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Case Western Reserve University

Articles 91 - 116 of 116

Full-Text Articles in Astrophysics and Astronomy

High-Resolution Rotation Curves Of Low Surface Brightness Galaxies. I. Data, Stacy S. Mcgaugh Jul 2001

High-Resolution Rotation Curves Of Low Surface Brightness Galaxies. I. Data, Stacy S. Mcgaugh

Faculty Scholarship

We present long-slit Hα observations of 50 low surface brightness galaxies. Of these, 36 are of sufficient quality to form rotation curves. These data provide a large increase in the number of low surface brightness galaxies for which accurate rotation curves are available. They also represent an order-of-magnitude improvement in spatial resolution over previous 21 cm studies (1″-2″ instead of 13″-45″). The improved resolution and accuracy of the data extend and strengthen the scientific conclusions previously inferred from 21 cm data.

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High-Resolution Rotation Curves Of Low Surface Brightness Galaxies. Ii. Mass Models, Stacy S. Mcgaugh Jul 2001

High-Resolution Rotation Curves Of Low Surface Brightness Galaxies. Ii. Mass Models, Stacy S. Mcgaugh

Faculty Scholarship

We present mass models for a sample of 30 high-resolution rotation curves of low surface brightness galaxies. We fit both pseudoisothermal (core dominated) and cold dark matter (CDM; cusp dominated) halos for a wide variety of assumptions about the stellar mass-to-light ratio. We find that the pseudoisothermal model provides superior fits. CDM fits show systematic deviations from the data and often have a small statistical likelihood of being the appropriate model. The distribution of concentration parameters is too broad, and has too low a mean, to be explained by low-density, flat CDM (ACDM). This failing becomes more severe as increasing …

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Mass Density Profiles Of Low Surface Brightness Galaxies, Stacy S. Mcgaugh Apr 2001

Mass Density Profiles Of Low Surface Brightness Galaxies, Stacy S. Mcgaugh

Faculty Scholarship

We derive the mass density profiles of dark matter halos that are implied by high spatial resolution rotation curves of low surface brightness galaxies. We find that, at small radii, the mass density distribution is dominated by a nearly constant density core with a core radius of a few kiloparsecs. For ρ(r) ∼ rα, the distribution of inner slopes α is strongly peaked around α = -0.2. This is significantly shallower than the cuspy α ≤ -1 halos found in cold dark matter simulations. While the observed distribution of α does have a tail toward such extreme values, the derived …

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Gas Mass Fractions And The Evolution Of Low Surface Brightness Dwarf Galaxies, Stacy S. Mcgaugh Jan 2001

Gas Mass Fractions And The Evolution Of Low Surface Brightness Dwarf Galaxies, Stacy S. Mcgaugh

Faculty Scholarship

The optical and H I properties are presented for a sample of low surface brightness (LSB) dwarf galaxies, cataloged from the Second Palomar Sky Survey. Gas mass fractions for LSB dwarfs reach the highest levels of any know galaxy type (fg = 95%), confirming that their low stellar densities are due to inefficient conversion of gas mass into stellar mass. Comparison with star formation models indicates that the blue optical colors of LSB dwarfs are not due to low metallicity or to recent star formation and can only be explained by a dominant stellar population less than 5 Gyr in …

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Boomerang Data Suggest A Purely Baryonic Universe, Stacy S. Mcgaugh Sep 2000

Boomerang Data Suggest A Purely Baryonic Universe, Stacy S. Mcgaugh

Faculty Scholarship

The amplitudes of peaks in the angular power spectrum of anisotropies in the microwave background radiation depend on the mass content of the universe. The second peak should be prominent when cold dark matter is dominant but is depressed when baryons dominate. Recent microwave background data are consistent with a purely baryonic universe with Ωm = Ωb and ΩA ∼ 1.

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The Baryonic Tully-Fisher Relation, Stacy S. Mcgaugh Mar 2000

The Baryonic Tully-Fisher Relation, Stacy S. Mcgaugh

Faculty Scholarship

We explore the Tully-Fisher relation over five decades in stellar mass in galaxies with circular velocities ranging over 30 ≲ Vc ≲ 300 km s-1. We find a clear break in the optical Tully-Fisher relation: field galaxies with Vc ≲ 90 km s-1 fall below the relation defined by brighter galaxies. These faint galaxies, however, are very rich in gas; adding in the gas mass and plotting the baryonic disk mass Md = M* + Mgas in place of luminosity restores the single linear relation. The Tully-Fisher relation thus appears fundamentally to be a relation between rotation velocity and total …

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Star Formation And Tidal Encounters With The Low Surface Brightness Galaxy Ugc 12695 And Companions, Stacy S. Mcgaugh Jan 2000

Star Formation And Tidal Encounters With The Low Surface Brightness Galaxy Ugc 12695 And Companions, Stacy S. Mcgaugh

Faculty Scholarship

We present VLA H I observations of the low surface brightness (LSB) galaxy UGC 12695 and its two companions, UGC 12687 and a newly discovered dwarf galaxy 2333 + 1234. UGC 12695 shows solid-body rotation but has a very lopsided morphology of the H I disk, with the majority of the H I lying in the southern arm of the galaxy. The H I column density distribution of this very blue LSB galaxy coincides in detail with its light distribution. Comparing the H I column density of UGC 12695 with the empirical (but not well-understood) value of Σc = 1021 …

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Distinguishing Between Cold Dark Matter And Modified Newtonian Dynamics: Predictions For The Microwave Background, Stacy S. Mcgaugh Aug 1999

Distinguishing Between Cold Dark Matter And Modified Newtonian Dynamics: Predictions For The Microwave Background, Stacy S. Mcgaugh

Faculty Scholarship

Two hypothesized solutions for the mass discrepancy problem are cold dark matter and modified Newtonian dynamics. The virtues and vices of these very different hypotheses are largely disjointed, making the process of distinguishing between them very dependent on how we weigh disparate lines of evidence. One clear difference is the nature of the principal mass constituent of the universe (cold dark matter or baryons). This difference in the baryon fraction (fb, ≈ 0.1 vs. 1) should leave a distinctive signature in the spectrum of fluctuations in the cosmic microwave background. Here I discuss some of the signatures that should be …

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The Molecular Ism In Low Surface Brightness Disk Galaxies, J. Christopher Mihos, Stacy S. Mcgaugh Apr 1999

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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Testing Modified Newtonian Dynamics With Low Surface Brightness Galaxies: Rotation Curve Fits, Stacy S. Mcgaugh May 1998

Testing Modified Newtonian Dynamics With Low Surface Brightness Galaxies: Rotation Curve Fits, Stacy S. Mcgaugh

Faculty Scholarship

We present modified Newtonian dynamics (MOND) fits to 15 rotation curves of low surface brightness (LSB) galaxies. Good fits are readily found, although for a few galaxies minor adjustments to the inclination are needed. Reasonable values for the stellar mass-to-light ratios are found, as well as an approximately constant value for the total (gas and stars) mass-to-light ratio. We show that the LSB galaxies investigated here lie on the one, unique Tully-Fisher relation, as predicted by MOND. The scatter on the Tully-Fisher relation can be completely explained by the observed scatter in the total mass-to-light ratio. We address the question …

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Hubble Space Telescope Wide Field Planetary Camera 2 Imaging Of Ugc 12695: A Remarkably Unevolved Galaxy At Low Redshift, Stacy S. Mcgaugh May 1998

Hubble Space Telescope Wide Field Planetary Camera 2 Imaging Of Ugc 12695: A Remarkably Unevolved Galaxy At Low Redshift, Stacy S. Mcgaugh

Faculty Scholarship

Utilizing the F814W and F300W filters, short-exposure Hubble Space Telescope Wide Field Planetary Camera 2 images were taken of UGC 12695, a nearby (z ∼ 0.021) low surface brightness disk galaxy. UGC 12695 has an unusual morphology, consisting of a Y-shaped nucleus surrounded by a faint spiral arm with a number of bright H II regions interspersed throughout the galaxy. Surface photometry indicates that the majority of recent star formation in this galaxy occurred in these very localized regions, most of which have a radius of ≲2″. This uneven stellar distribution, combined with the galaxy's overall extremely blue color and …

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Testing The Dark Matter Hypothesis With Low Surface Brightness Galaxies And Other Evidence, Stacy S. Mcgaugh Jan 1998

Testing The Dark Matter Hypothesis With Low Surface Brightness Galaxies And Other Evidence, Stacy S. Mcgaugh

Faculty Scholarship

The severity of the mass discrepancy in spiral galaxies is strongly correlated with the central surface brightness of their disks. Progressively lower surface brightness galaxies have ever larger mass discrepancies. No other parameter (luminosity, size, velocity, morphology) is so well correlated with the magnitude of the mass deficit. The rotation curves of low surface brightness disks thus provide a unique data set with which to probe the dark matter distribution in galaxies. The mass discrepancy is apparent from R = 0, giving a nearly direct map of the halo mass distribution. The luminous mass is insignificant. Interpreting the data in …

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Dynamical Stability And Environmental Influences In Low Surface Brightness Disk Galaxies, Stacy S. Mcgaugh Jan 1997

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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Gas Mass Fractions And The Evolution Of Spiral Galaxies, Stacy S. Mcgaugh Jan 1997

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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Could A Local Group X-Ray Halo Affect The X-Ray And Microwave Backgrounds?, Stacy S. Mcgaugh Aug 1996

Could A Local Group X-Ray Halo Affect The X-Ray And Microwave Backgrounds?, Stacy S. Mcgaugh

Faculty Scholarship

Suto et al. have suggested that an X-ray halo in the Local Group might explain both the observed low-energy excess in the X-ray background and the quadrupole anisotropy in the cosmic microwave background. Recent observations of poor groups of galaxies by the ROSAT PSPC set reasonable limits on how extensive and dense such a halo could be. The poor groups most similar to the Local Group do not have a detectable halo, and the upper limits of these observations suggest that any Local Group halo would be nearly 2 orders of magnitude too tenuous to produce the effects that Suto …

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Does Low Surface Brightness Mean Low Density?, Stacy S. Mcgaugh Jul 1996

Does Low Surface Brightness Mean Low Density?, Stacy S. Mcgaugh

Faculty Scholarship

We compare the dynamical properties of two galaxies at identical positions on the Tully-Fisher relation, but with different surface brightnesses. We find that the low surface brightness galaxy UGC 128 has a higher mass-to-light ratio, and yet has lower mass densities than the high surface brightness galaxy NGC 2403. This is true for the gas surface density, the stellar surface density, and the total mass density. © 1996. The American Astronomical Society. All rights reserved.

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Galaxy Selection And The Surface Brightness Distribution, Stacy S. Mcgaugh Aug 1995

Galaxy Selection And The Surface Brightness Distribution, Stacy S. Mcgaugh

Faculty Scholarship

Optical surveys for galaxies are biased against the inclusion of low surface brightness (LSB) galaxies. Disney [Nature, 263, 573 (1976)] suggested that the constancy of disk central surface brightness noticed by Freeman [ApJ, 160, 811 (1970)] was not a physical result, but instead was an artifact of sample selection. Since LSB galaxies do exist, the pertinent and still controversial issue is if these newly discovered galaxies constitute a significant percentage of the general galaxy population. In this paper, we address this issue by determining the space density of galaxies as a function of disk central surface brightness. Using the physically …

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The Contribution Of Low Surface Brightness Galaxies To Faint Galaxy Counts, Stacy S. Mcgaugh Feb 1995

The Contribution Of Low Surface Brightness Galaxies To Faint Galaxy Counts, Stacy S. Mcgaugh

Faculty Scholarship

Recent observations have revealed a population of blue galaxies at intermediate redshift with a space density well in excess of expectations from the local luminosity function and standard cosmology. The colors and luminosities of these faint blue galaxies are similar to nearby low surface brightness (LSB) galaxies. Such galaxies are severely underrepresented in surveys used to define the local luminosity function, but could in principle be detected in deep surveys. If LSB galaxy density is high enough, the faint-galaxy counts could be explained without requiring rapid galaxy evolution. To explore the consequences of including LSB galaxies, we construct catalogs of …

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The Morphology Of Low Surface Brightness Disk Galaxies, Stacy S. Mcgaugh Jan 1995

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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Oxygen Abundances In Low Surface Brightness Disk Galaxies, Stacy S. Mcgaugh May 1994

Oxygen Abundances In Low Surface Brightness Disk Galaxies, Stacy S. Mcgaugh

Faculty Scholarship

The oxygen abundances in the H II regions of a sample of low surface brightness (LSB) disk galaxies are presented. In general, LSB galaxies are found to be metal poor (Z < 1/3Z⊙). Indeed, some LSB galaxies rival the lowest abundance extragalactic objects known, and this sample greatly increases the number of very low metallicity systems known. These low metallicities indicate that LSB galaxies evolve slowly, forming relatively few stars over a Hubble time. The low metallicities of LSB galaxies occur even though many are comparable in size and mass to the prominent spirals which define the Hubble sequence. As well as being low in surface brightness, these galaxies tend to be isolated. This suggests that surface mass density and environment are more relevant to galaxy evolution than gross size. Despite the low surface brightness of the disks, massive (M > 60 M⊙) stars are inferred to be present and no abnormality of the IMF is indicated. Many low-excitation H II regions exist at low metallicity in LSB galaxies, and the ionization parameter is not tightly correlated with metallicity. However, there does seem to be a significant envelope of maximum ionization at a given metallicity.

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Structural Characteristics And Stellar Composition Of Low Surface Brightness Disk Galaxies, Stacy S. Mcgaugh Feb 1994

Structural Characteristics And Stellar Composition Of Low Surface Brightness Disk Galaxies, Stacy S. Mcgaugh

Faculty Scholarship

We present UBVI surface photometry of a sample of low surface brightness (LSB) disk galaxies. LSB disk galaxies are fairly well described as exponential disks with no preferred value for either scale length, central surface brightness, or rotational velocity. Indeed, the distribution of scale lengths is indistinguishable from that of high surface brightness spirals, indicating that dynamically similar galaxies (e.g., those with comparable v2R) exist over a large range in surface density. These LSB galaxies are strikingly blue. The complete lack of correlation between central surface brightness and color rules out any fading scenario. Similarly, the oxygen abundances inferred from …

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Star Formation Thresholds In Low Surface Brightness Galaxies, Stacy S. Mcgaugh Aug 1993

Star Formation Thresholds In Low Surface Brightness Galaxies, Stacy S. Mcgaugh

Faculty Scholarship

Low Surface Brightness (LSB) galaxies appear to have low star formation rates despite their often quite normal H I contents as judged from global H I properties such as MH I/L and MH I/MT ratios. H I imaging with the Very Large Array of the National Radio Astronomy Observatory (the NRAO is operated by Associated Universities Inc. under contract with the National Science Foundation) of eight LSB galaxies shows that the H I is extended compared with the optical size and has average surface densities which are about a factor 2 lower than in High Surface Brightness (HSB) galaxies of …

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The Small Scale Environment Of Low Surface Brightness Disk Galaxies, Stacy S. Mcgaugh Aug 1993

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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A Catalog Of Low Surface Brightness Galaxies. List Ii, Stacy S. Mcgaugh Apr 1992

A Catalog Of Low Surface Brightness Galaxies. List Ii, Stacy S. Mcgaugh

Faculty Scholarship

The second edition of a catalog of low surface brightness (LSB) galaxies is presented based on a visual search of the Second Palomar Sky Survey. This paper presents newly discovered LSB galaxies in 97 fields within the declination range of 0° to + 25°. The primary catalog (objects with diameters greater than 1 arcmin) contains 198 objects; the secondary catalog (30 arcsec < D < 1 arcmin) contains 140 objects. H I observations are presented for 171 of these newly discovered galaxies. Our 80% detection rate yields a fairly representative velocity distribution for a sample of LSB objects. The sample is dominated by LSB disks galaxies with 2000 < v < 9000 km s-1. Below 2000 km s-1 a few true dwarf galaxies are found. Approximately 10% of the sample has v > 10 000 km s-1 and thus represents additional examples of truly large LSB galaxies. The H I redshift distribution is consistent with the hypothesis that LSB and HSB disk galaxies, in general, are drawn from the same distribution …

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H Ii Region Abundances: Model Oxygen Line Ratios, Stacy S. Mcgaugh Oct 1991

H Ii Region Abundances: Model Oxygen Line Ratios, Stacy S. Mcgaugh

Faculty Scholarship

We present an extensive, homogeneous grid of H II region models. We find that the behavior of the strong oxygen lines can be modeled by taking proper account of the softening of the ionizing spectra pro duced by stars of increasing metallicity. This provides a calibration of the abundance indicating line ratio ([O II] λ3727 + [O III] λλ4959, 5007)/Hβ which is comparable in accuracy to that obtainable by direct methods when a temperature-sensitive line ratio is available. No metallicity dependence of the IMF is required to explain the systematic softening of stellar spectra with increasing metallicity. It results naturally …

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Stellar Populations In Shell Galaxies, Stacy S. Mcgaugh Oct 1990

Stellar Populations In Shell Galaxies, Stacy S. Mcgaugh

Faculty Scholarship

UBV surface photometry of the shell galaxies Arp 230, NGC 7010, and Arp 223 is presented. We find all to be the result of mergers. In Arp 230, the merger of two spirals reduced a burst of star formation which has declined in strength since the collision. The remnant seems likely to become elliptical. NGC 7010 has very red shells, perhaps redder than the galaxy itself. Evolution of the stellar content of these shells may be important. Arp 223 has shell colors which are slightly bluer than thee main body of the galaxy, consistent with an origin in an Sb. …

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