arXiv subjects

Mark Seibert

Publications and source records attributed to Mark Seibert.

139 records · Page 8Linked to original sources

arXiv2006

Suppression of star formation in early-type galaxies by feedback from supermassive black holes

Detailed high-resolution observations of the innermost regions of nearby galaxies have revealed the presence of supermassive black holes1. These black holes may interact with their host galaxies by means of 'feedback' in the form of energy and material jets; this feedback affects the evolution of the host and gives rise to observed relations between the black hole and the host. Here we report observations of the ultraviolet emissions of massive early-type galaxies. We derive an empirical relation for a critical black-hole mass (as a function of velocity dispersion) above which the outflows from these black holes suppress star formation in their hosts by heating and expelling all available cold gas. Supermassive black holes are negligible in mass compared to their hosts but nevertheless seem to play a critical role in the star formation history of galaxies.

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arXiv2006

GALEX and Optical Light Curves of EF Eridanus During a Low State: the Puzzling Source of UV Light

Low state optical photometry of EF Eri during an extended low accretion state combined with GALEX near and far UV time-resolved photometry reveals a source of UV flux that is much larger than the underlying 9500K white dwarf, and that is highly modulated on the orbital period. The near UV and optical light curves can be modeled with a 20,000K spot but no spot model can explain both the large amplitude FUV variations and the SED. The limitations of limb darkening, cyclotron and magnetic white dwarf models in explaining the observations are discussed.

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arXiv2006

The detection of M-dwarf UV flare events in the GALEX data archives

We present the preliminary results from implementing a new software tool that enables inspection of time-tagged photon data for the astronomical sources contained within individual GALEX ultraviolet images of the sky. We have inspected the photon data contained within 1802 GALEX images to reveal rapid, short-term (<500 sec) UV source variability in the form of stellar flares. The mean associated change in NUV magnitude due to this flaring activity is 2.7+/-0.3 mag. A list of 49 new UV variable-star candidates is presented, together with their associated Sloan Digital Sky Survey (SDSS) photometric magnitudes. From these data we can associate the main source of these UV flare events with magnetic activity on M-dwarf stars. Photometric parallaxes have been determined for 32 of these sources, placing them at distances ranging from approximately 25 to 1000pc. The average UV flare energy for these flare events is 2.5E30 ergs, which is of a similar energy to that of U-band, X-ray and EUV flares observed on many local M-dwarf stars. We have found that stars of classes M0 to M5 flare with energies spanning a far larger range and with an energy approximately 5 times greater than those of later (M6 to M8) spectral type.

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arXiv2005

GALEX Observations of an Energetic Ultraviolet Flare on the dM4e Star GJ 3685A

The Galaxy Evolution Explorer (GALEX) satellite has obtained high time resolution ultraviolet photometry during a large flare on the M4 dwarf star GJ 3685A. Simultaneous NUV (1750 - 2800A) and FUV (1350 - 1750A) time-tagged photometry with time resolution better than 0.1 s shows that the overall brightness in the FUV band increased by a factor of 1000 in 200 s. Under the assumption that the NUV emission is mostly due to a stellar continuum, and that the FUV flux is shared equally between emission lines and continuum, then there is evidence for two distinct flare components for this event. The first flare type is characterized by an exponential increase in flux with little or no increase in temperature. The other involves rapid increases in both temperature and flux. While the decay time for the first flare component may be several hours, the second flare event decayed over less than 1 minute, suggesting that there was little or no confinement of the heated plasma.

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arXiv2005

The Galex Ultraviolet Variability (GUVV) Catalog

We present Version 1.0 of the NASA Galaxy Evolution Explorer (GALEX) ultraviolet variability catalog (GUVV) that contains information on 84 time-variable and transient sources gained with simultaneous near and far ultraviolet photometric observations. These time-variable sources were serendipitously revealed in the various 1.2 degree star fields currently being surveyed by the GALEX satellite in two ultraviolet bands (NUV 1750-2750A, FUV 1350-1750A) with limiting AB magnitudes of 23-25. The largest-amplitude variable objects presently detected by GALEX are M-dwarf flare stars, which can brighten by 5-10 mag in both the NUV and FUV bands during short duration (< 500s) outbursts. Other types of large-amplitude ultraviolet variable objects include ab-type RR Lyrae stars, which can vary periodically by 2-5mag in the GALEX FUV band. This first GUVV catalog lists galactic positions and possible source identifications in order to provide the astronomical community with a list of time-variable objects that can now be repeatedly observed at other wavelengths. We expect the total number of time-variable source detections to increase as the GALEX mission progresses, such that later version numbers of the GUVV catalog will contain substantially more variable sources.

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arXiv2004

The Properties of Ultraviolet-Luminous Galaxies at the Current Epoch

We have used the first matched set of GALEX and SDSS data to investigate the properties of a sample of 74 nearby galaxies with far-ultraviolet luminosities chosen to overlap the luminosity range of typical high-z Lyman Break Galaxies (LBGs). GALEX deep surveys have shown that ultraviolet-luminous galaxies (UVLGs) similar to these are the fastest evolving component of the UV galaxy population. Model fits to the combined GALEX and SDSS photometry yield typical FUV extinctions similar to LBGs. The implied star formation rates are SFR ~ 3 to 30 solar mass per year. This overlaps the range of SFRs for LBGs. We find a strong inverse correlation between galaxy mass and far-ultraviolet surface brightness, and on this basis divide the sample into ``large'' and ``compact'' UVLGs. The compact UVLGs have half-light radii of a few kpc or less (similar to LBGs). They are relatively low mass galaxies (~10 billion solar masses) with typical velocity dispersions of 60 to 150 km/s. They span a range in metallicity from 0.3 to 1 times solar, have blue optical-UV colors, and are forming stars at a rate sufficient to build the present galaxy in ~a Gigayear. In all these respects they appear similar to the LBG population. These ``living fossils'' may therefore provide an opportunity for detailed investigation of the physical processes occurring in typical star forming galaxies in the early universe.

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arXiv2004

Number Counts of GALEX Sources in FUV (1530A) and NUV (2310A) Bands

Number Counts of galaxies in two GALEX bands (FUV: 1530A and NUV: 2310A, both in AB magnitudes) are reported. They provide for the first time in the literature homogeneously calibrated number counts of UV galaxies covering continuously a very wide range of UV magnitude (14 -- 23.8). Both the FUV and NUV counts are inconsistent with a non-evolution model, while they are in good agreement with evolution models (essentially luminosity evolution) derived from the high-z UV luminosity functions of Arnouts et al. (2004). It is found that the contribution from galaxies detected by GALEX to the UV background is 0.68+-0.10 nW m-2 sr-1 at 1530A and 0.99+-0.15 nW m-2 sr-1 at 2310A. These are 66+-9% and 44+-6% of the total contributions of galaxies to the the UV background at 1530A, respectively, as estimated using the evolution models. ...

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arXiv2004

The Star Formation Rate Function of the Local Universe

We have derived the bivariate luminosity function for the far ultraviolet (1530Angstroms) and far infrared (60 microns). We used matched GALEX and IRAS data, and redshifts from NED and PSC-z. We have derived a total star formation luminosity function phi(L_{tot}), with L_{tot} = L_{FUV}+L_{FIR}. Using these, we determined the cosmic ``star formation rate'' function and density for the local universe. The total SFR function is fit very well by a log-normal distribution over five decades of luminosity. We find that the bivariate luminosity function phi(L_{FUV},L_{FIR}) shows a bimodal behavior, with L_{FIR} tracking L_{FUV} for L_{TOT}< 10^10 L_sun, and L_{FUV} saturating at 10^10 L_sun, while L_{TOT} L_{FIR} for higher luminosities. We also calculate the SFR density and compare it to other measurements.

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arXiv2002

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

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

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arXiv2002

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

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

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arXiv2002

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

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

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arXiv2001

Far-Infrared Galaxies in the Far-Ultraviolet

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

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arXiv2001

How to Correct for Dust Absorption in Starbursts

We review new and published results to examine how well the bolometric flux of starbursts can be recovered from ultraviolet (UV) and optical observations. We show that the effective absorption of starbursts can be substantial, up to \~10 mag in the far UV, and ~5 mag in H-alpha, but apparently not as high as some claims in the literature (several tens to a thousand mag). The bolometric fluxes of an IUE sample of starbursts can be recovered to 0.14 dex accuracy using the UV flux and spectral slope. However, this relationship breaks down for Ultra Luminous Infrared Galaxies (ULIGs). The H-alpha flux combined with the Balmer decrement can be used to predict the bolometric flux to 0.5 dex accuracy for starbursts including most ULIGs. These results imply a foreground screen component to the dust distribution.

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