arXiv subjects

Luis C. Ho

Publications and source records attributed to Luis C. Ho.

At least 541 records · Page 30Linked to original sources

arXiv2012

A Uniformly Selected Sample of Low-mass Black Holes in Seyfert 1 Galaxies

We have conducted a systematic search of low-mass black holes (BHs) in active galactic nuclei (AGNs) with broad Halpha emission lines, aiming at building a homogeneous sample that is more complete than previous ones for fainter, less highly accreting sources. For this purpose, we developed a set of elaborate, automated selection procedures and applied it uniformly to the Fourth Data Release of the Sloan Digital Sky Survey. Special attention is given to AGN--galaxy spectral decomposition and emission-line deblending. We define a sample of 309 type 1 AGNs with BH masses in the range $8 \times 10^4$--$2 \times 10^6$ \msun (with a median of $1.2 \times 10^6$ solar mass), using the virial mass estimator based on the broad Halpha line. About half of our sample of low-mass BHs differs from that of Greene & Ho, with 61 of them discovered here for the first time. Our new sample picks up more AGNs with low accretion rates: the Eddington ratios of the present sample range from $<~0.01$ to ~1, with 30% below 0.1. This suggests that a significant fraction of low-mass BHs in the local Universe are accreting at low rates. The host galaxies of the low-mass BHs have luminosities similar to those of $L^*$ field galaxies, optical colors of Sbc spirals, and stellar spectral features consistent with a continuous star formation history with a mean stellar age of less than 1 Gyr.

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arXiv2012

Simultaneous Ultraviolet and Optical Emission-line Profiles of Quasars: Implications for Black Hole Mass Determination

The X-shooter instrument on the VLT was used to obtain spectra of seven moderate-redshift quasars simultaneously covering the spectral range 3000 Ang to 2.5 microns. At z ~ 1.5, most of the prominent broad emission lines in the ultraviolet to optical region are captured in their rest frame. We use this unique dataset, which mitigates complications from source variability, to intercompare the line profiles of C IV 1549, C III] 1909, Mg II 2800, and Halpha and evaluate their implications for black hole mass estimation. We confirm that Mg II and the Balmer lines share similar kinematics and that they deliver mutually consistent black hole mass estimates with minimal internal scatter (< 0.1 dex) using the latest virial mass estimators. Although no virial mass formalism has yet been calibrated for C III], this line does not appear promising for such an application because of the large spread of its velocity width compared to lines of both higher and lower ionization; part of the discrepancy may be due to the difficulty of deblending C III] from its neighboring lines. The situation for C IV is complex and, because of the limited statistics of our small sample, inconclusive. On the one hand, slightly more than half of our sample (4/7) have C IV line widths that correlate reasonably well with Halpha line widths, and their respective black hole mass estimates agree to within ~0.15 dex. The rest, on the other hand, exhibit exceptionally broad C IV profiles that overestimate virial masses by factors of 2-5 compared to Halpha. As C IV is widely used to study black hole demographics at high redshifts, we urgently need to revisit our analysis with a larger sample.

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arXiv2012

Chandra Observations of the Nuclear Star Cluster and Ultraluminous X-ray Sources in NGC 2139

We report Chandra observations of the Scd galaxy NGC 2139, which is known to host a recently formed (10^7.6 yrs) nuclear star cluster. The star cluster is undetected in X-rays, with an upper bound on 0.5-7 keV luminosity of L_X < 7.1 x 10^37 erg/s. This bound implies a bolometric accretion luminosity <0.3 percent of the Eddington luminosity for a black hole with the mass (approximately 3400 M_sun) expected from extrapolation of the M-sigma relation. The lack of X-ray emission indicates that a black hole, if present, is not undergoing significant accretion at the current time. While the central cluster is undetected, the data reveal a substantial population of bright X-ray point sources elsewhere in this galaxy, with eight qualifying as ultraluminous X-ray sources with L_X > 10^39 erg/s. We use archival Hubble Space Telescope images to identify candidate optical counterparts for seven Chandra sources, which in most cases have optical luminosities and spatial profiles consistent with star clusters. Compared with other galaxies, the number of luminous X-ray sources in NGC 2139 is larger by a factor of 4 - 10 than expected based on its present star formation rate and stellar mass. This finding can be understood if NGC 2139 has concluded a burst of star formation in the recent past, and suggests that this galaxy could be important for testing the use of X-ray source populations as a chronometer of star formation history.

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arXiv2012

Berkeley Supernova Ia Program I: Observations, Data Reduction, and Spectroscopic Sample of 582 Low-Redshift Type Ia Supernovae

In this first paper in a series we present 1298 low-redshift (z\leq0.2) optical spectra of 582 Type Ia supernovae (SNe Ia) observed from 1989 through 2008 as part of the Berkeley SN Ia Program (BSNIP). 584 spectra of 199 SNe Ia have well-calibrated light curves with measured distance moduli, and many of the spectra have been corrected for host-galaxy contamination. Most of the data were obtained using the Kast double spectrograph mounted on the Shane 3 m telescope at Lick Observatory and have a typical wavelength range of 3300-10,400 Ang., roughly twice as wide as spectra from most previously published datasets. We present our observing and reduction procedures, and we describe the resulting SN Database (SNDB), which will be an online, public, searchable database containing all of our fully reduced spectra and companion photometry. In addition, we discuss our spectral classification scheme (using the SuperNova IDentification code, SNID; Blondin & Tonry 2007), utilising our newly constructed set of SNID spectral templates. These templates allow us to accurately classify our entire dataset, and by doing so we are able to reclassify a handful of objects as bona fide SNe Ia and a few other objects as members of some of the peculiar SN Ia subtypes. In fact, our dataset includes spectra of nearly 90 spectroscopically peculiar SNe Ia. We also present spectroscopic host-galaxy redshifts of some SNe Ia where these values were previously unknown. [Abridged]

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arXiv2012

Early type galaxies with tidal debris and their scaling relations in the Spitzer Survey of Stellar Structure in Galaxies (S4G)

Tidal debris around galaxies can yield important clues on their evolution. We have identified tidal debris in 11 early type galaxies (T \leq 0) from a sample of 65 early types drawn from the Spitzer Survey of Stellar Structure in Galaxies (S4G). The tidal debris includes features such as shells, ripples and tidal tails. A variety of techniques, including two-dimensional decomposition of galactic structures, was used to quantify the residual tidal features. The tidal debris contributes ~3 - 10% to the total 3.6 μm luminosity of the host galaxy. Structural parameters of the galaxies were estimated using two-dimensional profile fitting. We investigate the locations of galaxies with tidal debris in the Fundamental Plane and Kormendy relation. We find that galaxies with tidal debris lie within the scatter of early type galaxies without tidal features. Assuming that the tidal debris is indicative of recent gravitational interaction or merger, this suggests that these galaxies have either undergone minor merging events so that the overall structural properties of the galaxies are not significantly altered, or they have undergone a major merging events but already have experienced sufficient relaxation and phase-mixing so that their structural properties become similar to those of the non-interacting early type galaxies.

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arXiv2012

Two Populations of Molecular Clouds in the Antennae Galaxies

Super star clusters --- extremely massive clusters found predominately in starburst environments --- are essential building blocks in the formation of galaxies and thought to dominate star formation in the high-redshift universe. However, the transformation from molecular gas into these ultra-compact star clusters is not well understood. To study this process, we used the Submillimeter Array and the Plateau de Bure Interferometer to obtain high angular resolution (~1.5" or 160 pc) images of the Antennae overlap region in CO(2--1) to search for the molecular progenitors of the super star clusters. We resolve the molecular gas distribution into a large number of clouds, extending the differential cloud mass function down to a 5σcompleteness limit of 3.8x10^5 M_sun. We identify a distinct break in the mass function around log M_mol/M_sun ~ 6.5, which separates the molecular clouds into two distinct populations. The smaller, less massive clouds reside in more quiescent areas in the region, while the larger, more massive clouds cluster around regions of intense star formation. A broken power-law fit to the mass function yields slopes of α= -1.39+/-0.10 and α= -1.44+/-0.14 for the low- and high-mass cloud population, well-matched to the mass function found for super star clusters in the Antennae galaxies. We find large velocity gradients and velocity dispersions at the locations of intense star formation, suggestive of compressive shocks. It is likely that these environmental factors contribute to the formation of the observed massive molecular clouds and super star clusters in the Antennae galaxies.

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arXiv2012

The S4G perspective on circumstellar dust extinction of AGB stars in M100

We examine the effect of circumstellar dust extinction on the near-IR contribution of asymptotic giant branch (AGB) stars in intermediate-age clusters throughout the disk of M100. For our sample of 17 AGB-dominated clusters we extract optical-to-mid-IR SEDs and find that NIR brightness is coupled to the mid-IR dust emission in such a way that a significant reduction of AGB light, of up to 1 mag in K-band, follows from extinction by the dust shell formed during this stage. Since the dust optical depth varies with AGB chemistry (C-rich or O-rich), our results suggest that the contribution of AGB stars to the flux from their host clusters will be closely linked to the metallicity and the progenitor mass of the AGB star, to which dust chemistry and mass-loss rate are sensitive. Our sample of clusters--each the analogue of a ~1 Gyr old post-starburst galaxy--has implications within the context of mass and age estimation via SED modelling at high z: we find that the average ~0.5 mag extinction estimated here may be sufficient to reduce the AGB contribution in (rest-frame) K-band from ~70%, as predicted in the latest generation of synthesis models, to ~35%. Our technique for selecting AGB-dominated clusters in nearby galaxies promises to be effective for discriminating the uncertainties associated with AGB stars in intermediate-age populations that plague age and mass estimation in high-z galaxies.

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arXiv2012

Cosmological Evolution of Supermassive Black Holes. II. Evidence for Downsizing of Spin Evolution

The spin is an important but poorly constrained parameter for describing supermassive black holes (SMBHs). Using the continuity equation of SMBH number density, we explicitly obtain the mass-dependent cosmological evolution of the radiative efficiency for accretion, which serves as a proxy for SMBH spin. Our calculations make use of the SMBH mass function of active and inactive galaxies (derived in the first paper of this series), the bolometric luminosity function of active galactic nuclei (AGNs), corrected for the contribution from Compton-thick sources, and the observed Eddington ratio distribution. We find that the radiative efficiency generally increases with increasing black hole mass at high redshifts (z>~1), roughly as η\propto M_bh^0.5, while the trend reverses at lower redshifts, such that the highest efficiencies are attained by the lowest mass black holes. Black holes with M_bh>~10^8.5M_sun maintain radiative efficiencies as high as η~0.3-0.4 at high redshifts, near the maximum for rapidly spinning systems, but their efficiencies drop dramatically (by an order of magnitude) by z~0. The pattern for lower mass holes is somewhat more complicated but qualitatively similar. Assuming that the standard accretion disk model applies, we suggest that the accretion history of SMBHs and their accompanying spins evolve in two distinct regimes: an early phase of prolonged accretion, plausibly driven by major mergers, during which the black hole spins up, then switching to a period of random, episodic accretion, governed by minor mergers and internal secular processes, during which the hole spins down. The transition epoch depends on mass, mirroring other evidence for "cosmic downsizing" in the AGN population; it occurs at z~2 for high-mass black holes, and somewhat later, at z~1, for lower-mass systems.

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arXiv2011

The Carnegie-Irvine Galaxy Survey. I. Overview and Atlas of Optical Images

The Carnegie-Irvine Galaxy Survey (CGS) is a long-term program to investigate the photometric and spectroscopic properties of a statistically complete sample of 605 bright (B_T < 12.9 mag), southern (Dec. < 0) galaxies using the facilities at Las Campanas Observatory. This paper, the first in a series, outlines the scientific motivation of CGS, defines the sample, and describes the technical aspects of the optical broadband (BVRI) imaging component of the survey, including details of the observing program, data reduction procedures, and calibration strategy. The overall quality of the images is quite high, in terms of resolution (median seeing 1"), field of view (8.9' X 8.9'), and depth (median limiting surface brightness 27.5, 26.9, 26.4, and 25.3 mag/arcsec2 in the B, V, R, and I bands, respectively). We prepare a digital image atlas showing several different renditions of the data, including three-color composites, star-cleaned images, stacked images to enhance faint features, structure maps to highlight small-scale features, and color index maps suitable for studying the spatial variation of stellar content and dust. In anticipation of upcoming science analyses, we tabulate an extensive set of global properties for the galaxy sample. These include optical isophotal and photometric parameters derived from CGS itself, as well as published information on multiwavelength (ultraviolet, U-band, near-infrared, far-infrared) photometry, internal kinematics (central stellar velocity dispersions, disk rotational velocities), environment (distance to nearest neighbor, tidal parameter, group or cluster membership), and H I content. The digital images and science-level data products will be made publicly accessible to the community.

astro-ph.CO↗

arXiv2011

The Carnegie-Irvine Galaxy Survey. II. Isophotal Analysis

The Carnegie-Irvine Galaxy Survey (CGS) is a comprehensive investigation of the physical properties of a complete, representative sample of 605 bright (B_T <= 12.9 mag) galaxies in the southern hemisphere. This contribution describes the isophotal analysis of the broadband (BVRI) optical imaging component of the project. We pay close attention to sky subtraction, which is particularly challenging for some of the large galaxies in our sample. Extensive crosschecks with internal and external data confirm that our calibration and sky subtraction techniques are robust with respect to the quoted measurement uncertainties. We present a uniform catalog of one-dimensional radial profiles of surface brightness and geometric parameters, as well as integrated colors and color gradients. Composite profiles highlight the tremendous diversity of brightness distributions found in disk galaxies and their dependence on Hubble type. A significant fraction of S0 and spiral galaxies exhibit non-exponential profiles in their outer regions. We perform Fourier decomposition of the isophotes to quantify non-axisymmetric deviations in the light distribution. We use the geometric parameters, in conjunction with the amplitude and phase of the m=2 Fourier mode, to identify bars and quantify their size and strength. Spiral arm strengths are characterized using the m=2 Fourier profiles and structure maps. Finally, we utilize the information encoded in the m=1 Fourier profiles to measure disk lopsidedness. The databases assembled here and in Paper I lay the foundation for forthcoming scientific applications of CGS.

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arXiv2011

Medium Resolution Spectral Library of Late-Type Stellar Templates in Near-Infrared Band

We present medium resolution (R = 5000-6000) spectra in the near-infrared band, 1.4-1.8 microns, for template stars in G, K, and M types observed by the echelle spectrometer, IRCS, at the SUBARU 8.2 m telescope. The identification of lines is based on the spectra of Arcturus (K2 III) in the literature. We measured the equivalent of widths and compared our results to those of Meyer et al. (1998). We conclude that our spectral resolution (R = 6000) data can investigate more accurately the properties of lines in stellar spectra. The library of the template stellar spectra in ASCII format are available for download on the World Wide Web.

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arXiv2011

Reconstructing the stellar mass distributions of galaxies using S4G IRAC 3.6 and 4.5 micron images: I. Correcting for contamination by PAH, hot dust, and intermediate age stars

With the aim of constructing accurate 2D maps of the stellar mass distribution in nearby galaxies from S4G 3.6 and 4.5 micron images, we report on the separation of the light from old stars from the emission contributed by contaminants (e.g. hot dust and the 3.3 micron PAH feature). Results for a small sample of six disk galaxies (NGC 1566, NGC 2976, NGC 3031, NGC 3184, NGC 4321, and NGC 5194) with a range of morphological properties, dust contents and star formation histories are presented to demonstrate our approach. We use an Independent Component Analysis (ICA) technique designed to separate statistically independent source distributions, maximizing the distinction in the [3.6]-[4.5] colors of the sources. The technique also removes emission from intermediate-age evolved red objects with a low mass-to-light ratio, such as asymptotic giant branch (AGB) and red supergiant (RSG) stars, revealing maps of the underlying old distribution of light with [3.6]-[4.5] colors consistent with the colors of K and M giants. Contaminants are identified via comparison to the non-stellar emission imaged at 8 microns, which is dominated by the broad PAH feature. Using the measured 3.6/8 micron ratio to select the individual contaminants, we find that hot dust and PAH together contribute between ~5-15% to the integrated light at 3.6 microns, while light from regions dominated by intermediate-age stars accounts for only 1-5%. Locally, however, the contribution from either contaminant can reach much higher levels; dust contributes on average 22% to the emission in star-forming regions throughout the sample, while intermediate age-stars contribute upwards of 50% in localized knots. The removal of these contaminants with ICA leaves maps of the old stellar disk that retain a high degree of structural information and are ideally suited for tracing the stellar mass, as will be the focus in a companion paper.

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arXiv2011

Cosmological evolution of supermassive black holes. I. mass function at 0<z<2

We present the mass function of supermassive black holes (SMBHs) over the redshift range z=0-2, using the latest deep luminosity and mass functions of field galaxies to constrain the masses of their spheroids, which we relate to SMBH mass through the empirical correlation between SMBH and spheroid mass (the M_bh-M_sph relation). In ddition to luminosity fading of the stellar content of the spheroids, we carefully consider the variation of the bulge-to-total luminosity ratio of the galaxy populations and the M_bh/M_sph ratio, which, according to numerous recent studies, evolves rapidly with redshift. The SMBH mass functions derived from the galaxy luminosity and mass functions show very good agreement, both in shape and in normalization. The resultant SMBH mass function and integrated mass density for the local epoch (z~0) match well those derived independently by other studies. Consistent with other evidence for cosmic downsizing, the upper end of the mass function remains roughly constant since $z\approx2$, while the space density of lower mass black holes undergoes strong evolution. We carefully assess the impact of various sources of uncertainties on our calculations. A companion paper uses the mass function derived in this work to determine the radiative efficiency of black hole accretion and constraints that can be imposed on the cosmological evolution of black hole spin.

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arXiv2011

Thick disks of edge-on galaxies seen through the Spitzer Survey of Stellar Structure in Galaxies (S4G): Lair of missing baryons?

Most, if not all, disk galaxies have a thin (classical) disk and a thick disk. In most models thick disks are thought to be a necessary consequence of the disk formation and/or evolution of the galaxy. We present the results of a study of the thick disk properties in a sample of carefully selected edge-on galaxies with types ranging from T=3 to T=8. We fitted one-dimensional luminosity profiles with physically motivated functions - the solutions of two stellar and one gaseous isothermal coupled disks in equilibrium - which are likely to yield more accurate results than other functions used in previous studies. The images used for the fits come from the Spitzer Survey of Stellar Structure in Galaxies (S4G). We found that thick disks are on average more massive than previously reported, mostly due to the selected fitting function. Typically, the thin and the thick disk have similar masses. We also found that thick disks do not flare significantly within the observed range in galactocentric radii and that the ratio of thick to thin disk scaleheights is higher for galaxies of earlier types. Our results tend to favor an in situ origin for most of the stars in the thick disk. In addition the thick disk may contain a significant amount of stars coming from satellites accreted after the initial build-up of the galaxy and an extra fraction of stars coming from the secular heating of the thin disk by its own overdensities. Assigning thick disk light to the thin disk component may lead to an underestimate of the overall stellar mass in galaxies, because of different mass to light ratios in the two disk components. On the basis of our new results, we estimate that disk stellar masses are between 10% and 50% higher than previously thought and we suggest that thick disks are a reservoir of "local missing baryons".

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arXiv2011

The unusual vertical mass distribution of NGC 4013 seen through the Spitzer Survey of Stellar Structure in Galaxies (S4G)

NGC 4013 is a nearby Sb edge-on galaxy known for its "prodigious" HI warp and its "giant" tidal stream. Previous work on this unusual object shows that it cannot be fitted satisfactorily by a canonical thin+thick disk structure. We have produced a new decomposition of NGC 4013, considering three stellar flattened components (thin+thick disk plus an extra and more extended component) and one gaseous disk. All four components are considered to be gravitationally coupled and isothermal. To do so, we have used the 3.6micron images from the Spitzer Survey of Stellar Structure in Galaxies (S4G). We find evidence for NGC4013 indeed having a thin and a thick disk and an extra flattened component. This smooth and extended component (scaleheight z_EC kpc) could be interpreted as a thick disk or as a squashed ellipsoidal halo and contains ~20% of the total mass of all three stellar components. We argue it is unlikely to be related to the ongoing merger or due to the off-plane stars from a warp in the other two disk components. Instead, we favor a scenario in which the thick disk and the extended component were formed in a two-stage process, in which an initially thick disk has been dynamically heated by a merger soon enough in the galaxy history to have a new thick disk formed within it.

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arXiv2011

The Host Galaxies of Low-mass Black Holes

Using HST observations of 147 host galaxies of low-mass black holes (BHs), we systematically study the structures and scaling relations of these active galaxies. Our sample is selected to have central BHs with virial masses ~10^5-10^6 solar mass. The host galaxies have total I-band magnitudes of -23.2<M_I<-18.8 mag and bulge magnitudes of -22.9<M_I<-16.1 mag. Detailed bulge-disk-bar decompositions with GALFIT show that 93% of the galaxies have extended disks, 39% have bars and 5% have no bulges at all at the limits of our observations. Based on the Sersic index and bulge-to-total ratio, we conclude that the majority of the galaxies with disks are likely to contain pseudobulges and very few of these low-mass BHs live in classical bulges. The fundamental plane of our sample is offset from classical bulges and ellipticals in a way that is consistent with the scaling relations of pseudobulges. The sample has smaller velocity dispersion at fixed luminosity in the Faber-Jackson plane, compared with classical bulges and elliptical galaxies. The galaxies without disks are structurally more similar to spheroidals than to classical bulges according to their positions in the fundamental plane, especially the Faber-Jackson projection. Overall, we suggest that BHs with mass < 10^6 solar mass live in galaxies that have evolved secularly over the majority of their history. A classical bulge is not a prerequisite to host a black hole.

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arXiv2011

Black Hole Mass and Bulge Luminosity for Low-mass Black Holes

We study the scaling between bulge magnitude and central black hole (BH) mass in galaxies with virial BH masses < 10^6 solar mass. Based on careful image decomposition of a snapshot Hubble Space Telescope I-band survey, we found that these BHs are found predominantly in galaxies with pseudobulges. Here we show that the \mbulge\ relation for the pseudobulges at low mass is significantly different from classical bulges with BH masses >10^7 solar mass. Specfically, bulges span a much wider range of bulge luminosity, and on average the luminosity is larger, at fixed black hole mass. The trend holds both for the active galaxies from Bentz et al. and the inactive sample of Gultekin et al. and cannot be explained by differences in stellar populations, as it persists when we use dynamical bulge masses. Put another way, the ratio between bulge and BH mass is much larger than $\sim 1000$ for our sample. This is consistent with recent suggestions that black hole mass does not scale with the pseudobulge luminosity. The low-mass scaling relations appear to flatten, consistent with predictions from Volonteri & Natarajan for massive seed BHs.

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arXiv2011

Exploring the Low-Mass End of The M-Sigma Relation with Active Galaxies

We present new measurements of stellar velocity dispersions, using spectra obtained with the Keck Echellette Spectrograph and Imager (ESI) and the Magellan Echellette (MagE), for 76 Seyfert 1 galaxies from the recent catalogue of Greene & Ho. These objects were selected from the Sloan Digital Sky Survey (SDSS) to have estimated black hole (BH) masses below 2\times10^6 M\odot. Combining our results with previous ESI observations of similar objects, we obtain an expanded sample of 93 galaxies and examine the relation between BH mass and velocity dispersion (the M-Sigma relation) for active galaxies with low BH masses. The low-mass active galaxies tend to follow the extrapolation of the M-Sigma relation of inactive galaxies. Including results for active galaxies of higher BH mass from the literature, we find a zero pointα= 7.68\pm0.08 and slope of β= 3.32\pm0.22 for the M-Sigma relation [in the form log Mbh=α+βlog(σ*/200 km s-1)], with intrinsic scatter of 0.46\pm0.03 dex. This result is consistent, within the uncertainties, with the slope of the M-Sigma relation for reverberation-mapped active galaxies with BH masses from 10^6 to 10^9 M\odot. For the subset of our sample having morphological information from Hubble Space Telescope images, we examine the slope of the M-Sigma relation separately for subsamples of barred and unbarred host galaxies, and find no significant evidence for a difference in slope. We do find a mild offset between low-inclination and high-inclination disk galaxies, such that more highly inclined galaxies tend to have larger σ* at a given value of BH mass, presumably due to the contribution of disk rotation within the spectroscopic aperture. We also find that the velocity dispersion of the ionized gas trace the stellar velocity dispersion well for this large sample of low-mass Seyfert 1 galaxies.

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