December 17, 2015
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In a previous post we wrote about the morphology of a galaxy’s star light. Most galaxies have matter that we can see in 3 forms: stars, gas, and dust.
Figure 1 shows a picture of M51 at optical wavelengths of light. The yellow, red, and blue parts of the picture are the regions hosting M51’s stars which are visible to us. Along the spiral arms we also see dark structures. The dark parts of the picture are the regions hosting M51’s stars which are invisible to us — the stars which are obscured by dust.
Dust grains in M51 absorb light from these stars and reemit that light at infrared wavelengths. Figure 2 shows a picture of M51 at an infrared wavelength of light. The spiral arms in the infrared picture line up with the dark structure in the optical picture. We know where the dust is in M51. What about the dust in other galaxies?
As we study galaxies that are further and further away from our own, we lose information on where the dust n these galaxies is. Infrared telescopes cannot produce pictures of a distant galaxy at the same resolution as pictures of M51. We can guess at where the dust is by looking at dark structures in pictures at optical wavelengths. Your eye is good at picking out dark structures in a many-color image. What is a dark structure in a two-color image?
Figure 2: M51 at an infrared wavelength of light. Credit: IRSA
Figure 3: a cropped and zoomed-in view of M51 at optical wavelengths of light. Credit: NASA, ESA, S. Beckwith (STScI),and The Hubble Heritage Team (STScI/AURA).
Figure 3 shows a zoomed in picture of M51. The dark structure is black in many places. In many other places the dark structure is adjacent to a red spot, a spot missing blue and yellow colors. Dust grains in M51 are good at obscuring blue and yellow light and less good at obscuring red light. If we measure the brightness of a spot in a red image, and the brightness at the same location in a blue image, many galaxies will have the same ratio between those brightnesses. The ratio comes from two aspects of the dust: the sizes of the grains and the number of grains. A dark structure in a distant galaxy might be a red spot with a weak blue spot.
The red color in the image of M51 is due to light from Hydrogen atoms. The Hubble Space Telescope [HST] has an instrument allowing us to see the light from Hydrogen atoms in distant galaxies; it has another instrument allowing us to see blue light from distant galaxies.
I wrote a paper using CANDELS data to compare the brightnesses of the light at the two wavelengths. We conclude that we need more data!
The ratio of brightnesses between red spots and blue spots for distant galaxies is different from the ratio for local galaxies. Dust grains in distant galaxies might have different sizes compared to their sizes in M51, which would make them more or less good at obscuring red light compared to how they obscure blue and yellow light. We cannot distinguish between this hypothesis and the one saying that the number of grains differs.
NASA has a plan to launch several telescopes into space and connect them, which would solve the problem of resolution that prevents us from having detailed pictures at infrared wavelengths of distant galaxies. You can find out more about the Far-IR Surveyor here:
See the full article here .
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About the CANDELS blog
In late 2009, the Hubble Space Telescope began an ambitious program to map five carefully selected areas of the sky with its sensitive near-infrared camera, the Wide-Field Camera 3. The observations are important for addressing a wide variety of questions, from testing theories for the birth and evolution of galaxies, to refining our understanding of the geometry of the universe.
This is a research blog written by people involved in the project. We aim to share some of the excitement of working at the scientific frontier, using one of the greatest telescopes ever built. We will also share some of the trials and tribulations of making the project work, from the complications of planning and scheduling the observations to the challenges of trying to understand the data. Along the way, we may comment on trends in astronomy or other such topics.
CANDELS stands for the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey. It builds on the legacy of the Hubble Deep Field, as well as the wider-area surveys called GOODS, AEGIS, COSMOS, and UKIDSS UDS. The CANDELS observations are designed to search for galaxies within about a billion years of the big bang, study galaxies at cosmic high-noon about 3 billion years after the big bang – when star-formation and black hole growth were at their peak intensity – and discover distant supernovae for refining our understanding of cosmic acceleration. You can find more details, and download the CANDELS data, from the CANDELS website.
You can also use the Hubble Legacy Archive to view the CANDELS images.