supermassive black hole,” Schödel says.

    If the Milky Way has a cusp, then it’s likely that other galaxies do as well. That’s good news for an observatory like LISA, which may be able to pick up waves from dozens or even hundreds of interactions between stellar mass and supermassive black holes each year.

    The work is a significant advance over previous methods and seems to support the existence of a cusp, says Tuan Do at the University of California, Los Angeles. “The galactic centre is always surprising us though, so I think it would be great to take more observations to verify that there is a cusp of faint old stars,” he says.

    The next generation of enormous observatories, like the Thirty Meter Telescope and Giant Magellan Telescope, will see an order of magnitude more stars than current observatories can.

    TMT-Thirty Meter Telescope, proposed for Mauna Kea, Hawaii, USA
    TMT-Thirty Meter Telescope, proposed for Mauna Kea, Hawaii, USA

    Giant Magellan Telescope, Las Campanas Observatory, to be built  some 115 km (71 mi) north-northeast of La Serena, Chile
    Giant Magellan Telescope, Las Campanas Observatory, to be built some 115 km (71 mi) north-northeast of La Serena, Chile

    They will almost certainly observe the cusp if it’s there, Schödel says.

    See the full article here .

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  • richardmitnick 8:47 pm on September 12, 2016 Permalink | Reply
    Tags: , , , Black Holes,   

    From New Scientist: “First glimpse of a black hole being born from a star’s remains” 

    NewScientist

    New Scientist

    12 September 2016
    Anna Nowogrodzki

    1
    Born phoenix-like from the ashes of a dying star? Science Photo Library/Getty

    We’ve received a birth announcement from 20 million light years away, in the form of our first ever glimpse of what seems to be the birth of a black hole.

    When massive stars run out of fuel, they die in a huge explosion, shooting out high-speed jets of matter and radiation. What’s left behind collapses into a black hole, which is so dense and has such strong gravity that not even light can escape it.

    Or so the theory goes, anyway. Now, a team led by Christopher Kochanek at Ohio State University in Columbus have glimpsed something very special in data from the Hubble Space Telescope, from when it was watching the red supergiant star N6946-BH1, which is about 20 million light years from Earth.

    Fading star

    This star, first observed in 2004, was once about 25 times the mass of our sun. Kochanek and his colleagues found that for some months in 2009, the star briefly flared a million times brighter than our sun, then steadily faded away. New Hubble images show that it has disappeared in visible wavelengths, but a fainter source in the same spot is detectable in the infrared, as a warm afterglow.

    These observations mesh with what theory predicts should happen when a star that size crumples into a black hole. First, the star spews out so many neutrinos that it loses mass. With less mass, the star lacks enough gravity to hold on to a cloud of hydrogen ions loosely bound around it. As this cloud of ions floats away, it cools off, allowing the detached electrons to reattach to the hydrogen. This causes a year-long bright flare – when it fades, only the black hole remains.

    There are two other potential explanations for the star’s disappearing act: it could have merged with another star, or be hidden by dust. But they don’t fit the data: a merger would shine more brightly than the original star for much longer than a few months, and dust wouldn’t hide it for so long.

    “It’s an exciting result and long anticipated,” says Stan Woosley at Lick Observatory in California.

    “This may be the first direct clue to how the collapse of a star can lead to the formation of a black hole,” says Avi Loeb at Harvard University.

    A dark life cycle

    The find needs further confirmation, but that may not be far off. Material falling into the black hole would emit X-rays in a particular spectrum, which could be spotted by the Chandra X-ray Observatory. Kochanek says his group will be getting new data from Chandra in the next two months or so.

    If Chandra sees nothing, that doesn’t mean it’s not a black hole. In any case, the team will continue to look with Hubble – the longer the star is not there, the more likely that it’s a black hole. “Patience proves it no matter what,” says Kochanek.

    This data will help describe the beginning of the life cycle of a black hole, and will inform simulations of how black holes form and what makes a massive star form a neutron star rather than a black hole.

    Despite calling himself a “nasty pessimist”, Kochanek thinks it’s quite likely this is indeed the formation of a black hole. “I’m not quite at ‘I’d bet my life on it’ yet,” he says, “but I’m willing to go for your life.”

    See the full article here .

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  • richardmitnick 11:07 am on September 5, 2016 Permalink | Reply
    Tags: , , Black Holes, ,   

    From Ethan Siegel: “What Is The Biggest Black Hole As Seen From Earth?” 

    From Ethan Siegel

    9.5.16

    1
    The supermassive black hole at the core of galaxy NGC 1277 weighs in at 17 billion solar masses. But it’s too distant to be resolved from Earth. Image credit: NASA / ESA / Andrew C. Fabian / Remco C. E. van den Bosch (MPIA).

    If you collapse a large enough mass into a small enough volume, you’ll create a black hole.

    2
    The anatomy of a very massive star throughout its life, culminating in a Type II Supernova. Image credit: Nicole Rager Fuller for the NSF.

    Every object has a gravitational field, and without enough speed, you can’t leave it; you can’t reach escape velocity.

    For black holes, where escape velocity is bigger than the speed of light at the event horizon, nothing can escape, not even light.

    3
    Black holes may still emit light from outside the event horizon, as accelerated matter either falls in or is funneled into jets, but nothing inside the event horizon can ever escape. Image credit: ESO/L. Calçada, of an illustration of the quasar SDSS J1106-1939.

    Black holes are formed from the collapse of incredibly massive objects: ultramassive stars imploding in supernovae at the end of their lives.

    But common, stellar mass black holes, at 1-100 times the Sun’s mass, are surpassed by rarer, supermassive ones.

    4
    The core of galaxy NGC 4261, like the core of a great many galaxies, show signs of a supermassive black hole in both infrared and X-ray observations. Image credit: NASA / Hubble and ESA.

    Almost every galaxy has one, including our Milky Way.

    Sag A*  NASA Chandra X-Ray Observatory 23 July 2014, the supermassive black hole at the center of the Milky Way
    Sag A* NASA Chandra X-Ray Observatory 23 July 2014, the supermassive black hole at the center of the Milky Way

    4
    The largest flare ever observed from the supermassive black hole at our galaxy’s center. Image credit NASA/CXC/Stanford/I. Zhuravleva et al.

    At 4 million solar masses, our black hole is only 26,000 light years away.


    Andrea Ghez, UCLA

    Other, larger, more distant galaxies, like Messier 87, have even larger black holes, reaching into the billions of solar masses.

    5
    Three views of the center of Messier 87 and its central, 6.6 billion solar mass black hole. Images credit: Top, optical, Hubble Space Telescope / NASA / Wikisky, via Wikimedia Commons user Friendlystar; lower left, radio, NRAO / Very Large Array (VLA); lower right, X-ray, NASA / Chandra X-ray telescope.

    Later this decade, an array of radio telescopes — the Event Horizon Telescope — comes online.

    Event Horizon Telescope Array

    Event Horizon Telescope map

    The locations of the radio dishes that will be part of the Event Horizon Telescope array. Image credit: Event Horizon Telescope sites, via University of Arizona at https://www.as.arizona.edu/event-horizon-telescope.

    Arizona Radio Observatory
    Arizona Radio Observatory/Submillimeter-wave Astronomy (ARO/SMT)

    ESO/APEX
    Atacama Pathfinder EXperiment (APEX)

    CARMA Array no longer in service
    Combined Array for Research in Millimeter-wave Astronomy (CARMA)

    Atacama Submillimeter Telescope Experiment (ASTE)
    Atacama Submillimeter Telescope Experiment (ASTE)

    Caltech Submillimeter Observatory
    Caltech Submillimeter Observatory (CSO)

    IRAM NOEMA interferometer
    Institut de Radioastronomie Millimetrique (IRAM) 30m

    James Clerk Maxwell Telescope interior, Mauna Kea, Hawaii, USA
    James Clerk Maxwell Telescope interior, Mauna Kea, Hawaii, USA

    Large Millimeter Telescope Alfonso Serrano
    Large Millimeter Telescope Alfonso Serrano

    CfA Submillimeter Array Hawaii SAO
    Submillimeter Array Hawaii SAO

    Future Array/Telescopes

    ESO/NRAO/NAOJ ALMA Array
    ESO/NRAO/NAOJ ALMA Array, Chile

    Plateau de Bure interferometer
    Plateau de Bure interferometer

    South Pole Telescope SPTPOL
    South Pole Telescope SPTPOL

    With a resolution of 10 micro-arc-seconds (μas), it should see the Milky Way’s supermassive black hole’s event horizon.

    6
    The expected view of the Milky Way’s supermassive black hole through the Event Horizon Telescope. It should be the only one directly visible. Image credit: S. Doeleman et al., via http://www.eventhorizontelescope.org/docs/Doeleman_event_horizon_CGT_CFP.pdf.

    With an angular size of 19 μas, no other black hole appears larger from Earth.

    See the full article here .

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    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
  • richardmitnick 6:44 am on August 23, 2016 Permalink | Reply
    Tags: , , , Black Holes   

    From AAS NOVA: ” When Charged Black Holes Merge” 

    AASNOVA

    American Astronomical Society

    22 August 2016
    Susanna Kohler

    1
    Simulated image of the two merging black holes. New research examines what happens when one of the black holes in a merger is charged. [Cornell SXS Lensing]

    Most theoretical models assume that black holes aren’t charged. But a new study shows that mergers of charged black holes could explain a variety of astrophysical phenomena, from fast radio bursts to gamma-ray bursts.

    No Hair

    The black hole “no hair” theorem states that all black holes can be described by just three things: their mass, their spin, and their charge. Masses and spins have been observed and measured, but we’ve never measured the charge of a black hole — and it’s widely believed that real black holes don’t actually have any charge.

    That said, we’ve also never shown that black holes don’t have charge, or set any upper limits on the charge that they might have. So let’s suppose, for a moment, that it’s possible for a black hole to be charged. How might that affect what we know about the merger of two black holes? A recent theoretical study by Bing Zhang (University of Nevada, Las Vegas) examines this question.

    2
    Intensity profile of a fast radio burst, a sudden burst of radio emission that lasts only a few milliseconds. [Swinburne Astronomy Productions]

    Driving Transients

    Zhang’s work envisions a pair of black holes in a binary system. He argues that if just one of the black holes carries charge — possibly retained by a rotating magnetosphere — then it may be possible for the system to produce an electromagnetic signal that could accompany gravitational waves, such as a fast radio burst or a gamma-ray burst!

    In Zhang’s model, the inspiral of the two black holes generates a global magnetic dipole that’s perpendicular to the plane of the binary’s orbit. The magnetic flux increases rapidly as the separation between the black holes decreases, generating an increasingly powerful magnetic wind. This wind, in turn, can give rise to a fast radio burst or a gamma-ray burst, depending on the value of the black hole’s charge.

    Zhang calculates lower limits on the charge necessary to produce each phenomenon. For a 10-solar-mass black hole, he finds that the merger can generate a fast radio burst if the black hole’s charge is more than ~1012 Coulombs (roughly one billion times the charge that travels through a AA battery from full to empty). If its charge is more than ~1016 Coulombs, it can generate a gamma-ray burst.

    Limits on Charge

    Zhang’s calculations are not just useful in the hypothetical scenario where black holes are charged. They could, in fact, be a way of testing whether black holes are charged.

    As we accumulate future gravitational-wave observations (and with two observations by LIGO already announced, it seems likely that there will be many more), we will grow a larger sample of follow-up observations in radio through gamma-ray wavelengths. Our detections — or our lack of detections — of fast radio bursts or gamma-ray bursts associated with these black-hole mergers will allow us to set some of the first real limits on the charge of black holes.

    Citation

    Bing Zhang 2016 ApJ 827 L31. doi:10.3847/2041-8205/827/2/L31

    See the full article here .

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