Friday, 20 February 2009

A role for self-gravity at multiple length scales in the process of star formation

Alyssa A. Goodman1,2, Erik W. Rosolowsky2,3, Michelle A. Borkin1,5, Jonathan B. Foster2, Michael Halle1,4, Jens Kauffmann1,2 & Jaime E. Pineda2

Nature 457, 63-66 (1 January 2009)

Fig: Observation and simulation of molecular cloud L1448. Most of the emission in the L1448 region is contained with large-scale self-gravitating structures, but only a low fraction of small-scale objects show signs of self-gravitation. In the L1448 observations, gravity is significant on all scales, but not in all regions. In contrast, the simulated map implies that nearly all scales, and all regions, should be influenced by gravity (which was ignored in the simulation).

Self-gravity plays a decisive role in the final stages of star formation, where dense cores (size 0.1 parsecs) inside molecular clouds collapse to form star-plus-disk systems1. But self-gravity's role at earlier times (and on larger length scales, such as 1 parsec) is unclear; some molecular cloud simulations that do not include self-gravity suggest that 'turbulent fragmentation' alone is sufficient to create a mass distribution of dense cores that resembles, and sets, the stellar initial mass function2. Here we report a 'dendrogram' (hierarchical tree-diagram) analysis that reveals that self-gravity plays a significant role over the full range of possible scales traced by 13CO observations in the L1448 molecular cloud, but not everywhere in the observed region. In particular, more than 90 per cent of the compact 'pre-stellar cores' traced by peaks of dust emission3 are projected on the sky within one of the dendrogram's self-gravitating 'leaves'. As these peaks mark the locations of already-forming stars, or of those probably about to form, a self-gravitating cocoon seems a critical condition for their existence. Turbulent fragmentation simulations without self-gravity—even of unmagnetized isothermal material—can yield mass and velocity power spectra very similar to what is observed in clouds like L1448. But a dendrogram of such a simulation4 shows that nearly all the gas in it (much more than in the observations) appears to be self-gravitating. A potentially significant role for gravity in 'non-self-gravitating' simulations suggests inconsistency in simulation assumptions and output, and that it is necessary to include self-gravity in any realistic simulation of the star-formation process on subparsec scales.

Explaining Lya Blobs as Cold Streams of Gas in the Halos of Galaxies

There have been several proposed mechanisms to power the extended diffuse Lyman-alpha emission (Lyman-alpha blobs, or LABs) that is sometimes associated with massive and active galaxies at high redshift.  One idea is that the we are observing cooling radiation from gas that is accreting onto the galaxies.  Dijkstra and Loeb endorse this idea, and specifically associate the radiation with cold accretion.

In their model, some fraction of the gravitational energy is converted into heat via weak shocks, which leads to the Ly-a emission.  By taking the cold gas fraction that comes from simulations, and putting in a few additional parameters, they are able to calculate the Ly-a emission as a function of halo mass.  This leads to the above figure.  The curves are different model predictions for how biased a region of space was probed by the survey that led to those data points (are there no good luminosity functions available for LABs?), so I think the comparison of the models and data is illustrative only.

Connecting LBGs to DRGs

Figure 12 from Stark et al.,

This paper presents an analysis of Lyman break galaxy candidates at
z=4, 5, and 6 (B, V, and i-dropouts respectively) from the GOODS
fields. In these fields they find 2443 B, 506 V, and 137 i dropouts;
reliable Spitzer data are available for about 35% of them, and they
use the Spitzer data to estimate stellar masses. The above figure
shows the number density of LBGs above log(M)=11 compared to the
Kriek et al. (2008) "red sequence" galaxies at z=2.3 (shown as the
red asterisk). Given the prevalence of massive star-forming galaxies
at higher redshifts, it appears plausible that these could account
for a significant fraction (the authors quote 50%) of the z=2
quiescent galaxy population.

Friday, 13 February 2009

Clustering of DRGs

Jeremy Tinker, Risa Wechsler and Zheng Zheng

We're right and Ryan is wrong.

A downturn in intergalactic CIV as redshift 6 is approached

From Ryan-Weber et al. ( Caption reads:
"Figure 5. Cosmological mass density of C IV as a function of redshift. The blue squares show the measurements by Songaila (2001), the red triangle is from Pettini et al. (2003), and the green triangle is the value deduced here. All values plotted have been reduced to the ‘737’ cosmology adopted in the present work. Error bars are 1-sigma. While this plot shows the actual values of C Iv measured, they are not strictly comparable because each of the three surveys had a different sensitivity limit. This issue is discussed in detail in the text (section 5)."

The authors claim that the this downturn of metals implies a deficiency of ionizing photons at high-z (that is, there would not be enough photons to keep the universe ionized).