584 research outputs found
ISM enrichment and local pollution in dwarf galaxies
The fate of metals after they are released in starburst episodes is still
unclear. What phases of the interstellar medium are involved, in which
timescales? Evidence has grown over the past few years that the neutral phase
of blue compact dwarf (BCD) galaxies may be metal- deficient as compared to the
ionized gas of their HII regions. These results have strong implications for
our understanding of the chemical evolution of galaxies. We review here the
main results and the main caveats in the abundance determination from far-UV
absorption-lines. We also discuss possible scenarios concerning the journey of
metals into the interstellar medium, or even their ejection from the galaxy
into the intergalactic medium.Comment: Long version of a proceeding for the conference "A Universe of Dwarf
Galaxies: Observations, Theories, Simulations" held in Lyon, France
(14th-18th, June 2010
Chemical enrichment and physical conditions in I Zw 18
Abridged.
Low-metallicity star-forming dwarf galaxies are prime targets to understand
the chemical enrichment of the interstellar medium. The HI region provides
important constraints on the dispersal and mixing of heavy elements released by
successive star-formation episodes. Our primary objective is to study the
enrichment of the HI region and the interplay between star-formation history
and metallicity evolution.
We observed the most metal-poor star-forming galaxy in the Local Universe, I
Zw 18, with Hubble/COS. The abundances in the neutral gas are derived from
far-UV absorption-lines (HI, CII, CII*, NI, OI, ...) and are compared to the
abundances in the HII region. Models are constructed to calculate the
ionization structure and the thermal processes. We investigate the gas cooling
in the HI region through physical diagnostics drawn from the fine-structure
level of C+.
We find that HI region abundances are lower by a factor of ~2 as compared to
the HII region. There is no differential depletion on dust between the HI and
HII region. Using sulfur as a metallicity tracer, we calculate a metallicity of
1/46 solar (vs. 1/31 in the HII region). From the study of abundance ratios, we
propose that C, N, O, and Fe are mainly produced in massive stars. We argue
that the HI envelope may contain pockets of pristine gas with a metallicity
essentially null. Finally, we derive the physical conditions in the HI region
by investigating the CII* absorption line. The cooling rate derived from CII*
is consistent with collisions with H atoms in the diffuse neutral gas. We
calculate the star-formation rate from the CII* cooling rate assuming that
photoelectric effect on dust is the dominant gas heating mechanism. Our
determination is in good agreement with the values in the literature if we
assume a low dust-to-gas ratio (~2000 times lower than the Milky Way value).Comment: Accepted for publication in A&A. Fixed typos and reference
CASSISjuice: open-source pipeline and offline complete atlas of Spitzer/IRS staring observations
Mid-infrared spectroscopy provides many important diagnostics on gas and dust
features in a wide variety of astrophysical objects. The Spitzer Infrared
Spectrograph observed more than 20000 targets with wavelengths as low as 5.2um
and as long as 38um, thereby complementing JWST/MIRI data for long wavelength
diagnostics and providing overall invaluable diagnostics together with JWST or
in view of future IR facilities. In order to maximize the science output of
Spitzer/IRS, the CASSIS atlas has provided reduced IRS spectra since 2011,
extracting and selecting the best spectrum from various methods.
We now present CASSISjuice, an offline version of the pipeline and atlas,
adding several hundred sources that had never cleared the pipeline in order to
make it complete for the first time. We updated the low- and high-resolution
pipelines in order to be able to process every IRS staring mode observation
(i.e., all observations but maps), and we also upgraded the high-resolution
pipeline to version 2. The new pipeline also associates the pointings within
"cluster" observations resulting in a single spectrum (possibly low- and
high-resolution) per position and therefore overall a single CASSISjuice ID per
targeted position.
The initial repositories are hosted at Zenodo, providing the open-source
pipeline code and the atlas itself with specific attention to producing the
smallest dataset possible. Version controlled repositories are also available
at GitLab, including Python notebooks to illustrate the offline manipulation of
the full atlas. The offline CASSISjuice atlas is meant to facilitate the
analysis of large samples and the identComment: arXiv-ony paper, please see suggested proper citations within pape
Infrared Classification and Luminosities For Dusty AGN and the Most Luminous Quasars
Mid-infrared spectroscopic measurements from the Infrared Spectrometer on
Spitzer (IRS) are given for 125 hard X-ray AGN (14-195 keV) from the Swift
Burst Alert Telescope sample and for 32 AGN with black hole masses from
reverberation mapping. The 9.7 um silicate feature in emission or absorption
defines an infrared AGN classification describing whether AGN are observed
through dust clouds, indicating that 55% of the BAT AGN are observed through
dust. The mid-infrared dust continuum luminosity is shown to be an excellent
indicator of intrinsic AGN luminosity, scaling closely with the hard X-ray
luminosity, log vLv(7.8 um)/L(X) = -0.31 +- 0.35 and independent of
classification determined from silicate emission or absorption. Dust luminosity
scales closely with black hole mass, log vLv(7.8 um) = (37.2 +- 0.5) + 0.87 log
BHM for luminosity in erg per sec and BHM in solar masses. The 100 most
luminous type 1 quasars as measured in vLv(7.8 um) are found by comparing Sloan
Digital Sky Survey optically discovered quasars with photometry at 22 um from
the Wide-Field Infrared Survey Explorer, scaled to rest frame 7.8 um using an
empirical template determined from IRS spectra. The most luminous SDSS/WISE
quasars have the same maximum infrared luminosities for all 1.5 < z < 5,
reaching total infrared luminosity L(IR) = 10^14.4 solar luminosities.
Comparing with Dust Obscured Galaxies from Spitzer and WISE surveys, we find no
evidence of hyperluminous obscured quasars whose maximum infrared luminosities
exceed the maximum infrared luminosities of optically discovered quasars.
Bolometric luminosities L(bol) estimated from rest frame optical or ultraviolet
luminosities are compared to L(IR).Comment: accepted for publication in The Astrophysical Journa
Advanced optimal extraction for the Spitzer/IRS
We present new advances in the spectral extraction of point-like sources
adapted to the Infrared Spectrograph onboard the Spitzer Space Telescope. For
the first time, we created a super-sampled point spread function of the
low-resolution modules. We describe how to use the point spread function to
perform optimal extraction of a single source and of multiple sources within
the slit. We also examine the case of the optimal extraction of one or several
sources with a complex background. The new algorithms are gathered in a plugin
called Adopt which is part of the SMART data analysis software.Comment: Accepted for publication in PAS
PAH Strength and the Interstellar Radiation Field around the Massive Young Cluster NGC3603
We present spatial distribution of polycyclic aromatic hydrocarbons and
ionized gas within the Galactic giant HII region NGC3603. Using the IRS
instrument on board the Spitzer Space Telescope, we study in particular the PAH
emission features at ~5.7, 6.2, 7.7, 8.6, and 11.3um, and the [ArII] 6.99um,
[NeII] 12.81um, [ArIII] 8.99um, and [SIV] 10.51um forbidden emission lines. The
observations probe both ionized regions and photodissociation regions. Silicate
emission is detected close to the central cluster while silicate absorption is
seen further away. We find no significant variation of the PAH ionization
fraction across the whole region. The emission of very small grains lies closer
to the central stellar cluster than emission of PAHs. The PAH/VSG ratio
anticorrelates with the hardness of the interstellar radiation field suggesting
a destruction mechanism of the molecules within the ionized gas, as shown for
low-metallicity galaxies by Madden et al. (2006).Comment: Accepted for publication in ApJ. Corrected typo
Elemental Abundances of Blue Compact Dwarfs from mid-IR Spectroscopy with Spitzer
We present a study of elemental abundances in a sample of thirteen Blue
Compact Dwarf (BCD) galaxies, using the 10--37m high resolution
spectra obtained with Spitzer/IRS. We derive the abundances of neon and sulfur
for our sample using the infrared fine-structure lines probing regions which
may be obscured by dust in the optical and compare our results with similar
infrared studies of starburst galaxies from ISO. We find a good correlation
between the neon and sulfur abundances, though sulfur is under-abundant
relative to neon with respect to the solar value. A comparison of the elemental
abundances (neon, sulfur) measured from the infrared data with those derived
from the optical (neon, sulfur, oxygen) studies reveals a good overall
agreement for sulfur, while the infrared derived neon abundances are slightly
higher than the optical values. This indicates that either the metallicities of
dust enshrouded regions in BCDs are similar to the optically accessible
regions, or that if they are different they do not contribute substantially to
the total infrared emission of the host galaxy.Comment: 11 pages, 6 figures, accepted by Ap
The Infrared Database of Extragalactic Observables from Spitzer I: the redshift catalog
This is the first of a series of papers on the Infrared Database of
Extragalactic Observables from Spitzer (IDEOS). In this work we describe the
identification of optical counterparts of the infrared sources detected in
Spitzer Infrared Spectrograph (IRS) observations, and the acquisition and
validation of redshifts. The IDEOS sample includes all the spectra from the
Cornell Atlas of Spitzer/IRS Sources (CASSIS) of galaxies beyond the Local
Group. Optical counterparts were identified from correlation of the extraction
coordinates with the NASA Extragalactic Database (NED). To confirm the optical
association and validate NED redshifts, we measure redshifts with unprecedented
accuracy on the IRS spectra ({\sigma}(dz/(1+z))=0.0011) by using an improved
version of the maximum combined pseudo-likelihood method (MCPL). We perform a
multi-stage verification of redshifts that considers alternate NED redshifts,
the MCPL redshift, and visual inspection of the IRS spectrum. The statistics is
as follows: the IDEOS sample contains 3361 galaxies at redshift 0<z<6.42 (mean:
0.48, median: 0.14). We confirm the default NED redshift for 2429 sources and
identify 124 with incorrect NED redshifts. We obtain IRS-based redshifts for
568 IDEOS sources without optical spectroscopic redshifts, including 228 with
no previous redshift measurements. We provide the entire IDEOS redshift catalog
in machine-readable formats. The catalog condenses our compilation and
verification effort, and includes our final evaluation on the most likely
redshift for each source, its origin, and reliability estimates.Comment: 11 pages, 6 figures, 1 table. Accepted for publication in MNRAS. Full
redshift table in machine-readable format available at
http://ideos.astro.cornell.edu/redshifts.htm
Chemical composition and mixing in giant HII regions: NGC3603, 30Doradus, and N66
We investigate the chemical abundances of NGC3603 in the Milky Way, of
30Doradus in the Large Magellanic Cloud, and of N66 in the Small Magellanic
Cloud. Mid-infrared observations with the Infrared Spectrograph onboard the
Spitzer Space Telescope allow us to probe the properties of distinct physical
regions within each object: the central ionizing cluster, the surrounding
ionized gas, photodissociation regions, and buried stellar clusters. We detect
[SIII], [SIV], [ArIII], [NeII], [NeIII], [FeII], and [FeIII] lines and derive
the ionic abundances. Based on the ionic abundance ratio (NeIII/H)/(SIII/H), we
find that the gas observed in the MIR is characterized by a higher degree of
ionization than the gas observed in the optical spectra. We compute the
elemental abundances of Ne, S, Ar, and Fe. We find that the alpha-elements Ne,
S, and Ar scale with each other. Our determinations agree well with the
abundances derived from the optical. The Ne/S ratio is higher than the solar
value in the three giant HII regions and points toward a moderate depletion of
sulfur on dust grains. We find that the neon and sulfur abundances display a
remarkably small dispersion (0.11dex in 15 positions in 30Doradus), suggesting
a relatively homogeneous ISM, even though small-scale mixing cannot be ruled
out.Comment: Accepted for submission to ApJ. The present version replaces the
submitted one. Changes: new title, new figure, the text was modified in the
discussio
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