An extended UV disc of NGC 300 ?

About a decade ago (and how scary that I can use that phrase !) extended UV (XUV) discs were the sexy topic in extragalactic astronomy. While the boring old optical data obviously traces light from stars, the high-energy ultraviolet emission only traces the emission from hot, bright stars. Since these only live for a few tens of millions of years, it's a good tracer of ongoing star formation. And since the stellar discs of galaxies are, though they don't have a definite edge, nevertheless reasonably well-defined structures, it came as quite a surprise to find some had very extended UV emission well beyond the stellar disc. How could stars be forming in such a low-density environment, and why weren't there any older stars out there as well ?

For a few years XUV discs provoked a host of interesting questions. Was this UV emission really tracing young stars ? Did this indicate very rapid stellar migration, or were stars actually forming way out there ? Did they form with the same mass distribution as other stars or does the initial mass function (IMF) vary depending on environment ? But somehow interest in the field seemed to die and it never provoked much beyond the initial burst of interest - I don't know why, or if these questions were ever satisfactorily answered.

This paper feels a bit like a blast from the past, or rather a small gust, presenting evidence of another XUV disc around the small spiral galaxy NGC 300. But it doesn't really tackle any of the major questions, and I'm rather skeptical of their main result anyway.

If you simply look at the optical and UV images of NGC 300, there's no obvious major difference between the two (but see below). Instead, to find the possible XUV emission they identify point sources in the GALEX data in two annuli centred on the disc : one close, one further away as their control background field. By taking a colour-magnitude diagram of the objects in the background field, they determine how to remove background objects in their target field.

The problem is that the distribution of both background and foreground candidate objects looks pretty well uniform across both their fields - there's no obvious overdensity associated with the galaxy. Granted, their CMDs do look different, but given that NGC 300 is a big, bright source, I'd expect to see it very clearly in the distribution of point sources. And they don't show the distribution of their candidate sources after removing the background ones, so if the overdensity is weak there's no way to see it.

I got irritated that they didn't present a comparison of the UV and optical, so here's a very quick-and-dirty stab at it - optical (DSS) on the left and GALEX UV on the right, aligned to the same scale :

The colour scheme may be ugly but it's really good for giving high contrast.

Don't be fooled by the similar colour scheme - the sensitivity of each image is probably very different, but it's more work that this post warrants for me to try and do it properly. But there are two reasonably clear points. First, the main disc is of equal size in both images. Second, there are hints of a more extended, irregular component in the UV disc. I tried smoothing the optical image to increase sensitivity, but I didn't find any sign of them. And these are only hints anyway - intriguing, but not compelling. Low surface brightness features can be tricksy.

Slightly more convincing than their contour plots (which don't tell you much since they don't compare the UV and optical) are their surface brightness profiles. These show that the shape of the profile gets flatter (i.e. more extended) at shorter wavelengths going from IR to optical to NUV and FUV. There's a lot of scatter there though so I'm not entirely persuaded. And at much longer radio wavelengths the emission is much flatter again, but that traces the gas rather than the stars so that's not terribly surprising.

So does this galaxy have XUV or not, and if so, why ? Sadly their only comment is that the low density of UV emission beyond the main stellar disc may be due to the lower density of gas, but this isn't very informative. They do a lot of other work looking for correlations between other components, including trying to determine the age of the UV regions, but they don't really exploit this very much. It would be interesting to see if there's a radial age dependence, but they don't plot this.

If the faint streaks visible in the UV image are real, then they don't seem to bear much resemblance to the optical structures. Nor is there any obvious relation to the hydrogen data from radio telescopes, but I had trouble plotting contours of the data cube (not sure why, and I'm not interested enough to investigate). The most I'd be willing to venture is to say there are hints that the UV is more extended than the optical and has a different morphology in the outskirts. But I really don't think we can say any more than that. I would not be so confident as the authors as claiming an XUV detection, though at least someone is working on this stuff again.

Tracing the outer disk of NGC 300: An ultraviolet view

We present an ultra-violet (UV) study of the galaxy NGC~300 using GALEX far-UV (FUV) and near-UV (NUV) observations. We studied the nature of UV emission in the galaxy and correlated it with optical, H~I and mid-infrared (3.6 $μ$m) wavelengths. Our study identified extended features in the outer disk, with the UV disk extending up to radius 12 kpc ($>$2R$_{25})$.