arXiv subjects

Timothy M. Heckman

Publications and source records attributed to Timothy M. Heckman.

At least 163 records · Page 9Linked to original sources

arXiv2004

Present-Day Growth of Black Holes and Bulges: the SDSS Perspective

We investigate the accretion-driven growth of supermassive black holes in the low-redshift Universe using 23,000 ``Type 2'' AGN and the complete sample of 123,000 galaxies in the SDSS from which they were drawn. We use the stellar velocity dispersions of the early type galaxies and AGN hosts to estimate their black hole masses and we use the AGN [OIII]5007 emission line luminosities to estimate black hole accretion rates. We find that most present-day accretion occurs onto black holes with masses < 10E8 Msun that reside in moderately massive galaxies (M ~ 10E10 to 10E11.5 Msun) with high stellar densities (~10E8.5 to 10E9.5 Msun/kpc^2) and young stellar populations. The volume-averaged accretion rates of low mass black holes (<10E7.5 Msun) imply that this population is growing on a timescale that is comparable to the age of the Universe. Around half this growth takes place in AGN that are radiating within a factor of five of the Eddington luminosity. Such systems are rare, making up only 0.2% of the low mass black hole population at the present day. The rest of the growth occurs in lower luminosity AGN. The growth timescale is more than two orders of magnitude longer for the population of the most massive black holes in our sample. The volume averaged ratio of star formation to black hole accretion in bulge-dominated galaxies is ~1000, in remarkable agreement with the observed ratio of stellar mass to black hole mass in nearby galaxy bulges. We conclude: a) that bulge formation and black hole formation are tightly coupled, even in present-day galaxies; and b) that the evolution of the AGN luminosity function documented in recent optical and x-ray surveys is driven by a decrease in the characteristic mass scale of accreting black holes.

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arXiv2004

Chandra & XMM-Newton Observations of NGC5253. Analysis of the X-ray Emission from a Dwarf Starburst Galaxy

We present Chandra and XMM-Newton X-ray data of NGC5253, a local starbursting dwarf elliptical galaxy, in the early stages of a starburst episode. Contributions to the X-ray emission come from discrete point sources and extended diffuse emission, in the form of what appear to be multiple superbubbles, and smaller bubbles probably associated with individual star clusters. Chandra detects 17 sources within the optical extent of NGC5253 down to a completeness level corresponding to a luminosity of 1.5E37 erg/s.The slope of the point source X-ray luminosity function is -0.54, similar to that of other nearby dwarf starburst galaxies. Several different types of source are detected within the galaxy, including X-ray binaries and the emission associated with star-clusters. Comparison of the diffuse X-ray emission with the observed Halpha emission shows similarities in their extent. The best spectral fit to the diffuse emission is obtained with an absorbed, two temperature model giving temperatures for the two gas components of ~0.24keV and ~0.75keV.The derived parameters of the diffuse X-ray emitting gas are as follows: a total mass of \~1.4E6 f^{1/2} Msun, where f is the volume filling factor of the X-ray emitting gas, and a total thermal energy content for the hot X-ray emitting gas of \~3.4E54 f^{1/2} erg. The pressure in the diffuse gas is P/k ~ 1E6f^{-1/2}K/cm3. We find that these values are broadly commensurate with the mass and energy injection from the starburst population. Analysis of the kinematics of the starburst region suggest that the stellar ejecta contained within it can escape the gravitational potential well of the galaxy, and pollute the surrounding IGM.

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arXiv2004

The Environmental Dependence of the Relations between Stellar Mass, Structure, Star Formation and Nuclear Activity in Galaxies

We use a complete sample of galaxies drawn from the SDSS to study how structure, star formation and nuclear activity depend on local density and on stellar mass. Local density is estimated by counting galaxies above a fixed absolute magnitude limit within cylinders 2 Mpc in projected radius and +-500 km/s in depth. The stellar mass distribution of galaxies shifts by nearly a factor of two towards higher masses between low and high density regions. At fixed stellar mass, both star formation and nuclear activity depend strongly on local density, while structural parameters such as size and concentration are almost independent of it. The galaxy property most sensitive to environment is specific star formation rate. For galaxies with stellar masses less than 3 x 10^10 M_sun, the median SFR/M* decreases by more than a factor of 10 from low to high densities. This decrease is less marked for massive galaxies. At fixed stellar mass, twice as many galaxies host AGN with strong [OIII] emission in low density regions as in high. Massive galaxies in low-density environments also contain more dust. We have analyzed correlations between spectroscopic indicators that probe SFH on different timescales (D4000, Hdelta_A and SFR/M*). The correlations do not depend on environment, suggesting that the decrease in star formation has ocurred over long (>1 Gyr) timescales. Since structure does not depend on environment for more massive galaxies, trends in recent SFH, dust and AGN for these systems cannot be driven by processes that alter structure, e.g. mergers. The SFH-density correlation is strongest for small scale (< 1 Mpc) estimates of local density. Finally, we highlight a striking similarity between changes in the galaxy population as a function of density and as a function of redshift and we interpret this using N-body simulations.

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arXiv2003

Chandra and XMM-Newton Observations of NGC 4214: The Hot Interstellar Medium and the Luminosity Function of Dwarf Starbursts

We present results from Chandra and XMM-Newton X-ray observations of NGC 4214, a nearby dwarf starburst galaxy containing several young regions of very active star-formation. Starburst regions are known to be associated with diffuse X-ray emission, and in this case the X-ray emission from the galaxy shows an interesting morphological structure within the galaxy, clearly associated with the central regions of active star-formation. Of the two main regions of star-formation in this galaxy, X-ray emission associated with the older is identified whereas little is detected from the younger, providing an insight into the evolutionary process of the formation of superbubbles around young stellar clusters. The spectra of the diffuse emission from the galaxy can be fitted with a two temperature component thermal model with kT=0.14keV and 0.52keV, and analysis of this emission suggests that NGC 4214 will suffer a blow-out in the future. The point source population of the galaxy has an X-ray luminosity function with a slope of -0.76. This result, together with those for other dwarf starburst galaxies (NGC 4449 and NGC 5253), was added to a sample of luminosity functions for spiral and starburst galaxies. The slope of the luminosity function of dwarf starbursts is seen to be similar to that of their larger counterparts and clearly flatter than those seen in spirals. Further comparisons between the luminosity functions of starbursts and spiral galaxies are also made.

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arXiv2003

Type II Quasars from the SDSS

Type II quasars are suggested by unification models of Active Galactic Nuclei as luminous analogs of Seyfert 2 galaxies. A number of different methods have been used hitherto to discover type II quasars, but only a handful have been found. We selected about 150 type II quasar candidates from the SDSS spectroscopic database; these objects have strong narrow high-ionization emission lines. We describe the selection procedure and estimate intrinsic luminosities of these optically obscured AGN. A campaign to perform a multi-wavelength follow-up of the sample is now underway.

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arXiv2003

Cooling in Coronal Gas in the M82 Starburst Superwind

We have used the Far Ultraviolet Spectroscopic Explorer to search for OVI 1031.926 Angstrom emission at four locations in the starburst superwind of M82. No OVI emission was detected at any of the four pointings, with upper limits less than or equal to the 0.3-2 keV X-ray flux. These observations limit the energy lost through radiative cooling of coronal phase (T~300,000 K) gas to roughly the same magnitude as that lost in the hot phase through X-ray emission, which has been shown to be small. The wind material retains most of its energy and should be able to escape from the gravitational potential of M82, enriching the intergalactic medium with energy and metals. The lack of coronal gas in the wind and observations of spatially correlated X-ray and H-alpha emission are consistent with a scenario in which the hot wind material over-runs cold clouds in the halo, or one where the H-alpha and X-ray emission arise at the interface between the hot wind and a cool shell of swept-up ISM, as long as the shock velocity is <150 km/s. The observed limits on the OVI/H-alpha and CIII/H-alpha flux ratios rule out shock heating as the source of the 10,000 K gas unless the shock velocity is <90 km/s.

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arXiv2003

The Host Galaxies of AGN

We examine the properties of the host galaxies of 22,623 narrow-line AGN with 0.02<z<0.3 selected from a complete sample of 122,808 galaxies from the Sloan Digital Sky Survey. We focus on the luminosity of the [OIII]$λ$5007 emission line as a tracer of the strength of activity in the nucleus. We study how AGN host properties compare to those of normal galaxies and how they depend on L[OIII]. We find that AGN of all luminosities reside almost exclusively in massive galaxies and have distributions of sizes, stellar surface mass densities and concentrations that are similar to those of ordinary early-type galaxies in our sample. The host galaxies of low-luminosity AGN have stellar populations similar to normal early-types. The hosts of high- luminosity AGN have much younger mean stellar ages. The young stars are not preferentially located near the nucleus of the galaxy, but are spread out over scales of at least several kiloparsecs. A significant fraction of high- luminosity AGN have strong H$δ$ absorption-line equivalent widths, indicating that they experienced a burst of star formation in the recent past. We have also examined the stellar populations of the host galaxies of a sample of broad-line AGN. We conclude that there is no significant difference in stellar content between type 2 Seyfert hosts and QSOs with the same [OIII] luminosity and redshift. This establishes that a young stellar population is a general property of AGN with high [OIII] luminosities.

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arXiv2003

Star Formation in Active Galaxies: A Spectroscopic Perspective

I review the relationship between star formation and black hole building, based on spectroscopic observations of the stellar population in active galactic nuclei (AGNs) and their host galaxies. My emphasis is on large, well-defined local samples of AGNs, whose optical continua are dominated by starlight. I summarize the spectroscopic tools used to characterize their stellar content. Surveys of the nuclei of the nearest galaxies show that the stellar population in most low-power AGNs is predominantly old. In contrast, young ($< 1$ Gyr) stars are detected in about half of the powerful type 2 Seyfert nuclei. I summarize Sloan Digital Sky Survey spectroscopy of the host galaxies of 22,000 AGNs. The AGN phenomenon is commonplace only in galaxies with high mass, high velocity dispersion, and high stellar surface mass density. Most normal galaxies with these properties have old stellar populations. However, the hosts of powerful AGNs (Seyfert 2s) have young stellar populations. The fraction of AGN hosts that have recently undergone a major starburst also increases with AGN luminosity. A powerful AGN presumably requires a massive black hole (host with a substantial bulge) plus an abundant fuel supply (a star-forming ISM). This combination is rare today, but would have been far more common at early epochs.

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arXiv2003

Chandra Observation of NGC4449. Analysis of the X-ray Emission from a Dwarf Starburst Galaxy

We present CHANDRA X-ray data of the nearby Magellanic Irregular dwarf starburst galaxy NGC4449. Contributions to the X-ray emission come from discrete point sources and extended diffuse emission. The extended emission has a complex morphology with an extent of 2.4x1.6kpc down to a flux density of 1.3E-13 erg/s/cm2/arcmin2. The best spectral fit to this emission is obtained with an absorbed, two temperature model giving temperatures for the two gas components of 0.28keV and 0.86keV, a total mass content of ~10^7 Msun compared with a galactic mass of several 10^{10} Msun and a total thermal energy content of ~2.5E55erg, with an average energy injection rate for the galaxy of a few 1E41 erg/s. Comparison of the morphology of the diffuse X-ray emission with that of the observed Halpha emission shows similarities in the two emissions. An expanding super-bubble is suggested by the presence of diffuse X-ray emission within what appears to be a cavity in the Halpha emission. The kinematics of this bubble suggest an expansion velocity of ~220km/s and a mass injection rate of 0.14Msun/yr, but the presence of NGC4449's huge HI halo (r~40kpc) may prevent the ejection, into the IGM, of the metal-enriched material and energy it contains. The arcsecond-resolution of CHANDRA has detected 24 X-ray point sources down to a completeness level corresponding to a flux of ~2E-14 erg/s/cm2, within the optical extent of NGC4449 and analysis of their spectra has shown them to be from at least 3 different classes of object. As well as the known SNR in this galaxy, it also harbours several X-ray binaries and super-soft sources. The point source X-ray luminosity function, for the higher luminosity sources, has a slope of ~-0.51, comparable to those of other starburst galaxies.

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arXiv2002

Stellar Masses and Star Formation Histories for 10^5 Galaxies from the Sloan Digital Sky Survey

We develop a new method to constrain the star formation histories, dust attenuation and stellar masses of galaxies. It is based on two stellar absorption line indices, the 4000 Angstrom break strength and the Balmer absorption line index Hdelta_A. Together, these indices allow us to constrain the mean stellar ages of galaxies and the fractional stellar mass formed in bursts over the past few Gyr. A comparison with broad band photometry then yields estimates of dust attenuation and of stellar mass. We generate a large library of Monte Carlo realizations of different star formation histories, including starbursts of varying strength and a range of metallicities. We use this library to generate median likelihood estimates of burst mass fractions, dust attenuation strengths, stellar masses and stellar mass-to-light ratios for a sample of 122,208 galaxies drawn from the Sloan Digital Sky Survey. The typical 95% confidence range in our estimated stellar masses is +-40%. We study how the stellar mass-to-light ratios of galaxies vary as a function of absolute magnitude, concentration index and photometric pass-band and how dust attenuation varies as a function of absolute magnitude and 4000 Angstrom break strength. We also calculate how the total stellar mass of the present Universe is distributed over galaxies as a function of their mass, size, concentration, colour, burst mass fraction and surface mass density. We find that most of the stellar mass in the local Universe resides in galaxies that have stellar masses \~5\times 10^10 M_sol, half light radii ~3 kpc, and half- light surface mass densities ~10^9 M_sol/kpc^2. The distribution of D(4000) is strongly bimodal, showing a clear division between galaxies dominated by old stellar populations and galaxies with more recent star formation.

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arXiv2002

The Dependence of Star Formation History and Internal Structure on Stellar Mass for 10^5 Low-Redshift Galaxies

We study the relations between stellar mass, star formation history, size and internal structure for a complete sample of 122,808 galaxies drawn from the Sloan Digital Sky Survey. We show that low-redshift galaxies divide into two distinct families at a stellar mass of 3 \times 10^10 M_sol. Lower mass galaxies have young stellar populations, low surface mass densities and the low concentrations typical of disks. A significant fraction of the lowest mass galaxies in our sample have experienced recent starbursts. At given stellar mass, the sizes of low mass galaxies are log- normally distributed with dispersion sigma(ln R_50) \sim 0.5, in excellent agreement with the idea that they form with little angular momentum loss through cooling and condensation in a gravitationally dominant dark matter halo. Their median stellar surface mass density scales with stellar mass as mu* propto M_*^0.54, suggesting that the stellar mass of a disk galaxy is proprtional to the three halves power of its halo mass. This suggests that the efficiency of the conversion of baryons into stars in low mass galaxies increases in propor- tion to halo mass, perhaps as a result of supernova feedback processes. At stellar masses above 3 \times 10^10 M_sol, there is a rapidly increasing frac- tion of galaxies with old stellar populations, high surface mass densities and high concentrations typical of bulges. In this regime, the size distribution is log-normal, but its dispersion decreases rapidly with increasing stellar mass and the median mass surface density is approximately constant. This suggests that the star formation efficiency decreases in the highest mass halos, and that little star formation occurs in massive galaxies once they have assembled.

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arXiv2002

Synthetic High-Resolution Line Spectra of Star-Forming Galaxies Below 1200A

We have generated a set of far-ultraviolet stellar libraries using spectra of OB and Wolf-Rayet stars in the Galaxy and the Large and Small Magellanic Cloud. The spectra were collected with the Far Ultraviolet Spectroscopic Explorer and cover a wavelength range from 1003.1 to 1182.7A at a resolution of 0.127A. The libraries extend from the earliest O- to late-O and early-B stars for the Magellanic Cloud and Galactic libraries, respectively. Attention is paid to the complex blending of stellar and interstellar lines, which can be significant, especially in models using Galactic stars. The most severe contamination is due to molecular hydrogen. Using a simple model for the H$_2$ line strength, we were able to remove the molecular hydrogen lines in a subset of Magellanic Cloud stars. Variations of the photospheric and wind features of CIII 1176, OVI 1032, 1038, PV 1118, 1128, and SIV 1063, 1073, 1074 are discussed as a function of temperature and luminosity class. The spectral libraries were implemented into the LavalSB and Starburst99 packages and used to compute a standard set of synthetic spectra of star-forming galaxies. Representative spectra are presented for various initial mass functions and star formation histories. The valid parameter space is confined to the youngest ages of less than 10 Myr for an instantaneous burst, prior to the age when incompleteness of spectral types in the libraries sets in. For a continuous burst at solar metallicity, the parameter space is not limited. The suite of models is useful for interpreting the restframe far-ultraviolet in local and high-redshift galaxies.

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arXiv2002

The Starburst Nature of Lyman-Break Galaxies: Testing UV Extinction with X-rays

We derive the bolometric to X-ray correlation for a local sample of normal and starburst galaxies and use it, in combination with several UV reddening schemes, to predict the 2--8 keV X-ray luminosity for a sample of 24 Lyman-break galaxies in the HDF/CDF-N. We find that the mean X-ray luminosity, as predicted from the Meurer UV reddening relation for starburst galaxies, agrees extremely well with the Brandt stacking analysis. This provides additional evidence that Lyman-break galaxies can be considered as scaled-up local starbursts and that the locally derived starburst UV reddening relation may be a reasonable tool for estimating the UV extinction at high redshift. Our analysis shows that the Lyman-break sample can not have far-IR to far-UV flux ratios similar to nearby ULIGs, as this would predict a mean X-ray luminosity 100 times larger than observed, as well as far-IR luminosities large enough to be detected in the sub-mm. We calculate the UV reddening expected from the Calzetti effective starburst attenuation curve and the radiative transfer models of Witt & Gordon for low metallicity dust in a shell geometry with homogeneous or clumpy dust distributions and find that all are consistent with the observed X-ray emission. Finally, we show that the mean X-ray luminosity of the sample would be under predicted by a factor of 6 if the the far-UV is unattenuated by dust.

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arXiv2001

Far-Infrared Galaxies in the Far-Ultraviolet

In an effort to better understand the UV properties of ultraluminous infrared galaxies (ULIGs), and compare them to the rest-frame UV properties of high redshift sub-mm and Lyman-break galaxies, we have obtained far- and near-UV imaging observations (lambda_{eff}=1457Å, lambda_{eff}=2364Å, respectively) of two luminous infrared galaxies (LIGs--VV 114 and IC 883) and five ULIGs (IRAS 08572+3915, Mrk 273, IRAS 15250+3609, Arp 220, and IRAS 19254--7245) using the Hubble Space Telescope. All the galaxies were detected in both channels. UV light, both diffuse and from star clusters, can be traced to within the inner kpc of the dominant near-IR nuclei. However, in general, the brightest UV sources are clearly displaced from the $I$-band and near-IR peaks by at least hundreds of pc. Further, only 0.07%-7.3% of the total near-UV light is projected within the inner 500 pc radius, even though this is the same region wherein most of the bolometric energy is generated. All nuclei are highly obscured by dust. (Abridged)

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arXiv2001

Evidence for Reionization at z ~ 6: Detection of a Gunn-Peterson Trough in a z=6.28 Quasar

We present moderate resolution Keck spectroscopy of quasars at z=5.82, 5.99 and 6.28, discovered by the Sloan Digital Sky Survey (SDSS). We find that the Ly Alpha absorption in the spectra of these quasars evolves strongly with redshift. To z~5.7, the Ly Alpha absorption evolves as expected from an extrapolation from lower redshifts. However, in the highest redshift object, SDSSp J103027.10+052455.0 (z=6.28), the average transmitted flux is 0.0038+-0.0026 times that of the continuum level over 8450 A < lambda < 8710 A (5.95 150, and is consistent with zero flux in the Ly Alpha forest region immediately blueward of the Ly Alpha emission line, compared with a drop by a factor of ~10 at z(abs)~5.3. A similar break is seen at Ly Beta; because of the decreased oscillator strength of this transition, this allows us to put a considerably stronger limit, tau(eff) > 20, on the optical depth to Ly Alpha absorption at z=6. This is a clear detection of a complete Gunn-Peterson trough, caused by neutral hydrogen in the intergalactic medium. Even a small neutral hydrogen fraction in the intergalactic medium would result in an undetectable flux in the Ly Alpha forest region. Therefore, the existence of the Gunn-Peterson trough by itself does not indicate that the quasar is observed prior to the reionization epoch. However, the fast evolution of the mean absorption in these high-redshift quasars suggests that the mean ionizing background along the line of sight to this quasar has declined significantly from z~5 to 6, and the universe is approaching the reionization epoch at z~6.

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arXiv2001

Galactic Superwinds Circa 2001

In this contribution I summarize our current knowledge of the nature and significance of starburst-driven galactic winds (``superwinds''). Superwinds are complex multiphase outflows of cool, warm, and hot gas, dust, and magnetized relativistic plasma. The observational manifestations of superwinds result from the hydrodynamical interaction between the primary energy-carrying wind fluid and the ambient interstellar medium. Superwinds are ubiquitous in galaxies in which the global star-formation rate per unit area exceeds roughly 10$^{-1}$ M$_{\odot}$ yr$^{-1}$ kpc$^{-2}$. This criterion is met by local starbursts and the high-z Lyman Break galaxies. Several independent datasets and techniques imply that the total mass and energy outflow rates in a superwind are comparable to the starburst's star-formation-rate and mechanical energy injection rate, respectively. Outflow speeds in interstellar matter entrained in the wind range from $\sim 10^2$ to $10^3$ km/s, but the primary wind fluid itself may reach velocities as high as $\sim$3000 km s$^{-1}$. The available X-ray and far-UV ($FUSE$) data imply that radiative losses in superwinds are not significant. Superwinds may have established the mass-metallicity relation in ellipticals and bulges, polluted the present-day inter-galactic medium to a metallicity of $\sim$ 10 to 30% solar, heated the inter-galactic medium, and ejected enough dust into the inter-galactic medium to have potentially observable consequences.

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arXiv2000

Galactic Superwinds at Low and High Redshift

In this contribution I summarize our current knowledge of the nature and significance of starburst-driven galactic superwinds. These flows are driven primarily by the kinetic energy supplied by supernovae. Superwinds are complex, multiphase phenomena requiring a panchromatic observational approach. They are ubiquitous in galaxies in which the global star-formation rate per unit area exceeds roughly 10$^{-1}$ M$_{\odot}$ yr$^{-1}$ kpc$^{-2}$ (a condition satisfied by local starbursts and high-z Lyman Break galaxies). Data on X-ray emission, optical line-emission, and optical/UV interstellar absorption-lines together imply that the mass outflow rates are comparable to the star-formation-rates and that the conversion of kinetic energy from supernovae to superwind is quite efficient ($\sim$ 30 to 100%). Measured/inferred outflow speeds range from a few $\times 10^2$ to 10$^3$ km/s and appear to be independent of the rotation speed of the ``host'' galaxy. The outflows are dusty (dust/gas ratios of $\sim$ 1% by mass). These properties imply that superwinds may have established the mass-metallicity relation in elliptical and bulges, polluted the inter-galactic medium to a metallicity of $\sim$ 10 to 30% solar, heated the inter-galactic medium by up to $\sim$1 kev per baryon, and ejected enough dust into the inter-galactic medium to have potentially observable consequences.

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arXiv2000

Chandra observations of NGC 253: New insights into the nature of starburst-driven superwinds

Arcsecond-resolution X-ray imaging of the nucleus of the nearby starburst galaxy NGC 253 with Chandra reveals a well-collimated, strongly limb-brightened, kiloparsec-scale conical outflow from the central starburst region. The outflow is very similar in morphology to the known H-alpha outflow cone, on scales down to <= 20 pc. This provides, for the first time, robust evidence that both X-ray and H-alpha emission come from low volume filling factor regions of interaction between the fast energetic wind of SN-ejecta and the denser ambient interstellar medium (ISM), and not from the wind fluid itself. We provide estimates of the (observationally and theoretically important) filling factor of the X-ray emitting gas, of between 4 and 40 per cent, consistent with an upper limit of ~40 per cent based directly on the observed limb-brightened morphology of the outflow. Only <= 20 per cent of the observed X-ray emission can come from the volume-filling, metal-enriched, wind fluid itself. Spatially-resolved spectroscopy of the soft diffuse thermal X-ray emission reveals that the predominant source of spectral variation along the outflow cones is due to strong variation in the absorption, on scales of < 60 pc, there being little change in the characteristic temperature of the emission. We show that these observations are easily explained by, and fully consistent with, the standard model of a superwind driven by a starburst of NGC 253's observed power. If these results are typical of all starburst-driven winds, then we do not directly see all the energy and gas (in particular the hot metal-enriched gas) transported out of galaxies by superwinds, even in X-ray emission.

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