Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Effects of Photoionization on Galaxy Formation   We investigate the effects of the observed UV background radiation on galaxy
formation. Photoionization by UV radiation decreases the cooling rate of the
gas in halos, so that objects with only large density contrasts can self-shield
against the background radiation, thereby allowing the shielded neutral cores
to cool and form stars. In the context of the CDM model, we use the criterion
that self-shielding is essential for star formation to calculate the mass
function of galaxies based on both Press-Schechter and the peaks formalisms.
The ionizing UV radiation results in the inhibition of galaxy formation --- we
show the decrease in the number density of the galaxies quantitatively. We also
find that the merging in general is made inefficient by the UV photons through
photoionization of the gas in the bigger system into which small objects are
incorporated. The latter means that, in a merging dominated region, where the
number density at the low-mass end ($M_b \la 10^{10}M_{\odot}$) is usually
expected to decrease with time, the trend is {\it reversed} (the number of
low-mass galaxies is more at lower redshifts $z$) due to the decreasing UV flux
with time after $z \sim 2$. We further discuss the implication of our results
for the number counts of galaxies and possible evolution of the luminosity
function of the galaxies.

31
The categories of this paper are: ao-sci
3
The categories of this paper are: physics
19
The categories of this paper are: eess
10
The categories of this paper are: q-bio
32
The categories of this paper are: atom-ph
21
The categories of this paper are: adap-org
18
The categories of this paper are: econ
36
The categories of this paper are: cmp-lg
28
The categories of this paper are: alg-geom
2
The categories of this paper are: math
1
The categories of this paper are: cs
15
The categories of this paper are: nucl-ex
20
The categories of this paper are: acc-phys
30
The categories of this paper are: q-alg
12
The categories of this paper are: nucl-th
4
The categories of this paper are: cond-mat