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Stacy McGaugh

Publications and source records attributed to Stacy McGaugh.

At least 73 records · Page 4Linked to original sources
arXiv2013

Challenges for Lambda-CDM and MOND

The Universe on large scales is well described by the Lambda-CDM cosmological model. There however remain some heavy clouds on our global understanding, especially on galaxy scales, which we review here. While some of these clouds might perhaps disappear through small compensatory adjustments of the model, such as changing the mass of the dark matter particles or accounting better for baryonic physics, others should rather be taken as strong indications that the physics of the dark sector is, at the very least, much richer and complex than currently assumed, and that our understanding of gravity and dynamics might also be at play. For instance, the empirically well-tested MOND phenomenology in galaxies, whatever its final explanation, should be understood in any model of galaxy formation and dynamics. Current alternatives to Lambda-CDM however bring with them many unsolved questions and challenges.

astro-ph.CO↗
arXiv2012

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 (or a characteristic surface density) of 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.

astro-ph.CO↗
arXiv2011

The Baryonic Tully-Fisher Relation of Gas Rich Galaxies as a Test of LCDM and MOND

The Baryonic Tully-Fisher Relation (BTFR) is an empirical relation between baryonic mass and rotation velocity in disk galaxies. It provides tests of galaxy formation models in LCDM and of alternative theories like MOND. Observations of gas rich galaxies provide a measure of the slope and normalization of the BTFR that is more accurate (if less precise) than that provided by star dominated spirals, as their masses are insensitive to the details of stellar population modeling. Recent independent data for such galaxies are consistent with Mb = AVf^4 with A = 47+/-6 Msun (km/s)^-4. This is equivalent to MOND with $a_0 = 1.3 +/- 0.3 A/s/s. The scatter in the data is consistent with being due entirely to observational uncertainties. It is unclear why the physics of galaxy formation in LCDM happens to pick out the relation predicted by MOND. We introduce a feedback efficacy parameter E to relate halo properties to those of the galaxies they host. E correlates with star formation rate and gas fraction in the sense that galaxies that have experienced the least star formation have been most impacted by feedback.

astro-ph.CO↗
arXiv2011

Stellar Populations and the Star Formation Histories of LSB Galaxies: I. Optical and H-alpha Imaging

This paper presents optical and H-alpha imaging for a large sample of LSB galaxies selected from the PSS-II catalogs (Schombert et. al 1992). As noted in previous work, LSB galaxies span a range of luminosities (-10 > M_V > -20) and sizes (0.3 kpc < R_V25 < 10 kpc), although they are consistent in their irregular morphology. Their H-alpha luminosities (L(H-alpha) range from 10^36 to 10^41 ergs s^-1 (corresponding to a range in star formation, using canonical prescriptions, from 10^-5 to 1 M_solar yr^-1). Although their optical colors are at the extreme blue edge for galaxies, they are similar to the colors of dwarf galaxies (van Zee 2001) and gas-rich irregulars (Hunter & Elmegreen 2006). However, their star formation rates per unit stellar mass are a factor of ten less than other galaxies of the same baryonic mass, indicating that they are not simply quiescent versions of more active star forming galaxies. This paper presents the data, reduction techniques and new philosophy of data storage and presentation. Later papers in this series will explore the stellar population and star formation history of LSB galaxies using this dataset.

astro-ph.CO↗
arXiv2011

Missing the Point - a Brief Reply to Foreman & Scott and Gnedin

In recent postings, Foreman & Scott and Gnedin criticize my work on the baryonic Tully-Fisher relation (BTFR) of gas rich galaxies as tests of MOND and LCDM (McGaugh 2011a,b). These criticisms are rather redundant, as they mostly rehash material I have already discussed. Gnedin is concerned with explaining away the problem of the apparently missing baryons with ionized gas. This is one hypothetical possibility that I have previously discussed (McGaugh et al. 2010; McGaugh & Wolf 2010; McGaugh 2011b). Foreman & Scott claim to find a difference between the acceleration scale that they measure and that I measure from the same data, but the apparent difference results simply from their failure to account for the well known fact (Binney & Tremaine 1987) that flattened disks rotate faster than the equivalent spherical mass distribution (as spelled out for this application in McGaugh 2011b). They further argue that the the intrinsic scatter in the BTFR is merely tiny rather than zero. This difference is within the uncertainty of the uncertainties, so confirms rather than refutes my result. Worse, these papers miss the basic point. Why does MOND have any predictions come true? Why does it provide such an effective and economical description of galaxy dynamics? They do nothing to address these deeper questions. One can imagine that the apparently MONDian behavior of galaxies is merely an approximation to some phenomenology that emerges in the context of LCDM, but we have yet to demonstrate how this occurs. Until we confront this issue seriously, and confirm the existence of non-baryonic cold dark matter in the laboratory, it is reasonable to remain skeptical of LCDM as well as of MOND.

astro-ph.CO↗
arXiv2010

Contradiction between strong lensing statistics and a feedback solution to the cusp/core problem

Standard cosmology has many successes on large scales, but faces some fundamental difficulties on small, galactic scales. One such difficulty is the cusp/core problem. High resolution observations of the rotation curves for dark matter dominated low surface brightness (LSB) galaxies imply that galactic dark matter halos have a density profile with a flat central core, whereas N-body structure formation simulations predict a divergent (cuspy) density profile at the center. It has been proposed that this problem can be resolved by stellar feedback driving turbulent gas motion that erases the initial cusp. However, strong gravitational lensing prefers a cuspy density profile for galactic halos. In this paper, we use the most recent high resolution observations of the rotation curves of LSB galaxies to fit the core size as a function of halo mass, and compare the resultant lensing probability to the observational results for the well defined combined sample of the Cosmic Lens All-Sky Survey (CLASS) and Jodrell Bank/Very Large Array Astrometric Survey (JVAS). The lensing probabilities based on such density profiles are too low to match the observed lensing in CLASS/JVAS. High baryon densities in the galaxies that dominate the lensing statistics can reconcile this discrepancy, but only if they steepen the mass profile rather than making it more shallow. This places contradictory demands upon the effects of baryons on the central mass profiles of galaxies.

astro-ph↗
arXiv2009

Baryons and Their Halos

Galaxies are composed of baryonic stars and gas embedded in dark matter halos. Here I briefly review two aspects of the connection between baryons and their halos. (1) The observed baryon content of galaxies falls short of the cosmic baryon fraction by an amount that varies systematically with mass. Where these missing baryons now reside is unclear. (2) The characteristic acceleration in disk galaxies correlates strongly with their baryonic mass surface density. This implies a close coupling between the gravitational dynamics, which is presumably dominated by dark matter, and the purely baryonic components of galaxies.

astro-ph.GA↗
arXiv2008

Milky Way Mass Models and MOND

Using the Tuorla-Heidelberg model for the mass distribution of the Milky Way, I determine the rotation curve predicted by MOND. The result is in good agreement with the observed terminal velocities interior to the solar radius and with estimates of the Galaxy's rotation curve exterior thereto. There are no fit parameters: given the mass distribution, MOND provides a good match to the rotation curve. The Tuorla-Heidelberg model does allow for a variety of exponential scale lengths; MOND prefers short scale lengths in the range 2.0 to 2.5 kpc. The favored value of scale length depends somewhat on the choice of interpolation function. There is some preference for the `simple' interpolation function as found by Famaey & Binney. I introduce an interpolation function that shares the advantages of the simple function on galaxy scales while having a much smaller impact in the solar system. I also solve the inverse problem, inferring the surface mass density distribution of the Milky Way from the terminal velocities. The result is a Galaxy with `bumps and wiggles' in both its luminosity profile and rotation curve that are reminiscent of those frequently observed in external galaxies.

astro-ph↗
arXiv2006

CO Detection and Millimeter Continuum Emission from Low Surface Brightness Galaxies

We present BIMA and IRAM CO(1--0) observations of seven low surface brightness (LSB) galaxies, including three large spirals with faint disks but prominent bulges, and four relatively small LSB galaxies with irregular disks. The giant LSB galaxies are UGC 5709, UGC 6614 and F568-6 (Malin2). The smaller LSB galaxies are NGC 5585, UGC 4115, UGC 5209 and F583-1. The galaxies were selected based on their relatively high metallicity and apparent signs of star formation in their disks. The BIMA maps suggested the presence of molecular gas in 2 of the giant LSB galaxies, F568-6 and UGC 6614. Using the 30m IRAM telescope we detected CO (1--0) emission in the disks of both galaxies and in the nucleus of F568-6. The molecular gas in these galaxies is clearly offset from the nucleus and definitely associated with the LSB disk. In addition we also detected a millimeter continuum source in the center of UGC 6614. When compared with VLA 1.5 GHz observations of the galaxy, the emission was found to have a flat spectrum indicating that the millimeter continuum emission is most likely due to an active galactic nucleus (AGN) in the galaxy. Our results show that giant LSB spirals may contain significant quantities of molecular gas in their disks and also harbor radio bright AGN in their centers.

astro-ph↗
arXiv2006

Observational Constraints on the Acceleration Discrepancy Problem

I review the data relating to the appearance of the missing mass problem at a particular acceleration scale. Rotation curves are examined in detail, with emphasis on the empirical connection between baryonic and total mass distributions. Other lines of evidence are also considered, including disk stability, dwarf spheroidals, giant ellipticals, clusters of galaxies, the microwave background, BBN, and the growth of large scale structure.

astro-ph↗
arXiv2005

Some Systematic Properties of Rotation Curves

The rotation curves of spiral galaxies obey strong scaling relations. These include the Tully-Fisher and baryonic Tully-Fisher relations, and the mass discrepancy--acceleration relation. These relations can be used to place constraints on the mass-to-light ratios of stars. Once the stellar mass is constrained, the distribution of dark matter follows. The shape of the dark matter distribution is consistent with the expectations of NFW halos exterior to 1 kpc, but the amplitude is wrong. This is presumably related to the long-standing problem of the normalization of the Tully-Fisher relation and may imply a downturn in the amplitude of the power spectrum at small scales. More fundamentally, the persistent success of MOND remains a troubling fact.

astro-ph↗
arXiv2005

The Balance of Dark and Luminous Mass in Rotating Galaxies

A fine balance between dark and baryonic mass is observed in spiral galaxies. As the contribution of the baryons to the total rotation velocity increases, the contribution of the dark matter decreases by a compensating amount. This poses a fine-tuning problem for \LCDM galaxy formation models, and may point to new physics for dark matter particles or even a modification of gravity.

astro-ph↗
arXiv2004

Confrontation of MOND Predictions with WMAP First Year Data

I present a model devoid of non-baryonic cold dark matter (CDM) which provides an acceptable fit to the WMAP data for the power spectrum of temperature fluctuations in the cosmic background radiation (CBR). An a priori prediction of such no-CDM models was a first-to-second peak amplitude ratio A1:2 = 2.4. WMAP measures A1:2 = 2.34 +/- 0.09. The baryon content is the dominant factor in fixing this ratio; no-CDM models which are consistent with the WMAP data are also consistent with constraints on the baryon density from the primordial abundances of 2H, 4He, and 7Li. However, in order to match the modest width of the acoustic peaks observed by WMAP, a substantial neutrino mass is implied: m(nu) ~ 1 eV. Even with such a heavy neutrino, structure is expected to form rapidly under the influence of MOND. Consequently, the epoch of reionization should occur earlier than is nominally expected in LCDM. This prediction is realized in the polarization signal measured by WMAP. An outstanding test is in the amplitude of the third acoustic peak. Experiments which probe high-L appear to favor a third peak which is larger than predicted by the no-CDM model.

astro-ph↗
arXiv2001

Galaxy Masses: Disks and Their Halos

I review what we currently do and do not know about the masses of disk galaxies and their dark matter halos. The prognosis for disks is good: the asymptotic rotation velocity provides a good indicator of total disk mass. The prognosis for halos is bad: cuspy halos provide a poor description of the data, and the total mass of individual dark matter halos remains ill-constrained.

astro-ph↗
arXiv2001

High-resolution rotation curves of LSB galaxies: Mass Models

We present mass models for a sample of 30 high-resolution rotation curves of low surface brightness (LSB) galaxies. We fit both pseudo-isothermal (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 pseudo-isothermal 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 $Λ$CDM. This failing becomes more severe as increasing allowance is made for stellar mass: NFW 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 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.

astro-ph↗
arXiv2001

High-resolution rotation curves of low surface brightness galaxies: Data

We present long slit Halpha 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" to 2" instead of 13" to 45"). The improved resolution and accuracy of the data extend and strengthen the scientific conclusions previously inferred from 21 cm data.

astro-ph↗
arXiv2000

Galaxy Dynamics and the Second Peak: Cold Dark Matter?

The amplitude of the second peak in the angular power spectrum of the cosmic microwave background radiation is constrained to be small by recent experiments like Boomerang. This is surprising in the context of the LambdaCDM model, which predicted a large second peak. However, this result is expected if CDM does not exist. The observed shape of the power spectrum was accurately predicted (before the fact) by a model motivated by the surprising recent successes of the modified dynamics (MOND) hypothesized by Milgrom.

hep-ph↗