Hot Stars and Cool Clouds: The Photodissociation Region M16

We present high-resolution spectroscopy and images of a photodissociation region (PDR) in M16 obtained during commissioning of NIRSPEC on the Keck II telescope. PDRs play a significant role in regulating star formation, and M16 offers the opportunity to examine the physical processes of a PDR in detail. We simultaneously observe both the molecular and ionized phases of the PDR and resolve the spatial and kinematic differences between them. The most prominent regions of the PDR are viewed edge-on. Fluorescent emission from nearby stars is the primary excitation source, although collisions also preferentially populate the lowest vibrational levels of H2. Variations in density-sensitive emission line ratios demonstrate that the molecular cloud is clumpy, with an average density n = 3x10^5 cm^(-3). We measure the kinetic temperature of the molecular region directly and find T_H2 = 930 K. The observed density, temperature, and UV flux imply a photoelectric heating efficiency of 4%. In the ionized region, n_i=5x10^3 cm^(-3) and T_HII = 9500 K. In the brightest regions of the PDR, the recombination line widths include a non-thermal component, which we attribute to viewing geometry.Comment: 5 pages including 2 Postscript figures. To appear in ApJ Letters, April 200

J-Band Infrared Spectroscopy of a Sample of Brown Dwarfs Using Nirspec on Keck II

Near-infrared spectroscopic observations of a sample of very cool, low-mass objects are presented with higher spectral resolution than in any previous studies. Six of the objects are L-dwarfs, ranging in spectral class from L2 to L8/9, and the seventh is a methane or T-dwarf. These new observations were obtained during commissioning of NIRSPEC, the first high-resolution near-infrared cryogenic spectrograph for the Keck II 10-meter telescope on Mauna Kea, Hawaii. Spectra with a resolving power of R=2500 from 1.135 to 1.360 microns (approximately J-band) are presented for each source. At this resolution, a rich spectral structure is revealed, much of which is due to blending of unresolved molecular transitions. Strong lines due to neutral potassium (K I), and bands due to iron hydride (FeH) and steam (H2O) change significantly throughout the L sequence. Iron hydride disappears between L5 and L8, the steam bands deepen and the K I lines gradually become weaker but wider due to pressure broadening. An unidentified feature occurs at 1.22 microns which has a temperature dependence like FeH but has no counterpart in the available FeH opacity data. Because these objects are 3-6 magnitudes brighter in the near-infrared compared to the I-band, spectral classification is efficient. One of the objects studied (2MASSW J1523+3014) is the coolest L-dwarf discovered so far by the 2-Micron All-Sky Survey (2MASS), but its spectrum is still significantly different from the methane-dominated objects such as Gl229B or SDSS 1624+0029.Comment: New paper, Latex format, 2 figures, accepted to ApJ Letter

The Rest-Frame Optical Spectrum of MS 1512-cB58

Moderate resolution, near-IR spectroscopy of MS1512-cB58 is presented, obtained during commissioning of the the Near IR Spectrometer (NIRSPEC) on the Keck II telescope. The strong lensing of this z=2.72 galaxy by the foreground cluster MS1512+36 makes it the best candidate for detailed study of the rest-frame optical properties of Lyman Break Galaxies. A redshift of z=2.7290+/-0.0007 is inferred from the emission lines, in contrast to the z=2.7233 calculated from UV observations of interstellar absorption lines. Using the Balmer line ratios, we find an extinction of E(B-V)=0.27. Using the line strengths, we infer an SFR=620+/-18 Msun/yr (H_0=75, q_0=0.1, Lambda =0), a factor of 2 higher than that measured from narrow-band imaging observations of the galaxy, but a factor of almost 4 lower than the SFR inferred from the UV continuum luminosity. The width of the Balmer lines yields a mass of M_vir=1.2x10^10 Msun. We find that the oxygen abundance is 1/3 solar, in good agreement with other estimates of the metallicity. However, we infer a high nitrogen abundance, which may argue for the presence of an older stellar population.Comment: 14 pages, including 3 figures. Accepted for publication in ApJ Letter

Infrared Spectroscopy of a Massive Obscured Star Cluster in the Antennae Galaxies (NGC 4038/4039) with NIRSPEC

We present infrared spectroscopy of the Antennae Galaxies (NGC 4038/4039) with NIRSPEC at the W. M. Keck Observatory. We imaged the star clusters in the vicinity of the southern nucleus (NGC 4039) in 0.39" seeing in K-band using NIRSPEC's slit-viewing camera. The brightest star cluster revealed in the near-IR (M_K(0) = -17.9) is insignificant optically, but coincident with the highest surface brightness peak in the mid-IR (12-18 micron) ISO image presented by Mirabel et al. (1998). We obtained high signal-to-noise 2.03 - 2.45 micron spectra of the nucleus and the obscured star cluster at R ~ 1900. The cluster is very young (4 Myr old), massive (16e6 M_sun), and compact (density ~ 115 M_sun pc^(-3) within a 32 pc half-light radius), assuming a Salpeter IMF (0.1 - 100 M_sun). Its hot stars have a radiation field characterized by T_eff ~ 39,000 K, and they ionize a compact H II region with n_e ~ 1e4 cm^(-3). The stars are deeply embedded in gas and dust (A_V ~ 9-10 mag), and their strong FUV field powers a clumpy photodissociation region with densities n_H >= 1e5 cm^(-3) on scales of up to 200 pc, radiating L[H_2 1-0 S(1)] = 9600 L_sun.Comment: 4 pages, 5 embedded figures. To appear in proceedings of 33d ESLAB Symposium: Star Formation from the Small to the Large Scale, held in Noordwijk, The Netherlands, Nov. 1999. Also available at http://astro.berkeley.edu/~agilber

Measurement of [OIII] Emission in Lyman Break Galaxies

Measurements of [OIII] emission in Lyman Break galaxies (LBGs) at z>3 are presented. Four galaxies were observed with narrow-band filters using the Near-IR Camera on the Keck I 10-m telescope. A fifth galaxy was observed spectroscopically during the commissioning of NIRSPEC, the new infrared spectrometer on Keck II. The emission-line spectrum is used to place limits on the metallicity. Comparing these new measurements with others available from the literature, we find that strong oxygen emission in LBGs may suggest sub-solar metallicity for these objects. The [OIII]5007 line is also used to estimate the star formation rate (SFR) of the LBGs. The inferred SFRs are higher than those estimated from the UV continuum, and may be evidence for dust extinction.Comment: 25 pages, including 6 figures. Accepted for publication in Ap

NIRSPEC brown dwarf spectroscopic survey

The NIRSPEC Brown Dwarf Spectroscopic Survey is a project to obtain a consistent set of high-quality near-IR spectra for each spectral class and sub-class of low-mass and/or sub- stellar objects to provide a new data base for models of the atmosphere of brown dwarfs and extra-solar giant planets. Most of the current targets are L-dwarfs and T-dwarfs discovered by the 2MASS. The survey is begin performed with the recently-commissioned near-IR spectrometer, NIRSPEC, a 1-5 micrometers cryogenic spectrograph at the WM Keck Observatory on Mauna Kea, using resolving powers of R equals 2,500-25,000. Preliminary results for four sources, three L-dwarfs and one T-dwarf, are presented here. Spectra from 1.13-2.33 micrometers at an average resolution of R equals 2,500 illustrate the development of deep steam bands and the weakening of FeH through the L-sequence, and the emergence of methane bands in the T-dwarfs. Complex detail in the spectra are the result of blending of numerous unresolved molecular transitions