454 research outputs found

    Rates of Observable Black Hole Emergence in Supernovae

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    A newly formed black hole may be directly identified if late-time accretion of material from the base of the ejected envelope generates a luminosity that is observable in the tail of the supernova light curve. In this work we estimate the rate at which events where the black hole ``emerges'' in the supernova light curve can be detected with present capabilities. Our investigation is based on an analytical model of the accretion luminosity at emergence as a function of progenitor mass, coupled to the inferred rate of observed Type II supernovae in nearby galaxies. We find through a parameter survey that under optimistic assumptions the potential rate of observable events can be as high as several per year. However, supernovae which produce black holes are also likely to be low energy explosions and therefore subluminous, as was the case for the best candidate to date, SN1997D. If black hole-forming supernovae are underdetected owing to lower luminosities, the rate of observing black hole emergence is probably not larger than once every few years. We therefore emphasize the importance of dedicated searches for nearby supernovae as well as faint supernovae projects for improving the prospects of observationally certifying the supernova--black hole connection.Comment: ApJ accepted, 13 pages, uses emulateapj

    Testing population synthesis models with globular cluster colors

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    We have measured an extensive set of UBVRIJHK colors for M31 globular clusters [Barmby et al. 2000]. We compare the predicted simple stellar population colors of three population synthesis models to the intrinsic colors of Galactic and M31 globular clusters. The best-fitting models fit the cluster colors very well -- the weighted mean color offsets are all < 0.05 mag. The most significant offsets between model and data are in the U and B passbands; these are not unexpected and are likely due to problems with the spectral libraries used by the models. The metal-rich clusters ([Fe/H] > -0.8) are best fit by young (8 Gyr) models, while the metal-poor clusters are best fit by older (12--16 Gyr) models. If this range of globular cluster ages is correct, it implies that conditions for cluster formation must have existed for a substantial fraction of the galaxies' lifetimes.Comment: To appear in ApJ Letters; 8 pages including 3 figures and 1 tabl

    On the Sensitivity of Massive Star Nucleosynthesis and Evolution to Solar Abundances and to Uncertainties in Helium Burning Reaction Rates

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    We explore the dependence of pre-supernova evolution and supernova nucleosynthesis yields on the uncertainties in helium burning reaction rates. Using the revised solar abundances of Lodders (2003) for the initial stellar composition, instead of those of Anders & Grevesse (1989), changes the supernova yields and limits the constraints that those yields place on the 12C(a,g)16O reaction rate. The production factors of medium-weight elements (A = 16-40) were found to be in reasonable agreement with observed solar ratios within the current experimental uncertainties in the triple alpha reaction rate. Simultaneous variations by the same amount in both reaction rates or in either of them separately, however, can induce significant changes in the central 12C abundance at core carbon ignition and in the mass of the supernova remnant. It therefore remains important to have experimental determinations of the helium burning rates so that their ratio and absolute values are known with an accuracy of 10% or better.Comment: Accepted for publication by the Astrophysical Journa

    Spectroscopy of Close Companions to QSOs and the Ages of Interaction-Induced Starbursts

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    We present low-resolution absorption-line spectra of three candidate close ( < 3 arcsec) companions to the low redshift QSOs 3CR 323.1, PG 1700+518, and PKS 2135-147. The spectra were obtained with LRIS on the Keck telescopes and with the Faint Object Spectrograph on the University of Hawaii 2.2 m telescope. For 3CR 323.1 and PG 1700+518, we measure relative velocities that are consistent with an association between the QSOs and their companion galaxies. The spectral features of the companion galaxy to 3CR 323.1 indicate a stellar population of intermediate age (approx. 2.3 Gyr). In contrast, the spectrum of the companion object to PG 1700+518 shows strong Balmer absorption lines from a relatively young stellar population, along with the Mg Ib absorption feature and the 4000 A break from an older population. By modeling the two stellar components of this spectrum, it is possible to estimate the time that has elapsed since the end of the most recent major starburst event: we obtain approx. 0.1 Gyr. This event may have coincided with an interaction that triggered the QSO activity. Finally, our spectroscopy shows conclusively that the supposed companion to PKS 2135-147 is actually a projected Galactic G star.Comment: 10 pages, 5 Postscript figures. Latex (AASTEX). To appear in ApJ. Letters, Volume 480 (1997

    The Stellar Mass Spectrum in the Young Populous Cluster NGC 1866

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    The young populous cluster NGC 1866 in the Large Magellanic Cloud LMC), which is probably one of the most massive object formed in the LMC during the last ~ 3 Gyr, appears to have an unexpectedly high mass-to-light ratio. From its velocity dispersion Fischer et al. (1992) find its mass to be (1.35 " 0.25) x 105 Mu. The luminosity of this cluster is MV = -8.93 " 0.13, corresponding to LV = (3.2 " 0.4) x 105 LV (u). This yields M/LV = 0.42 " 0.09 in solar units. For a cluster of age 0.1 Gyr such a relatively high mass-to-light ratio requires a mass spectrum with an exponent x = 1.72 " 0.09; or x = 1.75 " 0.09 if mass loss by evolving stars is taken into account.Comment: To be published in the October 1999 issue of the Publications of the Astronomical Society of the Pacifi

    The Galactic disk mass-budget : II. Brown dwarf mass-function and density

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    In this paper, we extend the calculations conducted previously in the stellar regime to determine the brown dwarf IMF in the Galactic disk. We perform Monte Carlo calculations taking into account the brown dwarf formation rate, spatial distribution and binary fraction. Comparison with existing surveys seems to exclude a power-law MF as steep as the one determined in the stellar regime below 1 \msol and tends to favor a more flatish behaviour. Comparison with methane-dwarf detections tends to favor an eventually decreasing form like the lognormal or the more general exponential distributions determined in the previous paper. We calculate predicting brown dwarf counts in near-infrared color diagrams and brown dwarf discovery functions. These calculations yield the presently most accurate determination of the brown dwarf census in the Galactic disk. The brown dwarf number density is comparable to the stellar one, nBD≃n⋆≃0.1n_{BD}\simeq n_\star\simeq 0.1 pc−3^{-3}. The corresponding brown dwarf mass density, however, represents only about 10% of the stellar contribution, i.e. \rho_{BD}\simle 5.0\times 10^{-3} \mvol. Adding up the local stellar density determined previously yields the density of star-like objects, stars and brown dwarfs, in the solar neighborhood \rho_\odot \approx 5.0\times 10^{-2} \mvol.Comment: 39 pages, Latex file, uses aasms4.sty, to be published in ApJ, corrected version with correct figure

    Self-Regulated Growth of Supermassive Black Holes in Galaxies as the Origin of the Optical and X-ray Luminosity Functions of Quasars

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    We postulate that supermassive black-holes grow in the centers of galaxies until they unbind the galactic gas that feeds them. We show that the corresponding self-regulation condition yields a correlation between black-hole mass (Mbh) and galaxy velocity dispersion (sigma) as inferred in the local universe, and recovers the observed optical and X-ray luminosity functions of quasars at redshifts up to z~6 based on the hierarchical evolution of galaxy halos in a Lambda-CDM cosmology. With only one free parameter and a simple algorithm, our model yields the observed evolution in the number density of optically bright or X-ray faint quasars between 2<z<6 across 3 orders of magnitude in bolometric luminosity and 3 orders of magnitude in comoving density per logarithm of luminosity. The self-regulation condition identifies the dynamical time of galactic disks during the epoch of peak quasar activity (z~2.5) as the origin of the inferred characteristic quasar lifetime of ~10 million years. Since the lifetime becomes comparable to the Salpeter e-folding time at this epoch, the model also implies that the Mbh-sigma relation is a product of feedback regulated accretion during the peak of quasar activity. The mass-density in black-holes accreted by that time is consistent with the local black-hole mass density of ~(0.8-6.3) times 10^5 solar masses per cubic Mpc, which we have computed by combining the Mbh-sigma relation with the measured velocity dispersion function of SDSS galaxies (Sheth et al.~2003). Applying a similar self-regulation principle to supernova-driven winds from starbursts, we find that the ratio between the black hole mass and the stellar mass of galactic spheroids increases with redshift as (1+z)^1.5 although the Mbh-sigma relation is redshift-independent.Comment: 10 pages, 5 figures, submitted to Ap

    Spectroscopic Confirmation of a Radio-Selected Galaxy Overdensity at z=1.11

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    We report the discovery of a galaxy overdensity at z=1.11 associated with the z=1.110 high-redshift radio galaxy MG0442+0202. The group, CL0442+0202, was found in a near-infrared survey of z>1 radio galaxies undertaken to identify spatially-coincident regions with a high density of objects red in I-K' color, typical of z>1 elliptical galaxies. Spectroscopic observations from the Keck telescope reveal five galaxies within 35" of MG0442+0202 at 1.10<z<1.11. These member galaxies have broad-band colors and optical spectra consistent with passively-evolving elliptical galaxies formed at high redshift. A 45ks Chandra X-Ray Observatory observation detects the radio galaxy and four point sources within 15" of the radio galaxy, corresponding to a surface density two orders of magnitude higher than average for X-ray sources at these flux levels, S(0.5-2keV) > 5e-16 erg/cm2/s. One of these point sources is identified with a radio-quiet, typeII quasar at z=1.863, akin to sources recently reported in deep Chandra surveys. The limit on an extended hot intracluster medium in the Chandra data is S(1-6keV) < 1.9e-15 erg/cm2/s (3-sigma, 30" radius aperture). Though the X-ray observations do not confirm the existence of a massive, bound cluster at z>1, the success of the optical/near-infrared targeting of early-type systems near the radio galaxy validates searches using radio galaxies as beacons for high-redshift large-scale structure. We interpret CL0442+0202 to be a massive cluster in the process of formation.Comment: 23 pages, 7 figure