218 research outputs found
The Baryon Content of Extremely Low Mass Dwarf Galaxies
We investigate the gas content and baryonic Tully-Fisher relationship for
extremely low luminosity dwarf galaxies in the absolute magnitude range -13.5 >
Mr > -16. The sample is selected from the Sloan Digital Sky Survey and consists
of 101 galaxies for which we have obtained follow-up HI observations using the
Arecibo Observatory and Green Bank Telescope. This represents the largest
homogeneous sample of dwarfs at low luminosities with well-measured HI and
optical properties. The sample spans a range of environments, from dense groups
to truly isolated galaxies. The average neutral gas fraction is f_gas=0.6,
significantly exceeding that of typical gas-rich galaxies at higher
luminosities. Dwarf galaxies are therefore less efficient at turning gas into
stars over their lifetimes. The strong environmental dependence of the gas
fraction distribution demonstrates that while internal processes can reduce the
gas fractions to roughly f_gas=0.4, external processes are required to fully
remove gas from a dwarf galaxy. The average rotational velocity of our sample
is vrot=50 km/s. Including more massive galaxies from the literature, we fit a
baryonic Tully-Fisher slope of M_baryon \propto vrot^(3.70+/- 0.15). This slope
compares well with CDM models that assume an equal baryon to dark matter ratio
at all masses. While gas stripping or other processes may modify the baryon to
dark matter ratio for dwarfs in the densest environments, the majority of dwarf
galaxies in our sample have not preferentially lost significant baryonic mass
relative to more massive galaxies.Comment: 33 pages, 8 figures. Accepted to ApJ. Data available at
http://www.ociw.edu/~mgeha/researc
What triggers galaxy transformations? The environments of post-starburst galaxies
(abridged) There are good observational reasons to believe that the
progenitors of red galaxies have undergone starbursts, followed by a
post-starburst phase. We investigate the environments of post-starburst
galaxies by measuring \textsl{(1)} number densities in
radius comoving spheres, \textsl{(2)} transverse distances to nearest
Virgo-like galaxy clusters, and \textsl{(3)} transverse distances to nearest
luminous-galaxy neighbors. We compare the post-starburst galaxies to currently
star-forming galaxies identified solely by A-star excess or \Halpha emission.
We find that post-starburst galaxies are in the same kinds of environments as
star-forming galaxies; this is our ``null hypothesis''. More importantly, we
find that at each value of the A-star excess, the star-forming and
post-starburst galaxies lie in very similar distributions of environment. The
only deviations from our null hypothesis are barely significant: a slight
deficit of post-starburst galaxies (relative to the star-forming population) in
very low-density regions, a small excess inside the virial radii of clusters,
and a slight excess with nearby neighbors. None of these effects is strong
enough to make the post-starburst galaxies a high-density phenomenon, or to
argue that the starburst events are primarily triggered by external tidal
impulses (e.g., from close passages of massive galaxies). The small excess
inside cluster virial radii suggests that some post-starbursts are triggered by
interactions with the intracluster medium, but this represents a very small
fraction of all post-starburst galaxies.Comment: ApJ in pres
Cross-correlation Weak Lensing of SDSS Galaxy Clusters III: Mass-to-light Ratios
We present measurements of the excess mass-to-light ratio measured
aroundMaxBCG galaxy clusters observed in the SDSS. This red sequence cluster
sample includes objects from small groups with masses ranging from ~5x10^{12}
to ~10^{15} M_{sun}/h. Using cross-correlation weak lensing, we measure the
excess mass density profile above the universal mean \Delta \rho(r) = \rho(r) -
\bar{\rho} for clusters in bins of richness and optical luminosity. We also
measure the excess luminosity density \Delta l(r) = l(r) - \bar{l} measured in
the z=0.25 i-band. For both mass and light, we de-project the profiles to
produce 3D mass and light profiles over scales from 25 kpc/ to 22 Mpc/h. From
these profiles we calculate the cumulative excess mass M(r) and excess light
L(r) as a function of separation from the BCG. On small scales, where \rho(r)
>> \bar{\rho}, the integrated mass-to-light profile may be interpreted as the
cluster mass-to-light ratio. We find the M/L_{200}, the mass-to-light ratio
within r_{200}, scales with cluster mass as a power law with index 0.33+/-0.02.
On large scales, where \rho(r) ~ \bar{\rho}, the M/L approaches an asymptotic
value independent of cluster richness. For small groups, the mean M/L_{200} is
much smaller than the asymptotic value, while for large clusters it is
consistent with the asymptotic value. This asymptotic value should be
proportional to the mean mass-to-light ratio of the universe . We find
/b^2_{ml} = 362+/-54 h (statistical). There is additional uncertainty in
the overall calibration at the ~10% level. The parameter b_{ml} is primarily a
function of the bias of the L <~ L_* galaxies used as light tracers, and should
be of order unity. Multiplying by the luminosity density in the same bandpass
we find \Omega_m/b^2_{ml} = 0.02+/-0.03, independent of the Hubble parameter.Comment: Third paper in a series; v2.0 incorporates ApJ referee's suggestion
A Search for the Most Massive Galaxies. II. Structure, Environment and Formation
We study a sample of 43 early-type galaxies, selected from the SDSS because
they appeared to have velocity dispersion > 350 km/s. High-resolution
photometry in the SDSS i passband using HRC-ACS on board the HST shows that
just less than half of the sample is made up of superpositions of two or three
galaxies, so the reported velocity dispersion is incorrect. The other half of
the sample is made up of single objects with genuinely large velocity
dispersions. None of these objects has sigma larger than 426 +- 30 km/s. These
objects define rather different relations than the bulk of the early-type
galaxy population: for their luminosities, they are the smallest, most massive
and densest galaxies in the Universe. Although the slopes of the scaling
relations they define are rather different from those of the bulk of the
population, they lie approximately parallel to those of the bulk "at fixed
sigma". These objects appear to be of two distinct types: the less luminous
(M_r>-23) objects are rather flattened and extremely dense for their
luminosities -- their properties suggest some amount of rotational support and
merger histories with abnormally large amounts of gaseous dissipation. The more
luminous objects (M_r<-23) tend to be round and to lie in or at the centers of
clusters. Their properties are consistent with the hypothesis that they are
BCGs. Models in which BCGs form from predominantly radial mergers having little
angular momentum predict that they should be prolate. If viewed along the major
axis, such objects would appear to have abnormally large sigma for their sizes,
and to be abnormally round for their luminosities. This is true of the objects
in our sample once we account for the fact that the most luminous galaxies
(M_r<-23.5), and BCGs, become slightly less round with increasing luminosity.Comment: 21 pages, 19 figures, accepted for publication in MNRA
The luminosity -- halo-mass relation for brightest cluster galaxies
We examine the central-galaxy luminosity -- host-halo mass relation for 54
Brightest Group Galaxies (BGGs) and 92 Brightest Cluster Galaxies (BCGs) at
z<0.1 and present the first measurement of this relation for a sample of known
BCGs at 0.1<z<0.8 (average z~0.3). At z<0.1 we find L_K \propto
M_{200}^{0.24\pm0.08} for the BCGs and the early-type BGGs in groups with
extended X-ray emission and L_K \propto M_{200}^{0.11\pm0.10} for the BCGs
alone. At 0.1<z<0.8 we find L_K \propto M_{200}^{0.28\pm0.11}. We conclude that
there is no evidence for evolution in this relationship between z<0.1 and
z<0.8: BCG growth appears to still be limited by the timescale for dynamical
friction at these earlier times, not proceeding according to the predictions of
current semi-analytic models.Comment: Accepted for publication in MNRA
Improved constraints on the expansion rate of the Universe up to z~1.1 from the spectroscopic evolution of cosmic chronometers
We present new improved constraints on the Hubble parameter H(z) in the
redshift range 0.15 < z < 1.1, obtained from the differential spectroscopic
evolution of early-type galaxies as a function of redshift. We extract a large
sample of early-type galaxies (\sim11000) from several spectroscopic surveys,
spanning almost 8 billion years of cosmic lookback time (0.15 < z < 1.42). We
select the most massive, red elliptical galaxies, passively evolving and
without signature of ongoing star formation. Those galaxies can be used as
standard cosmic chronometers, as firstly proposed by Jimenez & Loeb (2002),
whose differential age evolution as a function of cosmic time directly probes
H(z). We analyze the 4000 {\AA} break (D4000) as a function of redshift, use
stellar population synthesis models to theoretically calibrate the dependence
of the differential age evolution on the differential D4000, and estimate the
Hubble parameter taking into account both statistical and systematical errors.
We provide 8 new measurements of H(z) (see Tab. 4), and determine its change in
H(z) to a precision of 5-12% mapping homogeneously the redshift range up to z
\sim 1.1; for the first time, we place a constraint on H(z) at z \neq 0 with a
precision comparable with the one achieved for the Hubble constant (about 5-6%
at z \sim 0.2), and covered a redshift range (0.5 < z < 0.8) which is crucial
to distinguish many different quintessence cosmologies. These measurements have
been tested to best match a \Lambda CDM model, clearly providing a
statistically robust indication that the Universe is undergoing an accelerated
expansion. This method shows the potentiality to open a new avenue in constrain
a variety of alternative cosmologies, especially when future surveys (e.g.
Euclid) will open the possibility to extend it up to z \sim 2.Comment: 34 pages, 15 figures, 6 tables, published in JCAP. It is a companion
to Moresco et al. (2012b, http://arxiv.org/abs/1201.6658) and Jimenez et al.
(2012, http://arxiv.org/abs/1201.3608). The H(z) data can be downloaded at
http://www.physics-astronomy.unibo.it/en/research/areas/astrophysics/cosmology-with-cosmic-chronometer
Building galaxies by accretion and in-situ star formation
We examine galaxy formation in a cosmological AMR simulation, which includes
two high resolution boxes, one centered on a 3 \times 10^14 M\odot cluster, and
one centered on a void. We examine the evolution of 611 massive (M\ast >
10^10M\odot) galaxies. We find that the fraction of the final stellar mass
which is accreted from other galaxies is between 15 and 40% and increases with
stellar mass. The accreted fraction does not depend strongly on environment at
a given stellar mass, but the galaxies in groups and cluster environments are
older and underwent mergers earlier than galaxies in lower density
environments. On average, the accreted stars are ~2.5 Gyrs older, and ~0.15 dex
more metal poor than the stars formed in-situ. Accreted stellar material
typically lies on the outskirts of galaxies; the average half-light radius of
the accreted stars is 2.6 times larger than that of the in-situ stars. This
leads to radial gradients in age and metallicity for massive galaxies, in
qualitative agreement with observations. Massive galaxies grow by mergers at a
rate of approximately 2.6% per Gyr. These mergers have a median (mass-weighted)
mass ratio less than 0.26 \pm 0.21, with an absolute lower limit of 0.20, for
galaxies with M\ast ~ 10^12 M\odot. This suggests that major mergers do not
dominate in the accretion history of massive galaxies. All of these results
agree qualitatively with results from SPH simulations by Oser et al. (2010,
2012).Comment: 18 pages, 12 figures, submitted to MNRA
CANDELS Observations of the Structural Properties and Evolution of Galaxies in a Cluster at z=1.62
We discuss the structural and morphological properties of galaxies in a
z=1.62 proto-cluster using near-IR imaging data from Hubble Space Telescope
Wide Field Camera 3 data of the Cosmic Assembly Near-IR Deep Extragalactic
Legacy Survey (CANDELS). The cluster galaxies exhibit a clear color-morphology
relation: galaxies with colors of quiescent stellar populations generally have
morphologies consistent with spheroids, and galaxies with colors consistent
with ongoing star formation have disk-like and irregular morphologies. The size
distribution of the quiescent cluster galaxies shows a deficit of compact (<
1kpc), massive galaxies compared to CANDELS field galaxies at z=1.6. As a
result the cluster quiescent galaxies have larger average effective sizes
compared to field galaxies at fixed mass at greater than 90% significance.
Combined with data from the literature, the size evolution of quiescent cluster
galaxies is relatively slow from z~1.6 to the present, growing as
(1+z)^(-0.6+/-0.1). If this result is generalizable, then it implies that
physical processes associated with the denser cluster region seems to have
caused accelerated size growth in quiescent galaxies prior to z=1.6 and slower
subsequent growth at z<1.6 compared to galaxies in the lower density field. The
quiescent cluster galaxies at z=1.6 have higher ellipticities compared to lower
redshift samples at fixed mass, and their surface-brightness profiles suggest
that they contain extended stellar disks. We argue the cluster galaxies require
dissipationless (i.e., gas-poor or "dry") mergers to reorganize the disk
material and to match the relations for ellipticity, stellar mass, size, and
color of early-type galaxies in z<1 clusters.Comment: Accepted for publication in ApJ. 14 pages in emulateapj format.
Replacement includes improvements from referee report, and updates and
additions to reference
Nutrition and lung cancer: a case control study in Iran
Background: Despite many prospective and retrospective studies about the association of dietary habit and lung
cancer, the topic still remains controversial. So, this study aims to investigate the association of lung cancer with
dietary factors.
Method: In this study 242 lung cancer patients and their 484 matched controls on age, sex, and place of residence
were enrolled between October 2002 to 2005. Trained physicians interviewed all participants with standardized
questionnaires. The middle and upper third consumer groups were compared to the lower third according to the
distribution in controls unless the linear trend was significant across exposure groups.
Result: Conditional logistic regression was used to evaluate the association with lung cancer. In a multivariate
analysis fruit (Ptrend < 0.0001), vegetable (P = 0.001) and sunflower oil (P = 0.006) remained as protective factors and
rice (P = 0.008), bread (Ptrend = 0.04), liver (P = 0.004), butter (Ptrend = 0.04), white cheese (Ptrend < 0.0001), beef
(Ptrend = 0.005), vegetable ghee (P < 0.0001) and, animal ghee (P = 0.015) remained as risk factors of lung cancer.
Generally, we found positive trend between consumption of beef (P = 0.002), bread (P < 0.0001), and dairy
products (P < 0.0001) with lung cancer. In contrast, only fruits were inversely related to lung cancer (P < 0.0001).
Conclusion: It seems that vegetables, fruits, and sunflower oil could be protective factors and bread, rice, beef,
liver, dairy products, vegetable ghee, and animal ghee found to be possible risk factors for the development of
lung cancer in Iran
Evolution of the Far-Infrared-Radio Correlation and Infrared SEDs of Massive Galaxies over z = 0 - 2
We investigate the far-infrared-radio correlation (FRC) of
stellar-mass-selected galaxies in the Extended Chandra Deep Field South using
far-infrared imaging from Spitzer and radio imaging from the Very Large Array
and Giant Metre-Wave Radio Telescope. We stack in redshift bins to probe
galaxies below the noise and confusion limits. Radio fluxes are K-corrected
using observed flux ratios, leading to tentative evidence for an evolution in
spectral index. We compare spectral energy distribution (SED) templates of
local galaxies for K-correcting FIR fluxes, and show that the data are best fit
by a quiescent spiral template (M51) rather than a warm starburst (M82) or
ULIRG (Arp220), implying a predominance of cold dust in massive galaxies at
high redshift. In contrast we measure total infrared luminosities that are
consistent with high star-formation rates. We observe that the FRC index (q)
does not evolve significantly over z=0-2 when computed from K-corrected 24 or
160-mum photometry, but that using 70-mum fluxes leads to an apparent decline
in q beyond z~1. This suggests some change in the SED at high redshift, either
a steepening of the spectrum at rest-frame ~25-35mum or a deficiency at ~70mum
leading to a drop in the total infrared/radio ratios. We compare our results to
other work in the literature and find synergies with recent findings on the
high-redshift FRC, high specific star-formation rates of massive galaxies and
the cold dust temperatures in these galaxies.Comment: 21 pages, 13 figures, accepted for publication in MNRA
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