Stacy S. McGaugh

H II region abundances - Model oxygen line ratios

An extensive, homogeneous grid of H II region models, is presented. It is found that the behavior of the strong oxygen lines can be modeled by taking proper account of the softening of the ionizing spectra produced by stars of increasing metallicity. This provides a calibration of the abundance indicating line ratio ([OII] λ3727 + [OIII] 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.

MODIFIED NEWTONIAN DYNAMICS AS AN ALTERNATIVE TO DARK MATTER

▪ Abstract Modified Newtonian dynamics (MOND) is an empirically motivated modification of Newtonian gravity or inertia suggested by Milgrom as an alternative to cosmic dark matter. The basic idea is that at accelerations below ao ≈ 10−8 cm/s2 ≈ cHo/6 the effective gravitational attraction approaches , where gn is the usual Newtonian acceleration. This simple algorithm yields flat rotation curves for spiral galaxies and a mass-rotation velocity relation of the form M ∝ V4 that forms the basis for the observed luminosity–rotation velocity relation—the Tully-Fisher law. We review the phenomenological success of MOND on scales ranging from dwarf spheroidal galaxies to superclusters and demonstrate that the evidence for dark matter can be equally well interpreted as evidence for MOND. We discuss the possible physical basis for an acceleration-based modification of Newtonian dynamics as well as the extention of MOND to cosmology and structure formation.

THE BARYONIC TULLY-FISHER RELATION OF GALAXIES WITH EXTENDED ROTATION CURVES AND THE STELLAR MASS OF ROTATING GALAXIES

I investigate the baryonic Tully-Fisher relation for a sample of galaxies with extended 21 cm rotation curves spanning the range 20 km s-1 Vf ? 300 km s-1. A variety of scalings of the stellar mass-to-light ratio are considered. For each prescription for , I give fits of the form d = V. Presumably, the prescription that comes closest to the correct value will minimize the scatter in the relation. The fit with minimum scatter has = 50 ? km-4 s4 and x = 4. This relation holds over five decades in mass. Galaxy color, stellar fraction, and are correlated with each other and with d, in the sense that more massive galaxies tend to be more evolved. There is a systematic dependence of the degree of maximality of disks on surface brightness. High surface brightness galaxies typically have ~ of the maximum disk value, while low surface brightness galaxies typically attain ~ of this amount.

The Baryonic Tully-Fisher Relation

We explore the Tully-Fisher relation over five decades in stellar mass in galaxies with circular velocities ranging over 30 less, similarVc less, similar300 km s-1. We find a clear break in the optical Tully-Fisher relation: field galaxies with Vc less, similar90 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 baryonic mass of the form Md~V4c.

Mass Density Profiles of Low Surface Brightness Galaxies

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 , the distribution of inner a r(r) ∼ r slopes a is strongly peaked around . This is significantly shallower than the cuspy halos found a p 0.2 a ≤ 1 in cold dark matter simulations. While the observed distribution of a does have a tail toward such extreme values, the derived value of a is found to depend on the spatial resolution of the rotation curves: is found only a ≈ 1 for the least well resolved galaxies. Even for these galaxies, our data are also consistent with constant-density cores ( ) of modest (∼1 kpc) core radius, which can give the illusion of steep cusps when insufficiently a p 0 resolved. Consequently, there is no clear evidence for a cuspy halo in any of the low surface brightness galaxies observed. Subject headings: dark matter — galaxies: fundamental parameters — galaxies: kinematics and dynamicsWe 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 kpc. For ρ(r) ∼ r, 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 CDM simulations. While the observed distribution of α does have a tail towards such extreme values, the derived value of α is found to depend on the spatial resolution of the rotation curves: α ≈ −1 is found only for the least well resolved galaxies. Even for these galaxies, our data are also consistent with constant density cores (α = 0) of modest (∼ 1 kpc) core radius, which can give the illusion of steep cusps when insufficiently resolved. Consequently, there is no clear evidence for a cuspy halo in any of the low surface brightness galaxies observed. Subject headings: galaxies: kinematics and dynamics — galaxies: fundamental parameters — dark matter

Modified Newtonian Dynamics (MOND):Observational Phenomenology and Relativistic Extensions

A wealth of astronomical data indicate the presence of mass discrepancies in the Universe. The motions observed in a variety of classes of extragalactic systems exceed what can be explained by the mass visible in stars and gas. Either (i) there is a vast amount of unseen mass in some novel form — dark matter — or (ii) the data indicate a breakdown of our understanding of dynamics on the relevant scales, or (iii) both. Here, we first review a few outstanding challenges for the dark matter interpretation of mass discrepancies in galaxies, purely based on observations and independently of any alternative theoretical framework. We then show that many of these puzzling observations are predicted by one single relation — Milgrom’s law — involving an acceleration constant a0 (or a characteristic surface density Σ† = a0/G) on the order of the square-root of the cosmological constant in natural units. This relation can at present most easily be interpreted as the effect of a single universal force law resulting from a modification of Newtonian dynamics (MOND) on galactic scales. We exhaustively review the current observational successes and problems of this alternative paradigm at all astrophysical scales, and summarize the various theoretical attempts (TeVeS, GEA, BIMOND, and others) made to effectively embed this modification of Newtonian dynamics within a relativistic theory of gravity.

High-Resolution Rotation Curves of Low Surface Brightness Galaxies. II. Mass Models

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 (ΛCDM). This failing becomes more severe as increasing allowance is made for stellar mass: Navarro, Frenk, & White (NFW) model fits require uncomfortably low mass-to-light ratios. In contrast, the maximum disk procedure does often succeed in predicting the inner shape of the rotation curves, but it requires uncomfortably large stellar mass-to-light ratios. The data do admit reasonable stellar population mass-to-light ratios if halos have cores rather than cusps.

The dark and visible matter content of low surface brightness disc galaxies

We present mass models of a sample of 19 low surface brightness (LSB) galaxies and compare the properties of their constituent mass components with those of a sample of high surface brightness (HSB) galaxies. We find that LSB galaxies are dark matter dominated. Their halo parameters are only slightly affected by assumptions on stellar mass-to-light ratios. Comparing LSB and HSB galaxies we find that mass models derived using the maximum disk hypothesis result in the disks of LSB galaxies having systematically higher stellar mass-to-light ratios than HSB galaxies of similar rotation velocity. This is inconsistent with all other available evidence on the evolution of LSB galaxies. We argue therefore that the maximum disk hypothesis does not provide a representative description of the LSB galaxies and their evolution. Mass models with stellar mass-to-light ratios determined by the colors and stellar velocity dispersions of galactic disks imply that LSB galaxies have dark matter halos that are more extended and less dense than those of HSB galaxies. Surface brightness is thus related to the halo properties. LSB galaxies are slowly evolving, low density and dark matter dominated galaxies.

HI Observations of Low Surface Brightness Galaxies: Probing Low Density Galaxies

We present Very Large Array (VLA) and Westerbork Synthesis Radio Telescope (WSRT) 21-cm HI observations of 19 late-type low surface brightness (LSB) galaxies. Our main findings are that these galaxies, as well as having low surface brightnesses, have low HI surface densities, about a factor of similar to 3 lower than in normal late-type galaxies. We show that LSB galaxies in some respects resemble the outer parts of late-type normal galaxies, but may be less evolved. LSB galaxies are more gas-rich than their high surface brightness counterparts, The rotation curves of LSB galaxies rise more slowly than those of HSB galaxies of the same luminosity, with amplitudes between 50 and 120 km s(-1), and are often still increasing at the outermost measured point. The shape of the rotation curves suggests that LSB galaxies have low matter surface densities, We use the average total mass surface density of a galaxy as a measure for the evolutionary state, and show that LSB galaxies are among the least compact, least evolved galaxies. We show that both M(HI)/L(B) and M(dyn)/L(B) depend strongly on central surface brightness, consistent with the surface brightness-mass-to-light ratio relation required by the Tully-Fisher relation. LSB galaxies are therefore slowly evolving galaxies, and may well be low surface density systems in all respects.

Testing the dark matter hypothesis with low surface brightness galaxies and other evidence

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 terms of dark matter leads to troublesome fine-tuning problems. Different observations require contradictory amounts of dark matter. Structure formation theories are as yet far from able to explain the observations.