The Exceptionally Soft X-ray Spectrum of the Low-mass Starburst Galaxy NGC 1705

NGC 1705 is one of the optically brightest and best studied dwarf galaxies. It appears to be in the late stage of a major starburst and contains a young super star cluster. Type II supernovae are therefore likely to have been a major effect in the recent evolution of this galaxy and are likely to have produced a superbubble whose affects on the low-density ambient interstellar medium can be ideally studied. ROSAT PSPC observations of this galaxy reveal two striking blobs of X-ray emission embedded in \Ha loops which can be interpreted as both sides of the upper plumes of the same superbubble. These sources are a surprise. They are much softer than those observed from other starburst dwarf galaxies, and are so soft that they should have been blocked if the observed Galactic HI column density were uniformly distributed across NGC 1705 or if the sources were embedded in the HI disk of NGC 1705. In addition, the total X-ray luminosity in the ROSAT energy band of 1.2x10^{38} erg s^{-1} is low in comparison to similar objects. We discuss possible models for the two X-ray peaks in NGC 1705 and find that the sources most likely originate from relatively cool gas of one single superbubble in NGC 1705. The implications of the exceptional softness of these sources are addressed in terms of intrinsic properties of NGC 1705 and the nature of the foreground Galactic absorption.Comment: 7 pages, 2 ps-figures, LATEX-file; accepted for publication in ApJ.Letter

The Initial Mass Functions in the Super-Star-Clusters NGC 1569A and NGC 1705-1

I use recent photometric and stellar velocity dispersion measurements of the super-star-clusters (SSCs) NGC 1569A and NGC 1705-1 to determine their present-day luminosity/mass (L_V/M) ratios. I then use the inferred L_V/M ratios, together with population synthesis models of evolving star-clusters, to constrain the initial-mass-functions (IMFs) in these objects. I find that (L_V/M)_solar=28.9 in 1569A, and (L_V/M)_solar=126 in 1705-1. It follows that in 1569A the IMF is steep with alpha~2.5 for m**(-alpha)dm IMFs which extend to 0.1 M_sun. This implies that most of the stellar mass in 1569A is contained in low-mass (< 1 M_sun) stars. However, in 1705-1 the IMF is either flat, with alpha<2$, or it is truncated at a lower mass-limit between 1 and 3 M_sun. I compare the inferred IMFs with the mass functions (MFs) of Galactic globular clusters. It appears that 1569A has a sufficient reservoir of low-mass stars for it to plausibly evolve into an object similar to Galactic globular clusters. However, the apparent deficiency of low-mass stars in 1705-1 may make it difficult for this SSC to become a globular cluster. If low-mass stars do dominate the cluster mass in 1705-1, the large L_V/M ratio in this SSC may be evidence that the most massive stars have formed close to the cluster cores.Comment: ApJ, in press. 19 Pages, Latex; [email protected]

Observations of the rotational transitions of OH from the Orion molecular cloud

A summary of observed rotationally excited, far infrared OH line emissions from Orion-KL made using the Kuiper Airborne Observatory is given, together with a list of the resulting publications, talks, and lectures based on this data. In addition, a paper is appended, particularly addressing the (16)OH and (18)OH emission from Orion-KL. The first detections of the (16)OH (2)pi(1/2) to (2)pi(3/2) J = 3/2(-) to 3/2(+) rotational cross-ladder transition (53.351 micrometer) and the (18)OH (2)pi(3/2) J = 5/2(+) to 3/2(-) rotational ground-state transition (120.1719 micrometer). It is found that both of these lines exhibit a P-Cygni profile

SBS 0335-052W: The Lowest-Metallicity Star-Forming Galaxy Known

We present 4-meter Kitt Peak telescope and 6.5-meter MMT spectrophotometry of the extremely low-metallicity galaxy SBS 0335-052W, the western companion of the blue compact dwarf galaxy SBS 0335-052E. These observations have been combined with published 10-meter Keck data to derive for the brightest region of SBS 0335-052W an oxygen abundance 12+logO/H=7.12+/-0.03. This makes SBS 0335-052W the lowest metallicity star-forming galaxy known in the local universe. Using a Monte Carlo technique, we fit the spectral energy distribution of SBS 0335-052W to derive the age of the oldest stars contributing to its optical light. We find that star formation in SBS 0335-052W began less than 500 Myr ago, making it a likely nearby young dwarf galaxy.Comment: 13 pages, 3 figures, accepted for publication in the Astrophysical Journa

Metabolism of Nonessential N15-Labeled Amino Acids and the Measurement of Human Whole-Body Protein Synthesis Rates

Eight N-15 labeled nonessential amino acids plus (15)NH4Cl were administered over a 10 h period to four healthy adult males using a primed-constant dosage regimen. The amount of N-15 excreted in the urine and the urinary ammonia, hippuric acid, and plasma alanine N-15 enrichments were measured. There was a high degree of consistency across subjects in the ordering of the nine compounds based on the fraction of N-15 excreted (Kendall coefficient of concordance W = 0.83, P is less than 0.01). Protein synthesis rates were calculated from the urinary ammonia plateau enrichment and the cumulative excretion of N-15. Glycine was one of the few amino acids that gave similar values by both methods

UV Interstellar Absorption Lines towards the Starburst Dwarf Galaxy NGC 1705

Archival Goddard High Resolution Spectrograph low-resolution spectra of NGC 1705, with wavelength ranges 1170.3 to 1461.7 A and 1453.5 to 1740.1 A and a velocity resolution of about 100 km\s, have been used to derive the velocity structure and equivalent widths of the absorption lines of Si II 1190.42, 1260.42, 1304.37 and 1526.71 A, S II 1253 , Al II 1670.79 Aand Fe II 1608.45 A in this sightline. Three relatively narrow absorption components are seen at LSR velocities --20 km/s, 260 km/sand 540 km/s. Arguments are presented to show these absorption features are interstellar rather than stellar in origin based on a comparison with the C III 1175.7 A absorption feature. We identify the --20 km/s component with Milky Way disk/halo gas and the 260 km/s component with an isolated high-velocity cloud HVC 487. This small HVC is located about 10 degrees from the H I gas which envelops the Magellanic Clouds and the Magellanic Stream (MS). The (Si/H) ratio for this HVC is > 0.6 (Si/H)solar which together with velocity agreement, suggests association with the Magellanic Cloud and MS gas. H-alpha emission line kinematics of NGC 1705 show the presence of a kpc-scale expanding supershell of ionized gas centered on the central nucleus with a blue-shifted emission component at 540 km/s (Meurer et al. 1992). We identify the 540 km/s absorption component seen in the GHRS spectra with the front side of this expanding, ionized supershell. The most striking feature of this component is strong Si II and Al II absorption but weak Fe II 1608 A absorption. The low Fe II column density derived is most likely intrinsic since it cannot be accounted for by ionization corrections or dust depletion. Due to their shallow gravitational potential wells, dwarf galaxies have small gravitational binding energies and are vulnerable to largeComment: 15 pages, LaTEX, 1 figure. Accepted for publication in Astrophysical Journal Letter

On the Maximum Luminosity of Galaxies and Their Central Black Holes: Feedback From Momentum-Driven Winds

We investigate large-scale galactic winds driven by momentum deposition. Momentum injection is provided by (1) radiation pressure produced by the continuum absorption and scattering of UV photons on dust grains and (2) supernovae. UV radiation can be produced by a starburst or AGN activity. We argue that momentum-driven winds are an efficient mechanism for feedback during the formation of galaxies. We show that above a limiting luminosity, momentum deposition from star formation can expel a significant fraction of the gas in a galaxy. The limiting, Eddington-like luminosity is $L_{\rm M}\simeq(4f_g c/G) \sigma^4$, where $\sigma$ is the galaxy velocity dispersion and $f_g$ is the gas fraction. A starburst that attains $L_{\rm M}$ moderates its star formation rate and its luminosity does not increase significantly further. We argue that ellipticals attain this limit during their growth at $z \gtrsim 1$ and that this is the origin of the Faber-Jackson relation. We show that Lyman break galaxies and ultra-luminous infrared galaxies have luminosities near $L_{\rm M}$. Star formation is unlikely to efficiently remove gas from very small scales in galactic nuclei, i.e., scales much smaller than that of a nuclear starburst. This gas is available to fuel a central black hole (BH). We argue that a BH clears gas out of its galactic nucleus when the luminosity of the BH itself reaches $\approx L_{\rm M}$. This shuts off the fuel supply to the BH and may also terminate star formation in the surrounding galaxy. As a result, the BH mass is fixed to be $M_{\rm BH}\simeq (f_g \kappa_{\rm es}/\pi G^2)\sigma^4$, where $\kappa_{\rm es}$ is the electron scattering opacity. This limit is in accord with the observed $M_{\rm BH}-\sigma$ relation. (Abridged)Comment: 21 pages, emulateapj, accepted to ApJ, minor changes to discussio

Metal Abundances of KISS Galaxies III. Nebular Abundances for Fourteen Galaxies and the Luminosity-Metallicity Relationship for HII Galaxies

We report results from the third in a series of nebular abundance studies of emission-line galaxies from the KPNO International Spectroscopic Survey (KISS). Galaxies with coarse metallicity estimates of 12 + log(O/H) less than 8.2 dex were selected for observation. Spectra of 14 galaxies, which cover the full optical region from [OII]3727,3729 to beyond [SII]6717,6731, are presented, and abundance ratios of N, O, Ne, S, and Ar are computed. The auroral [OIII]4363 line is detected in all 14 galaxies. Oxygen abundances determined through the direct electron temperature T_e method confirm that the sample is metal-poor with 7.61 <= 12 + log(O/H) <= 8.32. By using these abundances in conjunction with other T_e-based measurements from the literature, we demonstrate that HII galaxies and more quiescent dwarf irregular galaxies follow similar metallicity-luminosity (L-Z) relationships. The primary difference is a zero-point shift between the correlations such that HII galaxies are brighter by an average of 0.8 B magnitudes at a given metallicity. This offset can be used as evidence to argue that low-luminosity HII galaxies typically undergo factor of two luminosity enhancements, and starbursts that elevate the luminosities of their host galaxies by 2 to 3 magnitudes are not as common. We also demonstrate that the inclusion of interacting galaxies can increase the scatter in the L-Z relation and may force the observed correlation towards lower metallicities and/or larger luminosities. This must be taken into account when attempting to infer metal abundance evolution by comparing local L-Z relations with ones based on higher redshift samples since the fraction of interacting galaxies should increase with look-back time.Comment: 36 pages, 5 figures. ApJ, in pres

Herschel observations of EXtra-Ordinary Sources (HEXOS): Observations of H_2O and its isotopologues towards Orion KL

We report the detection of more than 48 velocity-resolved ground rotational state transitions of H^(16)_2O, H^(18) _2O, and ^(17)_2O – most for the first time – in both emission and absorption toward Orion KL using Herschel/HIFI. We show that a simple fit, constrained to match the known emission and absorption components along the line of sight, is in excellent agreement with the spectral profiles of all the water lines. Using the measured H^(18)_2O line fluxes, which are less affected by line opacity than their H^(16)_2O counterparts, and an escape probability method, the column densities of H^(18)_2O associated with each emission component are derived. We infer total water abundances of 7.4 × 10^(−5), 1.0 × 10^(−5), and 1.6 × 10^(−5) for the plateau, hot core, and extended warm gas, respectively. In the case of the plateau, this value is consistent with previous measures of the Orion-KL water abundance as well as those of other molecular outflows. In the case of the hot core and extended warm gas, these values are somewhat higher than water abundances derived for other quiescent clouds, suggesting that these regions are likely experiencing enhanced water-ice sublimation from (and reduced freeze-out onto) grain surfaces due to the warmer dust in these sources

A dozen colliding wind X-ray binaries in the star cluster R136 in the 30Doradus region

We analyzed archival Chandra X-ray observations of the central portion of the 30 Doradus region in the Large Magellanic Cloud. The image contains 20 X-ray point sources with luminosities between $5 \times 10^{32}$ and $2 \times 10^{35}$ erg s$^{-1}$ (0.2 -- 3.5 keV). A dozen sources have bright WN Wolf-Rayet or spectral type O stars as optical counterparts. Nine of these are within $\sim 3.4$pc of R136, the central star cluster of NGC2070. We derive an empirical relation between the X-ray luminosity and the parameters for the stellar wind of the optical counterpart. The relation gives good agreement for known colliding wind binaries in the Milky Way Galaxy and for the identified X-ray sources in NGC2070. We conclude that probably all identified X-ray sources in NGC2070 are colliding wind binaries and that they are not associated with compact objects. This conclusion contradicts Wang (1995) who argued, using ROSAT data, that two earlier discovered X-ray sources are accreting black-hole binaries. Five of the eighteen brightest stars in R136 are not visible in our X-ray observations. These stars are either single, have low mass companions or very wide orbits. The resulting binary fraction among early type stars is then unusually high (at least 70%).Comment: 23 pages, To appear in August in Ap