Friday, May 26, 2006

Scientific American decries the gutting of NASA science

Scientific American's blog has a nice(ish) blog article about how NASA's budget woes are disemboweling the science program. Slightly old, but worth a read. One thing I'd grumble about is their example of how (non-solar system) astronomy is being hurt: TPF (terrestrial Planet Finder) being put on hold. TPF was not the highest priority mission to be mothballed, Constellation-X was, and in fact the whole Beyond Einstein initiative is in real danger. I'll post a rant about the shoddy TPF goings-on, and why I think TPF frankly isn't as much of a "most do" as Constellation-X, at some point in the future.

For some background to the cuts see the House Science Committee Democrats letter of 2006 May 11. The sad thing is only small amounts of money are required to prevent/roll back these cuts, small compared to the $250 million per week a certain unnecessary war is costing. Congress is penny-wise and pound foolish.

Busy busy busy

Well, I'm recovering from my deep disappointment at not finding out the true shape of the galaxy LF, but posting to the blog will be light as I'm busy preparing my talk for the Calgary AAS meeting - specifically the Warm-hot gas in and around Disk Galaxies topical session, at which I [unfortunately] speak near the end of the day.

Update: fixed link

Wednesday, May 24, 2006

Galaxy luminosity functions from 6dF: Schechter fit not ideal

Jones et al (2006, MNRAS, 369, 25) present optical and near-IR galaxy luminosity functions from a flux-limited sample of 138226 galaxies from the 6-degree Field Galaxy Survey. Here are the interesting results:

"a Schechter function is unable to decline rapidly enough at the bright end and remain as flat at the faint end [of the galaxy LF]... We do not find as steep a faint-end slope as Kochanek et al. (2001), and suspect this is due to their shallower depth and subsequent brighter faint-end limit. The 6dFGS rF-band LF most closely matches those of the Las Campanas Redshift Survey (Lin et al. 1996) and SDSS (Blanton et al. 2005), although we find only marginal evidence for the faint-end upturn claimed by Blanton et al. (2005). In bJ, the 6dFGS LF has a nearly identical faint-end slope to those obtained by 2dFGRS (Norberg et al. 2002) and the ESO Slice Project (Zucca et al. 1997), although the 6dFGS normalization is closer to that found by Blanton et al. (2005). Neither this survey nor 6dFGS used evolutionary corrections. Furthermore, we see no evidence for a faint-end upturn in any of the 6dFGS LFs. "
The faintest and brightest ends of the LF are where galaxy formation models have the most difficulty, and where "feedback" (either from supernovae or AGN) is often invoked as the solution. The figure below is Fig 10 from the Jones et al paper.

Figure 10. LFs for the 6dFGS, derived from the 1/Vmax (green open circles), SWML (red solid circles) and STY methods (blue dashed curve). The inset shows the 1, 2 and 3σ confidence contours of the STY fit. The upper panel shows the 1/Vmax and SWML residuals relative to STY (i.e. the deviations from the best-fitting Schechter function).

Jones, D. Heath, Peterson, Bruce A., Colless, Matthew & Saunders, Will
Near-infrared and optical luminosity functions from the 6dF Galaxy Survey.
Monthly Notices of the Royal Astronomical Society 369 (1), 25-42.
doi: 10.1111/j.1365-2966.2006.10291.x

Its nice to see we don't have to worry about large numbers of faint galaxies. But it doesn't appear to answer the question I was most interested in: what is the actual shape of the LF if its not a Schechter function?

What fuels star formation in galaxies? (part 1)

I've only just realized that back in when I did Physics with Astrophysics as an undergraduate student in Brum (1991-1994, in case you were wondering) we never touched on what fuels star formation in galaxies. Star formation requires interstellar gas (specifically cold dense molecular gas), and while we touched on the standard Jeans requirements for cloud collapse we never seem to have addressed the issues (a) where does the gas come from, and (b) what controls the star formation rate in a galaxy with a given amount of gas?

Back then we were taught about the monolithic collapse scenario for the formation of elliptical galaxies, and that spiral galaxies formed stars on a longer timescale, specifically a time scale long enough for their gas to have collapsed into a disk. Mergers may have been hot in the literature, but they weren't yet part of the intro astro courses I attended. A few years later and mergers were the shiznit (so to speak). Certainly I came away from conferences and the literature with the idea that it was mergers all the way down (i.e. galaxy evolution depended primarily on the merger history, and all growth was due to the accretion of satellite galaxies). The monolithic collapse folks were experiencing a hard time at conferences.

This is a lengthy introduction to a small series of posts that will discuss a recent paper by Keres et al (2005, MNRAS 363, 2) entitled "How do galaxies get their gas?", which is the culmination of work previously discussed in less detail by the authors in various conference proceedings over the last few years.

Tuesday, May 23, 2006

No more numarray

Python coding is hardly an auspicious topic on which to start a blog, but everything has to begin somewhere. I've finally fixed my python-based pgplot fits image plotter code to use Numeric and Andrew William's modules, replacing numarray and the STScI pyfits modules. The step back to Numeric was forced by pyhdf relying on Numeric, but I'm actually quite pleased with how things turned out. The image transpose weirdness I had to use with pyfits and pgplot is gone, so I no longer have to worry about the fortran vs C array order issues (except in pyfits). Its a pity the ST folks couldn't have made pyfits backwards compatible with Numeric, but I guess that is related to the whole Numeric vs numarray split.