1,988 research outputs found
A High Signal-to-Noise Ratio Composite Spectrum of Gamma-ray Burst Afterglows
We present a composite spectrum of 60 long duration gamma-ray burst (GRB)
afterglows with redshifts in the range 0.35<z<6.7 observed with low resolution
optical spectra. The composite spectrum covers the wavelength range 700-6600 A
in the rest frame and has a mean signal-to-noise ratio of 150 per 1 A pixel and
reaches a maximum of ~300 in the range 2500-3500 A. Equivalent widths are
measured from metal absorption lines from the Lya line to ~5200 A, and
associated metal and hydrogen lines are identified between the Lyman break and
Lya line. The average transmission within the Lyman forest is consistent with
that found along quasar lines of sight. We find a temporal variation in fine
structure lines when dividing the sample into bursts observed within 2 hours
from their trigger and those observed later. Other lines in the predominantly
neutral gas show variations too, but this is most likely a random effect caused
by weighting of individual strong absorption lines and which mimics a temporal
variation. Bursts characterized with high or low prompt GRB energy release
produce afterglows with similar absorption line strengths, and likewise for
bursts with bright or faint optical afterglows. Bursts defined as dark from
their optical to X-ray spectral index have stronger absorption lines relative
to the optically bright bursts. The composite spectrum has strong CaII and MgII
absorption lines as commonly found in dusty galaxies, however, we find no
evidence for dust or a significant molecular content based on the non-detection
of diffuse interstellar bands. Compared to starburst galaxy spectra, the GRB
composite has much stronger fine structure lines, while metal absorption lines
are weaker.Comment: Accepted for publication in ApJ, 24 page
Evidence for Ubiquitous Collimated Galactic-Scale Outflows along the Star-Forming Sequence at z~0.5
We present an analysis of the MgII 2796, 2803 and FeII 2586, 2600 absorption
line profiles in individual spectra of 105 galaxies at 0.3<z<1.4. The galaxies,
drawn from redshift surveys of the GOODS fields and the Extended Groth Strip,
fully sample the range in star formation rates (SFRs) occupied by the
star-forming sequence with stellar masses log M_*/M_sun > 9.5 at 0.3<z<0.7.
Using the Doppler shifts of the MgII and FeII absorption lines as tracers of
cool gas kinematics, we detect large-scale winds in 66+/-5% of the galaxies.
HST/ACS imaging and our spectral analysis indicate that the outflow detection
rate depends primarily on galaxy orientation: winds are detected in ~89% of
galaxies having inclinations (i) <30 degrees (face-on), while the wind
detection rate is only ~45% in objects having i>50 degrees (edge-on). Combined
with the comparatively weak dependence of the wind detection rate on intrinsic
galaxy properties, this suggests that biconical outflows are ubiquitous in
normal, star-forming galaxies at z~0.5. We find that the wind velocity is
correlated with host galaxy M_* at 3.4-sigma significance, while the equivalent
width of the flow is correlated with host galaxy SFR at 3.5-sigma significance,
suggesting that hosts with higher SFR may launch more material into outflows
and/or generate a larger velocity spread for the absorbing clouds. Assuming
that the gas is launched into dark matter halos with simple, isothermal density
profiles, the wind velocities measured for the bulk of the cool material
(~200-400 km/s) are sufficient to enable escape from the halo potentials only
for the lowest-M_* systems in the sample. However, the outflows typically carry
sufficient energy to reach distances of >50 kpc, and may therefore be a viable
source of cool material for the massive circumgalactic medium observed around
bright galaxies at z~0. [abridged]Comment: Submitted to ApJ. 61 pages, 25 figures, 4 tables, 4 appendices. Uses
emulateapj forma
Chemical Abundances of the Damped Lya Systems at z>1.5
We present chemical abundance measurements for 19 damped lya systems observed
with HIRES on the 10m W.M. Keck Telescope. Our principal goal is to investigate
the abundance patterns of the damped systems and thereby determine the
underlying physical processes which dominate their chemical evolution. We place
particular emphasis on gauging the relative importance of two complementary
effects often invoked to explain the damped lya abundances: (1) nucleosynthetic
enrichment from Type II supernovae and (2) an ISM-like dust depletion pattern.
Similar to the principal results of Lu et al. (1996), our observations lend
support both for dust depletion and Type II SN enrichment. Specifically, the
observed overabundance of Zn/Fe and underabundance of Ni/Fe relative to solar
abundances suggest significant dust depletion within the damped lya systems.
Meanwhile, the relative abundances of Al, Si, and Cr vs. Fe are consistent with
both dust depletion and Type II supernova enrichment. Our measurements of Ti/Fe
and the Mn/Fe measurements from Lu et al. (1996), however, cannot be explained
by dust depletion and indicate an underlying Type II SN pattern. Finally, the
observed values of [S/Fe] are inconsistent with the combined effects of dust
depletion and the nucleosynthetic yields expected for Type II supernovae. This
last result emphasizes the need for another physical process to explain the
damped lya abundance patterns.
We also examine the metallicity of the damped lya systems both with respect
to Zn/H and Fe/H. Our results confirm previous surveys by Pettini and
collaborators, i.e., [] = -1.15 +/- 0.15 dex. [abridged]Comment: 18 pages, 4 embedded figures, 20 additional figures. Accepted to the
Astrophysical Journal 10/20/98. Uses Latex2e, emualteapj.sty, and
onecolfloat.st
The host of the SN-less GRB 060505 in high resolution
The spiral host galaxy of GRB 060505 at z=0.089 was the site of a puzzling
long duration burst without an accompanying supernova. Studies of the burst
environment by Th\"one et al. (2008) suggested that this GRB came from the
collapse of a massive star and that the GRB site was a region with properties
different from the rest of the galaxy. We reobserved the galaxy in high spatial
resolution using the VIMOS integral-field unit (IFU) at the VLT with a spaxel
size of 0.67 arcsec. Furthermore, we use long slit high resolution data from
HIRES/Keck at two different slit positions covering the GRB site, the center of
the galaxy and an HII region next to the GRB region. We compare the properties
of different HII regions in the galaxy with the GRB site and study the global
and local kinematic properties of this galaxy. The resolved data show that the
GRB site has the lowest metallicity in the galaxy with around 1/3 Z_solar, but
its specific SFR (SSFR) of 7.4 M_solar/yr/L/L* and age (determined by the
Halpha EW) are similar to other HII regions in the host. The galaxy shows a
gradient in metallicity and SSFR from the bulge to the outskirts as it is
common for spiral galaxies. This gives further support to the theory that GRBs
prefer regions of higher star-formation and lower metallicity, which, in S-type
galaxies, are more easily found in the spiral arms than in the centre.
Kinematic measurements of the galaxy do not show evidence for large
perturbations but a minor merger in the past cannot be excluded. This study
confirms the collapsar origin of GRB060505 but reveals that the properties of
the HII region surrounding the GRB were not unique to that galaxy. Spatially
resolved observations are key to know the implications and interpretations of
unresolved GRB hosts observations at higher redshifts.Comment: 15 pages, 16 figures, 6 tables; resubmitted to MNRAS after minor
revision
No evidence for feedback: Unexceptional Low-ionization winds in Host galaxies of Low Luminosity Active Galactic Nuclei at Redshift z ~1
We study winds in 12 X-ray AGN host galaxies at z ~ 1. We find, using the
low-ionization Fe II 2586 absorption in the stacked spectra, that the
probability distribution function (PDF) of the centroid velocity shift in AGN
has a median, 16th and 84th percentiles of (-87, -251, +86) km/s respectively.
The PDF of the velocity dispersion in AGN has a median, 84th and 16th
percentile of (139, 253, 52) km/s respectively. The centroid velocity and the
velocity dispersions are obtained from a two component (ISM+wind) absorption
line model. The equivalent width PDF of the outflow in AGN has median, 84th and
16th percentiles of (0.4, 0.8, 0.1) Angstrom. There is a strong ISM component
in Fe II 2586 absorption with (1.2, 1.5, 0.8) Angstrom, implying presence of
substantial amount cold gas in the host galaxies. For comparison, star-forming
and X-ray undetected galaxies at a similar redshift, matched roughly in stellar
mass and galaxy inclination, have a centroid velocity PDF with percentiles of
(-74, -258, +90) km/s, and a velocity dispersion PDF percentiles of (150, 259,
57) km/s. Thus, winds in the AGN are similar to star-formation-driven winds,
and are too weak to escape and expel substantial cool gas from galaxies. Our
sample doubles the previous sample of AGN studied at z ~ 0.5 and extends the
analysis to z ~ 1. A joint reanalysis of the z ~ 0.5 AGN sample and our sample
yields consistent results to the measurements above.Comment: 24 pages, 11 figures, accepted in Ap
An Observational Determination of the Proton to Electron Mass Ratio in the Early Universe
In an effort to resolve the discrepancy between two measurements of the
fundamental constant mu, the proton to electron mass ratio, at early times in
the universe we reanalyze the same data used in the earlier studies. Our
analysis of the molecular hydrogen absorption lines in archival VLT/UVES
spectra of the damped Lyman alpha systems in the QSOs Q0347-383 and Q0405-443
yields a combined measurement of a (Delta mu)/mu value of (-7 +/- 8) x 10^{-6},
consistent with no change in the value of mu over a time span of 11.5
gigayears. Here we define (Delta mu) as (mu_z - mu_0) where mu_z is the value
of mu at a redshift of z and mu_0 is the present day value. Our null result is
consistent with the recent measurements of King et al. 2009, (Delta mu)/u =
(2.6 +/- 3.0) x 10^{-6}, and inconsistent with the positive detection of a
change in mu by Reinhold et al. 2006. Both of the previous studies and this
study are based on the same data but with differing analysis methods.
Improvements in the wavelength calibration over the UVES pipeline calibration
is a key element in both of the null results. This leads to the conclusion that
the fundamental constant mu is unchanged to an accuracy of 10^{-5} over the
last 80% of the age of the universe, well into the matter dominated epoch. This
limit provides constraints on models of dark energy that invoke rolling scalar
fields and also limits the parameter space of Super Symmetric or string theory
models of physics. New instruments, both planned and under construction, will
provide opportunities to greatly improve the accuracy of these measurements.Comment: Accepted for publication in the Astrophysical Journa
Uncovering strong MgII absorbing galaxies: Imaging below the Lyman limit
The nature of the galaxies that give rise to absorption lines, such as damped
Lyman-alpha systems (DLAs) or strong MgII lines, in quasar spectra is difficult
to investigate in emission. Taking advantage of the total absorption of the QSO
light bluewards of the Lyman limit of two DLAs at z>3.4, we look for the
continuum emission from intervening galaxies at z~2 that are identified via
strong metal absorption lines. The MgII absorbers have equivalent width large
enough to be potential DLA systems. Deep images are obtained with the FOcal
Reducer and Spectrograph (FORS1) on the Very Large Telescope for the fields
towards SDSS J110855+120953 and SDSS J140850+020522. These quasars have MgII
absorption lines at z=1.87 (W_r(MgII)=2.46 A) and z=1.98 (W_r(MgII)=1.89 A),
respectively, and each QSO has two intervening higher redshift DLAs at z>3. The
U and R bands of FORS1 lie blue and redwards of the Lyman limit of the
background DLAs, allowing us to search for emission from the foreground
galaxies directly along the lines of sight to the QSOs. No galaxies are found
close to the sight line of the QSO to a point source limit of U_AB=28.0. In
both fields, the closest objects lie at an impact parameter of 5 arcsec
corresponding to 40 kpc in projection at z=2, and have typical colours of star
forming galaxies at that redshift. However, the currently available data do not
allow us to confirm if the galaxies lie at the same redshifts as the absorption
systems. A more extended structure is visible in the SDSS J14085+020522 field
at an impact parameter of 0.8 arcsec or 7 kpc. If these objects are at z~2
their luminosities are 0.03-0.04 L* in both fields. The star formation rates
estimated from the UV flux are 0.5-0.6 M_sun yr^-1. (Abbreviated).Comment: 10 pages, Accepted for publication in A&
Probing the ISM Near Star Forming Regions with GRB Afterglow Spectroscopy: Gas, Metals, and Dust
We study the chemical abundances of the interstellar medium surrounding high
z gamma-ray bursts (GRBs) through analysis of the damped Lya systems (DLAs)
identified in afterglow spectra. These GRB-DLAs are characterized by large HI
column densities N(HI) and metallicities [M/H] spanning 1/100 to nearly solar,
with median [M/H]>-1. The majority of GRB-DLAs have [M/H] values exceeding the
cosmic mean metallicity of atomic gas at z>2, i.e. if anything, the GRB-DLAs
are biased to larger metallicity. We also observe (i) large [Zn/Fe] values
(>+0.6) and sub-solar Ti/Fe ratios which imply substantial differential
depletion, (ii) large a/Fe ratios suggesting nucleosynthetic enrichment by
massive stars, and (iii) low C^0/C^+ ratios (<10^{-4}). Quantitatively, the
observed depletion levels and C^0/C^+ ratios of the gas are not characteristic
of cold, dense HI clouds in the Galactic ISM. We argue that the GRB-DLAs
represent the ISM near the GRB but not gas directly local to the GRB (e.g. its
molecular cloud or circumstellar material). We compare these observations with
DLAs intervening background quasars (QSO-DLAs). The GRB-DLAs exhibit larger
N(HI) values, higher a/Fe and Zn/Fe ratios, and have higher metallicity than
the QSO-DLAs. We argue that the differences primarily result from
galactocentric radius-dependent differences in the ISM: GRB-DLAs preferentially
probe denser, more depleted, higher metallicity gaslocated in the inner few kpc
whereas QSO-DLAs are more likely to intersect the less dense, less enriched,
outer regions of the galaxy. Finally, we investigate whether dust obscuration
may exclude GRB-DLA sightlines from QSO-DLA samples; we find that the majority
of GRB-DLAs would be recovered which implies little observational bias against
large N(HI) systems.Comment: 16 pages, 9 figures. Submitted to Ap
Further Evidence for Cosmological Evolution of the Fine Structure Constant
We describe the results of a search for time variability of the fine
structure constant, alpha, using absorption systems in the spectra of distant
quasars. Three large optical datasets and two 21cm/mm absorption systems
provide four independent samples, spanning 23% to 87% of the age of the
universe. Each sample yields a smaller alpha in the past and the optical sample
shows a 4-sigma deviation: da/a = -0.72 +/- 0.18 x 10^{-5} over the redshift
range 0.5 < z < 3.5. We find no systematic effects which can explain our
results. The only potentially significant systematic effects push da/a towards
positive values, i.e. our results would become more significant were we to
correct for them.Comment: 5 pages, 1 figure. Published in Phys. Rev. Lett. Small changes to
discussion, added an acknowledgement and a referenc
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