101 research outputs found
Supernova dust for the extinction law in a young infrared galaxy at z = 1
We apply the supernova(SN) extinction curves to reproduce the observed
properties of SST J1604+4304 which is a young infrared (IR) galaxy at z = 1.
The SN extinction curves used in this work were obtained from models of unmixed
ejecta of type II supernovae(SNe II) for the Salpeter initial mass function
(IMF) with a mass range from 8 to 30 M_sun or 8 to 40 M_sun.
The effect of dust distributions on the attenuation of starlight is
investigated by performing the chi-square fitting method against various dust
distributions. These are the commonly used uniform dust screen, the clumpy dust
screen, and the internal dust geometry. We add to these geometries three
scattering properties, namely, no-scattering, isotropic scattering, and
forward-only scattering. Judging from the chi-square values, we find that the
uniform screen models with any scattering property provide good approximations
to the real dust geometry. Internal dust is inefficient to attenuate starlight
and thus cannot be the dominant source of the extinction.
We show that the SN extinction curves reproduce the data of SST J1604+4304
comparable to or better than the Calzetti extinction curve. The Milky Way
extinction curve is not in satisfactory agreement with the data unless several
dusty clumps are in the line of sight. This trend may be explained by the
abundance of SN-origin dust in these galaxies; SN dust is the most abundant in
the young IR galaxy at z = 1, abundant in local starbursts, and less abundant
in the Galaxy. If dust in SST J1604+4304 is dominated by SN dust, the dust
production rate is about 0.1 M_sun per SN.Comment: 12 pages, 8 figures, 1 tabl
Mass-Metallicity Relation for the Local Group Dwarf Spheroidal Galaxies: A New Picture for the Chemical Enrichment of Galaxies in the Lowest Mass Range
The virial mass ()-metallicity relation among the Local Group
dwarf spheroidal galaxies (dSphs) is examined. Hirashita, Takeuchi, & Tamura
showed that the dSphs can be divided into two distinct classes with respect to
the relation between their virial masses and luminosities: low-mass (M_{\rm
vir} \la 10^8 M_\odot) and high-mass (M_{\rm vir} \ga 10^8 M_\odot) groups.
We see that both the mass-metallicity and the mass-luminosity relations of the
high-mass dSphs are understood as a low-mass extension of giant ellipticals. On
the contrary, we find that the classical galactic-wind model is problematic to
apply to the low-mass dSphs, whose low binding energy is comparable to that
released by several supernova explosions. A strongly regulated star formation
in their formation phase is required to reproduce their observed metallicity.
Such regulation is naturally expected in a gas cloud with the primordial
elemental abundance according to Nishi & Tashiro. A significant scatter in the
mass-metallicity relation for the low-mass dSphs is also successfully explained
along with the scenario of Hirashita and coworkers. We not only propose a new
picture for a chemical enrichment of the dSphs, but also suggest that the
mass-metallicity and the mass-luminosity relations be understood in a
consistent context.Comment: 14 pages LaTeX, 1 PostScript figure, to appear in ApJ Lette
Extremely red galaxies: dust attenuation and classification
We re-address the classification criterion for extremely red galaxies (ERGs)
of Pozzetti and Mannucci (2000 -- PM00), which aims to separate, in the Ic-K
(or Rc-K) vs. J-K colour--colour diagram, passively evolving, old (> 1 Gyr)
stellar populations in a dust-free environment, associated with ellipticals
(Es), from dusty starburst galaxies (DSGs), both at 1 < z < 2. We explore a
category of objects not considered previously, i.e., galaxies forming in this
redshift range on short (0.1 Gyr) timescales and observed also in their early,
dusty post-starburst phase. We also investigate the impact of structure of the
dusty medium and dust amount on the observed optical/near-IR colours of high-z
DSGs/DPSGs, through multiple-scattering radiative transfer calculations for a
dust/stars configuration and an extinction function calibrated with nearby
dusty starbursts. As a main result, we find that dusty post-starburst galaxies
(DPSGs), with ages between 0.2 and 1 Gyr, at 1.3 < z < 2 mix with Es at 1 < z <
2 for a large range in dust amount. This ``intrusion'' is a source of concern
for the present two-colour classification of ERGs. On the other hand, we
confirm, in agreement with PM00, that DSGs are well separated from Es, both at
1 < z < 2, in the Ic-K vs. J-K colour--colour diagram, whatever the structure
(two-phase clumpy or homogeneous) of their dusty medium and their dust amount
are. This result holds under the new hypothesis of high-z Es being as dusty as
nearby ones. Thus the interpretation of the optical/near-IR colours of high-z
Es may suffer from a multiple degeneracy among age, metallicity, dust and
redshift. We also find that DPSGs at z around 1 mix with DSGs at 1 < z < 2, as
a function of dust amount and structure of the dusty medium. All these results
help explaining the complexity of the ERG classification... (Abridged)Comment: 17 pages, 19 figures, accepted for publication in MNRA
Stellar population and dust extinction in an ultraluminous infrared galaxy at z=1.135
We present the detailed optical to far-infrared observations of SST
J1604+4304, an ULIRG at z = 1.135. Analyzing the stellar absorption lines,
namely, the CaII H & K and Balmer H lines in the optical spectrum, we derive
the upper limits of an age for the stellar population. Given this constraint,
the minimum {chi}^2 method is used to fit the stellar population models to the
observed SED from 0.44 to 5.8um. We find the following properties. The stellar
population has an age 40 - 200 Myr with a metallicity 2.5 Z_{sun}. The
starlight is reddened by E(B-V) = 0.8. The reddening is caused by the
foreground dust screen, indicating that dust is depleted in the starburst site
and the starburst site is surrounded by a dust shell. The infrared (8-1000um)
luminosity is L_{ir} = 1.78 +/- 0.63 * 10^{12} L_{sun}. This is two times
greater than that expected from the observed starlight, suggesting either that
1/2 of the starburst site is completely obscured at UV-optical wavelengths, or
that 1/2 of L_{ir} comes from AGN emission. The inferred dust mass is 2.0 +/-
1.0 * 10^8 M_{sun}. This is sufficient to form a shell surrounding the galaxy
with an optical depth E(B-V) = 0.8. From our best stellar population model - an
instantaneous starburst with an age 40 Myr, we infer the rate of 19
supernovae(SNe) per year. Simply analytical models imply that 2.5 Z_{sun} in
stars was reached when the gas mass reduced to 30% of the galaxy mass. The gas
metallcity is 4.8 Z_{sun} at this point. The gas-to-dust mass ratio is then 120
+/- 73. The inferred dust production rate is 0.24 +/- 0.12 M_{sun} per SN. If
1/2 of L_{ir} comes from AGN emission, the rate is 0.48 +/- 0.24 M_{sun} per
SN. We discuss the evolutionary link of SST J1604+4304 to other galaxy
populations in terms of the stellar masses and the galactic winds.Comment: 11 pages, 9 figures, accepted for publication in MNRA
A General Formulation of the Source Confusion Statistics and Application to Infrared Galaxy Surveys
Source confusion has been a long-standing problem in the astronomical
history. In the previous formulation, sources are assumed to be distributed
homogeneously on the sky. This fundamental assumption is not realistic in many
applications. In this work, by making use of the point field theory, we derive
general analytic formulae for the confusion problems with arbitrary
distribution and correlation functions. As a typical example, we apply these
new formulae to the source confusion of infrared galaxies. We first calculate
the confusion statistics for power-law galaxy number counts as a test case.
When the slope of differential number counts, \gamma, is steep, the confusion
limits becomes much brighter and the probability distribution function (PDF) of
the fluctuation field is strongly distorted. Then we estimate the PDF and
confusion limits based on the realistic number count model for infrared
galaxies. The gradual flattening of the slope of the source counts makes the
clustering effect rather mild. Clustering effects result in an increase of the
limiting flux density with \sim 10%. In this case, the peak probability of the
PDF decreases up to \sim 15% and its tail becomes heavier.Comment: ApJ in press, 21 pages, 9 figures, using aastex.cls, emulateapj5.sty.
Abstract abridge
Highly-Ionized High-Velocity Gas in the Vicinity of the Galaxy
We report the results of an extensive FUSE study of high velocity OVI
absorption along 102 complete sight lines through the Galactic halo. The high
velocity OVI traces a variety of phenomena, including tidal interactions with
the Magellanic Clouds, accretion of gas, outflow from the Galactic disk,
warm/hot gas interactions in a highly extended Galactic corona, and
intergalactic gas in the Local Group. We identify 85 high velocity OVI features
at velocities of -500 < v(LSR) < +500 km/s along 59 of the 102 sight lines.
Approximately 60% of the sky (and perhaps as much as 85%) is covered by high
velocity H+ associated with the high velocity OVI. Some of the OVI is
associated with known high velocity HI structures (e.g., the Magellanic Stream,
Complexes A and C), while some OVI features have no counterpart in HI 21cm
emission. The smaller dispersion in the OVI velocities in the GSR and LGSR
reference frames compared to the LSR is necessary (but not conclusive) evidence
that some of the clouds are extragalactic. Most of the OVI cannot be produced
by photoionization, even if the gas is irradiated by extragalactic background
radiation. Collisions in hot gas are the primary OVI ionization mechanism. We
favor production of some of the OVI at the boundaries between warm clouds and a
highly extended [R > 70 kpc], hot [T > 10^6 K], low-density [n < 10^-4 cm^-3]
Galactic corona or Local Group medium. A hot Galactic corona or Local Group
medium and the prevalence of high velocity OVI are consistent with predictions
of galaxy formation scenarios. Distinguishing between the various phenomena
producing high velocity OVI will require continuing studies of the distances,
kinematics, elemental abundances, and physical states of the different types of
high velocity OVI features found in this study. (abbreviated)Comment: 78 pages of text/tables + 31 figures, AASTeX preprint format. All
figures are in PNG format due to astro-ph space restrictions. Bound copies of
manuscript and two accompanying articles are available upon request.
Submitted to ApJ
Search for cold gas in z>2 damped Lyman-alpha systems: 21-cm and H_2 absorption
(Abridged) We present the results of a systematic GBT and GMRT survey for
21-cm absorption in a sample of 10 DLAs at 2<z_abs<3.4. Analysis of L-band VLBA
images of the background QSOs are also presented. We detect 21-cm absorption in
only one DLA (at z_abs = 3.1745 towards J1337+3152). Combining our data with
the data from the literature (a sample of 28 DLAs) and assuming the measured
core fraction at milliarcsecond scale to represent the gas covering factor, we
find that the HI gas in DLAs at z> 2 is predominantly constituted by WNM. The
detection rate of 21-cm absorption seems to be higher for systems with higher
N(HI) or metallicity. However, no clear correlation is found between the
integrated 21-cm optical depth (or spin temperature) and either N(HI),
metallicity or velocity spread of the low ionization species. There are 13 DLAs
in our sample for which high resolution optical spectra covering the expected
wavelength range of H_2 absorption are available. We report the detection of
H_2 molecules in the z_abs = 3.3871 21-cm absorber towards J0203+1134 (PKS
0201+113). In 8 cases, neither H_2 nor 21-cm absorption are detected. The lack
of 21-cm and H_2 absorption in these systems can be explained if most of the HI
in these DLAs originate from low density high temperature gas. In one case we
have a DLA with 21-cm absorption not showing H_2 absorption. In two cases, both
species are detected but do not originate from the same velocity component. In
the remaining 2 cases 21-cm absorption is not detected despite the presence of
H_2 with evidence for the presence of cold gas. All this is consistent with the
idea that the H_2 components seen in DLAs are compact (with sizes of < 15 pc)
and contain only a small fraction (i.e typically <10%) of the total N(HI)
measured in the DLAs.Comment: Accepted for publication in MNRA
The fate of the interstellar medium in early-type galaxies I. First direct measurement of the timescale of dust removal
An important aspect of quenching star formation is the removal of the cold
interstellar medium (ISM; non-ionised gas and dust) from a galaxy. In addition,
dust grains can be destroyed in a hot or turbulent medium. The adopted
timescale of dust removal usually relies on uncertain theoretical estimates. It
is tricky to track the dust removal, because usually dust is constantly
replenished by consecutive generations of stars. Our objective is to measure
observationally the timescale of dust removal. We here explore an approach to
select galaxies which do have detectable amounts of dust and cold ISM but
exhibit a low current dust production rate. Any decrease of the dust and gas
content as a function of the age of such galaxies therefore must be attributed
to processes governing the ISM removal. We used a sample of galaxies detected
by Herschel in the far-infrared with visually assigned early-type morphology or
spirals with red colours. We also obtained JCMT/SCUBA-2 observations for five
of them. We discovered an exponential decline of the dust-to-stellar mass ratio
with age, which we interpret as an evolutionary trend of dust removal from
these galaxies. For the first time we directly measure the dust removal
timescale in such galaxies to be tau=(2.5+-0.4) Gyr (the corresponding
half-life time is (1.75+-0.25) Gyr). This quantity may be used in models in
which it must be assumed a priori and cannot be derived. Any process which
removes dust in these galaxies, such as dust grain destruction, cannot happen
on shorter timescales. The timescale is comparable to the quenching timescales
found in simulations for galaxies with similar stellar masses. The dust is
likely of internal, not external origin. It was either formed in the past
directly by supernovae, or from seeds produced by SNe and with grain growth in
the ISM contributing substantially to the dust mass accumulation.Comment: Astronomy & Astrophysics, accepted; 13 pages, 9 figures, 1 tabl
Are galactic disks dynamically influenced by dust?
Dynamically cold components are well known to destabilize hotter, even much
more massive components. In this paper we studied the dynamical influence of a
cold dust component on the gaseous phase in the central regions of galactic
disks. We performed two-dimensional hydrodynamical simulations for flat
multi-component disks embedded in a combined static stellar and dark matter
potential. The pressure-free dust component is coupled to the gas by a drag
force depending on their velocity difference.
It turned out that the most unstable regions are those with either a low or
near to minimum Toomre parameter or with rigid rotation, i.e. the central area.
In that regions the dust-free disks become most unstable for high azimuthal
modes (m~8), whereas in dusty disks all modes have a similar amplitude
resulting in a patchy appearance. The structures in the dust have a larger
contrast between arm and inter-arm regions than those of the gas. The dust
peaks are frequently correlated with peaks of the gas distribution, but they do
not necessarily coincide with them. Therefore, a large scatter in the
dust-to-gas ratios is expected. The appearance of the dust is more cellular
(i.e. sometimes connecting different spiral features), whereas the gas is
organized in a multi-armed spiral structure.
An admixture of 2% dust destabilizes gaseous disks substantially, whereas
dust-to-gas ratios below 1% have no influence on the evolution of the gaseous
disk. For a high dust-to-gas ratio of 10% the instabilities reach a saturation
level already after 30 Myr.Comment: 21 pages including 24 figures (some figures degraded in quality), in
press in Astronomy & Astrophysics 418, 959(2004), A&A version available at
http://www.edpsciences.org/articles/aa/full/2004/18/aa0047/aa0047.htm
A complete sample of 21-cm absorbers at z~1.3: Giant Metrewave Radio Telescope Survey Using MgII Systems
We present the results of a systematic Giant Metrewave Radio Telescope (GMRT)
survey of 21-cm absorption in a representative and unbiased sample of 35 strong
MgII systems in the redshift range: zabs~1.10-1.45, 33 of which have W_r>1 \AA.
The survey using ~400hrs of telescope time has resulted in 9 new 21-cm
detections and stringent 21-cm optical depth upper limits (median 3-sigma
optical depth per 10 km/s of 0.017) for the remaining 26 systems. This is by
far the largest number of 21-cm detections from any single survey of
intervening absorbers. Prior to our survey no intervening 21-cm system was
known in the above redshift range and only one system was known in the redshift
range 0.7<z<1.5. We discuss the relation between the detectability of 21-cm
absorption and various properties of UV absorption lines. We show that if MgII
systems are selected with the following criteria, MgII doublet ratio <1.3 and
W_r(MgI)/W_r(MgII)>0.3, then a detection rate of 21-cm absorption up to 90% can
be achieved. We estimate n_{21}, the number per unit redshift of 21-cm
absorbers with W_r(Mg(II)>W_o and integrated optical depth Tau_{21}>Tau_o and
show that n_{21} decreases with increasing redshift. In particular, for W_o=1.0
\AA and Tau_o>0.3 km\s, n_{21} falls by a factor 4 from =0.5 to =1.3. The
evolution seems to be stronger for stronger MgII systems. Using a subsample of
systems for which high frequency VLBA images are available, we show that the
effect is not related to the structure of the background radio sources and is
most probably due to the evolution of the cold neutral medium filling factor in
MgII systems. We find no correlation between the velocity spread of the 21-cm
absorption feature and W_r(MgII) at z~1.3.Comment: 22 pages, 8 tables, 12 figures, accepted for publication in MNRA
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