arXiv subjects

T. Heckman

Publications and source records attributed to T. Heckman.

54 records · Page 3Linked to original sources

arXiv2004

The Ultraviolet Galaxy Luminosity Function from GALEX data: Color Dependent Evolution at Low Redshift

We present measurements of the FUV (1530A) and NUV (2310A) galaxy luminosity functions (LF) at low redshift (z<0.2) from GALEX observations matched to the 2dF Galaxy Redshift Survey. We split our FUV and NUV samples into two UV-bj color bins and two redshift bins. As observed at optical wavelengths, the local LF of the bluest galaxies tend to have steeper faint end slopes and fainter characteristic magnitudes M* than the reddest subsamples. We find evidence for color dependent evolution at very low redshift in both bands, with bright blue galaxies becoming dominant in the highest redshift bin. The evolution of the total LF is consistent with an 0.3 magnitude brightening between z=0 and 0.13, in agreement with the first analysis of deeper GALEX fields probing adjacent and higher redshifts.

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arXiv2004

Discovery of six Ly alpha emitters near a radio galaxy at z ~ 5.2

We present the results of narrow-band and broad-band imaging with the Very Large Telescope of the field surrounding the radio galaxy TN J0924-2201 at z = 5.2. Fourteen candidate Ly alpha emitters with a rest-frame equivalent width of > 20 A were detected. Spectroscopy of 8 of these objects showed that 6 have redshifts similar to that of the radio galaxy. The density of emitters at the redshift of the radio galaxy is estimated to be a factor 1.5-6.2 higher than in the field, and comparable to the density of Ly alpha emitters in radio galaxy protoclusters at z = 4.1, 3.1 and 2.2. The Ly alpha emitters near TN J0924-2201 could therefore be part of a structure that will evolve into a massive cluster. These observations confirm that substantial clustering of Ly alpha emitters occurs at z > 5 and support the idea that radio galaxies pinpoint high density regions in the early Universe.

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arXiv2004

The physical properties of star forming galaxies in the low redshift universe

(modified) We present a comprehensive study of the physical properties of \~10^5 galaxies with measurable star formation in the SDSS. By comparing physical information extracted from the emission lines with continuum properties, we build up a picture of the nature of star-forming galaxies at z<0.2. We take out essentially all aperture bias using resolved imaging, allowing an accurate estimate of the total SFRs in galaxies. We determine the SFR density to be 1.915^{+0.02}_{-0.01}(rand.)^{+0.14}_{-0.42} (sys.) h70 10^{-2} Msun/yr/Mpc^3 at z=0.1 (for a Kroupa IMF) and we study the distribution of star formation as a function of various physical parameters. The majority of the star formation in the low redshift universe takes place in moderately massive galaxies (10^10-10^11 Msun), typically in HSB disk galaxies. Roughly 15% of all star formation takes place in galaxies that show some sign of an active nucleus. About 20% occurs in starburst galaxies. We show that the present to past-average star formation rate, the Scalo b-parameter; is almost constant over almost three orders of magnitude in mass, declining only at M*>10^10 Msun. The volume averaged b parameter is 0.408^{+0.005}_{-0.002} (rand).^{+0.029}_{-0.090} (sys.) h70^{-1}. We use this value constrain the star formation history of the universe. In agreement with other work we find a correlation between $b$ and morphological type, as well as a tight correlation between the 4000AA break (D4000) and b. We discuss how D4000 can be used to estimate b parameters for high redshift galaxies.

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arXiv2003

A Chandra Survey of the Nearest ULIRGs: Obscured AGN or Super-Starbursts?

We present initial results from a Chandra survey of a complete sample of the 8 nearest (z <= 0.04) ultraluminous IR galaxies (ULIRGs), and also include the IR-luminous galaxy NGC 6240 for comparison. In this paper we use the hard X-rays (2-8 kev) to search for the possible presence of an obscured AGN. In every case, a hard X-ray source is detected in the nuclear region. If we divide the sample according to the optical/IR spectroscopic classification (starburst vs. AGN), we find that the 5 ``starburst'' ULIRGs have hard X-ray luminosities about an order-of-magnitude smaller than the 3 ``AGN'' ULIRGs. NGC 6240 has an anomalously high hard X-ray luminosity compared to the ``starburst'' ULIRGs. The Fe Kalpha line is convincingly detected in only two ULIRGs. The weakness of the Fe-K emission in these ULIRGs generally suggests that the hard X-ray spectrum is not dominated by reflection from high N_H neutral material. The hard X-ray continuum flux ranges from a few X 10^3 to a few X 10^-5 of the far-IR flux, similar to values in pure starbursts, and several orders-of-magnitude smaller than in Compton-thin AGN. The upper limits on the ratio of the Fe Kalpha to far-IR flux are below the values measured in Compton-thick type 2 Seyfert galaxies. While very large column densities of molecular gas are observed in the nuclei of these galaxies, we find no evidence that the observed X-ray sources are obscured by Compton-thick material. Thus, our new hard X-ray data do not provide direct evidence that powerful ``buried quasars'' dominate the overall energetics of most ultraluminous infrared galaxies.

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arXiv2002

Stellar population gradients in Seyfert 2 galaxies

We study the variation of the stellar population properties as a function of distance from the nucleus for a sample of 35 Seyfert 2 galaxies using the technique of stellar population synthesis. We sample regions at the galaxies with dimensions in the range 200 x 200 to 400 x 400 pc and compare the synthesis results with those of a control sample of non-Seyfert galaxies. We find that both at the nucleus and up to 3 kpc from it the oldest age component (10 Gyr) presents a smaller contribution to the total flux in the Seyfert than in the non-Seyfert galaxies of the same Hubble type, while the components younger than 100 Myr present a larger contribution in the Seyfert's than in non-Seyferts. In addition, while for the non-Seyferts clear gradients are present, in which the contribution of the oldest components decrease with distance from the nucleus and the contribution of the 1 Gyr component increases -- we do not find such gradients in most Seyferts. These results suggest that the AGN-starburst connection is a large scale phenomenon affecting not only the inner few hundred parsecs, but the inner kiloparsecs.

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arXiv2002

Stellar population gradients in Seyfert 2 galaxies. Northern sample

We use high signal-to-noise ratio long-slit spectra in the 3600-4700A range of the twenty brightest northern Seyfert 2 galaxies to study the variation of the stellar population properties as a function of distance from the nucleus. In order to characterize the stellar population and other continuum sources (e.g. featureless continuum FC) we have measured equivalent widths Ws of six absorption features, four continuum colours and their radial variations, and performed spectral population synthesis as a function of distance from the nucleus. About half the sample has CaIIK and G-band W values smaller at the nucleus than at 1 kpc from it, due to a younger population and/or FC. The stellar population synthesis shows that, while at the nucleus, 75% of the galaxies present contribution > 20% of ages younger or equal than 100Myr and/or of a FC, this proportion decreases to 45% at 3 kpc. In particular, 55% of the galaxies have contribution > 10% of the 3 Myr/FC component (a degenerate component in which one cannot separate what is due to a FC or to a 3 Myr stellar population) at the nucleus, but only 25% of them have this contribution at 3 kpc. As reference, the stellar population of 10 non-Seyfert galaxies, spanning the Hubble types of the Seyfert (from S0 to Sc) was also studied. A comparison between the stellar population of the Seyferts and that of the non-Seyferts shows systematic differences: the contribution of ages younger than 1 Gyr is in most cases larger in the Seyfert galaxies than in non-Seyferts, not only at the nucleus but up to 1 kpc from it.

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arXiv2002

The most distant structure of galaxies known: a protocluster at z=4.1

Imaging and spectroscopy with the Very Large Telescope have revealed 20 Lyman-alpha emitters within a projected distance of 1.3 Mpc and 600 km/s of the luminous radio galaxy TN J1338-1942 at z=4.1. Compared to the field density of Lyman-alpha emitters, this implies an overdensity on the order of 15. The structure has a projected size of at least 2.7 Mpc x 1.8 Mpc and a velocity dispersion of 325 km/s, which makes it the most distant structure known. Using the galaxy overdensity and assuming a bias parameter b = 3 - 5, the mass is estimated to be ~10^15 M_sun. The radio galaxy itself is surrounded by an uniquely asymmetric Lyman-alpha halo. Taken together with our previous data on PKS 1138-262 at z ~ 2.16, these results suggest that luminous radio sources are excellent tracers of high density regions in the early Universe, which evolve into present-day clusters. The statistics of bright radio sources and of concentrations in the Lyman break galaxy population are consistent with the picture that each of those concentrations harbours an active or passive luminous radio source.

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arXiv2002

VLT observations of the z=6.28 quasar SDSS 1030+0524

We present new VLT spectroscopic observations of the most distant quasar known, SDSS J1030+0524 at z=6.28 which was recently discovered by the Sloan Digital Sky Survey. We confirm the presence of a complete Gunn-Peterson trough caused by neutral hydrogen in the intergalactic medium. There is no detectable flux over the wavelength range from 8450 to 8710 A. We set an improved limit on the drop of the flux level blueward of the Ly alpha line: a factor of > 200. Below 8450 A the spectrum shows a rise in flux, with a large fraction (> 60 %) of the total emission produced by few narrow features of transmitted flux. We discuss the obvious proximity effect around this quasar, with the presence of transmitted flux with many absorption features in a region of about 23h^{-1} comoving Mpc. If assuming the surrounding medium were completely neutral, the size of this region would imply a quasar lifetime of \~1.3x10^7 years. We also present near-IR spectroscopy of both SDSS J1030+0524 and of SDSS J1306+05, the second most distant quasar known at redshift 6.0. We combine measurements of the CIV line and limits on the HeII emission with the NV line measurements from the optical spectra to derive line ratios, and by implication the abundances of these early quasar environments. The results are indistinguishable from those of lower redshift quasars and indicate little or no evolution in the abundances from z ~ 6 to z ~ 2. The line ratios suggest supersolar metallicities, implying that the first stars around the quasars must have formed at least a few hundreds of Myrs prior to the observation, i.e. at redshift higher than 8.

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arXiv2001

Early-type galaxies in the SDSS

A sample of nearly 9000 early-type galaxies, in the redshift range 0.01<z<0.3, was selected from the Sloan Digital Sky Survey using morphological and spectral criteria. The sample was used to study how early-type galaxy observables, including luminosity L, effective radius R, surface brightness I, color, and velocity dispersion V, are correlated with one another. Measurement biases are understood with mock catalogs which reproduce all of the observed scaling relations. At any given redshift, the intrinsic distribution of luminosities, sizes and velocity dispersions in our sample are all approximately Gaussian. In the r* band L ~ V^3.91, L ~ R^1.58, R ~ I^(-0.75), and the Fundamental Plane relation is R ~ V^(1.49) I^(-0.75). These relations are approximately the same in the g*, i* and z* bands. At fixed luminosity, the mass-to-light ratio scales as M/L ~ L^0.14. The g*-r* color scales as V^0.25. Color also correlates with magnitude and size, but these correlations are entirely due to the L-V and R-V relations. Chemical evolution and star formation histories are investigated using co-added spectra of similar objects in our sample. Chemical abundances correlate primarily with velocity dispersion. At fixed V, the higher redshift population is bluer, is weaker in Mg2, and is stronger in Hbeta than the population nearby. In addition, the population at higher redshifts is slightly more luminous. These differences are consistent with that of a passively evolving population which formed the bulk of its stars about 9 Gyrs ago. The Fundamental Plane suggests that galaxies in dense regions are slightly different from those in less dense regions, but the co-added spectra and color--magnitude relations show no statistically significant dependence on environment.

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arXiv2001

Circumnuclear stellar population, morphology and environment of Seyfert 2 galaxies: an evolutionary scenario

We investigate the relation between the characteristics of the circumnuclear stellar population and both the galaxy morphology and the presence of close companions for a sample of 35 Seyfert 2 nuclei. Fifteen galaxies present unambiguous signatures of recent episodes of star formation within $\approx$300 pc from the nucleus. When we relate this property with the Hubble type of the host galaxy, we find that the incidence of recent circumnuclear star formation increases along the Hubble sequence, and seems to be larger than in non-Seyfert galaxies for the early Hubble types S0 and Sa, but similar to that in non-Seyfert galaxies for later Hubble types. Both in early-type and late-type Seyferts, the presence of recent star-formation is related to the galaxy morphology in the inner few kiloparsecs, as observed in HST images through the filter F606W by Malkan et al., who has assigned a late ``inner Hubble type'' to most Seyfert 2s with recent nuclear star-formation. This new classification is due to the presence of dust lanes and spiral structures in the inner region. The presence of recent star formation in Seyfert 2 nuclei is also related to interactions: among the 13 galaxies of the sample with close companions or in mergers, 9 have recent star formation in the nucleus. These correlations between the presence of companions, inner morphology and the incidence of recent star formation suggest an evolutionary scenario in which the interaction is responsible for sending gas inwards which both feeds the AGN and triggers star-formation. The starburst then fades with time and the composite Seyfert 2 + Starburst nucleus evolves to a ``pure'' Seyfert 2 nucleus with an old stellar population.

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arXiv2001

On the Escape of Ionizing Radiation from Starbursts

Far-ultraviolet spectra obtained with $FUSE$ show that the strong $CIIλ$1036 interstellar absorption-line is essentially black in five of the UV-brightest local starburst galaxies. Since the opacity of the neutral ISM below the Lyman-edge will be significantly larger than in the $CII$ line, these data provide strong constraints on the escape of ionizing radiation from these starbursts. Interpreted as a a uniform absorbing slab, the implied optical depth at the Lyman edge is huge ($τ_0 \geq 10^2$). Alternatively, the areal covering factor of opaque material is typically $\geq$ 94%. Thus, the fraction of ionizing stellar photons that escape the ISM of each galaxy is small: our conservative estimates typically yield $f_{esc} \leq 6%$. Inclusion of extinction due to dust will further decrease $f_{esc}$. An analogous analysis of the rest-UV spectrum of the star-forming galaxy $MS 1512-CB58$ at $z$ =2.7 leads to similar constraints on $f_{esc}$. These new results agree with the constraints provided by direct observations below the Lyman edge in a few other local starbursts. However, they differ from the recently reported properties of star-forming galaxies at $z \geq$ 3. We assess the idea that the strong galactic winds seen in many powerful starbursts clear channels through their neutral ISM. We show empirically that such outflows may be a necessary - but not sufficient - part of the process for creating a relatively porous ISM. We note that observations will soon document the cosmic evolution in the contribution of star-forming galaxies to the metagalactic ionizing background, with important implications for the evolution of the IGM.

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arXiv2001

Empirical diagnostics of the starburst-AGN connection

We examine a representative sample of 35 Seyfert 2 nuclei. Previous work has shown that nearly 1/2 of these nuclei show the direct (but difficult-to-detect) spectroscopic signatures at optical-UV wavelengths of the hot massive stars that power circum-nuclear starbursts. In this paper we examine a variety of more-easily-measured quantities for this sample, such as the equivalent widths of strong absorption features, continuum colors, emission line equivalent widths, ratios and profiles, far-IR luminosities and near-UV surface brightness. We compare the composite starburst+Seyfert 2 nuclei to ``pure'' Seyfert 2's, Starburst galaxies and normal galactic nuclei. Our goals are to verify whether these properties in composite nuclei are consistent with the expected impact of a starburst, and to investigate alternative less-demanding methods to infer the presence of starbursts in Seyfert 2's, applicable to larger or more distant samples. We show that starbursts do indeed leave clear and easily quantifiable imprints on the near-UV to optical continuum range. Composite starburst+Seyfert 2 systems can be recognized by: (1) a strong ``Featureless Continuum'' which dilutes the CaII K line from the host's bulge to W_K < 10 A; (2) emission lines whose equivalent widths are intermediate between Starburst galaxies and ``pure'' Seyfert 2's; (3) relatively low excitation line-ratios, which indicate that part of the gas ionization in these Seyfert 2's (typically \sim 50% of Hbeta) is due to photoionization by OB stars; (4) large far IR luminosities (> 10^10 Lsun); (5) High near-UV surface brightness (~10^3 Lsun/pc^2). These characteristics are all consistent with the expected impact of circum-nuclear starbursts on the observed properties of Seyfert 2's. (abridged)

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arXiv2000

The Nuclear and Circum-nuclear Stellar Population in Seyfert 2 Galaxies: Implications for the Starburst-AGN Connection

We report the results of a spectroscopic investigation of a sample of 20 of the brightest type 2 Seyfert nuclei. Our goal is to search for the direct spectroscopic signature of massive stars, and thereby probe the role of circumnuclear starbursts in the Seyfert phenomenon. The method used is based on the detection of the higher order Balmer lines and HeI lines in absorption and the Wolf-Rayet feature at $\sim$4680 Åin emission. These lines are strong indicators of the presence of young (a few Myrs) and intermediate-age (a few 100 Myrs) stellar populations. In over half the sample, we have detected HeI and/or strong stellar absorption features in the high-order (near-UV) Balmer series together with relatively weak lines from an old stellar population. In three others we detect a broad emission feature near 4680 Åthat is most plausibly ascribed to a population of Wolf-Rayet stars (the evolved descendants of the most massive stars). We therefore conclude that the blue and near-UV light of over half of the sample is dominated by young and/or intermediate age stars. The ``young'' Seyfert 2's have have larger far-IR luminosities, cooler mid/far-IR colors, and smaller [OIII]/H$β$ flux ratios than the ``old'' ones. These differences are consistent with a starburst playing a significant energetic role in the former class. We consider the possibility that there may be two distinct sub-classes of Seyfert 2 nuclei (``starbursts'' and ``hidden BLR''). However, the fact that hidden BLRs have been found in three of the ``young'' nuclei argues against this, and suggests that nuclear starbursts may be a more general part of the Seyfert phenomenon.

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arXiv2000

A search for clusters at high redshift - II. A proto cluster around a radio galaxy at z=2.16

VLT spectroscopic observations of Ly alpha--excess objects in the field of the clumpy radio galaxy 1138-262 at z=2.16 have led to the discovery of 14 galaxies and one QSO at approximately the same distance as the radio galaxy. All galaxies have redshifts in the range 2.16 \pm 0.02, centered around the redshift of the radio galaxy, and are within a projected physical distance of 1.5 Mpc from it. The velocity distribution suggests that there are two galaxy subgroups having velocity dispersions of ~500 km/s and ~300 km/s and a relative velocity of 1800 km/s. If these are virialized structures, the estimated dynamical masses for the subgroups are ~9 and ~3x10E13 solar masses respectively, implying a total mass for the structure of more than 10E14 solar masses. The new observations, together with previous results, suggest that the structure of galaxies around 1138-262 is likely to be a forming cluster.

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arXiv2000

Search for clusters at high redshift - I. Candidate Lya emitters near 1138-262 at z=2.2

Radio, optical and X-ray observations of the powerful radio galaxy PKS 1138-262 at z=2.156 have suggested that this galaxy is a massive galaxy in the center of a forming cluster. We have imaged 1138-262 and the surrounding 38 square arcminute field with the Very Large Telescope in a broad band and a narrow band encompassing the redshifted Lya emission. We detect 50 objects with rest equivalent width larger than 20 A and a luminous, highly extended Lya halo around 1138-262. If the radio galaxy is at the center of a forming cluster, these objects are candidate Lya emitting cluster galaxies.

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arXiv1999

An X-ray Mini-survey of Nearby Edge-on Starburst Galaxies II. The Question of Metal Abundance

(abbreviated) We have undertaken an X-ray survey of a far-infrared flux limited sample of seven nearby edge-on starburst galaxies. Here, we examine the two X-ray-brightest sample members NGC 253 and M 82 in a self-consistent manner, taking account of the spatial distribution of the X-ray emission in choosing our spectral models. There is significant X-ray absorption in the disk of NGC 253. When this is accounted for we find that multi-temperature thermal plasma models with significant underlying soft X-ray absorption are more consistent with the imaging data than single-temperature models with highly subsolar abundances or models with minimal absorption and non-equilibrium thermal ionization conditions. Our models do not require absolute abundances that are inconsistent with solar values or unusually supersolar ratios of the alpha-burning elements with respect to Fe (as claimed previously). We conclude that with current data, the technique of measuring abundances in starburst galaxies via X-ray spectral modeling is highly uncertain. Based on the point-like nature of much of the X-ray emission in the PSPC hard-band image of NGC 253, we suggest that a significant fraction of the ``extended'' X-ray emission in the 3-10 keV band seen along the disk of the galaxy with ASCA and BeppoSAX (Cappi et al.) is comprised of discrete sources in the disk, as opposed to purely diffuse, hot gas. This could explain the low Fe abundances of ~1/4 solar derived for pure thermal models.

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arXiv1999

HST NICMOS Imaging of z ~ 2 to 3 radio-quiet quasars

We will present the results of a NICMOS H-band imaging survey of a small sample of z ~ 2 to 3 radio-quiet quasars. We have resolved extension in at least 4 of 5 objects and find evidence for a wide range in the morphologies and magnitudes of these hosts. The host galaxy luminosities range from sub-L* to about 4 L*, with most of the hosts having luminosities about L*. These host galaxies have magnitudes and sizes consistent with those of the Ly break galaxies at similar redshifts and at similar rest wavelengths, but are about a magnitude fainter than the comparable 6C radio galaxies. One residual host component is not centered on the quasar nucleus, and several have close companions (within ~10 kpc), indications that these systems are possibly in some phase of a merger process.

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arXiv1999

The Emergence of the Modern Universe: Tracing the Cosmic Web

This is the report of the Ultraviolet-Optical Working Group (UVOWG) commissioned by NASA to study the scientific rationale for new missions in ultraviolet/optical space astronomy approximately ten years from now, when the Hubble Space Telescope (HST) is de-orbited. The UVOWG focused on a scientific theme, The Emergence of the Modern Universe, the period from redshifts z = 3 to 0, occupying over 80% of cosmic time and beginning after the first galaxies, quasars, and stars emerged into their present form. We considered high-throughput UV spectroscopy (10-50x throughput of HST/COS) and wide-field optical imaging (at least 10 arcmin square). The exciting science to be addressed in the post-HST era includes studies of dark matter and baryons, the origin and evolution of the elements, and the major construction phase of galaxies and quasars. Key unanswered questions include: Where is the rest of the unseen universe? What is the interplay of the dark and luminous universe? How did the IGM collapse to form the galaxies and clusters? When were galaxies, clusters, and stellar populations assembled into their current form? What is the history of star formation and chemical evolution? Are massive black holes a natural part of most galaxies? A large-aperture UV/O telescope in space (ST-2010) will provide a major facility in the 21st century for solving these scientific problems. The UVOWG recommends that the first mission be a 4m aperture, SIRTF-class mission that focuses on UV spectroscopy and wide-field imaging. In the coming decade, NASA should investigate the feasibility of an 8m telescope, by 2010, with deployable optics similar to NGST. No high-throughput UV/Optical mission will be possible without significant NASA investments in technology, including UV detectors, gratings, mirrors, and imagers.

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