30 research outputs found
Dust in the early Universe: Evidence for non-stellar dust production or observational errors?
Observations have revealed unexpectedly large amounts of dust in
high-redshift galaxies and its origin is still much debated. Valiante et al.
(2009, MNRAS, 397, 1661) suggested the net stellar dust production of the
quasar host galaxy SDSS J1148+5251 may be sufficient to explain the large dust
mass detected in this galaxy, albeit under some very special assumptions (e.g.,
'closed box' evolution and a rather high gas mass). Here it is shown that since
accretion of essentially pristine material may lower the efficiency of dust
formation significantly, and the observationally derived dust-to-gas ratios for
these high-redshift galaxies are remarkably high, stellar dust production is
likely insufficient. A model including metallicity-dependent, non-stellar dust
formation ('secondary dust') is presented. The required contribution from this
non-stellar dust component appears too large, however. If all observational
constraints are to be met, the resultant dust-to-metals ratio is close to
unity, which means that almost all interstellar metals exist in the form dust.
This is a very unlikely situation and suggests the large dust-to-gas ratios at
high-redshifts may be due to observational uncertainties and/or or incorrect
calibration of conversion factors for gas and dust tracers.Comment: 16 pages, 11 figures. Accepted for publication in MNRA
On the dust abundance gradients in late-type galaxies: I. Effects of destruction and growth of dust in the interstellar medium
We present basic theoretical constraints on the effects of destruction by
supernovae (SNe) and growth of dust grains in the interstellar medium (ISM) on
the radial distribution of dust in late-type galaxies. The radial gradient of
the dust-to-metals ratio is shown to be essentially flat (zero) if interstellar
dust is not destroyed by SN shock waves and all dust is produced in stars. If
there is net dust destruction by SN shock waves, the dust-to-metals gradient is
flatter than or equal to the metallicity gradient (assuming the gradients have
the same sign). Similarly, if there is net dust growth in the ISM, then the
dust-to-metals gradient is steeper than or equal to the metallicity gradient.
The latter result implies that if dust gradients are steeper than metallicity
gradients, i.e., the dust-to-metals gradients are not flat, then it is unlikely
dust destruction by SN shock waves is an efficient process, while dust growth
must be a significant mechanism for dust production. Moreover, we conclude that
dust-to-metals gradients can be used as a diagnostic for interstellar dust
growth in galaxy discs, where a negative slope indicates dust growth.Comment: 12 pages, 4 figures. Accepted for publication in MNRA
The origin of dust in galaxies revisited: the mechanism determining dust content
The origin of cosmic dust is a fundamental issue in planetary science. This
paper revisits the origin of dust in galaxies, in particular, in the Milky Way,
by using a chemical evolution model of a galaxy composed of stars, interstellar
medium, metals (elements heavier than helium), and dust. We start from a review
of time-evolutionary equations of the four components, and then, we present
simple recipes for the stellar remnant mass and yields of metal and dust based
on models of stellar nucleosynthesis and dust formation. After calibrating some
model parameters with the data from the solar neighborhood, we have confirmed a
shortage of the stellar dust production rate relative to the dust destruction
rate by supernovae if the destruction efficiency suggested by theoretical works
is correct. If the dust mass growth by material accretion in molecular clouds
is active, the observed dust amount in the solar neighborhood is reproduced. We
present a clear analytic explanation of the mechanism for determining dust
content in galaxies after the activation of accretion growth: a balance between
accretion growth and supernova destruction. Thus, the dust content is
independent of the uncertainty of the stellar dust yield after the growth
activation. The timing of the activation is determined by a critical metal mass
fraction which depends on the growth and destruction efficiencies. The solar
system formation seems to have occurred well after the activation and plenty of
dust would have existed in the proto-solar nebula.Comment: 12 pages, 11 figure
Dust formation in the ejecta of the Type II-P supernova 2004dj
Core-collapse supernovae (CC SNe), especially Type II-Plateau ones, are
thought to be important contributors to cosmic dust production. SN 2004dj, one
of the closest and brightest SN since 1987A, offered a good opportunity to
examine dust formation processes. To find signs of newly formed dust, we
analyze all available mid-infrared (MIR) archival data from the Spitzer Space
Telescope. We re-reduce and analyze data from IRAC, MIPS and IRS instruments
obtained between +98 and +1381 days after explosion, generating light curves
and spectra for each epoch. Observed spectral energy distributions are fitted
with both analytic and numerical models, using the radiative-transfer code
MOCASSIN for the latter ones. We also use imaging polarimetric data obtained at
+425 days by the Hubble Space Telescope. We present convincing evidence of dust
formation in the ejecta of SN 2004dj from MIR light curves and spectra.
Significant MIR excess flux is detected in all bands between 3.6 and 24 um. In
the optical, a ~0.8% polarization is also detected at a 2-sigma level, which
exceeds the interstellar polarization in that direction. Our analysis shows
that the freshly-formed dust around SN 2004dj can be modeled assuming a nearly
spherical shell containing amorphous carbon grains, cooling from ~700 K to ~400
K between +267 and +1246 days. Persistent excess flux has been found above 10
um, which is explained by a cold (~115 K) dust component. If this cold dust is
of circumstellar origin, it is likely to be condensed in a cool, dense shell
between the forward and reverse shocks. Pre-existing circumstellar dust is less
likely, but cannot be ruled out. An upper limit of ~8x10^{-4} M_sun was derived
for the dust mass, which is similar to previously published values for other
dust-producing SNe.Comment: 15 pages, 11 figures, 8 tables; accepted for publication in A&A; new
version after minor corrections and language editio
Life Satisfaction and the UK Citizenship Process: Do Tests and Ceremonies Enhance Immigrants' Lives?
Gaining citizenship in the UK requires applicants to pass a âLife in the UKâ test and (if successful) attend a citizenship ceremony. Critics of this policy agenda assert that it exacerbates exclusion of an already vulnerable and disadvantaged population. The UK government justifies the requirements in part on the basis that they facilitate integration, thus enhancing immigrantsâ lives. This article, using data from the UK longitudinal household survey (âUnderstanding Societyâ) considers outcomes for immigrants by investigating whether gaining citizenship in the current period is associated with immigrantsâ subjective wellâbeing. Results from regression models and matching analyses show that participating in the citizenship process (or not) is not generally associated with individualsâ life satisfaction