383 research outputs found
Galaxy evolution in the William Herschel deep field
In this Thesis we investigate the evolutionary histories of faint field galaxies using extremely deep optical and near-infrared photometry. Our work is centred on a 50 arcmin(^2) region at high galactic latitude which we call "The William Herschel Deep Field" (WHDF). In this work we describe three new near-infrared surveys of this field. In considering both this infrared data and the existing optical data, our broad aims are to increase our understanding of both the growth of galaxy clustering in the Universe and also to determine the star-formation histories of the field galaxy population. We consider our observations primarily in the context of luminosity evolution models in low density universes, but alternative scenarios are considered. Near-infrared galaxy counts derived from our catalogues are consistent with the predictions of our models, without the need for a steep faint-end slope for the galaxy luminosity function. We find that optical-infrared colour distributions of infrared-selected galaxies in the WHDF are deficient in red, early-type galaxies. This is consistent with the predictions of evolutionary models in which these systems have a small amount of on-going star-formation. We measure the amplitude of galaxy clustering in the WHDF for galaxies selected in optical and near-infrared bandpasses using the projected two-point correlation function. By comparing our measured clustering amplitudes with the predictions of our models we find that in all bandpasses the growth of galaxy clustering is approximately fixed in proper co-ordinates, again assuming a low-density Universe. Finally, an analysis of errors on the correlation function measurements suggest that discrepancies between our work and those of other authors may be explained by an underestimation of statistical errors
Real Laboratories for Distance Education
Providing distance laboratory-based courses is becoming critical for distance technical education. In this work, we describe remote laboratories in digital system courses. While the hardware is based on widely used programmable logic, the Internet interfaces include those for remote development, testing and debugging as well as the cooperative work environment. Special attention has been paid to the objectivity of evaluating the remote cooperative work. The web tools for project progress evaluation, self- and group- assessment and the automated hardware support are being developed. Previous work consisted mainly of providing simulated environments or prefabricated circuits. The productivity and accessibility of these tools was greatly enhanced by using off-the-shelf hardware, software and networking elements
The Evolution of the Stellar Mass Functions of Star-Forming and Quiescent Galaxies to z = 4 from the COSMOS/UltraVISTA Survey
We present measurements of the stellar mass functions (SMFs) of star-forming
and quiescent galaxies to z = 4 using a sample of 95 675 galaxies in the
COSMOS/UltraVISTA field. Sources have been selected from the DR1 UltraVISTA
K_{s}-band imaging which covers a unique combination of a wide area (1.62
deg^2), to a significant depth (K_{s,tot} = 23.4). The SMFs of the combined
population are in good agreement with previous measurements and show that the
stellar mass density of the universe was only 50%, 10% and 1% of its current
value at z ~ 0.75, 2.0, and 3.5, respectively. The quiescent population drives
most of the overall growth, with the stellar mass density of these galaxies
increasing by 2.71^{+0.93}_{-0.22} dex since z = 3.5. At z > 2.5, star-forming
galaxies dominate the total SMF at all stellar masses, although a nonzero
population of quiescent galaxies persists to z = 4. Comparisons of the
K_{s}-selected star-forming galaxy SMFs to UV-selected SMFs at 2.5 < z < 4 show
reasonable agreement and suggests UV-selected samples are representative of the
majority of the stellar mass density at z > 3.5. We estimate the average mass
growth of individual galaxies by selecting galaxies at fixed cumulative number
density. The average galaxy with Log(M_{*}/M_{sun}) = 11.5 at z = 0.3 has grown
in mass by only 0.2 dex (0.3 dex) since z = 2.0(3.5), whereas those with
Log(M_{*}/M_{sun}) = 10.5 have grown by > 1.0 dex since z = 2. At z < 2, the
time derivatives of the mass growth are always larger for lower-mass galaxies,
which demonstrates that the mass growth in galaxies since that redshift is
mass-dependent and primarily bottom-up. Lastly, we examine potential sources of
systematic uncertainties on the SMFs and find that those from photo-z
templates, SPS modeling, and the definition of quiescent galaxies dominate the
total error budget in the SMFs.Comment: 18 pages paper, 12 pages appendix, 23 figures. Accepted for
publication in the Ap
The VLA-COSMOS Survey: V. 324 MHz continuum observations
We present 90 cm VLA imaging of the COSMOS field, comprising a circular area
of 3.14 square degrees at 8.0"x6.0" angular resolution with an average rms of
0.5 mJy/beam. The extracted catalog contains 182 sources (down to 5.5sigma), 30
of which are multi-component sources. Using Monte Carlo artificial source
simulations we derive the completeness of the catalog, and we show that our 90
cm source counts agree very well with those from previous studies. Using X-ray,
NUV-NIR and radio COSMOS data to investigate the population mix of our 90 cm
radio sample, we find that our sample is dominated by active galactic nuclei
(AGN). The average 90-20 cm spectral index (S_nu~nu**alpha, where S_nu is the
flux density at frequency nu, and alpha the spectral index) of our 90 cm
selected sources is -0.70, with an interquartile range of -0.90 to -0.53. Only
a few ultra-steep-spectrum sources are present in our sample, consistent with
results in the literature for similar fields. Our data do not show clear
steepening of the spectral index with redshift. Nevertheless, our sample
suggests that sources with spectral indices steeper than -1 all lie at z>1, in
agreement with the idea that ultra-steep-spectrum radio sources may trace
intermediate-redshift galaxies (z>1).Comment: 10 pages, 12 figures, accepted for publication in MNRA
Encoding the infrared excess (IRX) in the NUVrK color diagram for star-forming galaxies
We present an empirical method of assessing the star formation rate (SFR) of
star-forming galaxies based on their locations in the rest-frame color-color
diagram (NUV-r) vs (r-K). By using the Spitzer 24 micron sample in the COSMOS
field (~16400 galaxies with 0.2 < z < 1.3) and a local GALEX-SDSS-SWIRE sample
(~700 galaxies with z = <
L_IR / L_UV > can be described by a single vector, NRK, that combines the two
colors. The calibration between and NRK allows us to recover the IR
luminosity, L_IR, with an accuracy of ~0.21 dex for the COSMOS sample and ~0.27
dex for the local one. The SFRs derived with this method agree with the ones
based on the observed (UV+IR) luminosities and on the spectral energy
distribution fitting for the vast majority (~85 %) of the star-forming
population. Thanks to a library of model galaxy SEDs with realistic
prescriptions for the star formation history, we show that we need to include a
two-component dust model (i.e., birth clouds and diffuse ISM) and a full
distribution of galaxy inclinations in order to reproduce the behavior of the
stripes in the NUVrK diagram. In conclusion, the NRK method, based only
on rest-frame UV and optical colors available in most of the extragalactic
fields, offers a simple alternative of assessing the SFR of star-forming
galaxies in the absence of far-IR or spectral diagnostic observations.Comment: 21 pages, 22 figures, in publication in Astronomy & Astrophysic
Clustering, host halos and environment of z2 galaxies as a function of their physical properties
Using a sample of 25683 star-forming and 2821 passive galaxies at ,
selected in the COSMOS field following the BzK color criterion, we study the
hosting halo mass and environment of galaxies as a function of their physical
properties. Spitzer and Herschel provide accurate SFR estimates for starburst
galaxies. We measure the auto- and cross-correlation functions of various
galaxy sub-samples and infer the properties of their hosting halos using both
an HOD model and the linear bias at large scale. We find that passive and
star-forming galaxies obey a similarly rising relation between the halo and
stellar mass. The mean host halo mass of star forming galaxies increases with
the star formation rate between 30 and 200 M.yr, but flattens
for higher values, except if we select only main-sequence galaxies. This
reflects the expected transition from a regime of secular co-evolution of the
halos and the galaxies to a regime of episodic starburst. We find similar large
scale biases for main-sequence, passive, and starburst galaxies at equal
stellar mass, suggesting that these populations live in halos of the same mass.
We detect an excess of clustering on small scales for passive galaxies and
showed, by measuring the large-scale bias of close pairs, that this excess is
caused by a small fraction () of passive galaxies being hosted by
massive halos ( M) as satellites. Finally,
extrapolating the growth of halos hosting the z2 population, we show that
M M galaxies at z2 will evolve, on average,
into massive (M M), field galaxies in the local
Universe and M M galaxies at z=2 into local,
massive, group galaxies. The most massive main-sequence galaxies and close
pairs of massive, passive galaxies end up in today's clusters.Comment: 18 pages, 16 figures, Accepted by A&
Extragalactic Fields Optimized for Adaptive Optics
In this paper we present the coordinates of 67 55' x 55' patches of sky which
have the rare combination of both high stellar surface density (>0.5
arcmin^{-2} with 13<R<16.5 mag) and low extinction (E(B-V)<0.1). These fields
are ideal for adaptive-optics based follow-up of extragalactic targets. One
region of sky, situated near Baade's Window, contains most of the patches we
have identified. Our optimal field, centered at RA: 7h24m3s, Dec: -1deg27'15",
has an additional advantage of being accessible from both hemispheres. We
propose a figure of merit for quantifying real-world adaptive optics
performance, and use this to analyze the performance of multi-conjugate
adaptive optics in these fields. We also compare our results to those that
would be obtained in existing deep fields. In some cases adaptive optics
observations undertaken in the fields given in this paper would be orders of
magnitude more efficient than equivalent observations undertaken in existing
deep fields.Comment: 28 pages, 15 figures, 1 table; accepted for publication in PAS
Brightest group galaxies-II : the relative contribution of BGGs to the total baryon content of groups at z <1.3
We performed a detailed study of the evolution of the star formation rate (SFR) and stellar mass of the brightest group galaxies (BGGs) and their relative contribution to the total baryon budget within (). The sample comprises 407 BGGs selected from X-ray galaxy groups () out to identified in the COSMOS, XMM-LSS, and AEGIS fields. We find that BGGs constitute two distinct populations of quiescent and star-forming galaxies and their mean SFR is dex higher than the median SFR at dex. The mean (median) of stellar mass of BGGs has grown by dex since to the present day. We show that up to of the stellar mass growth in a star-forming BGG can be due to its star-formation activity. With respect to , we find it to increase with decreasing redshift by dex while decreasing with halo mass in a redshift dependent manner. We show that the slope of the relation between and halo mass increases negatively with decreasing redshift. This trend is driven by an insufficient star-formation in BGGs, compared to the halo growth rate. We separately show the BGGs with the 20\% highest are generally non-star-forming galaxies and grow in mass by processes not related to star formation (e.g., dry mergers and tidal striping). We present the and relations and compare them with semi-analytic model predictions and a number of results from the literature. We quantify the intrinsic scatter in stellar mass of BGGs at fixed halo mass () and find that increases from 0.3 dex at to 0.5 dex at due to the bimodal distribution of stellar mass
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