7.5. The Role of Dust

Since most of our observations sample rest-frame UV wavelengths, derived quantities such as star formation rates are sensitive to gas and dust extinction. As noted earlier, Ly is particularly vulnerable since it is a resonance line whose escape is dependent upon the distribution of neutral gas.

One method of estimating the amount of reddening, used by Meurer et al. (1997), Pettini et al. (1999), Dickinson (1998), Bunker et al. (1999), and Steidel et al. (1999), is to assume a very young ( 10⁷ yr age) starburst and ascribe any color excess in the emitted UV to extinction. The uncertainty, of course, is that aging of a starburst will also cause reddening. Typical extinction corrections are factors of 2-7 near 1500 Å, depending upon the extinction law applied. Meurer et al. (1997) derive a correction factor of 15, in part due to different assumptions regarding the spectral slope of the underlying, unreddened population.

Near-infrared photometry offers the possibility of more reliable estimates of reddening, since at these wavelengths photometry is insensitive to the details and ages of the hottest stars. The ground-based (Keck/NIRC) J-band detection of 0140+326 RD1 by Armus et al. (1998) suggests substantial reddening (A_V > 0.5, for Bruzual & Charlot (1993) models with ages less than 10⁸ yr), which is somewhat surprising given its nondetection in deep sub-mm observations, strong Ly emission, and similar star formation rates inferred from its rest-frame UV continuum and Ly flux density.

HST/NICMOS imaging offers deep, reliable near-infrared photometry and will be a valuable asset for measuring reddening in Lyman-break galaxies. For example, Weymann et al. (1998) use NICMOS F110W and F160W photometry to limit the reddening and star formation rate of HDF 4-473.0 (z = 5.60). They find that the spectral energy distribution limits the reddening to 0.00 E(B-V) 0.12 and the star formation rate to 8 M yr^-1 19 M yr^-1. This modest amount of extinction is consistent with the rather strong Ly emission line emerging from this distant galaxy.

Near-infrared spectroscopy offers a potent tool for studying the dust content, mass, age, and kinematics of distant galaxies. Pettini et al. (1998) recently reported a pilot program of near-infrared spectra of the well-studied, rest-frame optical, nebular emission lines from H II regions in five Lyman-break galaxies at z 3. The observations used the CGS4 spectrometer on the United Kingdom Infrared Telescope and targeted the redshifted Balmer and [O III] emission lines. H luminosities, uncorrected for intrinsic dust extinction, imply star formation rates of 20-270 h₇₀^-2 M yr^-1 (q₀ = 0.1); that is, typically a factor of several larger than that inferred from the UV continuum of these galaxies. The implication is that an extinction of 1-2 mag at 1500 Å may be typical of the Lyman-break population. Velocity dispersions of 70 km s^-1 were reported in four out of the five galaxies, suggesting virial masses M_vir (1-5) × 10¹⁰ M. The relative redshifts of Ly emission, interstellar absorption, and nebular emission lines vary by several hundred km s^-1, suggestive of large-scale outflows. Similar scale outflows are common in regions of rapid star formation locally.

Future higher resolution (spectral and spatial) observations of these and related rest-frame optical transitions redshifted into the near-infrared should better address the kinematics and possibly the light-element abundances of young, forming galaxies.