378 research outputs found
The Ultraviolet Detection of Diffuse Gas in Galaxy Groups
A small survey of the UV-absorbing gas in 12 low- galaxy groups has been
conducted using the Cosmic Origins Spectrograph (COS) on-board the Hubble Space
Telescope (HST). Targets were selected from a large, homogeneously-selected
sample of groups found in the Sloan Digital Sky Survey (SDSS). A critical
selection criterion excluded sight lines that pass close ( virial radii)
to a group galaxy, to ensure absorber association with the group as a whole.
Deeper galaxy redshift observations are used both to search for closer galaxies
and also to characterize these to groups, the
most massive of which are highly-virialized with numerous early-type galaxies
(ETGs). This sample also includes two spiral-rich groups, not yet
fully-virialized. At group-centric impact parameters of 0.3-2 Mpc, these
-30 spectra detected HI absorption in 7 of 12 groups; high
(OVI) and low (SiIII) ion metal lines are present in 2/3 of the absorption
components. None of the three most highly-virialized, ETG-dominated groups are
detected in absorption. Covering fractions % are seen at all impact
parameters probed, but do not require large filling factors despite an enormous
extent. Unlike halo clouds in individual galaxies, group absorbers have radial
velocities which are too low to escape the group potential well without doubt.
This suggests that these groups are "closed boxes" for galactic evolution in
the current epoch. Evidence is presented that the cool and warm group absorbers
are not a pervasive intra-group medium (IGrM), requiring a hotter (
to K) IGrM to be present to close the baryon accounting.Comment: Resubmitted to ApJS after first review; 82 pages (27 for main text,
rest are Appendices and supplemental figures and tables), 47 figures, 21
table
Galaxy Bias and its Effects on the Baryon Acoustic Oscillations Measurements
The baryon acoustic oscillation (BAO) feature in the clustering of matter in
the universe serves as a robust standard ruler and hence can be used to map the
expansion history of the universe. We use high force resolution simulations to
analyze the effects of galaxy bias on the measurements of the BAO signal. We
apply a variety of Halo Occupation Distributions (HODs) and produce biased mass
tracers to mimic different galaxy populations. We investigate whether galaxy
bias changes the non-linear shifts on the acoustic scale relative to the
underlying dark matter distribution presented by Seo et al (2009). For the less
biased HOD models (b < 3), we do not detect any shift in the acoustic scale
relative to the no-bias case, typically 0.10% \pm 0.10%. However, the most
biased HOD models (b > 3) show a shift at moderate significance (0.79% \pm
0.31% for the most extreme case). We test the one-step reconstruction technique
introduced by Eisenstein et al. (2007) in the case of realistic galaxy bias and
shot noise. The reconstruction scheme increases the correlation between the
initial and final (z = 1) density fields achieving an equivalent level of
correlation at nearly twice the wavenumber after reconstruction. Reconstruction
reduces the shifts and errors on the shifts. We find that after reconstruction
the shifts from the galaxy cases and the dark matter case are consistent with
each other and with no shift. The 1-sigma systematic errors on the distance
measurements inferred from our BAO measurements with various HODs after
reconstruction are about 0.07% - 0.15%.Comment: Accepted by ApJ. 21 pages, 10 figure
The Optical Afterglow of GRB 011211
We present early-time optical photometry and spectroscopy of the optical
afterglow of the gamma-ray burst GRB 011211. The spectrum of the optical
afterglow contains several narrow metal lines which are consistent with the
burst occurring at a redshift of 2.140 +/- 0.001. The optical afterglow decays
as a power law with a slope of alpha = 0.83 +/- 0.04 for the first
approximately two days after the burst at which time there is evidence for a
break. The slope after the break is at least 1.4. There is evidence for rapid
variations in the R-band light approximately 0.5 days after the burst. These
variations suggest that there are density fluctuations near the gamma-ray burst
on spatial scales of approximately 40--125 AU. The magnitude of the break in
the light curve, the spectral slope, and the rate of decay in the optical,
suggest that the burst expanded into an ambient medium that is homogeneous on
large scales. We estimate that the local particle density is between
approximately 0.1 and 10 cm^{-3} and that the total gamma-ray energy in the
burst was 1.2--1.9 x 10^{50} erg. This energy is smaller than, but consistent
with, the ``standard'' value of (5 +/- 2) x 10^{50} erg. Comparing the observed
color of the optical afterglow with predictions of the standard beaming model
suggests that the rest-frame V-band extinction in the host galaxy is less than
approximately 0.03 mag.Comment: 17 pages, 4 figures, AASTeX 5.02, to appear in AJ Referee's report
incorporated, minor changes in the tex
Spectroscopic Discovery of the Supernova 2003dh Associated with GRB 030329
We present early observations of the afterglow of the Gamma-Ray Burst (GRB)
030329 and the spectroscopic discovery of its associated supernova SN 2003dh.
We obtained spectra of the afterglow of GRB 030329 each night from March 30.12
(0.6 days after the burst) to April 8.13 (UT) (9.6 days after the burst). The
spectra cover a wavelength range of 350 nm to 850 nm. The early spectra consist
of a power-law continuum (F_nu ~ nu^{-0.9}) with narrow emission lines
originating from HII regions in the host galaxy, indicating a low redshift of
z=0.1687. However, our spectra taken after 2003 Apr. 5 show broad peaks in flux
characteristic of a supernova. Correcting for the afterglow emission, we find
the spectrum of the supernova is remarkably similar to the type Ic `hypernova'
SN 1998bw. While the presence of supernovae have been inferred from the light
curves and colors of GRB afterglows in the past, this is the first direct,
spectroscopic confirmation that a subset of classical gamma-ray bursts
originate from supernovae.Comment: published by ApJ Letters; additional material avilable at
http://cfa-www.harvard.edu/cfa/oir/Research/GRB
Deep Photometry of GRB 041006 Afterglow: Hypernova Bump at Redshift z=0.716
We present deep optical photometry of the afterglow of gamma-ray burst (GRB)
041006 and its associated hypernova obtained over 65 days after detection (55
R-band epochs on 10 different nights). Our early data (t<4 days) joined with
published GCN data indicates a steepening decay, approaching F_nu ~t^{-0.6} at
early times (<<1 day) and F_nu ~t^{-1.3} at late times. The break at
t_b=0.16+-0.04 days is the earliest reported jet break among all GRB
afterglows. During our first night, we obtained 39 exposures spanning 2.15
hours from 0.62 to 0.71 days after the burst that reveal a smooth afterglow,
with an rms deviation of 0.024 mag from the local power-law fit, consistent
with photometric errors. After t~4 days, the decay slows considerably, and the
light curve remains approximately flat at R~24 mag for a month before decaying
by another magnitude to reach R~25 mag two months after the burst. This
``bump'' is well-fitted by a k-corrected light curve of SN1998bw, but only if
stretched by a factor of 1.38 in time. In comparison with the other GRB-related
SNe bumps, GRB 041006 stakes out new parameter space for GRB/SNe, with a very
bright and significantly stretched late-time SN light curve. Within a small
sample of fairly well observed GRB/SN bumps, we see a hint of a possible
correlation between their peak luminosity and their ``stretch factor'', broadly
similar to the well-studied Phillips relation for the type Ia supernovae.Comment: ApJ Letters, accepted. Additional material available at
ftp://cfa-ftp.harvard.edu/pub/kstanek/GRB041006
Distribution function approach to redshift space distortions. Part IV: perturbation theory applied to dark matter
We develop a perturbative approach to redshift space distortions (RSD) using
the phase space distribution function approach and apply it to the dark matter
redshift space power spectrum and its moments. RSD can be written as a sum over
density weighted velocity moments correlators, with the lowest order being
density, momentum density and stress energy density. We use standard and
extended perturbation theory (PT) to determine their auto and cross
correlators, comparing them to N-body simulations. We show which of the terms
can be modeled well with the standard PT and which need additional terms that
include higher order corrections which cannot be modeled in PT. Most of these
additional terms are related to the small scale velocity dispersion effects,
the so called finger of god (FoG) effects, which affect some, but not all, of
the terms in this expansion, and which can be approximately modeled using a
simple physically motivated ansatz such as the halo model. We point out that
there are several velocity dispersions that enter into the detailed RSD
analysis with very different amplitudes, which can be approximately predicted
by the halo model. In contrast to previous models our approach systematically
includes all of the terms at a given order in PT and provides a physical
interpretation for the small scale dispersion values. We investigate RSD power
spectrum as a function of \mu, the cosine of the angle between the Fourier mode
and line of sight, focusing on the lowest order powers of \mu and multipole
moments which dominate the observable RSD power spectrum. Overall we find
considerable success in modeling many, but not all, of the terms in this
expansion.Comment: 37 pages, 13 figures, published in JCA
Mc Neil's Nebula in Orion: The Outburst History
We present a sequence of I-band images obtained at the Venezuela 1m Schmidt
telescope during the outburst of the nebula recently discovered by J.W. McNeil
in the Orion L1630 molecular cloud. We derive photometry spanning the
pre-outburst state and the brightening itself, a unique record including 14
epochs and spanning a time scale of ~5 years. We constrain the beginning of the
outburst at some time between Oct. 28 and Nov. 15, 2003. The light curve of the
object at the vertex of the nebula, the likely exciting source of the outburst,
reveals that it has brightened ~5 magnitudes in about 4 months. The time scale
for the nebula to develop is consistent with the light travel time, indicating
that we are observing light from the central source scattered by the ambient
cloud into the line of sight. We also show recent FLWO optical spectroscopy of
the exciting source and of the nearby HH 22. The spectrum of the source is
highly reddened; in contrast, the spectrum of HH 22 shows a shock spectrum
superimposed on a continuum, most likely due to reflected light from the
exciting source reaching the HH object through a much less reddened path. The
blue portion of this spectrum is consistent with an early B spectral type,
similar to the early outburst spectrum of the FU Ori variable V1057 Cyg; we
estimate a luminosity of L ~219 Lsun. The eruptive behavior of the McNeil
nebula source, its spectroscopic characteristics and luminosity, suggest we may
be witnessing an FU Ori event on its way to maximum. Further monitoring of this
object will decide whether it qualifies as a member of this rare class of
objects.Comment: 13 pages, 4 figures, accepted to ApJ Letter
Cosmological Constraints from Galaxy Clustering and the Mass-to-Number Ratio of Galaxy Clusters
We place constraints on the average density (Omega_m) and clustering
amplitude (sigma_8) of matter using a combination of two measurements from the
Sloan Digital Sky Survey: the galaxy two-point correlation function, w_p, and
the mass-to-galaxy-number ratio within galaxy clusters, M/N, analogous to
cluster M/L ratios. Our w_p measurements are obtained from DR7 while the sample
of clusters is the maxBCG sample, with cluster masses derived from weak
gravitational lensing. We construct non-linear galaxy bias models using the
Halo Occupation Distribution (HOD) to fit both w_p and M/N for different
cosmological parameters. HOD models that match the same two-point clustering
predict different numbers of galaxies in massive halos when Omega_m or sigma_8
is varied, thereby breaking the degeneracy between cosmology and bias. We
demonstrate that this technique yields constraints that are consistent and
competitive with current results from cluster abundance studies, even though
this technique does not use abundance information. Using w_p and M/N alone, we
find Omega_m^0.5*sigma_8=0.465+/-0.026, with individual constraints of
Omega_m=0.29+/-0.03 and sigma_8=0.85+/-0.06. Combined with current CMB data,
these constraints are Omega_m=0.290+/-0.016 and sigma_8=0.826+/-0.020. All
errors are 1-sigma. The systematic uncertainties that the M/N technique are
most sensitive to are the amplitude of the bias function of dark matter halos
and the possibility of redshift evolution between the SDSS Main sample and the
maxBCG sample. Our derived constraints are insensitive to the current level of
uncertainties in the halo mass function and in the mass-richness relation of
clusters and its scatter, making the M/N technique complementary to cluster
abundances as a method for constraining cosmology with future galaxy surveys.Comment: 23 pages, submitted to Ap
The CIDA Variability Survey of Orion OB1. I: the low-mass population of Ori OB 1a and 1b
We present results of a large scale, multi-epoch optical survey of the Ori
OB1 association, carried out with the QuEST camera at the Venezuela National
Astronomical Observatory. We identify for the first time the widely spread
low-mass, young population in the Orion OB1a and OB1b sub-associations.
Candidate members were picked up by their variability in the V-band and
position in color-magnitude diagrams. We obtained spectra to confirm
membership. In a region spanning ~ 68 deg^2 we found 197 new young stars; of
these, 56 are located in the Ori OB1a subassociation and 142 in Ori OB1b.
Comparison with the spatial extent of molecular gas and extinction maps
indicates that the subassociation Ori 1b is concentrated within a ring-like
structure of radius ~2 deg (~15 pc at 440 pc), centered roughly on the star
epsilon Ori in the Orion belt. The ring is apparent in 13CO and corresponds to
a region with an extinction Av>=1. The stars exhibiting strong Ha emission, an
indicator of active accretion, are found along this ring, while the center is
populated with weak Ha emitting stars. In contrast, Ori OB1a is located in a
region devoid of gas and dust. We identify a grouping of stars within a ~3
deg^2 area located in 1a, roughly clustered around the B2 star 25 Ori. The
Herbig Ae/Be star V346 Ori is also associated with this grouping, which could
be an older analog of sigma Ori. Using using several sets of evolutionary
tracks we find an age of 7 - 10 Myr for Ori 1a and of ~4 - 6 Myr for Ori OB1b,
consistent with previous estimates from OB stars. Indicators such as the
equivalent width of Ha and near-IR excesses show that while a substantial
fraction of accreting disks remain at ages ~5 Myr, inner disks are essentially
dissipated by 10 Myr.Comment: 44 pages, 12 figures, to appear in the Astronomical Journal.
(Abridged abstract - to fit length limit in astroph) Full resolution figures
in http://www.cida.ve/~briceno/publications
The clustering of massive galaxies at z~0.5 from the first semester of BOSS data
We calculate the real- and redshift-space clustering of massive galaxies at
z~0.5 using the first semester of data by the Baryon Oscillation Spectroscopic
Survey (BOSS). We study the correlation functions of a sample of 44,000 massive
galaxies in the redshift range 0.4<z<0.7. We present a halo-occupation
distribution modeling of the clustering results and discuss the implications
for the manner in which massive galaxies at z~0.5 occupy dark matter halos. The
majority of our galaxies are central galaxies living in halos of mass
10^{13}Msun/h, but 10% are satellites living in halos 10 times more massive.
These results are broadly in agreement with earlier investigations of massive
galaxies at z~0.5. The inferred large-scale bias (b~2) and relatively high
number density (nbar=3e-4 h^3 Mpc^{-3}) imply that BOSS galaxies are excellent
tracers of large-scale structure, suggesting BOSS will enable a wide range of
investigations on the distance scale, the growth of large-scale structure,
massive galaxy evolution and other topics.Comment: 11 pages, 12 figures, matches version accepted by Ap
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