94 research outputs found
Dust extinction bias in the column density distribution of gamma-ray bursts; high column density, low redshift GRBs are more heavily obscured
The afterglows of gamma-ray bursts (GRBs) have more soft X-ray absorption
than expected from the foreground gas column in the Galaxy. While the redshift
of the absorption can in general not be constrained from current X-ray
observations, it has been assumed that the absorption is due to metals in the
host galaxy of the GRB. The large sample of X-ray afterglows and redshifts now
available allows the construction of statistically meaningful distributions of
the metal column densities. We construct such a sample and show, as found in
previous studies, that the typical absorbing column density (N_HX) increases
substantially with redshift, with few high column density objects found at low
to moderate redshifts. We show, however, that when highly extinguished bursts
are included in the sample, using redshifts from their host galaxies, high
column density sources are also found at low to moderate redshift. We infer
from individual objects in the sample and from observations of blazars, that
the increase in column density with redshift is unlikely to be related to
metals in the intergalactic medium or intervening absorbers. Instead we show
that the origin of the apparent increase with redshift is primarily due to dust
extinction bias: GRBs with high X-ray absorption column densities found at
typically have very high dust extinction column densities, while
those found at the highest redshifts do not. It is unclear how such a strongly
evolving N_HX/A_V ratio would arise, and based on current data, remains a
puzzle.Comment: 7 pages, 3 figures. Accepted for publication in ApJ, 1 August 201
The metals-to-dust ratio to very low metallicities using GRB and QSO absorbers; extremely rapid dust formation
Among the key parameters defining the ISM of galaxies is the fraction of the
metals that are locked up in dust: the metals-to-dust ratio. This ratio bears
not only on the ISM and its evolution, but particularly on the origin of cosmic
dust. We combine extinction and abundance data from GRB afterglows, from QSO
absorbers, as well as from galaxy-lensed QSOs, to determine the metals-to-dust
ratios for lines-of-sight through a wide diversity of galaxies from blue, dwarf
starbursts to massive ellipticals, across a vast range in redshift z=0.1-6.3,
and nearly three orders of magnitude in column density and metal abundance. We
thus determine the metals-to-dust ratio in a unique way, providing direct
determinations of in situ gas and dust columns without recourse to assumptions
with large uncertainties. We find that the metals-to-dust ratios in these
systems are surprisingly close to the value for the local group (10^{21.3} cm-2
A_V mag-1), with a mean value of 10^{21.2} cm-2 A_V mag-1 and a standard
deviation of 0.3 dex. There is no evidence of deviation from this mean ratio as
a function of metallicity, even down to our lowest metallicity of 0.01 Z/Z_sun.
The lack of any obvious dependence of the metals-to-dust ratio on either column
density, galaxy type or age, redshift, or metallicity indicates a close
correspondence between the formation of the metals and the formation of dust.
Any delay between the formation of metals and dust must be shorter than the
typical metal-enrichment times of these galaxies. Formation of the bulk of the
dust in low mass stars is therefore ruled out by these data at any cosmic
epoch. Furthermore, dust destruction must not dominate over formation/growth in
virtually any galaxy environment. The correlation between metals and dust is a
natural consequence of the formation of the bulk of dust in SNe [Abridged].Comment: 6 pages, 3 figures, 1 tabl
The Galactic dust-to-metals ratio and metallicity using gamma-ray bursts
The metallicity and dust-to-metals ratio of the Galaxy are fundamental
parameters in understanding the ISM, but there is still uncertainty surrounding
these parameters. In this paper, the dust-to-metals ratio in the Galaxy is
determined using the photoelectric absorption of the X-ray afterglows of a
sample of several hundred gamma-ray bursts (GRBs) to determine the metal column
density in combination with Galactic dust maps to determine the line-of-sight
dust extinction through the Galaxy in the direction of the GRB. GRB afterglows
often have large extragalactic soft X-ray absorptions and therefore the GRB
sample's upper-bound will define the Galactic dust-to-metals relation. Using a
two-dimensional two-sample KS test, we determine this upper-bound and so derive
the dust-to-metals ratio of the Galaxy. We find N_H = 2.2^{+0.3}_{-0.4}e21
cm^-2 A_V assuming solar, Anders & Grevesse (1989), metallicity. This result is
consistent with previous findings using bright X-ray sources in the Galaxy.
Using the same technique but substituting the HI maps from the
Leiden-Argentine-Bonn survey for the dust maps, allows us to place a limit on
the metallicity in the Galaxy. We find a metallicity consistent with the Anders
& Grevesse (1989) solar values often used in X-ray fitting. Based on this and
previous studies, we suggest that the metallicity of a typical ISM sightline
through the Galaxy is ~0.25 dex higher than the current best estimate of the
solar metallicity. We further show that the dust-to-gas ratio seems to be
correlated with the total gas column density, and that this may be due to the
metallicity gradient observed toward the Galactic centre. Based on the
non-constant nature of the dust-to-gas ratio, we propose that the dust column
density, at N_H = 2.2e21 cm^-2 A_V, represents a better proxy for the soft
X-ray absorption column density than HI maps.Comment: A&A in press, 6 pages, 3 figure
On inferring extinction laws in z~6 quasars as signatures of supernova dust
Unusual extinction curves of high-redshift QSOs have been taken as evidence
that dust is primarily produced by supernovae at high redshift. In particular,
the 3000 A Todini-Ferrara-Maiolino kink in the extinction curve of the z = 6.20
SDSS J1048+4637 has been attributed to supernova dust. Here we discuss the
challenges in inferring robust extinction curves of high-redshift QSOs and
critically assess previous claims of detection of supernova dust. In
particular, we address the sensitivity to the choice of intrinsic QSO spectrum,
the need for a long wavelength baseline, and the drawbacks in fitting
theoretical extinction curves. In a sample of 21 QSOs at z ~ 6 we detect
significant ultraviolet extinction using existing broad-band optical,
near-infrared, and Spitzer photometry. The median extinction curve is
consistent with a Small Magellanic Cloud curve with A_1450 ~ 0.7 mag and does
not exhibit any conspicuous (restframe) 2175 A or 3000 A features. For two
QSOs, SDSS J1044-0125 at z = 5.78 and SDSS J1030+0524 at z = 6.31, we further
present X-shooter spectra covering the wavelength range 0.9-2.5 um. The
resulting non-parametric extinction curves do not exhibit the 3000 A kink.
Finally, in a re-analysis of literature spectra of SDSS J1048+4637, we do not
find evidence for a conspicuous kink. We conclude that the existing evidence
for a 3000 A feature is weak and that the overall dust properties at high and
low redshift show no significant differences. This, however, does not preclude
supernovae from dominating the dust budget at high redshift.Comment: 13 pages, 13 figures, ApJ, in pres
Dusting off the diffuse interstellar bands: DIBs and dust in extragalactic SDSS spectra
Using over a million and a half extragalactic spectra we study the properties
of the mysterious Diffuse Interstellar Bands (DIBs) in the Milky Way. These
data provide us with an unprecedented sampling of the skies at high
Galactic-latitude and low dust-column-density. We present our method, study the
correlation of the equivalent width of 8 DIBs with dust extinction and with a
few atomic species, and the distribution of four DIBs - 5780.6A, 5797.1A,
6204.3A, and 6613.6A - over nearly 15000 squared degrees. As previously found,
DIBs strengths correlate with extinction and therefore inevitably with each
other. However, we show that DIBs can exist even in dust free areas.
Furthermore, we find that the DIBs correlation with dust varies significantly
over the sky. DIB under- or over-densities, relative to the expectation from
dust, are often spread over hundreds of square degrees. These patches are
different for the four DIBs, showing that they are unlikely to originate from
the same carrier, as previously suggested.Comment: MNRAS accepte
On Inferring Extinction Laws in Z -approximately 6 Quasars as Signatures of Supernova Dust
Unusual extinction curves of high-redshift QSOs have been taken as evidence that dust is primarily produced by supernovae at high redshift. In particular, the 3000 A Todini-Ferrara-Maiolino kink in the extinction curve of the z = 6.20 SDSS J1048+4637 has been attributed to supernova dust. Here we discuss the challenges in inferring robust extinction curves of high-redshift QSOs and critically assess previous claims of detection of supernova dust. In particular, we address the sensitivity to the choice of intrinsic QSO spectrum, the need for a long wavelength baseline, and the drawbacks in fitting theoretical extinction curves. In a sample of 21 QSOs at z 6 we detect significant ultraviolet extinction using existing broad-band optical, near-infrared, and Spitzer photometry. The median extinction curve is consistent with a Small Magellanic Cloud curve with A1450 0.7 mag and does not exhibit any conspicuous (restframe) 2175 A or 3000 A features. For two QSOs, SDSS J10440125 at z = 5.78 and SDSS J1030+0524 at z = 6.31, we further present X-shooter spectra covering the wavelength range 0.9-2.5 m. The resulting non-parametric extinction curves do not exhibit the 3000 A kink. Finally, in a re-analysis of literature spectra of SDSS J1048+4637, we do not find evidence for a conspicuous kink. We conclude that the existing evidence for a 3000 A feature is weak and that the overall dust properties at high and low redshift show no significant differences. This, however, does not preclude supernovae from dominating the dust budget at high redshift
A merger in the dusty, galaxy A1689-zD1?
The gravitationally-lensed galaxy A1689-zD1 is one of the most distant
spectroscopically confirmed sources (). It is the earliest known galaxy
where the interstellar medium (ISM) has been detected; dust emission was
detected with the Atacama Large Millimetre Array (ALMA). A1689-zD1 is also
unusual among high-redshift dust emitters as it is a sub-L* galaxy and is
therefore a good prospect for the detection of gaseous ISM in a more typical
galaxy at this redshift. We observed A1689-zD1 with ALMA in bands 6 and 7 and
with the Green Bank Telescope (GBT) in band . To study the structure of
A1689-zD1, we map the mm thermal dust emission and find two spatial components
with sizes about \,kpc (lensing-corrected). The rough spatial
morphology is similar to what is observed in the near-infrared with {\it HST}
and points to a perturbed dynamical state, perhaps indicative of a major merger
or a disc in early formation. The ALMA photometry is used to constrain the
far-infrared spectral energy distribution, yielding a dust temperature (--\,K for ). We do not detect the CO(3-2) line
in the GBT data with a 95\% upper limit of 0.3\,mJy observed. We find a slight
excess emission in ALMA band~6 at 220.9\,GHz. If this excess is real, it is
likely due to emission from the [CII] 158.8\,m line at . The stringent upper limits on the [CII]/ luminosity ratio
suggest a [CII] deficit similar to several bright quasars and massive
starbursts.Comment: 9 pages, accepted to MNRAS, in pres
Measuring the Hubble constant with kilonovae using the Expanding Photosphere Method
While gravitational wave (GW) standard sirens from neutron star (NS) mergers
have been proposed to offer good measurements of the Hubble constant, we show
in this paper how a variation of the expanding photosphere method (EPM) or
spectral-fitting expanding atmosphere method, applied to the kilonovae (KNe)
associated with the mergers, can provide an independent and potentially
percent-accurate distance measurement to individual mergers. The KN-EPM
overcomes the major uncertainties commonly associated with this method in
supernovae for four reasons: 1) the early continuum is very well-reproduced by
a blackbody spectrum, 2) the dilution effect from electron scattering opacity
is likely negligible, 3) the explosion times are exactly known due to the GW
detection, and 4) the ejecta geometry is, at least in some cases, highly
spherical and can be constrained from line-shape analysis. We provide an
analysis of the early VLT/X-shooter spectra AT2017gfo showing how the
luminosity distance can be determined, and find a luminosity distance of Mpc in agreement with, but more precise than, previous methods. We
investigate the dominant systematic uncertainties, but our simple framework,
which assumes a blackbody photosphere, does not account for the full
time-dependent, three-dimensional radiative transfer effects, so this distance
should be treated as preliminary. The luminosity distance corresponds to an
estimated Hubble constant of kmsMpc,
where the dominant uncertainty is due to the modelling of the host peculiar
velocity. We also estimate the expected constraints on from future
KN-EPM-analysis with the upcoming O4 and O5 runs of the LIGO collaboration
GW-detectors, where 5-10 similar KNe would yield 1% precision cosmological
constraints.Comment: Submitted to A&A. Comments are welcom
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