From Harvard-Smithsonian Center for Astrophysics: “Dark Matter and Massive Galaxies”

Harvard Smithsonian Center for Astrophysics

From Harvard-Smithsonian Center for Astrophysics

March 6, 2020

A dark matter map, created by Japanese astronomers using weak lensing.
The background image of a wide field of galaxies was analyzed for weak lensing effects and the inferred dark matter distribution is indicated with the contours. Satoshi Miyazaki.

About eighty-five percent of the matter in the universe is in the form of Dark Matter, whose nature remains a mystery, and the rest is of the kind found in atoms. Dark matter exhibits gravity but otherwise does not interact with normal matter, nor does it emit light. Astronomers studying the evolution of galaxies find that because it is so abundant dark matter does, however, dominate the formation in the universe of large-scale structures like clusters of galaxies.

Despite being hard to detect directly, dark matter can be traced by modeling sensitive observations of the distributions of galaxies across a range of scales. Galaxies generally reside at the centers of vast clumps of dark matter called haloes because they surround the galaxies. Gravitational lensing of more distant galaxies by foreground dark matter haloes offers a particularly unique and powerful probe of the detailed distribution of dark matter.

Gravitational Lensing

Gravitational Lensing NASA/ESA

“Weak lensing” results in modestly yet systematically deforming shapes of background galaxies and can provide robust constraints on the distribution of dark matter within the clusters; “strong lensing,” in contrast, creates highly distorted, magnified and occasionally multiple images of a single source.

In the past decade, observations and hydrodynamic simulations have significantly furthered our understanding of how massive galaxies develop, with a two-phase scenario now favored. In the first step, the massive cores of today’s galaxies form at cosmological times from the gravitational collapse of matter into a galaxy, together with their surrounding dark matter halo. Star-formation then boosts the stellar mass of the galaxy. The most massive galaxies, however, have a second phase in which they capture stars from the outer regions of other galaxies, and once their own star formation subsides this phase dominates their assembly. Computer models and some observational results appear to confirm this scenario.

CfA astronomer Joshua Speagle was a member of a team that used ultra-sensitive, wide-field-of-view imaging at optical and near infrared wavelength on the Subaru telescope to study massive galaxy assembly.

NAOJ/Subaru Telescope at Mauna Kea Hawaii, USA,4,207 m (13,802 ft) above sea level

Their technique took advantage of weak lensing effects because massive galaxies also tend to have more massive, dark matter haloes that distort light. The astronomers studied about 3200 galaxies whose stellar masses are more than that of the Milky Way (roughly about four hundred billion solar masses). Using weak lensing analyses, they found that information about the assembly history of massive dark matter halos is encoded in the stellar mass distributions of massive central galaxies. Among other implications, the scientists show that for galaxies of the same mass, those with more extended shapes tend to have more massive dark matter halos. The results open a new window for exploring how massive galaxies form and evolve over cosmic time.

Science paper:
Weak Lensing Reveals a Tight Connection between Dark Matter Halo Mass and the Distribution of Stellar Mass in Massive Galaxies


Dark Matter Background

Fritz Zwicky discovered Dark Matter in the 1930s when observing the movement of the Coma Cluster., Vera Rubin a Woman in STEM denied the Nobel, did most of the work on Dark Matter.

Fritz Zwicky from http://

Coma cluster via NASA/ESA Hubble

In modern times, it was astronomer Fritz Zwicky, in the 1930s, who made the first observations of what we now call dark matter. His 1933 observations of the Coma Cluster of galaxies seemed to indicated it has a mass 500 times more than that previously calculated by Edwin Hubble. Furthermore, this extra mass seemed to be completely invisible. Although Zwicky’s observations were initially met with much skepticism, they were later confirmed by other groups of astronomers.

Thirty years later, astronomer Vera Rubin provided a huge piece of evidence for the existence of dark matter. She discovered that the centers of galaxies rotate at the same speed as their extremities, whereas, of course, they should rotate faster. Think of a vinyl LP on a record deck: its center rotates faster than its edge. That’s what logic dictates we should see in galaxies too. But we do not. The only way to explain this is if the whole galaxy is only the center of some much larger structure, as if it is only the label on the LP so to speak, causing the galaxy to have a consistent rotation speed from center to edge.

Vera Rubin, following Zwicky, postulated that the missing structure in galaxies is dark matter. Her ideas were met with much resistance from the astronomical community, but her observations have been confirmed and are seen today as pivotal proof of the existence of dark matter.

Astronomer Vera Rubin at the Lowell Observatory in 1965, worked on Dark Matter (The Carnegie Institution for Science)

Vera Rubin measuring spectra, worked on Dark Matter (Emilio Segre Visual Archives AIP SPL)

Vera Rubin, with Department of Terrestrial Magnetism (DTM) image tube spectrograph attached to the Kitt Peak 84-inch telescope, 1970.

The Vera C. Rubin Observatory currently under construction on the El Peñón peak at Cerro Pachón Chile, a 2,682-meter-high mountain in Coquimbo Region, in northern Chile, alongside the existing Gemini South and Southern Astrophysical Research Telescopes.

LSST Data Journey, Illustration by Sandbox Studio, Chicago with Ana Kova

See the full article here .

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