Formation of late-type spiral galaxies: Gas return from stellar populations regulates disk destruction and bulge growth.

In astro-ph/0911.0891, Marie Martig and Frederic Bournaud report on the growth of bulges in disk like galaxies in a cosmological environment. The zoom in on a Milky-Way like halo in cosmological box that had a quiet merger history, to make it prone to disk formation. They include baryonic physics, including star formation, but excluding supernova feedback. In one simulation they add the mass loss of older stellar populations in a relatively simple way. They let the stars loose an amount of mass that is typical for a Salpeter IMF (~45% of the SSP mass is returned in total). This lost gas mass adds to the disk and makes disk survival (and a smaller bulge fraction) a lot easier. The disk becomes more stable to both internal instabilities and to minor mergers.

Clustered star formation as a natural explanation of the Halpha cutoff in disc galaxies

From Pflamm-Altenburg and Kroupa (arXiv:0905.0898v1)
Caption reads:
"The Hα-luminosity surface density versus the total gas surface density observed for seven disc galaxies15 averaged over annuli at different galactocentric radii is plotted (black squares) after correcting for photon leakage from H ii regions (see Supplementary Discussion). These galaxies have a mean star formation rate of SFR=6.9 M⊙ yr−1 (3.2 – 16.4 M⊙ yr−1 )2, 15 , a mean total gas mass of Mgas = 2.1 · 1010 M⊙ (0.6 – 3.6 · 1010 M⊙ )2, 15 and a mean scale length of rd = 4.4 kpc (3.9 – 5.2 kpc)25–28 . These mean values define our model standard disc galaxy. For a choice of γ = 2 the LIGIMF-theory predicts an ΣHα -Σgas relation which matches the observations excellently (solid line). Note that the underlying true star-formation density as derived from UV observations1 is directly proportional to the gas surface density (N = 1) and is shown after converting it into an Hα surface luminosity using the wrong linear Kennicutt Hα-SFR relation2, 29 (dashed line) and shows the expected ΣHα -Σgas relation based on the classical picture which is in disagreement with the observations."
Basically, since stars form in clusters, and because you have lower mass star clusters at lower densities, you expect relatively less massive stars and therefore a Hα cutoff

Reconstruction of a z=3.07 lensed galaxy

from Stark et al., http://arxiv.org/abs/0810.1471

This Nature paper describes integral field (OSIRIS on Keck) observations of a strongly lensed z=3.07 Lyman break galaxy. With AO corrections and the high magnification, these observations provide an effective physical resolution of 150 kpc. This figure shows (a) the reconstructed HST image with the lensing caustic overlaid, (b) [OIII] and Hbeta emission (bluescale and contours respectively), (c) velocity field and best-fit disk model, (d) [OIII] velocity dispersion, (e-h) 1-d profiles of the left panels taken along the "slit" shown in panel (d). The velocity field is well-fit by a disk model, so the authors conclude that this is a disk galaxy with v_r=67 km/s and M=2e9 Msun. However, the central velocity dispersion is large (v/sigma=1.2), so it's most likely still at an early stage of formation.