arXiv subjects

T. M. Heckman

Publications and source records attributed to T. M. Heckman.

107 records · Page 6Linked to original sources

arXiv2001

Extreme BAL Quasars from the Sloan Digital Sky Survey

The Sloan Digital Sky Survey has discovered a population of broad absorption line quasars with various extreme properties. Many show absorption from metastable states of FeII with varying excitations; several objects are almost completely absorbed bluewards of MgII; at least one shows stronger absorption from FeIII than FeII, indicating temperatures T>35000 K in the absorbing region; and one object even seems to have broad H-beta absorption. Many of these extreme BALs are also heavily reddened, though `normal' BALs (particularly LoBALs) from SDSS also show evidence for internal reddening.

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arXiv2001

Another intermediate mass black hole in a starburst galaxy?: The luminous X-ray source in NGC 3628 reappears

In a 52 ks-long Chandra ACIS-S observation of the nearby starburst galaxy NGC 3628, obtained to study the starburst-driven outflow from this galaxy, we have detected a very luminous (L_X = 1.1e40 erg/s in the 0.3-8.0 keV energy band) point source located at least 20 arcsec (~970 pc) from the nucleus of the galaxy. No radio, optical or near-IR counterpart to this source has been found. This is most probably the reappearance of the strongly-variable X-ray-luminous source discovered by Dahlem et al (1995), which faded by a factor >27 between December 1991 and March 1994 (at which point it had faded below the detection limit in a ROSAT HRI observation). This source is clearly a member of an enigmatic class of X-ray sources that are considerably more luminous than conventional X-ray binaries but less luminous than AGN, and which are not found at the dynamical center of the host galaxy. The Chandra spectrum is best-fit by an absorbed power law model with a photon index of Gamma = 1.8+/-0.2, similar to that seen in Galactic BH binary candidates in their hard state. Bremsstrahlung models or multi-color disk models (the favored spectral model for objects in this class based on ASCA observations) can provide statistically acceptable fits only if the data at energies E > 5 keV is ignored. This is one of the first X-ray spectra of such an object that is unambiguously that of the source alone, free from the spectral contamination by X-ray emission from the rest of the galaxy that affects previous spectral studies of these objects using ASCA.

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arXiv2001

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

We present NGC 6221 as a case study of ``X-ray loud composite galaxies,'' 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 K alpha 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 N_H = 10^{22} cm^{-2} combined with relatively weak intrinsic nuclear activity can produce an optical spectrum characteristic of the surrounding starburst alone. While optical surveys would not identify the active nuclei that make these galaxies significant X-ray sources, such galaxies may in fact be important contributors to the X-ray background.

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arXiv2001

FUSE Observations of Outflowing OVI in the Dwarf Starburst Galaxy NGC1705

We report FUSE far-UV spectroscopy of the prototypical dwarf starburst galaxy NGC 1705. These data allow us for the first time to probe the coronal-phase gas (T = 10E5 to 10E6 K) that may dominate the radiative cooling of the supernova-heated ISM and thereby determine the dynamical evolution of starburst-driven outflows. We detect a broad (100 km/s) and blueshifted (by 80 km/s) OVI absorption-line arising in the previously-known galactic outflow. The properties of the OVI absorption are inconsistent with the standard superbubble model in which this gas arises in a conductive interface inside the outer shell. We show that the superbubble in NGC 1705 is blowing out of the galaxy ISM. During blow-out, coronal-phase gas can be created by hydrodynamical mixing as hot gas rushes out through fissures in the fragmenting shell of cool gas. As the coronal gas cools radiatively, it can naturally produce the observed OVI column density and outflow speed. The OVI data show that the cooling rate in the coronal-phase gas is less than about 10% of the supernova heating rate. Since the X-ray luminosity from hotter gas is even smaller, we conclude that radiative losses are insignificant. The outflow should be able to vent its metals and kinetic energy out of the galaxy. This process has potentially important implications for the evolution of dwarf galaxies and the IGM.

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arXiv2001

Star Formation in the Field and Clusters of NGC 5253

We investigate the star formation history of both the bright star clusters and the diffuse `field star' population in the dwarf starburst galaxy NGC 5253 using STIS longslit ultraviolet spectroscopy. Our slit covers a physical area of 370 x 1.6 pc and includes 8 apparent clusters and several inter-cluster regions of diffuse light which we take to be the field. The diffuse light spectrum lacks the strong O-star wind features which are clearly visible in spectra of the brightest clusters. This discrepancy provides compelling evidence that the diffuse light is not reflected light from nearby clusters, but originates in a UV-bright field star population, and it raises the issue of whether the star formation process may be operating differently in the field than in clusters. We compare our spectra to STARBURST99 evolutionary synthesis models which incorporate a new low metallicity atlas of O-star spectra. We favor a scenario which accounts for the paucity of O-stars in the field without requiring the field to have a different IMF than the clusters: stellar clusters form continuously and then dissolve on ~10 Myr timescales and disperse their remaining stars into the field. We consider the probable contribution of an O-star deficient field population to the spatially unresolved spectra of high redshift galaxies. (Abridged)

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arXiv2000

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] 5007 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 (N_H > 10^{23} 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 starbursts and active galactic nuclei may be part of a common evolutionary sequence.

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arXiv2000

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 twelve observed galaxies are significantly resolved with the ROSAT HRI, while six of the eight sources observed with the lower-resolution PSPC 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 (N_H > 10^{23} 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 K alpha lines, which are typical of AGNs.

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arXiv2000

NICMOS Imaging of the Host Galaxies of z ~ 2 - 3 Radio-Quiet Quasars

We have made a deep NICMOS imaging study of a sample of 5 z ~ 2 - 3 radio-quiet quasars with low absolute nuclear luminosities, and we have detected apparent host galaxies in all of these. Most of the hosts have luminosities approximately equal to present-day L*, with a range from 0.2 L* to about 4 L*. These host galaxies have magnitudes and sizes consistent with those of the Lyman break galaxies at similar redshifts and at similar rest wavelengths, but are about two magnitudes fainter than high-z powerful radio galaxies. The hosts of our high-z sample are comparable to or less luminous than the hosts of the low-z RQQs with similar nuclear absolute magnitudes. However, the high z galaxies are more compact than the hosts of the low z quasars, and probably have only 10 - 20% of the stellar mass of their low-z counterparts. Application of the M(bulge)/M(BH) relation found for present-day spheroids to the stellar masses implied for the high z host galaxies would indicate that they contain black holes with masses around 10^8 Msolar. Comparison to their nuclear magnitudes implies accretion rates that are near or at the Eddington limit. Although these high z hosts already contain supermassive black holes, the galaxies will need to grow significantly to evolve into present-day L* galaxies. These results are basically consistent with theoretical predictions for the hierarchical buildup of the galaxy host and its relation to the central supermassive black hole.

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arXiv2000

Hiden Star Formation: The Ultraviolet Perspective

Many recent estimates of the star formation rate density at high redshift rely on rest-frame ultraviolet (UV) data. These are highly sensitive to dust absorption. Applying a correlation between the far-infrared (FIR) to UV flux ratio and UV color found in a local starbursts to galaxy samples out to z ~ 3, one can account for most of the FIR background. However, the correlation is based on a sample that does not include the most extreme starbursts, Ultra Luminous Infrared Galaxies (ULIGs). Our new UV images of ULIGs show that their FIR fluxes are underpredicted by this correlation by factors ranging from 7 to 70. We discuss how ULIGs compare to the various types of high-z galaxies: sub-mm sources, Lyman Break Galaxies, and Extremely Red Objects.

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arXiv2000

Five High-Redshift Quasars Discovered in Commissioning Imaging Data of the Sloan Digital Sky Survey

We report the discovery of five quasars with redshifts of 4.67 - 5.27 and z'-band magnitudes of 19.5-20.7 M_B ~ -27. All were originally selected as distant quasar candidates in optical/near-infrared photometry from the Sloan Digital Sky Survey (SDSS), and most were confirmed as probable high-redshift quasars by supplementing the SDSS data with J and K measurements. The quasars possess strong, broad Lyman-alpha emission lines, with the characteristic sharp cutoff on the blue side produced by Lyman-alpha forest absorption. Three quasars contain strong, broad absorption features, and one of them exhibits very strong N V emission. The amount of absorption produced by the Lyman-alpha forest increases toward higher redshift, and that in the z=5.27 object (D_A ~ 0.7) is consistent with a smooth extrapolation of the absorption seen in lower redshift quasars. The high luminosity of these objects relative to most other known objects at z >~ 5 makes them potentially valuable as probes of early quasar properties and of the intervening intergalactic medium.

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arXiv2000

Ambiguities in fits to the complex X-ray spectra of starburst galaxies

Spectral fits to X-ray data from both NGC 253 and M 82 provide ambiguous results. The so-called ``best fit'' results depend on the instrument with which the data were obtained and obviously on the choice of spectral model composition. We show that different spectral models can be fit equally well to BeppoSAX data of both galaxies. Metallicities are unreliable in general, with a strong dependence on the choice of model. Preference to one particular spectral model can only be given by combining spectroscopic and imaging X-ray data from all available satellites (ROSAT, ASCA, and BeppoSAX). Based on spectra of NGC 253, we demonstrate that a model consisting of two or more thermal plasma components plus a hard power law continuum and Fe K alpha line emission can explain all observations. These model components represent the integral spectrum of thermal gas and compact sources in starburst galaxies that are most likely supernova remnants and X-ray binaries. The same model can fit the X-ray data of M 82, but there the evidence, from ROSAT imaging, of the existence of compact sources which might represent high-mass X-ray binaries is weaker. This implies that its hard X-ray emission, which is extended in ROSAT images, might - if truly diffuse - be dominated by a very hot (several keV energy) thermal gas component.

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arXiv1999

The Evolution of the Interstellar Medium Around Young Stellar Clusters

The interplay between the ISM and the massive stars formed in clusters and, more generally, in recent events of star formation is reviewed via the global effects each has on the other. The pre-existing environment affects the properties of the massive stars, the duration of the star-forming event and could potentially affect the IMF. The collective effect of massive-star winds and supernova explosions creates a structured ISM by forming bubbles, supershells and, in more extreme cases, by inducing large-scale gas outflows. Gas/dust removal may quench star formation in young stellar clusters. Conversely, supernova-driven shocks may trigger star formation in molecular clouds surrounding the stellar clusters. Metal ejection from the massive stars is responsible for the pollution of the ISM and, if the metal-rich gas can escape the galaxy's gravitational potential, of the IGM. The environment where stellar clusters form is populated by a diffuse stellar population which contributes between 50% and 80% of the total UV light. The investigation of the nature of the diffuse UV light is the subject of a study employing HST STIS spectroscopy, whose preliminary results are presented and briefly discussed.

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arXiv1999

Very Extended X-ray and H-alpha Emission in M82: Implications for the Superwind Phenomenon

We discuss the properties and implications of a 3.7x0.9 kpc region of spatially-coincident X-ray and H-alpha emission about 11.6 kpc to the north of the galaxy M82 previously discussed by Devine and Bally (1999). The PSPC X-ray spectrum is fit by thermal plasma (kT=0.80+-0.17 keV) absorbed by only the Galactic foreground column density. We evaluate the relationship of the X-ray/H-alpha ridge to the M82 superwind. The main properties of the X-ray emission can all be explained as being due to shock-heating driven as the superwind encounters a massive ionized cloud in the halo of M82. This encounter drives a slow shock into the cloud, which contributes to the excitation of the observed H-alpha emission. At the same time, a fast bow-shock develops in the superwind just upstream of the cloud, and this produces the observed X-ray emission. This interpretation would imply that the superwind has an outflow speed of roughly 800 km/s, consistent with indirect estimates based on its general X-ray properties and the kinematics of the inner kpc-scale region of H-alpha filaments. The gas in the M82 ridge is roughly two orders-of-magnitude hotter than the minimum "escape temperature" at this radius, so this gas will not be retained by M82. (abridged)

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arXiv1998

An X-ray and Optical Investigation of the Starburst-driven Superwind in the Galaxy Merger Arp 299

We present a detailed investigation of the X-ray and optical properties of the starburst-merger system Arp299 (NGC 3690, Mrk 171), with an emphasis on its spectacular gaseous nebula. We analyse \rosat and \asca X-ray data and optical spectra and narrow-band images. We suggest that the on-going galaxy collision has tidally-redistributed the ISM of the merging galaxies. The optical emission-line nebula results as this gas is photoionized by radiation that escapes from the starburst, and is shock-heated, accelerated, and pressurized by a `superwind' driven by the collective effect of the starburst supernovae and stellar winds. The X-ray nebula in Arp 299 is is plausibly a mass-loaded flow of adiabatically-cooling gas that carries out a substantial fraction of the energy and metals injected by the starburst at close to the escape velocity from Arp 299. The mass outflow rate likely exceeds the star-formation rate in this system. We conclude that powerful starbursts are able to heat (and possibly eject) a significant fraction of the ISM in merging galaxies.

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arXiv1998

The Evolution of Dust Opacity in Galaxies

(Abridged) We investigate the evolution of the opacity of galaxies as a function of redshift, using simple assumptions about the metal and dust enrichment of the gas and the distribution of dust in galaxies. We use an iterative procedure to reconstruct the intrinsic Star Formation Rate (SFR) density of galaxies with redshift, by applying dust obscuration corrections to the observed UV emission. The iterative procedure converges to multiple solutions for the intrinsic SFR density, divided into two basic classes. The first class of solutions predicts relatively large UV attenuation at high redshift, with A(1500 A)=1.9 mag at z~3, and smaller attenuations at z<1, with A(2800 A)=1.25 mag. The SFR density of this set of solutions is constant for z>~1.2 and declines for z<1.2; it resembles in shape the ``monolithic collapse'' scenario for star formation. The second class of solutions predicts relatively low UV attenuations at high redshift, with A(1500 A)=0.75 mag at z~3, and larger attenuations at z<1, with A(2800 A)=1.50 mag. The SFR density in this case has a peak at z~1.2. The advantages and shortcomings of both classes are analyzed in the light of available observational constraints, including the opacity of galaxies at 0<z<1 and the intensity and spectral energy distribution of the cosmic infrared background from the COBE DIRBE and FIRAS data. We conclude that both classes of models are acceptable within the current uncertainties, but the ``monolithic collapse'' class matches the available observations better than the other one. We also investigate the dependence of our solutions on the different model assumptions.

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arXiv1998

ASCA Observations of the Starburst-Driven Superwind Galaxy NGC 2146: Broad Band (0.6 - 9 keV) Spectral Properties

We report ASCA GIS and SIS observations of the nearby (D = 11.6 Mpc), nearly edge-on, starburst galaxy NGC 2146. These X-ray spectral data complement ROSAT PSPC and HRI imaging discussed by Armus et al., 1995. The broad band (0.6-9 keV) X-ray spectrum of NGC 2146 is best described by a two component model: the soft X-ray emission with a Raymond-Smith thermal plasma model having a temperature of kT $\sim 0.8$ keV; the hard X-ray emission with a thermal plasma model having kT $\sim 8$ keV or a power-law model having a photon index of $\sim 1.7$. We do not find compelling evidence of substantial excess absorption above the Galactic value. The soft (hard) thermal component provides about 30% (70%) of the total luminosity in the 0.5 - 2.0 keV energy band, while in the 2-10 keV energy range only the hard component plays a major role. The spectral results allow us to set tighter constraints on the starburst-driven superwind model, which we show can satisfactorily account for the luminosity, mass, and energy content represented by the soft X-ray spectral component. We estimate that the mass outflow rate ($\sim$ 9 M$_{\odot}$ per year) is about an order of magnitude greater than the predicted rate at which supernovae and stellar winds return mass into the interstellar medium and, therefore, argue that the flow is strongly "mass-loaded" with material in and around the starburst. The estimated outflow velocity of the hot gas is close to the escape velocity from the galaxy, so the fate of the gas is not clear. We suggest that the hard X-ray spectral component is due to the combined emission of X-ray binaries and/or young supernovae remnants associated with the starburst.

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arXiv1995

Starbursts and Star Clusters in the Ultraviolet

Hubble Space Telescope ultraviolet (UV) images of nine starburst galaxies reveal them to be highly irregular, even after excluding compact sources (clusters and resolved stars). Most (7/9) are found to have a similar intrinsic effective surface brightnesses, suggesting that a negative feedback mechanism is setting an upper limit to the star formation rate per unit area. All starbursts in our sample contain UV bright star clusters indicating that cluster formation is an important mode of star formation in starbursts. On average about 20% of the UV luminosity comes from these clusters. The brightest clusters, or super star clusters (SSC), are preferentially found at the very heart of starbursts. The size of the nearest SSCs are consistent with those of Galactic globular clusters. The luminosity function of SSCs is well represented by a power law with a slope alpha ~ -2. There is a strong correlation between the far infrared excess and the UV spectral slope. The correlation is well modeled by a geometry where much of their dust is in a foreground screen near to the starburst, but not by a geometry of well mixed stars and dust.

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