arXiv subjects

Timothy M. Heckman

Publications and source records attributed to Timothy M. Heckman.

At least 109 records · Page 6Linked to original sources

arXiv2009

Feast and Famine: Regulation of Black Hole Growth in Low Redshift Galaxies

We analyze the observed distribution of Eddington ratios as a function of supermassive black hole mass for a large sample of nearby galaxies drawn from the Sloan Digital Sky Survey. We demonstrate that there are two distinct regimes of black hole growth in nearby galaxies. The first is associated with galaxies with significant star formation in their central kiloparsec regions, and is characterized by a broad log-normal distribution of accretion rates peaked at about one percent of the Eddington limit. In this regime, the Eddington ratio distribution is independent of the mass of the black hole and shows no further dependence on the central stellar population of the galaxy. The second regime is associated with galaxies with old central stellar populations, and is characterized by a power-law distribution function of Eddington ratios. In this regime, the time-averaged mass accretion rate onto black holes is proportional to the mass of stars in the galaxy bulge, with a constant of proportionality that depends on the mean stellar age of the stars. This result is once again independent of black hole mass. We show that both the slope of the power-law and the decrease in the accretion rate onto black holes in old galaxies are consistent with population synthesis model predictions of the decline in stellar mass loss rates as a function of mean stellar age. Our results lead to a very simple picture of black hole growth in the local Universe. If the supply of cold gas in a galaxy bulge is plentiful, the black hole regulates its own growth at a rate that does not further depend on the properties of the interstellar medium. Once the gas runs out, black hole growth is regulated by the rate at which evolved stars lose their mass.

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arXiv2009

Spatial Clustering from GALEX-SDSS samples: Star Formation History and large-scale clustering

We measure the projected spatial correlation function w_p(r_p) from a large sample combining GALEX ultraviolet imaging with the SDSS spectroscopic sample. We study the dependence of the clustering strength for samples selected on (NUV - r)_abs color, specific star formation rate (SSFR), and stellar mass. We find that there is a smooth transition in the clustering of galaxies as a function of this color from weak clustering among blue galaxies to stronger clustering for red galaxies. The clustering of galaxies within the "green valley" has an intermediate strength, and is consistent with that expected from galaxy groups. The results are robust to the correction for dust extinction. The comparison with simple analytical modeling suggests that the halo occupation number increases with older star formation epochs. When splitting according to SSFR, we find that the SSFR is a more sensitive tracer of environment than stellar mass.

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arXiv2009

Supernova feedback efficiency and mass loading in the starburst and galactic superwind exemplar M82

We measure the net energy efficiency of supernova and stellar wind feedback in the starburst galaxy M82 and the degree of mass-loading of the hot gas piston driving its superwind by comparing a large suite of 1 and 2-dimensional hydrodynamical models to a set of observational constraints derived from hard X-ray observations of the starburst region (the fluxes of the He-alpha and Ly-alpha-like lines of S, Ar, Ca and Fe, along with the total diffuse E=2--8 keV X-ray luminosity). These are the first direct measurements of the feedback efficiency and mass-loading of supernova heated and enriched plasma in a starburst galaxy. All the models that satisfy the existing observational constraints have medium to high thermalization efficiencies (30% <= epsilon <= 100$%) and the volume-filling wind fluid that flows out of the starburst region is only mildly centrally mass loaded (1.0 <= beta <= 2.8). These results imply a temperature of the plasma within the starburst region in the range 30--80 million Kelvin, a mass flow rate of the wind fluid out of the starburst region of Mdot_tot ~ 1.4--3.6 Msol/yr and a terminal velocity of the wind in the range v_infinity = 1410--2240 km/s. This velocity is considerably larger than the escape velocity from M82 (v_esc <~ 460 km/s) and the velocity of the H-alpha emitting clumps and filaments within M82's wind (v_H-alpha ~ 600 km/s). Drawing on these results we provide a prescription for implementing starburst-driven superwinds in cosmological models of galaxy formation and evolution. (Abridged)

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arXiv2009

The Star Formation Law at Low Surface Density

We investigate the nature of the star formation law at low gas surface densities using a sample of 19 low surface brightness (LSB) galaxies with existing HI maps in the literature, UV imaging from the Galaxy Evolution Explorer satellite, and optical images from the Sloan Digital Sky Survey. All of the LSB galaxies have (NUV-r) colors similar to those for higher surface brightness star-forming galaxies of similar luminosity indicating that their average star formation histories are not very different. Based upon four LSB galaxies with both UV and FIR data, we find FIR/UV ratios significantly less than one, implying low amounts of internal UV extinction in LSB galaxies. We use the UV images and HI maps to measure the star formation rate and hydrogen gas surface densities within the same region for all of the galaxies. The LSB galaxy star formation rate surface densities lie below the extrapolation of the power law fit to the star formation rate surface density as a function of the total gas density for higher surface brightness galaxies. Although there is more scatter, the LSB galaxies also lie below a second version of the star formation law in which the star formation rate surface density is correlated with the gas density divided by the orbital time in the disk. The downturn seen in both star formation laws is consistent with theoretical models that predict lower star formation efficiencies in LSB galaxies due to the declining molecular fraction with decreasing density.

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arXiv2008

SDSSJ092712.65+294344.0: NGC 1275 at z=0.7?

SDSSJ092712.65+294344.0 was identified by the SDSS as a quasar, but has the unusual property of having two emission line systems offset by 2650 km/s. One of these contains the usual combination of broad and narrow lines, the other only narrow lines. In the first paper commenting on this system (Komossa et al. 2008), it was interpreted as a galaxy in which a pair of black holes had merged, imparting a several thousand km/s recoil to the new, larger black hole. In two other papers (Bogdanovic, Eracleous & Sigurdsson 2008; Dotti et al. 2008), it was interpreted as a small-separation binary black hole. We propose a new interpretation: that this system is a more distant analog of NGC1275, a large and a small galaxy interacting near the center of a rich cluster.

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arXiv2008

The Lopsidedness of Present-Day Galaxies: Connections to the Formation of Stars, the Chemical Evolution of Galaxies, and the Growth of Black Holes

We have used the Sloan Digital Sky Survey (SDSS) to undertake an investigation of lopsidedness in a sample of ~25,000 nearby galaxies (z < 0.06). We use the m=1 azimuthal Fourier mode between the 50% and 90% light radii as our measure of lopsidedness. The SDSS spectra are used to measure the properties of the stars, gas, and black hole in the central-most few-kpc-scale region. We show that there is a strong link between lopsidedness in the outer parts of the galactic disk and the youth of the stellar population in the central region. This link is independent of the other structural properties of the galaxy. These results provide a robust statistical characterization of the connections between accretion/interactions/mergers and the resulting star formation. We also show that residuals in the galaxy mass-metallicity relation correlate with lopsidedness (at fixed mass, the more metal-poor galaxies are more lopsided). This suggests that the events causing lopsidedness and enhanced star formation deliver lower metallicity gas into the galaxy's central region. Finally, we find that there is a trend for the more powerful active galactic nuclei to be hosted by more lopsided galaxies (at fixed galaxy mass, density, or concentration). However if we compare samples matched to have both the same structures and central stellar populations, we then find no difference in lopsidedness between active and non-active galaxies. This leads to the following picture. The presence of cold gas in the central region of a galaxy (irrespective of its origin) is essential for both star-formation and black hole growth. The delivery of cold gas is aided by the processes that produce lopsidedness. Other processes on scales smaller than we can probe with our data are required to transport the gas to the black hole.

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arXiv2008

Lyman alpha emitting galaxies at 0.2 < z < 0.35 from GALEX spectroscopy

The GALEX (Galaxy Evolution Explorer) spectroscopic survey mode, with a resolution of about 8 A in the FUV (1350 - 1750 A) and about 20 A in the NUV (1950 - 2750 A) is used for a systematic search of Ly-a emitting galaxies at low redshift. This aims at filling a gap between high-redshift surveys and a small set of objects studied in detail in the nearby universe. A blind search of 7018 spectra extracted in 5 deep exposures (5.65 sq.deg) has resulted in 96 Ly-a emitting galaxy candidates in the FUV domain, after accounting for broad-line AGNs. The Ly-a EWs (equivalent width) are consistent with stellar population model predictions and show no trends as a function of UV color or UV luminosity, except a possible decrease in the most luminous that may be due to small-number statistics. Their distribution in EW is similar to that at z about 3 but their fraction among star-forming galaxies is smaller. Avoiding uncertain candidates, a sub-sample of 66 objects in the range 0.2 < z < 0.35 has been used to build a Ly-a LF (luminosity function). The incompleteness due to objects with significant Ly-a emission but a UV continuum too low for spectral extraction has been evaluated. A comparison with H-a LF in the same redshift domain is consistent with an average Ly-a/H-a of about 1 in about 15 % of the star-forming galaxies. A comparison with high-redshift Ly-a LFs implies an increase of the Ly-a luminosity density by a factor of about 16 from z about 0.3 to z about 3. By comparison with the factor 5 increase of the UV luminosity density in the same redshift range, this suggests an increase of the average Ly-a escape fraction with redshift.

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arXiv2008

Interactions, star formation and AGN activity

It has long been known that galaxy interactions are associated with enhanced star formation. In a companion paper, we explored this connection by applying a variety of statistics to SDSS data. In particular, we showed that specific star formation rates of galaxies are higher if they have close neighbours. Here we apply exactly the same techniques to AGN in the survey, showing that close neighbours are not associated with any similar enhancement of nuclear activity. Star formation is enhanced in AGN with close neighbours in exactly the same way as in inactive galaxies, but the accretion rate onto the black hole, as estimated from the extinction-corrected [O III] luminosity, is not influenced by the presence or absence of companions. Previous work has shown that galaxies with more strongly accreting black holes contain more young stars in their inner regions. This leads us to conclude that star formation induced by a close companion and star formation associated with black hole accretion are distinct events. These events may be part of the same physical process, for example a merger, provided they are separated in time. In this case, accretion onto the black hole and its associated star formation would occur only after the two interacting galaxies have merged. The major caveat in this work is our assumption that the extinction-corrected [O III] luminosity is a robust indicator of the bolometric luminosity of the central black hole. It is thus important to check our results using indicators of AGN activity at other wavelengths.

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arXiv2007

HST morphologies of local Lyman break galaxy analogs I: Evidence for starbursts triggered by merging

Heckman et al. (2005) used the Galaxy Evolution Explorer (GALEX) UV imaging survey to show that there exists a rare population of nearby compact UV-luminous galaxies (UVLGs) that closely resembles high redshift Lyman break galaxies (LBGs). We present HST images in the UV, optical, and Ha, and resimulate them at the depth and resolution of the GOODS/UDF fields to show that the morphologies of UVLGs are also similar to those of LBGs. Our sample of 8 LBG analogs thus provides detailed insight into the connection between star formation and LBG morphology. Faint tidal features or companions can be seen in all of the rest-frame optical images, suggesting that the starbursts are the result of a merger or interaction. The UV/optical light is dominated by unresolved (~100-300 pc) super starburst regions (SSBs). A detailed comparison with the galaxies Haro 11 and VV 114 at z=0.02 indicates that the SSBs themselves consist of diffuse stars and (super) star clusters. The structural features revealed by the new HST images occur on very small physical scales and are thus not detectable in images of high redshift LBGs, except in a few cases where they are magnified by gravitational lensing. We propose, therefore, that LBGs are mergers of gas-rich, relatively low-mass (~10^10 Msun) systems, and that the mergers trigger the formation of SSBs. If galaxies at high redshifts are dominated by SSBs, then the faint end slope of the luminosity function is predicted to have slope alpha~2. Our results are the most direct confirmation to date of models that predict that the main mode of star formation in the early universe was highly collisional.

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arXiv2007

Ultraviolet through Infrared Spectral Energy Distributions from 1000 SDSS Galaxies: Dust Attenuation

The meaningful comparison of models of galaxy evolution to observations is critically dependent on the accurate treatment of dust attenuation. To investigate dust absorption and emission in galaxies we have assembled a sample of ~1000 galaxies with ultraviolet (UV) through infrared (IR) photometry from GALEX, SDSS, and Spitzer and optical spectroscopy from SDSS. The ratio of IR to UV emission (IRX) is used to constrain the dust attenuation in galaxies. We use the 4000A break as a robust and useful, although coarse, indicator of star formation history (SFH). We examine the relationship between IRX and the UV spectral slope (a common attenuation indicator at high-redshift) and find little dependence of the scatter on 4000A break strength. We construct average UV through far-IR spectral energy distributions (SEDs) for different ranges of IRX, 4000A break strength, and stellar mass (M_*) to show the variation of the entire SED with these parameters. When binned simultaneously by IRX, 4000A break strength, and M_* these SEDs allow us to determine a low resolution average attenuation curve for different ranges of M_*. The attenuation curves thus derived are consistent with a lambda^{-0.7} attenuation law, and we find no significant variations with M_*. Finally, we show the relationship between IRX and the global stellar mass surface density and gas-phase-metallicity. Among star forming galaxies we find a strong correlation between IRX and stellar mass surface density, even at constant metallicity, a result that is closely linked to the well-known correlation between IRX and star-formation rate.

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arXiv2007

A Search for Extended Ultraviolet Disk (XUV-disk) Galaxies in the Local Universe

We have initiated a search for extended ultraviolet disk (XUV-disk) galaxies in the local universe. Herein, we compare GALEX UV and visible--NIR images of 189 nearby (D$<$40 Mpc) S0--Sm galaxies included in the GALEX Atlas of Nearby Galaxies and present the first catalogue of XUV-disk galaxies. We find that XUV-disk galaxies are surprisingly common but have varied relative (UV/optical) extent and morphology. Type~1 objects ($\ga$20% incidence) have structured, UV-bright/optically-faint emission features in the outer disk, beyond the traditional star formation threshold. Type~2 XUV-disk galaxies ($\sim$10% incidence) exhibit an exceptionally large, UV-bright/optically-low-surface-brightness (LSB) zone having blue $UV-K_s$ outside the effective extent of the inner, older stellar population, but not reaching extreme galactocentric distance. If the activity occuring in XUV-disks is episodic, a higher fraction of present-day spirals could be influenced by such outer disk star formation. Type~1 disks are associated with spirals of all types, whereas Type~2 XUV-disks are predominantly found in late-type spirals. Type~2 XUV-disks are forming stars quickly enough to double their [presently low] stellar mass in the next Gyr (assuming a constant SF rate). XUV-disk galaxies of both types are systematically more gas-rich than the general galaxy population. Minor external perturbation may stimulate XUV-disk incidence, at least for Type~1 objects. XUV-disks are the most actively evolving galaxies growing via inside-out disk formation in the current epoch, and may constitute a segment of the galaxy population experiencing significant, continued gas accretion from the intergalactic medium or neighboring objects.

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arXiv2007

Ultraviolet, Optical, and Infrared Constraints on Models of Stellar Populations and Dust Attenuation

The color of galaxies is a fundamental property, easily measured, that constrains models of galaxies and their evolution. Dust attenuation and star formation history (SFH) are the dominant factors affecting the color of galaxies. Here we explore the empirical relation between SFH, attenuation, and color for a wide range of galaxies, including early types. These galaxies have been observed by GALEX, SDSS, and Spitzer, allowing the construction of measures of dust attenuation from the ratio of infrared (IR) to ultraviolet (UV) flux and measures of SFH from the strength of the 4000A break. The empirical relation between these three quantities is compared to models that separately predict the effects of dust and SFH on color. This comparison demonstrates the quantitative consistency of these simple models with the data and hints at the power of multiwavelength data for constraining these models. The UV color is a strong constraint; we find that a Milky Way extinction curve is disfavored, and that the UV emission of galaxies with large 4000A break strengths is likely to arise from evolved populations. We perform fits to the relation between SFH, attenuation, and color. This relation links the production of starlight and its absorption by dust to the subsequent reemission of the absorbed light in the IR. Galaxy models that self-consistently treat dust absorption and emission as well as stellar populations will need to reproduce these fitted relations in the low-redshift universe.

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arXiv2007

The UV-Optical Color Magnitude Diagram II: Physical Properties and Morphological Evolution On and Off of a Star-Forming Sequence

We use the UV-optical color magnitude diagram in combination with spectroscopic and photometric measurements derived from the SDSS spectroscopic sample to measure the distribution of galaxies in the local universe (z<0.25) and their physical properties as a function of specific star formation rate (SSFR) and stellar mass. Throughout this study our emphasis is on the properties of galaxies on and off of a local "star-forming sequence." We discuss how the physical characteristics of galaxies along this sequence are related to scaling relations typically derived for galaxies of different morphological types. We find, among other trends that our measure of the star formation rate surface density is nearly constant along this sequence. We discuss this result and implications for galaxies at higher redshift. For the first time, we report on measurements of the local UV luminosity function versus galaxy structural parameters as well as inclination. We also split our sample into disk-dominated and bulge-dominated subsamples using the i-band Sersic index and find that disk-dominated galaxies occupy a very tight locus in SSFR vs. stellar mass space while bulge-dominated galaxies display a much larger spread of SSFR at fixed stellar mass. A significant fraction of galaxies with SSFR and SF surface density above those on the "star-forming sequence" are bulge-dominated. We can use our derived distribution functions to ask whether a significant fraction of these galaxies may be experiencing a final episode of star formation (possibly induced by a merger or other burst), soon to be quenched, by determining whether this population can explain the growth rate of the non-star-forming galaxies on the "red sequence." (Abridged)

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arXiv2007

The Diverse Properties of the Most Ultraviolet Luminous Galaxies Discovered by the Galaxy Evolution Explorer

We report on the properties of a sample of ultraviolet luminous galaxies (UVLGs) selected by matching the Galaxy Evolution Explorer (GALEX) Surveys with the Sloan Digital Sky Survey Third Data Release. Out of 25362 galaxies between 0.0 2x10^10 L_solar at 1530 Angstroms (observed wavelength). The properties of this population are well correlated with ultraviolet surface brightness. We find that the galaxies with low UV surface brightness are primarily large spiral systems with a mixture of old and young stellar populations, while the high surface brightness galaxies consist primarily of compact starburst systems. In terms of the behavior of surface brightness with luminosity, size with luminosity, the mass-metallicity relation, and other parameters, the compact UVLGs clearly depart from the trends established by the full sample of galaxies. The subset of compact UVLGs with the highest surface brightness (``supercompact UVLGs'') have characteristics that are remarkably similar to Lyman Break Galaxies at higher redshift. They are much more luminous than typical local ultraviolet-bright starburst galaxies and blue compact dwarf galaxies. They have metallicities that are systematically lower than normal galaxies of the same stellar mass, indicating that they are less chemically evolved. In all these respects, they are the best local analogs for Lyman Break Galaxies.

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arXiv2007

Clustering Properties of restframe UV selected galaxies I: the correlation length derived from GALEX data in the local Universe

We present the first measurements of the angular correlation function of galaxies selected in the far (1530 A) and near (2310 A) Ultraviolet from the GALEX survey fields overlapping SDSS DR5 in low galactic extinction regions. The area used covers 120 sqdeg (GALEX - MIS) down to magnitude AB = 22, yielding a total of 100,000 galaxies. The mean correlation length is ~ 3.7 \pm 0.6 Mpc and no significant trend is seen for this value as a function of the limiting apparent magnitude or between the GALEX bands. This estimate is close to that found from samples of blue galaxies in the local universe selected in the visible, and similar to that derived at z ~ 3 for LBGs with similar rest frame selection criteria. This result supports models that predict anti-biasing of star forming galaxies at low redshift, and brings an additional clue to the downsizing of star formation at z<1.

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arXiv2007

The Lopsidedness of Present-Day Galaxies: Results from the Sloan Digital Sky Survey

Large-scale asymmetries in the stellar mass distribution in galaxies are believed to trace non-equilibrium situations in the luminous and/or dark matter component. These may arise in the aftermath of events like mergers, accretion, and tidal interactions. These events are key in the evolution of galaxies. In this paper we quantify the large-scale lopsidedness of light distributions in 25155 galaxies at z < 0.06 from the Sloan Digital Sky Survey Data Release 4 using the m = 1 azimuthal Fourier mode. We show that the lopsided distribution of light is primarily due to a corresponding lopsidedness in the stellar mass distribution. Observational effects, such as seeing, Poisson noise, and inclination, introduce only small errors in lopsidedness for the majority of this sample. We find that lopsidedness correlates strongly with other basic galaxy structural parameters: galaxies with low concentration, stellar mass, and stellar surface mass density tend to be lopsided, while galaxies with high concentration, mass, and density are not. We find that the strongest and most fundamental relationship between lopsidedness and the other structural parameters is with the surface mass density. We also find, in agreement with previous studies, that lopsidedness tends to increase with radius. Both these results may be understood as a consequence of several factors. The outer regions of galaxies and low-density galaxies are more susceptible to tidal perturbations, and they also have longer dynamical times (so lopsidedness will last longer). They are also more likely to be affected by any underlying asymmetries in the dark matter halo.

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arXiv2007

Radio Jets in Galaxies with Actively Accreting Black Holes: new insights from the SDSS

The majority of nearby radio-loud AGN are found in massive, old elliptical galaxies with weak emission lines. At high redshifts,however, most known radio AGN have strong emission lines. In this paper, we examine a subset of radio AGN with emission lines selected from the Sloan Digital Sky Survey. The probability for a nearby radio AGN to have emission lines is a strongly decreasing function of galaxy mass and an increasing function of radio luminosity above 10^25 W/Hz. Emission line and radio luminosities are correlated, but with large dispersion. At a given radio power, AGN with small black holes have higher [OIII] luminosities (which we interpret as higher accretion rates) than AGN with big black holes. However, if we scale both radio and emission line luminosities by the black hole mass, we find a correlation between normalized radio power and accretion rate in Eddington units that is independent of black hole mass. There is also a clear correlation between normalized radio power and the age of the stellar population in the galaxy. Present-day AGN with the highest normalized radio powers are confined to galaxies with small black holes. High-redshift, high radio-luminosity AGN could be explained if big black holes were similarly active at earlier cosmic epochs. To investigate why only a small fraction of emission line AGN become radio loud, we create matched samples of radio-loud and radio-quiet AGN and compare their host galaxy properties and environments. The main difference lies in their environments; our local density estimates are a factor 2 larger around the radio-loud AGN. We propose a scenario in which radio-loud AGN with emission lines are located in galaxies where accretion of both cold and hot gas can occur simultaneously. (Abridged)

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arXiv2007

The Star Formation and Extinction Co-Evolution of UV-Selected Galaxies over 0.05<z<1.2

We use a new stacking technique to obtain mean mid IR and far IR to far UV flux ratios over the rest near-UV/near-IR color-magnitude diagram. We employ COMBO-17 redshifts and COMBO-17 optical, GALEX far and near UV, Spitzer IRAC and MIPS Mid IR photometry. This technique permits us to probe infrared excess (IRX), the ratio of far IR to far UV luminosity, and specific star formation rate (SSFR) and their co-evolution over two orders of magnitude of stellar mass and redshift 0.1<z<1.2. We find that the SSFR and the characteristic mass (M_0) above which the SSFR drops increase with redshift (downsizing). At any given epoch, IRX is an increasing function of mass up to M_0. Above this mass IRX falls, suggesting gas exhaustion. In a given mass bin below M_0 IRX increases with time in a fashion consistent with enrichment. We interpret these trends using a simple model with a Schmidt-Kennicutt law and extinction that tracks gas density and enrichment. We find that the average IRX and SSFR follows a galaxy age parameter which is determined mainly by the galaxy mass and time since formation. We conclude that blue sequence galaxies have properties which show simple, systematic trends with mass and time such as the steady build-up of heavy elements in the interstellar media of evolving galaxies and the exhaustion of gas in galaxies that are evolving off the blue sequence. The IRX represents a tool for selecting galaxies at various stages of evolution.

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