1,164 research outputs found
Galaxy Morphology from NICMOS Parallel Imaging
We present high resolution NICMOS images of random fields obtained in
parallel to other HST observations. We present galaxy number counts reaching
H=24. The H-band galaxy counts show good agreement with the deepest I- and
K-band counts obtained from ground-based data. We present the distribution of
galaxies with morphological type to H<23. We find relatively fewer irregular
galaxies compared to an I-band sample from the Hubble Deep Field, which we
attribute to their blue color, rather than to morphological K-corrections. We
conclude that the irregulars are intrinsically faint blue galaxies at z<1.Comment: 13 pages, including 4 figures. Accepted for publication in ApJ
Letter
Galaxy bimodality versus stellar mass and environment
We analyse a z<0.1 galaxy sample from the Sloan Digital Sky Survey focusing
on the variation of the galaxy colour bimodality with stellar mass and
projected neighbour density Sigma, and on measurements of the galaxy stellar
mass functions. The characteristic mass increases with environmental density
from about 10^10.6 Msun to 10^10.9 Msun (Kroupa IMF, H_0=70) for Sigma in the
range 0.1--10 per Mpc^2. The galaxy population naturally divides into a red and
blue sequence with the locus of the sequences in colour-mass and
colour-concentration index not varying strongly with environment. The fraction
of galaxies on the red sequence is determined in bins of 0.2 in log Sigma and
log mass (12 x 13 bins). The red fraction f_r generally increases continuously
in both Sigma and mass such that there is a unified relation: f_r =
F(Sigma,mass). Two simple functions are proposed which provide good fits to the
data. These data are compared with analogous quantities in semi-analytical
models based on the Millennium N-body simulation: the Bower et al. (2006) and
Croton et al. (2006) models that incorporate AGN feedback. Both models predict
a strong dependence of the red fraction on stellar mass and environment that is
qualitatively similar to the observations. However, a quantitative comparison
shows that the Bower et al. model is a significantly better match; this appears
to be due to the different treatment of feedback in central galaxies.Comment: 19 pages, 17 figures; accepted by MNRAS, minor change
The History of Galaxies and Galaxy Number Counts
(Abridged) A simple quantitative model is presented for the history of
galaxies to explain galaxy number counts, redshift distributions and some other
related observations. We first infer that irregular galaxies and the disks of
spiral galaxies are young, probably formed at from a
simultaneous consideration of colours and gas content under a moderate
assumption on the star formation history. Assuming that elliptical galaxies and
bulges of spiral galaxies, both called spheroids in the discussion, had formed
early in the universe, the resulting scenario is that spiral galaxies formed as
intergalactic gas accreting onto pre-existing bulges mostly at ;
irregular galaxies as seen today formed by aggregation of clouds at . Taking the formation epochs thus estimated into account, we construct
a model for the history of galaxies employing a stellar population synthesis
model. We assume that the number of galaxies does not change except that some
of them (irregulars) were newly born, and use a morphology-dependent local
luminosity function to constrain the number of galaxies. The predictions of the
model are compared with the observation of galaxy number counts and redshift
distributions for the , and colour bands. It is shown that young
irregular galaxies cause the steep slope of the -band counts. The fraction
of irregular galaxies increases with decreasing brightness: at mag, they
contribute as much as spiral galaxies. Thus, ``the faint blue galaxy problem''
is solved by invoking young galaxies. This interpretation is corroborated by a
comparison of our prediction with the morphologically-classified galaxy counts
in the band.Comment: 25 pages, LaTeX (aaspp4), 24 PostScript figures. Submitted to ApJ in
February 199
The Near-Infrared Number Counts and Luminosity Functions of Local Galaxies
This study presents a wide-field near-infrared (K-band) survey in two fields;
SA 68 and Lynx 2. The survey covers an area of 0.6 deg., complete to
K=16.5. A total of 867 galaxies are detected in this survey of which 175 have
available redshifts. The near-infrared number counts to K=16.5 mag. are
estimated from the complete photometric survey and are found to be in close
agreement with other available studies. The sample is corrected for
incompleteness in redshift space, using selection function in the form of a
Fermi-Dirac distribution. This is then used to estimate the local near-infrared
luminosity function of galaxies. A Schechter fit to the infrared data gives:
M, and Mpc (for H Km/sec/Mpc and q). When
reduced to , this agrees with other available estimates of the local
IRLF. We find a steeper slope for the faint-end of the infrared luminosity
function when compared to previous studies. This is interpreted as due to the
presence of a population of faint but evolved (metal rich) galaxies in the
local Universe. However, it is not from the same population as the faint blue
galaxies found in the optical surveys. The characteristic magnitude
() of the local IRLF indicates that the bright red galaxies ( mag.) have a space density of Mpc and hence,
are not likely to be local objects.Comment: 24 pages, 8 figures, AASTEX 4.0, published in ApJ 492, 45
Questions on pure luminosity evolution for ellipticals
The explanation for the existence of an excess population of faint blue
galaxies (FBGs) has been a mystery for nearly two decades, and remains one of
the grand astronomical issues to date. Existing models cannot explain all of
the observational data such as galaxy number counts in the optical and infrared
passbands and the redshift distributions of galaxies. Here, by modelling the
morphological number counts derived from the Hubble Space Telescope, as well as
the number counts in optical and infrared passbands, and the redshift and color
distributions of galaxies obtained from ground-based observations, we show that
the `FBG problem' cannot be resolved if elliptical galaxies are assumed to have
formed in an instantaneous burst of star formation at high redshift with no
subsequent star formation events, which is just the conventional scenario for
formation and evolution of ellipticals. There exist great discrepancies between
the observed color distribution and the predicted distribution for
ellipticals by such a pure luminosity evolution (PLE) model in the context of
the conventional scenario. Neither can the mild evolution (i.e., the star
formation events have lasted for a longer time than those of the instantaneous
burst and passive evolution since the formation of galaxies) for ellipticals be
accepted in the context of PLE assumption. The introduction of dust extinction
also cannot save the PLE models. This conclusion holds for each of the three
cosmological models under consideration: flat, open and -dominated.
Hence, our investigation suggests that PLE assumption for elliptical galaxies
is questionable, and number evolution may be essential for ellipticals.Comment: 20pages (AASTeX), 10 eps figures, 4 tables, to appear in ApJ, 1999,
Vol. 51
Pair Analysis of Field Galaxies from the Red-Sequence Cluster Survey
We study the evolution of the number of close companions of similar
luminosities per galaxy (Nc) by choosing a volume-limited subset of the
photometric redshift catalog from the Red-Sequence Cluster Survey (RCS-1). The
sample contains over 157,000 objects with a moderate redshift range of 0.25 < z
< 0.8 and absolute magnitude in Rc (M_Rc) < -20. This is the largest sample
used for pair evolution analysis, providing data over 9 redshift bins with
about 17,500 galaxies in each. After applying incompleteness and projection
corrections, Nc shows a clear evolution with redshift. The Nc value for the
whole sample grows with redshift as (1+z)^m, where m = 2.83 +/- 0.33 in good
agreement with N-body simulations in a LCDM cosmology. We also separate the
sample into two different absolute magnitude bins: -25 < M_Rc < -21 and -21 <
M_Rc < -20, and find that the brighter the absolute magnitude, the smaller the
m value. Furthermore, we study the evolution of the pair fraction for different
projected separation bins and different luminosities. We find that the m value
becomes smaller for larger separation, and the pair fraction for the fainter
luminosity bin has stronger evolution. We derive the major merger remnant
fraction f_rem = 0.06, which implies that about 6% of galaxies with -25 < M_Rc
< -20 have undergone major mergers since z = 0.8.Comment: ApJ, in pres
Probing the evolution of early-type galaxies using multi-colour number counts and redshift distributions
We investigate pure luminosity evolution models for early-type (elliptical
and S0) galaxies (i.e., no number density change or morphology transition), and
examine whether these models are consistent with observed number counts in the
B, I and K bands and redshift distributions of two samples of faint galaxies
selected in the I and K bands. The models are characterized by the star
formation time scale and the time when galactic wind blows
in addition to several other conventional parameters. We find the single-burst
model (=0.1 Gyr and =0.353 Gyr), which is known to reproduce
the photometric properties of early-type galaxies in clusters, is inconsistent
with redshift distributions of early-type galaxies in the field environment due
to overpredictions of galaxies at z\gsim1.4 even with strong extinction which
is at work until . In order for dust extinction to be more effective,
we change and as free parameters, and find that models
with \tau_{SF}\gsim0.5 Gyr and Gyr can be made consistent with
both the observed redshift distributions and number counts, if we introduce
strong extinction as a peak value). These results suggest that
early-type galaxies in the field environment do not have the same evolutionary
history as described by the single-burst model.Comment: 6 pages including 4 PS figures; accepted for publication in MNRA
The K-Band Galaxy Luminosity Function
We measured the K-band luminosity function using a complete sample of 4192
morphologically-typed 2MASS galaxies with 7 < K < 11.25 mag spread over 2.12
str. Early-type (T -0.5) galaxies have similarly
shaped luminosity functions, alpha_e=-0.92+/-0.10 and alpha_l=-0.87+/-0.09. The
early-type galaxies are brighter, M_*e=-23.53+/-0.06 mag compared to
M_*l=-22.98\pm0.06 mag, but less numerous, n_*e=(0.0045+/-0.0006)h^3/Mpc^3
compared to n_*l=(0.0101+/-0.0013)h^3/Mpc^3 for H_0=100h km/s Mpc, such that
the late-type galaxies slightly dominate the K-band luminosity density,
j_late/j_early=1.17+/-0.12. Our morphological classifications are internally
consistent, consistent with previous classifications and lead to luminosity
functions unaffected by the estimated uncertainties in the classifications.
These luminosity functions accurately predict the K-band number counts and
redshift distributions for K < 18 mag, beyond which the results depend on
galaxy evolution and merger histories.Comment: submitted to ApJ, 25 pages, 6 figures, complete redshift survey.
Table 1 included in sourc
Explorations in Hubble Space: A Quantitative Tuning Fork
In order to establish an objective framework for studying galaxy morphology,
we have developed a quantitative two-parameter description of galactic
structure that maps closely on to Hubble's original tuning fork. Any galaxy can
be placed in this "Hubble space", where the x-coordinate measures position
along the early-to-late sequence, while the y-coordinate measures in a
quantitative way the degree to which the galaxy is barred. The parameters
defining Hubble space are sufficiently robust to allow the formation of
Hubble's tuning fork to be mapped out to high redshifts. In the present paper,
we describe a preliminary investigation of the distribution of local galaxies
in Hubble space, based on the CCD imaging atlas of Frei et al. (1996). We find
that barred, weakly-barred, and unbarred galaxies are remarkably well-separated
on this diagnostic diagram. The spiral sequence is clearly bimodal and indeed
approximates a tuning fork: strongly-barred and unbarred spirals do not simply
constitute the extrema of a smooth unimodal distribution of bar strength, but
rather populate two parallel sequences. Strongly barred galaxies lie on a
remarkably tight sequence, strongly suggesting the presence of an underlying
unifying physical process. Rather surprisingly, weakly barred systems do not
seem to correspond to objects bridging the parameter space between unbarred and
strongly barred galaxies, but instead form an extension of the regular spiral
sequence. This relation lends support to models in which the bulges of
late-type spirals originate from secular processes driven by bars.Comment: Accepted for publication in The Astronomical Journal. Figure 2 is too
large to be embedded in the paper, and has been included as a JPEG imag
Observational Constraints on Exponential Gravity
We study the observational constraints on the exponential gravity model of
f(R)=-beta*Rs(1-e^(-R/Rs)). We use the latest observational data including
Supernova Cosmology Project (SCP) Union2 compilation, Two-Degree Field Galaxy
Redshift Survey (2dFGRS), Sloan Digital Sky Survey Data Release 7 (SDSS DR7)
and Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP7) in our analysis.
From these observations, we obtain a lower bound on the model parameter beta at
1.27 (95% CL) but no appreciable upper bound. The constraint on the present
matter density parameter is 0.245< Omega_m^0<0.311 (95% CL). We also find out
the best-fit value of model parameters on several cases.Comment: 14pages, 3 figures, accepted by PR
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