Shining a Light on Massive Galaxies

Ellipticals are the class containing the Universe’s most massive galaxies. In the local universe, these galaxies have tens of billions of solar masses of stars, but are also usually inactive – that is to say, not forming any new stars. Given that star formation seems to have ceased in these giants, some big questions remain: what caused the star formation to end, and how did these galaxies get so big in the first place? This paper seeks to at least start investigating these questions by looking at galaxies in the high redshift (z~2) universe that will probably evolve into modern-day elliptical galaxies.


This image shows a background quasar spectrum being used to probe the gas in a foreground luminous red galaxy. Despite the quasar being much farther away from us, the two appear close on the sky, and thus the light from the quasar passes through the gas surrounding the foreground galaxy, causing absorption lines in the quasar spectrum. Image from

If you’re not an astronomer, that last sentence may have confused you. What does it mean to look at a galaxy that will eventually be something we see today? The answer has to do with a common strategy in astronomy. The Universe evolves on a grand scale – structures like galaxies take billions of years to form, and most timescales in astronomy are much greater than a human lifespan. However, because light has a finite speed, it takes time to get to us. Sometimes billions of years. So by looking at objects that are very distant (high redshift) we can get an idea of what things that we see nearby may have looked like billions of years earlier.

Now on to the paper. Quasars are a special type of active galactic nucleus – the region in the center of a galaxy associated with energy emitted from the accretion disk around a black hole. What makes quasars special is that they are incredibly bright, so we can see them out to vast distances. As such, they provide a lighthouse-like beam that illuminates anything in the path between the galaxy hosting the quasar and us. The authors of this paper take advantage of that fact by using background quasars to study foreground objects. To do this, they find pairs of quasars that are in close proximity on the sky, despite the fact that the quasar in the foreground galaxy may be much closer to us than the background one. The distance between the sight line to the background quasar and the center of the foreground galaxy is called the impact parameter. See the figure at right.

The spectrum of the foreground quasar gives the authors a precise measurement of the distance to the galaxy it is in. The authors can then look for characteristic absorption lines in the spectrum of the background quasar at a location that indicates that the material doing the absorbing is actually gas surrounding the foreground galaxy. Thus, by getting a large sample of quasar pairs that cover a range of impact parameters, the authors can probe the regions around the foreground galaxies. Because these galaxies host quasars, we know that they are among the most massive in the Universe at that time, and thus, we are essentially taking a look at the conditions surrounding the systems that will become the most massive galaxies today.

This figure compares the equivalent widths of several absorption lines, as a function of impact parameter, for the quasars in this study (black points), with Lyman Break Galaxies at a similar redshift, and with Milky-Way type galaxies in the local universe.


So what do the authors find? After combining the spectra from a sample of 74 quasar pairs with impact parameters ranging from around 50 kiloparsecs (kpc) to about 300 kpc, the authors can pull out some key information. First, they find that there is a large quantity of cool (10^4 K) gas surrounding most of these galaxies that drops off with radius. This is a surprising result, because current theory predicts that much of the gas around these massive systems should be hot (10^7 K), and here’s why. The galaxies hosting these quasars sit in the center of massive gravitational potential wells. As the gas falls into this well, it should shock to a very high temperature when it gets to the virial radius (in this case ~160 kpc). Therefore, the presence of a large amount of cool gas around these systems, if corroborated in other studies, may indicate a the need for a big shift in the current formation theories for these most massive systems, and a fundamental difference with observations of smaller systems at around the same time.

The latter point is made in the plot on the right. This plot shows, as a function of impact parameter on the x-axis, the equivalent widths of the three absorption lines used in the study: Lyman alpha, CII, and CIV. Essentially, a larger equivalent width means more gas. The authors plot both the binned points from their quasar study (black points), as well as points from studies of Lyman Break Galaxies at similar redshift (red points), and galaxies about the size of the Milky Way in the local universe (blue points). As the plots show, not only does the amount of gas drop off with radius, but there is around twice as much surrounding the quasar hosts in comparison to the other samples. The trick now is for theorists and observers to figure out why these massive systems appear to have so much cool gas surrounding them, and what shuts off the flow of that gas, since we no longer observe it around massive systems today.

About Evan Schneider

I am a graduate student at the University of Arizona working on high resolution simulations of galactic winds, run with my new hydrodynamics code, Cholla. When I'm not doing astronomy, I enjoy essentially any outdoor activity, including hiking, rock climbing, and walking my dogs. On indoor days I can often be found reading, and of course, drinking coffee.

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