Gravitationally Lensed Dusty Galaxies
[Words by Justin Spilker]
About half of the light from newborn stars is invisible to human eyes - it gets absorbed by interstellar dust between the young star and us, and re-emitted at wavelengths much longer than our eyes can see. Some of the most intense star formation in the history of the universe takes place in galaxies that existed when the universe was about 1/4th of its current age, making new stars hundreds or thousands of times faster than our own Milky Way galaxy does today. These galaxies contain vast amounts of dust, but are extremely faint due to their great distances from us. One way of getting around this problem is to take advantage of a natural phenomenon called gravitational lensing - if there is another galaxy between us and the distant, dusty galaxy we're interested in, the intervening galaxy distorts and magnifies the light of the background galaxy. This makes the background galaxy appear much brighter than it actually is, and allows us to get better spatial resolution images essentially for free, letting us study the dusty galaxies in more detail than otherwise possible.
Steward graduate student Justin Spilker, Steward professor Dan Marrone, and their colleagues used the Atacama Large Millimeter/submillimeter Array (ALMA), a new observatory high in the Chilean Andes, to observe about 50 gravitationally lensed dusty galaxies. ALMA is very sensitive to the re-emitted light coming from dusty galaxies, and uses 66 individual telescope dishes working together to create the highest resolution, most sensitive images ever made at the wavelengths it observes. We used the telescope while it was still under construction, with "only" 14-25 dishes in operation, but this was already enough to make impressive images of our target galaxies. Using an advanced modeling technique to account for the distortions caused by gravitational lensing, we recovered the intrinsic properties of these dusty galaxies, which are magnified by about a factor of 6 on average, and up to a factor of 30. Importantly, we found that there is no noticeable difference between the intrinsic sizes of our lensed galaxies and similar galaxies which are not lensed - in other words, the results are not biased by looking at lensed objects. Lastly, we looked at how the brightness of a spectral emission line of ionized carbon - one of the most important but enigmatic ways for the gas in galaxies to cool - compared across our sample of distant galaxies and very nearby galaxies. The brightness of this line is closely related to the amount of dust emission per unit area (the dust surface brightness). These objects lie on exactly the same relationship as nearby galaxies. Whatever the physics are that drive the emission of this carbon line, it's clear that the compactness of the galaxies plays an important role in setting the relationship.
All four figures show ALMA data in red and HST data in blue. ALMA shows the background galaxies while HST shows the foreground lens galaxies. The three photos not used as the cover photo can be seen HERE, HERE, and HERE.
This research was also featured in the National Radio Astronomy Observatories monthly newsletter.
Photos courtesy of Justin Spilker.
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