Mark Harry Seibert

The Origin of the Mass-Metallicity Relation: Insights from 53,000 Star-forming Galaxies in the Sloan Digital Sky Survey

We utilize Sloan Digital Sky Survey imaging and spectroscopy of ~53,000 star-forming galaxies at z ~ 0.1 to study the relation between stellar mass and gas-phase metallicity. We derive gas-phase oxygen abundances and stellar masses using new techniques that make use of the latest stellar evolutionary synthesis and photoionization models. We find a tight (?0.1 dex) correlation between stellar mass and metallicity spanning over 3 orders of magnitude in stellar mass and a factor of 10 in metallicity. The relation is relatively steep from 108.5 to 1010.5 M? h, in good accord with known trends between luminosity and metallicity, but flattens above 1010.5 M?. We use indirect estimates of the gas mass based on the H? luminosity to compare our data to predictions from simple closed box chemical evolution models. We show that metal loss is strongly anticorrelated with baryonic mass, with low-mass dwarf galaxies being 5 times more metal depleted than L* galaxies at z ~ 0.1. Evidence for metal depletion is not confined to dwarf galaxies but is found in galaxies with masses as high as 1010 M?. We interpret this as strong evidence of both the ubiquity of galactic winds and their effectiveness in removing metals from galaxy potential wells.

Stellar masses and star formation histories for 105 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-A break strength and the Balmer absorption-line index Hδ 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 122 808 galaxies drawn from the Sloan Digital Sky Survey. The typical 95 per cent confidence range in our estimated stellar masses is ′40 per cent. We study how the stellar mass-to-light ratios of galaxies vary as a function of absolute magnitude, concentration index and photometric passband and how dust attenuation varies as a function of absolute magnitude and 4000-A 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, to within a factor of approximately 2, stellar masses ∼5 x 10 1 0 M O ., half-light radii ∼3 kpc and half-light surface mass densities ∼10 9 M O .kpc - 2 . The distribution of D n (4000) is strongly bimodal, showing a clear division between galaxies dominated by old stellar populations and galaxies with more recent star formation.

The dependence of star formation history and internal structure on stellar mass for 105 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 x 10 1 0 M O .. Lower-mass galaxies have young stellar populations, low surface mass densities and the low concentrations typical of discs. Their star formation histories are more strongly correlated with surface mass density than with stellar mass. 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 lognormally distributed with dispersion σ(In R 5 0 ) ∼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 μ * M * 0.54, suggesting that the stellar mass of a disc galaxy is proportional to the three halves power of its halo mass. All of this suggests that the efficiency of the conversion of baryons into stars in low-mass galaxies increases in proportion to halo mass, perhaps as a result of supernova feedback processes. At stellar masses above 3 x 10 1 0 M O ., there is a rapidly increasing fraction of galaxies with old stellar populations, high surface mass densities and the high concentrations typical of bulges. In this regime, the size distribution remains lognormal, but its dispersion decreases rapidly with increasing mass and the median stellar mass surface density is approximately constant. This suggests that the star formation efficiency decreases in the highest-mass haloes, and that little star formation occurs in massive galaxies after they have assembled.

Disk Accretion onto High-Mass Planets

We analyze the nonlinear, two-dimensional response of a gaseous, viscous protoplanetary disk to the presence of a planet of one Jupiter mass (1 MJ) and greater that orbits a 1 M☉ star by using the ZEUS hydrodynamics code with high resolution near the planets Roche lobe. The planet is assumed to be in a circular orbit around the central star and is not allowed to migrate. A gap is formed about the orbit of the planet, but there is a nonaxisymmetric flow through the gap and onto the planet. The gap partitions the disk into an inner (outer) disk that extends inside (outside) the planets orbit. For a 1 MJ planet and typical disk parameters, the accretion through the gap onto the planet is highly efficient. That is, the rate is comparable to the accretion rate toward the central star that would occur in the absence of the planet (at the location of the planet). For typical disk parameters, the mass-doubling timescale is less than 105 yr, considerably shorter than the disk lifetime. Following shocks near the L1 and L2 Lagrangian points, disk material enters the Roche lobe in the form of two gas streams. Shocks occur within the Roche lobe as the gas streams collide, and shocks lead to rapid inflow toward the planet within much of planets Roche lobe. Shocks also propagate in the inner and outer disks that orbit the star. For higher mass planets (of order 6 MJ), the flow rate onto the planet is considerably reduced, which suggests an upper mass limit to planets in the range of 10 MJ. This rate reduction is related to the fact that the gap width increases relative to the Roche (Hill sphere) radius with increasing planetary mass. The flow in the gap affects planetary migration. For the 1 MJ planet case, mass can penetrate from the outer disk to the inner disk, so that the inner disk is not depleted. The results suggest that most of the mass in gas giant planets is acquired by flows through gaps.

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 submillimeter and Lyman-break galaxies, we have obtained far- and near-UV imaging observations (λeff = 1457 and 2364 A, 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 kiloparsec 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 parsecs. Furthermore, 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 where most of the bolometric energy is generated. All nuclei are highly obscured by dust. Even after correction for dust reddening, the global UV emission fails to account for the total bolometric luminosities of these systems by factors of 3-75. The discrepancy is much worse if only the central regions, where the bolometric luminosities are generated, are included. In two cases (VV 114 and IRAS 08572+3915), the merging companion galaxies are more prominent in the UV than the more IR luminous member. While all our galaxies show possible signatures of active galactic nucleus (AGN) activity, only IRAS 19254-7245 yields even a possible detection of an AGN in our UV images. Simple calculations show that all but one of our galaxies would be expected to drop below the detection thresholds of, e.g., the Hubble Deep Fields at redshifts between 1.5 and 3, and we find that ~2 of our five ULIGs would be selected as extremely red objects in this redshift range. A typical ULIG in our sample would be too faint to be detected at high redshift in the deepest current optical or submillimeter deep surveys. Only VV 114 has UV luminosity and color similar to Lyman-break galaxies at z ~ 3; the other galaxies would be too faint and/or red to be selected by current surveys. The low UV brightnesses of our ULIGs mean that they would not appear as optically bright (or bright ERO) submillimeter galaxy counterparts, although they might be similar to the fainter submillimeter galaxy counterparts.

Massive star formation within the Leo 'primordial' ring

Few intergalactic, plausibly primordial clouds of neutral atomic hydrogen (Hu2009i) have been found in the local Universe, suggesting that such structures have either dispersed, become ionized or produced a stellar population on gigayear timescales. The Leo ring, a massive (MHu2009iu2009≈u20091.8u2009×u2009109, denoting the solar mass), 200-kpc-wide structure orbiting the galaxies M105 and NGCu20093384 with a 4-Gyr period, is a candidate primordial cloud. Despite repeated atttempts, it has previously been seen only from Hu2009i emission, suggesting the absence of a stellar population. Here we report the detection of ultraviolet light from gaseous substructures of the Leo ring, which we attribute to recent massive star formation. The ultraviolet colour of the detected complexes is blue, implying the onset of a burst of star formation or continuous star formation of moderate (∼108-yr) duration. Measured ultraviolet–visible photometry favours models with low metallicity (Zu2009≈u2009/50–/5, denoting the solar metallicity), that is, a low proportion of elements heavier than helium, although spectroscopic confirmation is needed. We speculate that the complexes are dwarf galaxies observed during their formation, but distinguished by their lack of a dark matter component. In this regard, they resemble tidal dwarf galaxies, although without the enrichment preceding tidal stripping. If structures like the Leo ring were common in the early Universe, they may have produced a large, yet undetected, population of faint, metal-poor, halo-lacking dwarf galaxies.

The Starburst Nature of Lyman Break Galaxies: Testing Ultraviolet 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 Hubble Deep Field and Chandra Deep Field North. 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 cannot have far-IR to far-UV flux ratios similar to nearby ultraluminous infrared galaxies, since 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 submillimeter. 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 underpredicted by a factor of 6 if the far-UV is unattenuated by dust.

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 Hα, 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 Hα 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.

Hidden Star Formation: The Ultraviolet Perspective

Many recent estimates of the star formation rate density at high redshift rely on rest-frame ultraviolet (UV) data. These are highly sensitive to dust absorption. Applying a correlation between the far-infrared (FIR) to UV flux ratio and UV color found in a local starbursts to galaxy samples out to z ~ 3, one can account for most of the FIR background. However, the correlation is based on a sample that does not include the most extreme starbursts, Ultra Luminous Infrared Galaxies (ULIGs). Our new UV images of ULIGs show that their FIR fluxes are underpredicted by this correlation by factors ranging from 7 to 70. We discuss how ULIGs compare to the various types of high-z galaxies: sub-mm sources, Lyman Break Galaxies, and Extremely Red Objects.