by Sarah Scoles

Let's talk about extragalactic radio sources.

The things that we can see emitting radio waves outside of our galaxy are other galaxies, which can be divided up into "ones that have black holes that are feeding" and "ones that are forming stars."

Here is a picture of what the sky looks like in radio waves, as pictured by the 300-foot telescope (RIP) in Green Bank. The smudgy objects are inside the Milky Way (like supernova remnants), and the tiny white dots are not stars, like they would be in an optical picture: each one is a whole other galaxy.

Notice, though, that the blank sky--the parts of the image that don't contain discrete dots--isn't 100% dark.

What an astute observation, right?

It turns out that the question "How not-dark is that not-dark sky?" can shed light on the content and evolution of the universe.

A map of the background brightnesses as measured by the VLA. In other words, a map of confusion. It's a real beauty, isn't it? (Condon, et al., 2012)First, why is it so bright and cheerful out there?

In short, because most of the universe is really, really far away. So far away that many extragalactic galaxies aren't resolved into individual galaxies. Their light just contributes, diffusely, to the general background glow.

It's like when you look up at the sky and see the band of the Milky Way--it looks hazy and cloudy, but definitely white like light. That's because your eye can't resolve the distant stars into individual ones--the pixels your eyes make are bigger than the size of one of those stars, so each eye-pixel contains two, three, four, 100 stars. It's the same for distant radio galaxies.

When this happens in astronomy, it's called "confusion," as the sources cannot be separated from each other.

How not-dark is that not-dark sky?

Well, as usual, it depends on who you ask. If you ask the people who sent a balloon called ARCADE2 into space, they'll say it's about five times brighter than if you asked people who used the VLA, like Jim Condon (and collaborators), who have a forthcoming paper called "Resolving the Radio Source Background: Deeper Understanding through Confusion." Condon, et al., would say that ARCADE2 is probably wrong, even if it is a balloon.

Condon's paper asserts that his observations and analysis have resolved 96% of the radio background into the expected two types of galaxies.

But both the VLA's and ARCADE2's measurements exist, so the instruments/analyzers either have to be flawed, or there has to be some un-accounted-for source of background radio waves.

How do you tell what's in the background?



In this paper, Condon and collaborators set out to determine how many sources--galaxies both active and star-forming--it would take to produce the background seen in the previous two images. The paper goes into detai This is an evolutionary model (cafepress.com).l about the statistical methods used to get source counts--specifically, using probability distributions--, but the basic summary is

You can measure some flux density (or brightness) from the background. You have to figure out how many, and which different, galaxies' radio waves had to combine to produce that flux density. You have to figure out the relationship between "some amount of brightness" and "some number of galaxies" in "some part of the sky." This is complicated because Galaxies are not all the same distance from us. So we get fainter radio waves from a farther-away galaxy than we do acloser one, even though they each still count as one galaxy. The farther away the galaxy is, the farther back in the universe's history you're looking (if you're looking at a galaxy 5 billion light-years away, you're looking 5 billion years in the past, because that's how long the light took to get to us). Galaxies have changed over time, so the amount of radio waves galaxies emit has changed over time. You gotsta take evolutionary models into account.

So what's the word?

Well, here's the plot:

The most important parts of this plot are the longer-dashes dashed line and the smaller-dashes dashed line. The longer dashes represent the contribution that active galaxies should make toward the background; the shorter one represents the contribution that star-forming galaxies should make toward the background. The solid line is the result from the Condon paper, which looks like a fair combination of the dashed lines. This plot, essentially (in spite of and because of the long, parentheses-heavy axis labels) shows how many individual sources (vertical height) there are that each put out a certain brightness of radio waves. (horizontal position).

So, then, the large bump toward the right side of the plot shows that there are lots of sources that, as individuals, are pretty bright; these are active galaxies. And the small bump toward the left shows that there are fewer sources that are, as individuals, pretty dim; these are star-forming galaxies.

But wait, there's more controversy.

You may have noticed that the magenta counts from Owen&Morrison (2008) are much higher than other results, specifically "This Paper's." Owen&Morrison's count is 4 times This Paper's. Why?

Well, the answer that the Condon paper gives is a lovely barrage of questions, rhetorically unusual for a journal article:

What might cause this discrepancy and, for that matter, the surprisingly large scatter among all published faint-source counts? Are most of the faint Owen & Morrison sources spurious, or did we miss a large fraction of real sources that Owen & Morrison counted? The Owen & Morrison image has much higher [angular resolution] than our...resolution and the Mitchell & Condon resolution. How often have we blended into one source" what Owen & Morrison resolved into two or more sources?...We suspect that most of the count difference is caused by count corrections made for partial resolution of extended sources in the high-resolution 1.4 GHz beam.

To add to and summarize: Based on the apparent number of faint extragalactic radio sources, we know that in the past there were more radio sources than there are now. But to tell these faint sources, which are also small-looking to us because we are so far away, apart, you have to make the field-of-view (beam) of your telescope smaller and smaller. Which essentially means using a bigger telescope. But you can make your beam so small that you only see part of a galaxy, so then you only measure part of its brightness, so then you have to make corrections.

But sometimes you correct incorrectly, leading to an increased estimate of sources. In the Condon versus Owen&Morrison smackdown, that's the punch Condon is throwing.

And what about ARCADE2's brighter-background-than-expected measurements?

One possible explanation is the existence of a new population of faint extragalactic sources.

What?! New population?! I love new populations.

It appears that the smooth ARCADE2 background cannot be produced by galaxies or by objects located in or near individual galaxies...or by star-forming galaxies that are more radio-loud than expected...If [long detailed conditional statement], a very numerous (N > 1013 over the whole sky) and unexpected population of radio sources not associated with galaxies has been discovered.

But is a big number, and 1013 some are dubious that we would have missed that many of something.

So what conclusive results did Condon, et al., gain in their "deeper understanding through confusion" paper?

The number of galaxies needed to account for the radio background is the same as the number required to explain the infrared background.

63% of the radio background is due to active galaxies with feeding black holes.

37% of the radio background is due to spiral galaxies forming stars.

Though it is more likely that ARCADE2's measurements are off somehow than that we haven't discovered a whole population of new, non-galaxy objects, the latter possibility remain.

Why care?

It's interesting to know how many other galaxies are out there, and we can't find that out unless we find out how many contribute to the radio background. Anything that gives us more of a sense of our insignificance is worthwhile, in my opinion.

So the next time you go outside, think about how you're being bathed in the gentle glow of distant galaxies, ones with activity going on, and some of that activity is the kind that led to the Sun's birth, and, much later, our own.

I would have guessed that star-forming galaxies contributed 39% of the background. Huh.