THE SPITZER VIEW OF LOW-METALLICITY STAR FORMATION. III. FINE-STRUCTURE LINES, AROMATIC FEATURES, AND MOLECULES

Journal Article · Sat Mar 20 04:00:00 EDT 2010 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/712/1/164· OSTI ID:21394281

Hunt, Leslie K ^[1]; Thuan, Trinh X ^[2]; Izotov, Yuri I ^[3]; Sauvage, Marc ^[4]

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1. INAF-Osservatorio Astrofisico di Arcetri, Largo Fermi 5, I-50125 Firenze (Italy)
2. Department of Astronomy, University of Virginia, P.O. Box 400325, Charlottesville, VA 22904-4325 (United States)
3. Main Astronomical Observatory, National Academy of Sciences of Ukraine, 03680 Kiev (Ukraine)
4. CEA/DSM/DAPNIA/Service d'Astrophysique, UMR AIM, CE Saclay, 91191 Gif sur Yvette Cedex (France)

We present low- and high-resolution Spitzer/IRS spectra, supplemented by Infrared Array Camera and Multiband Imaging Photometer for Spitzer measurements, of 22 blue compact dwarf (BCD) galaxies. The BCD sample spans a wide range in oxygen abundance (12+log(O/H) between 7.4 and 8.3), and hardness of the interstellar radiation field (ISRF). The Infrared Spectrograph (IRS) spectra provide us with a rich set of diagnostics to probe the physics of star and dust formation in very low metallicity environments. We find that metal-poor BCDs have harder ionizing radiation than metal-rich galaxies: [O IV] emission is {approx}>4 times as common as [Fe II] emission. They also have a more intense ISRF, as indicated by the 71 to 160 {mu}m luminosity ratio. Two-thirds of the sample (15 BCDs) show polycyclic aromatic hydrocarbon (PAH) features, although the fraction of PAH emission normalized to the total infrared (IR) luminosity is considerably smaller in metal-poor BCDs ({approx}0.5%) than in metal-rich star-forming galaxies ({approx}10%). We find several lines of evidence for a deficit of small PAH carriers at low metallicity, and attribute this to destruction by a hard, intense ISRF, only indirectly linked to metal abundance. Our IRS spectra reveal a variety of H{sub 2} rotational lines, and more than a third of the objects in our sample (eight BCDs) have {approx}>3sigma detections in one or more of the four lowest-order transitions. The warm gas masses in the BCDs range from 10{sup 3} M{sub sun} to 10{sup 8} M{sub sun}, and can be comparable to the neutral hydrogen gas mass; relative to their total IR luminosities, some BCDs contain more H{sub 2} than Spitzer Nearby Galaxy Survey galaxies.

• OSTI ID: 21394281
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 1 Vol. 712; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
ABUNDANCE
AROMATICS
CAMERAS
DUSTS
ELEMENTS
EMISSION
FINE STRUCTURE
GALAXIES
HYDROCARBONS
HYDROGEN
IONIZING RADIATIONS
LUMINOSITY
MASS
MEASURING INSTRUMENTS
METALS
NONMETALS
OPTICAL PROPERTIES
ORGANIC COMPOUNDS
OXYGEN
PHOTOMETERS
PHYSICAL PROPERTIES
POLYCYCLIC AROMATIC HYDROCARBONS
RADIATIONS
SPECTRA
STARS