2,201 research outputs found
The LBDS Hercules sample of mJy radio sources at 1.4 GHz - II. Redshift distribution, radio luminosity function, and the high-redshift cut-off
{Abridged} A combination of spectroscopy and broadband photometric redshifts
has been used to find the complete redshift distribution of the Hercules sample
of millijansky radio sources. These data have been used to examine the
evolution of the radio luminosity function (RLF) and its high-redshift cut-off.
New redshifts have been measured for eleven sources, and a further ten upper
limits are given. The total number of sources with known redshifts in the
sample is now 47 (65%). We calculated broadband photometric redshifts for the
remaining one-third of the sample.
For the luminosity range probed by the present study (P_1.4 > 10^24.5
W/Hz/sr), we use the V/V_max test to show conclusively that there is a deficit
of high-redshift (z > 2-2.5) objects. Comparison with the model RLFs of Dunlop
& Peacock (1990) shows that our data can now exclude pure luminosity evolution.
Two of the models of DP90, and the RLF deduced by direct binning of the data,
both favour a luminosity dependence for the high-redshift cut-off, with
lower-luminosity sources (P_1.4 \simeq 10^24 W/Hz/sr) in decline by z \simeq
1-1.5 while higher-luminosity sources (P_1.4 \simeq 10^{25-26} W/Hz/sr) decline
in comoving number density beyond z \simeq 2-2.5.Comment: Revised version submitted to MNRAS. 16 pages, 12 figure
The Evolving Faint-End of the Luminosity Function
We investigate the evolution of the faint-end slope of the luminosity
function, , using semi-analytical modeling of galaxy formation. In
agreement with observations, we find that the slope can be fitted well by
, with a=-1.13 and b=-0.1. The main driver for the evolution
in is the evolution in the underlying dark matter mass function.
Sub-L_* galaxies reside in dark matter halos that occupy a different part of
the mass function. At high redshifts, this part of the mass function is steeper
than at low redshifts and hence is steeper. Supernova feedback in
general causes the same relative flattening with respect to the dark matter
mass function. The faint-end slope at low redshifts is dominated by field
galaxies and at high redshifts by cluster galaxies. The evolution of
in each of these environments is different, with field galaxies
having a slope b=-0.14 and cluster galaxies b=-0.05. The transition from
cluster-dominated to field-dominated faint-end slope occurs roughly at a
redshift , and suggests that a single linear fit to the overall
evolution of might not be appropriate. Furthermore, this result
indicates that tidal disruption of dwarf galaxies in clusters cannot play a
significant role in explaining the evolution of at z< z_*. In
addition we find that different star formation efficiencies a_* in the
Schmidt-Kennicutt-law and supernovae-feedback efficiencies generally
do not strongly influence the evolution of .Comment: 4 pages, replaced with version accepted to ApJL, minor changes to
figure
Candidates of z ~ 5.5--7 Galaxies in the HST Ultra Deep Field
We report results from our --7 galaxy search in the HST Ultra
Deep Field (UDF). Using the 400-orbit of ACS data, we found 108 plausible
(or for short) candidates to
mag. The contamination to the sample, either due to
image artifacts or known types of astronomical objects, is likely negligible.
The inferred surface densities of galaxies are consistent with our
earlier predictions from to 28.5 mag. After correcting
for detection incompleteness, the counts of candidates to
mag suggests that the faint-end slope of the galaxy
luminosity function (LF) at this redshift is likely between and
-1.9, which is sufficient to account for the entire Lyman photon budget
necessary to complete the reionization of the universe at . We also
searched for --7 candidates using the UDF NICMOS data, and have
found four candidates to mag. However, the infrared colors of
three candidates cannot be easily explained by galaxies in this redshift range.
We tentatively derive an upper limit to the cumulative surface density of
galaxies at of 0.36 per arcmin to mag, which
suggest a noticeable drop in the LF amplitude from to .Comment: accepted for publication in ApJL (submitted March 17 2004, accepted
July 20 2004
The Contribution of Late-type/Irregulars to the Faint Galaxy Counts from HST Medium Deep Survey Images
We present a complete morphologically classified sample of 144 faint field
galaxies from the HST Medium Deep Survey with 20.0 < I <22.0 mag. We compare
the global properties of the ellipticals, early and late-type spirals, and find
a non-negligible fraction (13/144) of compact blue [(V-I) < 1.0 mag] systems
with -profiles. We give the differential galaxy number counts for
ellipticals and early-type spirals independently, and find that the data are
consistent with no-evolution predictions based on conventional flat Schechter
luminosity functions (LF's) and a standard cosmology.
Conversely, late-type/Irregulars show a steeply rising differential number
count with slope . No-evolution
models based on the Loveday et al. (1992) and Marzke et al. (1994b) {\it local}
luminosity functions under-predict the late-type/Irregular counts by 1.0 and
0.5 dex, respectively, at I = 21.75 mag. Examination of the Irregulars alone
shows that % appear inert and the remainder have multiple cores. If
the inert galaxies represent a non-evolving late-type population, then a
Loveday-like LF () is ruled out for these types, and a LF
with a steep faint-end () is suggested. If multiple core
structure indicates recent star-formation, then the observed excess of faint
blue field galaxies is likely due to {\it evolutionary} processes acting on a
{\it steep} field LF for late-type/Irregulars. The evolutionary mechanism is
unclear, but 60% of the multiple-core Irregulars show close companions. To
reconcile a Marzke-like LF with the faint redshift surveys, this evolution must
be preferentially occurring in the brightest late-type galaxies with z > 0.5 at
I = 21.75 mag.Comment: 29 pages, 1 catalog and 10 figures. The figures and catalog can be
found at http://www.phys.unsw.edu.au/~spd/bib.htm
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