Leonard E Parker

Center for Gravitation, Cosmology & Astrophysics

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Leonard E Parker Center for Gravitation, Cosmology and Astrophysics

A Closer Look at a Young, Distant Galaxy

Physical Conditions in a Young, Unreddened, Low-Metallicity Galaxy at High Redshift Dawn K. Erb, Max Pettini, Alice E. Shapley, Charles C. Steidel, David R. Law, and Naveen A. Reddy (Paper)
Galaxy image

BX418 is shown in this rest-frame optical image, captured by the Wide Field Camera 3 on the Hubble Space Telescope. The area shown is 3" x 3". BX418 is a compact galaxy, with a half light radius of 1.5 kpc. Credit: Erb, Pettini, Shapley, Steidel, Law, and Reddy.

Many galaxies have recently been discovered at very high redshift (z>5), although detailed investigations of these are not yet possible. The properties of the galaxy Q2343-BX418, found at redshift z=2.3, are broadly similar to those inferred for the young, low mass galaxies now being discovered at higher redshifts. The detailed spectral properties of BX418 may shed light on the likely physical conditions in these distant objects, well before facilities are available to measure them directly.

BX418 is an L* galaxy with a very low mass-to-light ratio, (M/L)B = .03. It is also a highly unusual galaxy. It is rather young (<100 Myr), low mass (109 Msun), low in metallicity (Z ~ 1/6 Zsun) and unreddened (E(B-V)~.02, UV continuum slope β = -2.1 where ƒλ is proportional to λβ). Its ionization parameter, representing the ratio of ionizing photons to particle density, is very high. The source of ionization in BX418 is unlikely to be an AGN.

The rest-frame ultraviolet and optical spectra indicate that BX418 contains distinctive spectral features when compared to other more typical z ~ 2-3 galaxies. As expected, the UV spectrum contains strong high ionization interstellar absorption lines from outflowing gas, however BX418 is rare in that it has a few additional very strong emission lines and its low ionization lines are extremely weak. This indicates that a larger than average fraction of the outflow is highly ionized.

BX418's Lyman alpha emission line is extremely broad and shows significant blueshifted emission in addition to the usual asymmetric redshifted profile. The is likely due to low optical depth in the outflowing gas, which allows substantial escape of Lyman alpha photons from the approaching gas on the near side of the galaxy.

The broad portion of the strong stellar He II emission peak indicates that there may be a large number of very young Wolf-Rayet stars in the galaxy. The narrow component of the He II peak suggests that unresolved nebular emission is also present in BX418. 

Since BX418 is young and undergoing extreme star formation, it is a good place to study trends in the C/O ratio. The primary cause of the increase in C/O with O/H is thought to be metallicity-dependent stellar yields. Evidence of delayed carbon production is not seen, and carbon production is not dominated by low mass stars.

Strong stellar wind lines of C IV and Si IV are present for BX418 and show distinctive P Cygni profiles, meaning that redshifted emission and blueshifted absorption appear on the spectrum because gas is expanding away from the star. These lines depend sensitively on the properties of the massive stellar population, so interpreting them is difficult. The emission and absorption peaks cannot yet be matched with models.

Observations used in this analysis of BX418 were performed on a number of instruments, including the Low Resolution Imaging Spectrometer on the Keck I telescope, NIRSPEC on the Keck II telescope, the Large Format Camera and Wide-Field Infrared Camera on the Palomar 20-in telescope, IRAC on the Spitzer Space Telescope, and the Hubble Space Telescope. It remains to be determined how many of BX418's unique spectral features are due to its global properties and how many are indicative of a short-lived phase in the early evolution of an otherwise normal star-forming galaxy.

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A portion of the BX418 spectrum is shown in black, while a composite spectrum representing the average of 966 z=2 galaxies is shown in red. BX418 has a strong He II emission line at a wavelength of 1640 angstroms. The broad part of the He II emission peak is attributed to Wolf-Rayet stars, while the narrow component is likely from nebular emission. Credit: Erb, Pettini, Shapley, Steidel, Law, and Reddy.

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