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

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

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

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

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 $\sim$10--37$\mu$m 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

Interstellar abundances in the neutral and ionized gas of NGC604

We present FUSE spectra of the giant HII region NGC604 in the spiral galaxy M33. Chemical abundances are derived from far-UV absorption lines and are compared to those derived from optical emission lines. We derived the column densities of HI, NI, OI, SiII, PII, ArI, and FeII, fitting the line profiles with either a single component or several components. Our net results, assuming a single component, show that N, O, Si, and Ar are apparently underabundant in the neutral phase by a factor of 10 or more with respect to the ionized phase, while Fe is the same. However, we discuss the possibility that the absorption lines are made of individual unresolved components, and find that only PII, ArI, and FeII lines should not be affected by the presence of hidden saturated components, while NI, OI, and SiII might be much more affected. If N, O, and Si are actually underabundant in the neutral gas of NGC604 with respect to the ionized gas, this would confirm earlier results obtained for the blue compact dwarfs. However, a deeper analysis focused on P, Ar, and Fe mitigates the above conclusion and indicates that the neutral gas and ionized gas could have similar abundances.Comment: Accepted for publication in A&

The extraordinary mid-infrared spectral properties of FeLoBAL Quasars

We present mid-infrared spectra of six FeLoBAL QSOs at 1<z<1.8, taken with the Spitzer space telescope. The spectra span a range of shapes, from hot dust dominated AGN with silicate emission at 9.7 microns, to moderately obscured starbursts with strong Polycyclic Aromatic Hydrocarbon (PAH) emission. The spectrum of one object, SDSS 1214-0001, shows the most prominent PAHs yet seen in any QSO at any redshift, implying that the starburst dominates the mid-IR emission with an associated star formation rate of order 2700 solar masses per year. With the caveats that our sample is small and not robustly selected, we combine our mid-IR spectral diagnostics with previous observations to propose that FeLoBAL QSOs are at least largely comprised of systems in which (a) a merger driven starburst is ending, (b) a luminous AGN is in the last stages of burning through its surrounding dust, and (c) which we may be viewing over a restricted line of sight range.Comment: ApJ, accepte