N. A. Levenson

The Rest-Frame Optical Spectrum of MS 1512–cB58*

Moderate-resolution, near-IR spectroscopy of MS 1512-cB58 is presented, obtained during commissioning of the near-infrared spectrometer (NIRSPEC) on the Keck II telescope. The strong lensing of this z=2.72 galaxy by the foreground cluster MS 1512+36 makes it the best candidate for detailed study of the rest-frame optical properties of Lyman-break galaxies. In 80 minutes of on-source integration, we have detected Halpha, [N ii] lambdalambda6583, 6548, [O i] lambda6300, He i lambda5876, [O iii] lambdalambda5007, 4959, Hbeta, Hgamma, [O ii] lambda3727, and a strong continuum signal in the range of 1.29-2.46 µm. 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 a star formation rate (SFR) of 620+/-18 M middle dot in circle yr-1 (H0=75, q0=0.1, and Lambda=0), which is a factor of 2 higher than that measured from narrowband imaging observations of the galaxy but is 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 Mvir=1.2x1010 M middle dot in circle. We find that the oxygen abundance is 13 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.

Performance and results with the NIRSPEC echelle spectrograph on the Keck II telescope

This paper describes the performance of NIRSPEC, the cryogenic cross-dispersed IR echelle spectrograph for the Keck II telescope on Mauna Kea. NIRSPEC employs a 1024 by 1024 InSb array, diamond-machined metal optics and closed- cycle refrigeration on achieve high throughput and low backgrounds. The instrument operates directly at the f/15 Nasmyth focus, but can also be used in conjunction with the Keck adaptive optics system. First Light was obtained on April 25, 1999. As expected, the performance is detector- limited at short wavelengths and background-limited at longer wavelengths. All of the design goals have been met and result illustrating the optical performance and sensitivity are reported.

The Seyfert-Starburst Connection in X-Rays. II. Results and Implications

We present the results of X-ray imaging and spectroscopic analysis of a sample of Seyfert 2 galaxies that contain starbursts, based on their optical and UV characteristics. These composite galaxies exhibit extended, soft, thermal X-ray emission, which we attribute to their starburst components. Comparing their X-ray and far-infrared properties with ordinary Seyfert and starburst galaxies, we identify the spectral characteristics of their various intrinsic emission sources. The observed far-infrared emission of the composite galaxies may be associated almost exclusively with star formation, rather than the active nucleus. The ratio of the hard X-ray luminosity to the far-infrared and [O III] λ5007 luminosity distinguishes most of these composite galaxies from pure Seyfert 2 galaxies, while their total observed hard X-ray luminosity distinguishes them from pure starbursts. The hard nuclear X-ray source is generally heavily absorbed (NH > 1023 cm-2) in the composite galaxies. Based on these results, we suggest that the interstellar medium of the nuclear starburst is a significant source of absorption. The majority of the sample are located in groups or are interacting with other galaxies, which may trigger the starburst or allow rapid mass infall to the central black hole or both. We conclude that starbursts are energetically important in a significant fraction of active galaxies and that starbursts and active galactic nuclei may be part of a common evolutionary sequence.

Extreme X-Ray Iron Lines in Active Galactic Nuclei

We analyze X-ray spectra of heavily obscured (NH > 1024 cm-2) active galaxies obtained with Chandra, concentrating on the iron Kα fluorescence line. We measure very large equivalent widths in most cases, up to 5 keV in the most extreme example. The geometry of an obscuring torus of material near the active galactic nucleus (AGN) determines the Fe emission, which we model as a function of torus opening angle, viewing angle, and optical depth. The starburst/AGN composite galaxies in this sample require small opening angles. Starburst/AGN composite galaxies in general therefore present few direct lines of sight to their central engines. These composite galaxies are common, and their large covering fractions and heavy obscuration effectively hide their intrinsically bright X-ray continua. While few distant obscured AGNs have been identified, we propose to exploit their signature large Fe Kα equivalent widths to find more examples in X-ray surveys.

Measurement of [O III] emission in Lyman-break galaxies

Measurements of [O III] emission in Lyman-break galaxies (LBGs) at z > 3 are presented. Four galaxies were observed with narrowband 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 subsolar metallicity for these objects. The [O III] λ5007 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.

The Seyfert-Starburst Connection in X-Rays. I. The Data

We analyze X-ray spectra and images of a sample of Seyfert 2 galaxies that unambiguously contain starbursts, based on their optical and UV characteristics. Although all sample members contain active galactic nuclei (AGNs), supermassive black holes or other related processes at the galactic centers alone cannot account for the total X-ray emission in all instances. Eleven of the 12 observed galaxies are significantly resolved with the ROSAT High Resolution Imager, while six of the eight sources observed with the lower resolution Position Sensitive Proportional Counter also appear extended on larger scales. The X-ray emission is extended on physical scales of 10 kpc and greater, which we attribute to starburst-driven outflows and supernova heating of the interstellar medium. Spectrally, a physically motivated composite model of the X-ray emission that includes a heavily absorbed (NH > 1023 cm-2) nuclear component (the AGN), power-law-like scattered AGN flux, and a thermal starburst describes this sample well. Half the sample exhibit iron Kα lines, which are typical of AGNs.

Infrared Spectroscopy of a Massive Obscured Star Cluster in the Antennae Galaxies (NGC 4038/9) with NIRSPEC

We present infrared spectroscopy of the Antennae galaxies (NGC 4038/9) with the near-infrared spectrometer (NIRSPEC) at the W. M. Keck Observatory. We imaged the star clusters in the vicinity of the southern nucleus (NGC 4039) with 0&farcs;39 seeing in the K band using NIRSPECs slit-viewing camera. The brightest star cluster revealed in the near-IR [MK&parl0;0&parr0; approximately -17.9] is insignificant optically but is coincident with the highest surface brightness peak in the mid-IR (12-18 µm) Infrared Space Observatory image presented by Mirabel et al. We obtained high signal-to-noise ratio 2.03-2.45 µm spectra of the nucleus and the obscured star cluster at R approximately 1900. The cluster is very young ( approximately 4 Myr), massive (M approximately 16x106 M middle dot in circle), and compact (with a density of approximately 115 M middle dot in circle pc-3 within a 32 pc half-light radius), assuming a Salpeter initial mass function (0.1-100 M middle dot in circle). Its hot stars have a radiation field characterized by Teff approximately 39,000 K, and they ionize a compact H ii region with ne approximately 104 cm-3. The stars are deeply embedded in gas and dust (AV approximately 9-10 mag), and their strong far-ultraviolet field powers a clumpy photodissociation region with densities nH greater, similar105 cm-3 on scales of approximately 200 pc, radiating LH21-0S&parl0;1&parr0;=9600 L middle dot in circle.

The Obscuring Starburst of NGC 6221 and Implications for the Hard X-Ray Background

We present NGC 6221 as a case study of ii X-rayloud composite galaxies,ˇˇ which appear similar to starbursts at optical wavelengths and resemble traditional active galactic nuclei in X-rays. The net optical spectrum of NGC 6221 is properly characterized as a starburst galaxy, but in X-rays, NGC 6221 is similar to Seyfert 1 galaxies, exhibiting a power-law continuum spectrum, a broad Fe Ka line, and continuum variability on timescales of days and years. High-resolution images reveal that the detected active nucleus is relatively weak, not only at optical, but also at near-infrared wavelengths. An obscuring starburst, in which the interstellar gas and dust associated with the starburst conceal the active nucleus, accounts for these peculiar features. We demonstrate quantitatively that obscuration by column density cm~2 combined with relatively weak intrinsic nuclear activity can produce an optical spec- N H 1022 trum that is characteristic of the surrounding starburst alone. While optical surveys would not identify the active nuclei that make these galaxies signi—cant X-ray sources, such galaxies may, in fact, be impor- tant contributors to the X-ray background. Subject headings: galaxies: individual (NGC 6221) ¨ galaxies: Seyfertgalaxies: starburst ¨ X-rays: diUuse backgroundX-rays: galaxies

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 the near-infrared spectrometer (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=3x105 cm-3. We measure the kinetic temperature of the molecular region directly and find that TH2=930 K. The observed density, temperature, and UV flux imply a photoelectric heating efficiency of 4%. In the ionized region, ni=5x103 cm-3 and THii=9500 K. In the brightest regions of the PDR, the recombination line widths include a nonthermal component, which we attribute to viewing geometry.

Shocking Clouds in the Cygnus Loop

With Hubble Space Telescope Wide-Field Planetary Camera 2 observations of the Cygnus Loop supernova remnant, we examine the interaction of an interstellar cloud with the blast wave on physical scales of 1015 cm. The shock front is distorted, revealing both edge-on and face-on views of filaments and diffuse emission, similar to those observed on larger scales at lower resolution. We identify individual shocks in the cloud of density n ≈ 15 cm-3 having velocity vs ≈ 170 km s-1. We also find the morphologically unusual diffuse Balmer-dominated emission of faster shocks in a lower density region. The obstacle diffracts these shocks, so they propagate at oblique angles with respect to the primary blast wave. The intricate network of diffuse and filamentary Hα emission arises during the early stage of interaction between the cloud and blast wave, demonstrating that complex shock propagation and emission morphology occur before the onset of instabilities that destroy clouds completely.