Quasar - GRB Oddity

It's been awhile since I've posted anything about some real, solid astronomy. The main reason is that I've been extremely busy this semester (two 500 level classes and a 600 level) and writing on astronomical topics takes a good deal of reading for most topics that I feel like writing about.

And I've done enough research and writing recently. I have a twenty page paper on planetary nebulae due in my astrophysics class on Tuesday. Fortunately, aside from a few revisions, it's mostly finished. I'll post a copy when I'm finished.

But in the meantime, one of my friends pointed me to an article that's currently in pre-print that I found interesting.

Many times in science, some of the most routine investigations can produce the most surprising results. It was the rather innocuous study of the rotation curves that lead to the discovery of dark matter.

In this study a group of astronomers were looking at how many galaxies were along the line of sight from Gamma Ray Bursts (GRBs) and Quasars. Both GRBs and Quasars are thought to be extremely energetic events that are, for the most part, only observed at extremely great distances.

One of the ways that astronomers have detected galaxies that are too faint to discover optically is to look for signs of them in the spectra of these distant objects. Namely, they looked for absorption lines from MgII (ionized magnesium) present in the light from the star.

Those that aren't familiar with astronomical techniques might well ask why MgII just can't be present in the spectra in the first place and that astronomers infer the presence of galaxies. The answer is that due to the quasar or GRB is highly doppler shifted whereas, galaxies that are closer to us are not. At least not to the same extent. Thus, the MgII lines would not be in the correct place in the redshifted spectrum.

So this means that if we look at a distant quasar or GRB, and see MgII lines taken out, we can infer the presence of galaxies in the intervening distance.

Given that quasars and GRBs are thought to both be at cosmological distances (ie, really really far away), we should see roughly the same number of galaxies in the intervening distance.

But according to this study, that's not how things are ending up. It turns out that for some reason, quasars are far less likely to be observed with galaxies along the line of sight. So there's a few possibilities the authors propose to solve this dilemma:

1. The galaxies are just blocking out the lower luminosity quasars, thus producing a selection effect.

2. The MgII lines observed in the GRBs aren't truly caused by intervening galaxies are are somehow tied to the GRBs themselves.

3. Light from the GRBs is being magnified by the galaxies it tends to pass through (an effect known as gravitational lensing).

Fortunately or unfortunately depending on how you look at it, none of these explanations seems to stand above the rest. But that's how science works. It's not looking at things and having the answers immediately. The evidence is never complete, and it's startling discoveries like this that can either add to our understanding or overhaul it altogether.

For those interested, the article can be found: here.