PHYSICAL CONDITIONS IN A YOUNG, UNREDDENED, LOW-METALLICITY GALAXY AT HIGH REDSHIFT
- Department of Physics, University of California, Santa Barbara, CA 93106-9530 (United States)
- Institute of Astronomy, Madingley Road, Cambridge, CB3 0HA (United Kingdom)
- Department of Astronomy, University of California Los Angeles, Los Angeles, CA 90095-1547 (United States)
- Department of Astronomy, California Institute of Technology, MS 249-17, Pasadena, CA 91125 (United States)
- National Optical Astronomy Observatory, 950 North Cherry Avenue, Tucson, AZ 85719 (United States)
Increasingly large samples of galaxies are now being discovered at redshifts z {approx} 5-6 and higher. Many of these objects are inferred to be young, low in mass, and relatively unreddened, but detailed analysis of their high quality spectra will not be possible until the advent of future facilities. In this paper, we shed light on the physical conditions in a plausibly similar low-mass galaxy by presenting the analysis of the rest-frame optical and UV spectra of Q2343-BX418, an L* galaxy at z = 2.3 with a very low mass-to-light ratio and unusual properties: BX418 is young (<100 Myr), low mass (M {sub *} {approx} 10{sup 9} M{sub sun}), low in metallicity (Z {approx} 1/6 Z{sub sun}), and unreddened (E(B - V) {approx_equal} 0.02, UV continuum slope {beta} = -2.1). We infer a metallicity 12 + log(O/H) = 7.9 {+-} 0.2 from the rest-frame optical emission lines. We also determine the metallicity via the direct, electron temperature method, using the ratio O III] {lambda}{lambda}1661, 1666/[O III] {lambda}5007 to determine the electron temperature and finding 12 + log(O/H) = 7.8 {+-} 0.1. These measurements place BX418 among the most metal-poor galaxies observed in emission at high redshift. The rest-frame UV spectrum, which represents {approx}12 hr of integration with the Keck telescope, contains strong emission from Ly{alpha} (with rest-frame equivalent width 54 A), He II {lambda}1640 (both stellar and nebular), C III] {lambda}{lambda}1907, 1909 and O III] {lambda}{lambda}1661, 1666. The C IV/C III] ratio indicates that the source of ionization is unlikely to be an active galactic nucleus. Analysis of the He II, O III], and C III] line strengths indicates a very high ionization parameter log U {approx} -1, while Ly{alpha} and the interstellar absorption lines indicate that outflowing gas is highly ionized over a wide range of velocities. It remains to be determined how many of BX418's unique spectral features are due to its global properties, such as low metallicity and dust extinction, and how many are indicative of a short-lived phase in the early evolution of an otherwise normal star-forming galaxy.
- OSTI ID:
- 21457101
- Journal Information:
- Astrophysical Journal, Vol. 719, Issue 2; Other Information: DOI: 10.1088/0004-637X/719/2/1168; ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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