Thursday, July 4, 2013

1307.0943 (Daniel Ceverino et al.)

Radiative feedback and the low efficiency of galaxy formation in low-mass halos at high redshift    [PDF]

Daniel Ceverino, Anatoly Klypin, Elizabeth Klimek, Sebastian Trujillo-Gomez, Christopher W. Churchill, Joel Primack
Any successful model of galaxy formation needs to explain the low rate of star formation in the small progenitors of today's normal galaxies. The low efficiency of star formation is necessary for reproducing the low stellar-to-halo mass fractions, as suggested by current abundance matching models. We found that the main driver of this low efficiency is the radiation pressure exerted by ionizing photons from massive and young stars. We model the effect of radiation pressure in cosmological, zoom-in galaxy formation simulations, as a non-thermal pressure that acts locally around dense and optically thick star-forming regions. We also include the effect of photoionization and photoheating on the gas cooling and heating rates. In some conditions, the full photoionization of HI reduces the HI peak of the cooling curve, effectively preventing cooling in the 10^4-10^4.5 K regime. We also consider a simple model for the boosting of radiation pressure due to the trapping of infrared radiation. The main effect of the local UV radiation is to regulate and limit the high values of the gas density distribution and the amount of gas available for star formation. Radiation pressure is the main driver that regulates star formation, stellar and galaxy growth in halos of masses around 10^11 Msun at high redshifts, z=3. Infrared trapping and photoionization/photoheating processes are secondary effects in this mass range. The typical galaxies residing in these halos contain only 0.6% of the total virial mass in stars and form stars at a small rate of 1 Msun/yr at z=3, roughly consistent with current abundance matching models. The effect of the local UV radiation ultimately translates into a lighter and more extended galaxy and it yields a raising circular velocity profile.
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