Astrophysics and Astronomy | Open Access Articles

The Molecular Ism In Low Surface Brightness Disk Galaxies, J. Christopher Mihos, Stacy S. Mcgaugh

Faculty Scholarship

We present models for the interstellar medium in disk galaxies. In particular, we investigate whether the ISM in low surface brightness galaxies can support a significant fraction of molecular gas given their low metallicity and surface density. It is found that the abundance and line brightness of CO in LSB galaxies is small and typically below current observational limits. Still, depending on physical details of the ISM, the fraction of gas in the form of molecular hydrogen can be significant in the inner few kilo-parsecs of a low surface brightness galaxy. This molecular gas would be at temperatures of ∼ …

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The Peculiar Motions Of Early-Type Galaxies In Two Distant Regions -- V. The Mg-- Relation, Age And Metallicity, M. Colless, D. Burstein, R. L. Davies, R. K. Mcmahan, R. P. Saglia, G. Wegner

Dartmouth Scholarship

We have examined the Mg—σ relation for early-type galaxies in the EFAR sample and its dependence on cluster properties. A comprehensive maximum likelihood treatment of the sample selection and measurement errors gives fits to the global Mg—σ relation of Mg b^′=0.131 log σ −0.131 and Mg₂=0.257 log σ −0.305. The slope of these relations is 25 per cent steeper than that obtained by most other authors owing to the reduced bias of our fitting method. The intrinsic scatter in the global Mg— σ relation is estimated to be 0.016 mag in Mg b^′ and 0.023 …

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Near-Infrared Galaxy Counts To J And K ∼ 24 As A Function Of Image Size, Matthew A. Bershady, James D. Lowenthal, David C. Koo

Astronomy: Faculty Publications

We have used the Keck 10 m telescope to count objects as a function of image size in two high Galactic latitude Ðelds covering 1.5 arcmin2 and reaching 50% completeness depths of K \ 24 and J \ 24.5 for stellar sources. Our counts extend D1 mag deeper in K than those of surveys with other telescopes; complement other Keck surveys in the K-band that provide counts at comparable or shallower depths but that have not utilized image structure; and extend by several magnitudes the J-band counts from brighter surveys using smaller telescopes that cover larger areas. We Ðnd the …

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Far-Ultraviolet Spectra Of Starburst Galaxies: Stellar Population And The Kinematics Of The Interstellar Medium, Rosa M. Gonzalez Delgado, Claus Leitherer, Timothy Heckman, James D. Lowenthal, Henry C. Ferguson, Carmelle Robert

Astronomy: Faculty Publications

The far-ultraviolet spectra of the four starburst galaxies NGC 6090, Mrk 66, Mrk 1267, and IRAS 0833 + 6517 were observed with the Hopkins Ultraviolet Telescope during the Astro-2 mission. Additional data were obtained for IRAS 0833 + 6517 with the Goddard High-Resolution Spectrograph (GHRS) on the Hubble Space Telescope. We analyze the observations in terms of the stellar content and the kinematics of the interstellar medium, and we discuss the implications of these results for the interpretation of the ultraviolet spectra of high-redshift galaxies. Evolutionary synthesis models are used to constrain the star formation history from the absolute ultraviolet …

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Gas Mass Fractions And The Evolution Of Spiral Galaxies, Stacy S. Mcgaugh

Faculty Scholarship

We show that the gas mass fraction of spiral galaxies is strongly correlated with luminosity and surface brightness. It is not correlated with linear size. Gas fraction varies with luminosity and surface brightness at the same rate, indicating evolution at fixed size. Dim galaxies are clearly less evolved than bright ones, having consumed only ∼1/2 of their gas. This resolves the gas consumption paradox, since there exist many galaxies with large gas reservoirs. These gas-rich galaxies must have formed the bulk of their stellar populations in the last half of a Hubble time. The existence of such immature galaxies at …

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The Nature Of Compact Galaxies In The Hubble Deep Field. Ii. Spectroscopic Properties And Implications For The Evolution Of The Star Formation Rate Density Of The Universe, Rafael Guzmán, Jesús Gallego, David C. Koo, Andrew C. Phillips, James D. Lowenthal, S. M. Faber, Garth D. Illingworth, Nicole P. Vogt

Astronomy: Faculty Publications

We present a spectroscopic study of 51 compact field galaxies with redshifts z < 1.4 and apparent magnitudes I < 23.74 in the flanking fields of the Hubble Deep Field. These galaxies are compact in the sense that they have small apparent half-light radii (r ≤ 0″.5) and high surface brightnesses (μ ≤ 22.2 mag arcsec ). The spectra, taken at the Keck telescope, show emission lines in 88% of our sample, and only absorption lines in the remaining 12%. Emission-line profiles are roughly Gaussian with velocity widths that range from the measurement limit of σ ∼ 35 km s to 150 km s . Rest frame [O II] λ3727 equivalent widths range from 5 to 94 Å, yielding star formation rates (SFRs) of ∼0.1 to 14 M yr . The analysis of various line diagnostic diagrams reveals that ∼60% of compact emission-line galaxies have velocity widths, excitations, Hβ luminosities, SFRs, and mass-to-light ratios characteristic of young star-forming H II galaxies. The remaining 40% form a more heterogeneous class of evolved starbursts, similar to local starburst disk galaxies. We find that, although the compact galaxies at z > 0.7 have similar SFRs per unit mass to those at z < 0.7, they are on average ∼10 times more massive. Our sample implies a lower limit for the global comoving SFR density of ∼0.004 M yr Mpc at z = 0.55, and ∼0.008 M yr Mpc at z = 0.85 (assuming Salpeter IMF, H = 50 km s Mpc , and q = 0.5). These values, when compared to estimates for a sample of local compact galaxies selected in a similar fashion, support a history of the universe in which the SFR density declines by a factor ∼10 from z = 1 to today. From the comparison with the SFR densities derived for magnitude-limited samples of field galaxies, we conclude that compact emission-line galaxies, though only ∼20% of the general field population, may contribute as much as ∼45% to the global SFR of the universe at 0.4 < z < 1. © 1997. The American Astronomical Society. All rights reserved. 814 1/2 I814 ⊙ ⊙ ⊙ 0 o -2 -1 -1 -1 -1 -3 -1 -3 -1 -1

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Keck Spectroscopy Of Redshift Z ∼ 3 Galaxies In The Hubble Deep Field, James Lowenthal, David C. Koo, Rafael Guzmán, Jesús Gallego, Andrew C. Phillips, S. M. Faber, Nicole P. Vogt, Garth D. Illingworth, Caryl Gronwall

Astronomy: Faculty Publications

We have obtained spectra with the 10 m Keck telescope of a sample of 24 galaxies having colors consistent with star-forming galaxies at redshifts 2 ≲ z ≲ 4.5 in the Hubble deep field (HDF). Eleven of these galaxies are confirmed to be at high redshift (z = 3.0), one is at z = 0.5, and the other 12 have uncertain redshifts but have spectra consistent with their being at z > 2. The spectra of the confirmed high-redshift galaxies show a diversity of features, including weak Lyα emission, strong Lyα breaks or damped Lyα absorption profiles, and the stellar and …

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The Nature Of Compact Galaxies In The Hubble Deep Field. I. Global Properties, Andrew C. Phillips, Rafael Guzmán, Jesús Gallego, David C. Koo, James D. Lowenthal, Nicole P. Vogt, S. M. Faber, Garth D. Illingworth

Astronomy: Faculty Publications

We present 10 m Keck spectroscopy and photometry for a sample of 61 small (r1/2 ≤ 0″.5), faint (I814 ≤ 23.74), high-surface brightness (μI814 < 22.2 mag arcsec-2) galaxies in fields flanking the Hubble Deep Field. The majority of this empirically defined sample of compact galaxies lies at redshifts 0.4 ≲ z ≲ 1 (88% completeness in redshift identifications), ruling out a large component of low-redshift galaxies. The number of such galaxies in the range 1.4 ≲ z ≲ 2.2 is also constrained to ≲ 10%. The majority of the observed galaxies are emission-line systems, while a significant fraction (23%-34%) appear to be normal ellipticals or otherwise early-type systems. One object is an active galactic nucleus, and two are at high redshift (z > 2). The Keck redshift and photometric data are combined with Hubble Space Telescope images to derive luminosities and physical sizes. We also use emission-line widths, where available, to estimate masses. About two-thirds of the emission-line galaxies, or roughly one-half the sample, are small, low-mass, relatively luminous systems with properties resembling those of local H II galaxies. We compare the properties and numbers of these galaxies to the "bursting dwarf" model of Babul & Ferguson. Our sample …

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Dynamical Stability And Environmental Influences In Low Surface Brightness Disk Galaxies, Stacy S. Mcgaugh

Faculty Scholarship

Using analytic stability criteria we demonstrate that, because of their low surface mass density and large dark matter content, low surface brightness (LSB) disks are quite stable against the growth of global nonaxisymmetric modes such as bars. However, depending on their (poorly constrained) stellar velocity dispersions, they may be only marginally stable against local instabilities. We simulate a collision between an LSB and a high surface brightness (HSB) galaxy and find that, while the HSB galaxy forms a strong bar, the response of the LSB disk is milder, manifesting weaker rings and spiral features. The lack of sufficient disk self-gravity …

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The Morphology Of Low Surface Brightness Disk Galaxies, Stacy S. Mcgaugh

Faculty Scholarship

We present UBVI and Ha images of a sample of Low Surface Brightness (LSB) disk galaxies. These galaxies are generally late types, if they can be sensibly classified at all. However, they are not dwarfs, being intrinsically large and luminous. The morphology of LSB galaxies is discussed in terms of the physical interpretation of the Hubble sequence. Galaxies with high contrast relative to the sky background are subject to being more finely typed than those that appear merely as fuzzy blobs on photographic plates. This causes the stages of the Hubble sequence to be nonlinear in the sense that large …

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The Small Scale Environment Of Low Surface Brightness Disk Galaxies, Stacy S. Mcgaugh

Faculty Scholarship

We use a sample of ≈340 low surface brightness (LSB) disk galaxies with measured redshifts in combination with the Center for Astrophysics redshift survey to test the hypothesis that LSB galaxies have a deficit of nearby companion galaxies compared to high surface brightness (HSB) disk galaxies. We find a very strong statistical deficit of galaxies located within a projected radius of 0.5 Mpc and within a velocity of 500 km s-1 around LSB disk galaxies compared to HSB ones. Further, comparing LSB and HSB disk galaxies which are located in the same portion of the sky indicates that the average …

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Microjansky Source Counts And Spectral Indices At 8.44 Ghz, Rogier A. Windhorst, Edward B. Fomalont, R. Bruce Partridge, James D. Lowenthal

Astronomy: Faculty Publications

We used the VLA to make deep images of two 7′ × 7′ fields at 8.44 GHz with 10″ resolution. With an rms noise of 3.2 and 5.1 μJy, respectively, in the two fields, we compiled a catalog of 82 sources. From the complete sample of 20 sources with S ≥ 14.5 μJy, the differential 8.44 GHz source count is dN(S)/ dS = (-4.6 ± 0.7) × S-2.3 ± 0.2 Jy-1 sr-1 in the range 14.5-1000 mJy. Analysis of statistical image fluctuations from weak sources (Fomalont et al. 1993) suggests that this slope remains unchanged at γ = 2.3 ± …

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Full-Text Articles in Astrophysics and Astronomy

The Molecular Ism In Low Surface Brightness Disk Galaxies, J. Christopher Mihos, Stacy S. Mcgaugh

Faculty Scholarship

The Peculiar Motions Of Early-Type Galaxies In Two Distant Regions -- V. The Mg-- Relation, Age And Metallicity, M. Colless, D. Burstein, R. L. Davies, R. K. Mcmahan, R. P. Saglia, G. Wegner

Dartmouth Scholarship

Near-Infrared Galaxy Counts To J And K ∼ 24 As A Function Of Image Size, Matthew A. Bershady, James D. Lowenthal, David C. Koo

Astronomy: Faculty Publications

Far-Ultraviolet Spectra Of Starburst Galaxies: Stellar Population And The Kinematics Of The Interstellar Medium, Rosa M. Gonzalez Delgado, Claus Leitherer, Timothy Heckman, James D. Lowenthal, Henry C. Ferguson, Carmelle Robert

Astronomy: Faculty Publications

Gas Mass Fractions And The Evolution Of Spiral Galaxies, Stacy S. Mcgaugh

Faculty Scholarship

Astronomy: Faculty Publications

Keck Spectroscopy Of Redshift Z ∼ 3 Galaxies In The Hubble Deep Field, James Lowenthal, David C. Koo, Rafael Guzmán, Jesús Gallego, Andrew C. Phillips, S. M. Faber, Nicole P. Vogt, Garth D. Illingworth, Caryl Gronwall

Astronomy: Faculty Publications

The Nature Of Compact Galaxies In The Hubble Deep Field. I. Global Properties, Andrew C. Phillips, Rafael Guzmán, Jesús Gallego, David C. Koo, James D. Lowenthal, Nicole P. Vogt, S. M. Faber, Garth D. Illingworth

Astronomy: Faculty Publications

Dynamical Stability And Environmental Influences In Low Surface Brightness Disk Galaxies, Stacy S. Mcgaugh

Faculty Scholarship

The Morphology Of Low Surface Brightness Disk Galaxies, Stacy S. Mcgaugh

Faculty Scholarship

The Small Scale Environment Of Low Surface Brightness Disk Galaxies, Stacy S. Mcgaugh

Faculty Scholarship

Microjansky Source Counts And Spectral Indices At 8.44 Ghz, Rogier A. Windhorst, Edward B. Fomalont, R. Bruce Partridge, James D. Lowenthal

Astronomy: Faculty Publications