arXiv subjects

Stacy McGaugh

Publications and source records attributed to Stacy McGaugh.

At least 55 records · Page 3Linked to original sources

arXiv2018

A Precise Milky Way Rotation Curve Model for an Accurate Galactocentric Distance

I provide a model rotation curve for the Milky Way that matches the details of the terminal velocity curve normalized to the Galactocentric distance $R_0 = 8.122$ kpc obtained by the GRAVITY collaboration and the corresponding circular speed of the LSR $Θ_0 = 233.3$ km/s. The model provides a numerical representation of the azimuthally averaged radial run of the gravitational potential of each mass component of the Galaxy (bulge-bar, stellar disk, gas disk, and dark matter) as represented by the rotation curve of each. It provides precise estimates of quantities like the stellar mass of the Galaxy ($6.16 \pm 0.31 \times 10^{10}\;\mathrm{M}_{\odot}$) and the local density of dark matter ($ρ_{DM}(R_0) = 6.76^{+0.08}_{-0.14} \times 10^{-3}\; \mathrm{M}_{\odot}\,\mathrm{pc}^{-3} = 0.257^{+0.003}_{-0.005}\; \mathrm{GeV}\,\mathrm{cm}^{-3}$). The dark matter density implied by the radial force is less than that found in many studies of the vertical force, perhaps indicating that the usual assumption of a spherical dark matter halo is no longer adequate.

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arXiv2018

Predictions for the sky-averaged depth of the 21cm absorption signal at high redshift in cosmologies with and without non-baryonic cold dark matter

We consider the 21cm absorption signal expected at high redshift in cosmologies with and without non-baryonic cold dark matter. The expansion of the early universe decelerates strongly with dark matter, but approximately coasts without it. This results in a different path length across the epochs when absorption is expected, with the consequence that the absorption is predicted to be a factor of $\sim 2$ greater without dark matter than with it. Observation of such a signal would motivate consideration of extended theories of gravity in lieu of dark matter.

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arXiv2018

Stellar Feedback and the Energy Budget of Late-Type Galaxies: Missing Baryons and Core Creation

In a $Λ$CDM cosmology, galaxy formation is a globally inefficient process: it is often the case that far fewer baryons are observed in galaxy disks than expected from the cosmic baryon fraction. The location of these "missing baryons" is unclear. By fitting halo profiles to the rotation curves of galaxies in the SPARC data set, we measure the "missing baryon" mass for individual late-type systems. Assuming that haloes initially accrete the cosmological baryon fraction, we show that the maximum energy available from supernovae is typically not enough to completely eject these "missing baryons" from a halo, but it is often sufficient to heat them to the virial temperature. The energy available from supernovae has the same scaling with galaxy mass as the energy needed to heat or eject the "missing baryons", indicating that the coupling efficiency of the feedback to the ISM may be constant with galaxy virial mass. We further find that the energy available from supernova feedback is always enough to convert a primordial cusp into a core and has magnitude consistent with what is required to heat the "missing baryons" to the virial temperature. Taking a census of the baryon content of galaxies with ${\rm 10^9<M_{vir}/M_{\odot}<10^{12}}$ reveals that $\sim86\%$ of baryons are likely to be in a hot phase surrounding the galaxies and possibly observable in the X-ray, $\sim7\%$ are in the form of cold gas, and $\sim7\%$ are in stars.

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arXiv2018

A New Algorithm to Quantify Maximum Discs in Galaxies

Maximum disc decompositions of rotation curves place a dynamical upper limit to the mass attributable to stars in galaxies. The precise definition of this term, however, can be vague and varies in usage. We develop an algorithm to robustly quantify maximum-disc mass models and apply it to 153 galaxies from the SPARC database. Our automatic procedure recovers classic results from manual decompositions. High-mass, high-surface-brightness galaxies have mean maximum-disc mass-to-light ratios of $\sim 0.7 \;{\mathrm{M}_\odot}/{\mathrm{L}_\odot}$ in the Spitzer 3.6 $μ$m band, which are close to the expectations from stellar population models, suggesting that these galaxies are nearly maximal. Low-mass, low-surface-brightness galaxies have very high maximum-disc mass-to-light ratios (up to 10 $\mathrm{M}_\odot/\mathrm{L}_\odot$), which are unphysical for standard stellar population models, confirming they are sub-maximal. The maximum-disc mass-to-light ratios are more closely correlated with surface brightness than luminosity. The mean ratio between baryonic and observed velocity at the peak of the baryonic contribution is $\mathrm{V}_{\mathrm{bar}}/{\mathrm{V}_\mathrm{p}} \approx 0.88$, but correlates with surface brightness, so it is unwise to use this mean value to define the maximum disc concept. Our algorithm requires no manual intervention and could be applied to large galaxy samples from future HI surveys with Apertif, Askap, and SKA.

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arXiv2018

Strong Hydrogen Absorption at Cosmic Dawn: the Signature of a Baryonic Universe

The recently reported detection of redshifted 21cm absorption at $z \approx 17$ is a significant advance in the exploration of the cosmic dark ages. The observed signal ($T_{\mathrm{21}} \approx -0.5$ K with the limit $T_{\mathrm{21}} < -0.3$ K at 99\% c.l.) is anomalously strong for $Λ$CDM, which predicts at most $-0.24$ K. Here I show that the strong observed signal is expected in a purely baryonic universe.

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arXiv2017

The Star Forming Main Sequence of Dwarf Low Surface Brightness Galaxies

We explore the star forming properties of late type, low surface brightness (LSB) galaxies. The star forming main sequence (SFR-$M_*$) of LSB dwarfs has a steep slope, indistinguishable from unity ($1.04 \pm 0.06$). They form a distinct sequence from more massive spirals, which exhibit a shallower slope. The break occurs around $M_* \approx 10^{10}\;M_{\odot}$, and can also be seen in the gas mass-stellar mass plane. The global Kennicutt-Schmidt law (SFR-$M_g$) has a slope of $1.47 \pm 0.11$ without the break seen in the main sequence. There is an ample supply of gas in LSB galaxies, which have gas depletion times well in excess of a Hubble time, and often tens of Hubble times. Only $\sim 3\%$ of this cold gas need be in the form of molecular gas to sustain the observed star formation. In analogy with the faint, long-lived stars of the lower stellar main sequence, it may be appropriate to consider the main sequence of star forming galaxies to be defined by thriving dwarfs (with $M_* < 10^{10}\;M_{\odot}$) while massive spirals (with $M_* > 10^{10}\;M_{\odot}$) are weary giants that constitute more of a turn-off population.

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arXiv2016

The Radial Acceleration Relation in Rotationally Supported Galaxies

We report a correlation between the radial acceleration traced by rotation curves and that predicted by the observed distribution of baryons. The same relation is followed by 2693 points in 153 galaxies with very different morphologies, masses, sizes, and gas fractions. The correlation persists even when dark matter dominates. Consequently, the dark matter contribution is fully specified by that of the baryons. The observed scatter is small and largely dominated by observational uncertainties. This radial acceleration relation is tantamount to a natural law for rotating galaxies.

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arXiv2015

Comment on "Evidence for dark matter in the inner Milky Way"

This is a brief rebuttal to arXiv:1502.03821, which claims to provide the first observational proof of dark matter interior to the solar circle. We point out that this result is not new, and can be traced back at least a quarter century.

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arXiv2015

Weighing Galaxy Disks with the Baryonic Tully-Fisher Relation

We estimate the stellar masses of disk galaxies with two independent methods: a photometrically self-consistent color$-$mass-to-light ratio relation (CMLR) from population synthesis models, and the Baryonic Tully-Fisher relation (BTFR) calibrated by gas rich galaxies. These two methods give consistent results. The CMLR correctly converts distinct Tully-Fisher relations in different bands into the same BTFR. The BTFR is consistent with $M_b \propto V_f^4$ over nearly six decades in mass, with no hint of a change in slope over that range. The intrinsic scatter in the BTFR is negligible, implying that the IMF of disk galaxies is effectively universal. The gas rich BTFR suggests an absolute calibration of the stellar mass scale that yields nearly constant mass-to-light ratios in the near-infrared (NIR): $0.57\;M_{\odot}/L_{\odot}$ in $K_s$ and $0.45\;M_{\odot}/L_{\odot}$ at $3.6μ$. There is only modest intrinsic scatter ($\sim 0.12$ dex) about these typical values. There is no discernible variation with color or other properties: the NIR luminosity is a good tracer of stellar mass.

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arXiv2014

Stellar Populations and the Star Formation Histories of LSB Galaxies: III. Stellar Population Models

A series of population models are designed to explore the star formation history of gas-rich, low surface brightness (LSB) galaxies. LSB galaxies are unique in having properties of very blue colors, low H$α$ emission and high gas fractions that indicated a history of constant star formation (versus the declining star formation models used for most spirals and irregulars). The model simulations use an evolving multi-metallicity composite population that follows a chemical enrichment scheme based on Milky Way observations. Color and time sensitive stellar evolution components (i.e., BHB, TP-AGB and blue straggler stars) are included, and model colors are extended into the Spitzer wavelength regions for comparison to new observations. In general, LSB galaxies are well matched to the constant star formation scenario with the variation in color explained by a fourfold increase/decrease in star formation over the last 0.5 Gyrs (i.e., weak bursts). Early-type spirals, from the S$^4$G sample, are better fit by a declining star formation model where star formation has decreased by 40% in the last 12 Gyrs.

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arXiv2014

Color--Mass-to-Light Ratio Relations for Disk Galaxies

We combine Spitzer $3.6μ$ observations of a sample of disk galaxies spanning over 10 magnitudes in luminosity with optical luminosities and colors to test population synthesis prescriptions for computing stellar mass. Many commonly employed models fail to provide self-consistent results: the stellar mass estimated from the luminosity in one band can differ grossly from that of another band for the same galaxy. Independent models agree closely in the optical ($V$-band), but diverge at longer wavelengths. This effect is particularly pronounced in recent models with substantial contributions from TP-AGB stars. We provide revised color--mass-to-light ratio relations that yield self-consistent stellar masses when applied to real galaxies. The $B-V$ color is a good indicator of the mass-to-light ratio. Some additional information is provided by $V-I$, but neither it nor $J-K_s$ are particularly useful for constraining the mass-to-light ratio on their own. In the near-infrared, the mass-to-light ratio depends weakly on color, with typical values of $0.6\; \mathrm{M}_{\odot}/\mathrm{L}_{\odot}$ in the $K_s$-band and $0.47\; \mathrm{M}_{\odot}/\mathrm{L}_{\odot}$ at $3.6μ$.

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arXiv2014

The Stellar Masses of Disk Galaxies and the Calibration of Color-Mass to Light Ratio Relations

We present new Spitzer 3.6 micron observations of a sample of disk galaxies spanning over 10 magnitudes in luminosity and ranging in gas fraction from ~10% to over 90%. We use these data to test population synthesis prescriptions for computing stellar mass. Many commonly employed models fail to provide self-consistent stellar masses in the sense that the stellar mass estimated from the optical luminosity typically exceeds that estimated from the near-infrared (NIR) luminosity. This problem is present in models both with and without TP-AGB stars, but is more severe in the former. Self-consistency can be achieved if NIR mass-to-light ratios are approximately constant with a mean value near 0.5 Msun/Lsun at 3.6 microns. We use the Baryonic Tully-Fisher relation calibrated by gas rich galaxies to provide an independent estimate of the color-mass to light ratio relation. This approach also suggests that the typical 3.6 micron mass-to-light ratio is 0.5 (0.65 in the K band) for rotationally supported galaxies. These values are consistent with a Kroupa IMF.

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arXiv2014

Stellar Populations and the Star Formation Histories of LSB Galaxies: IV Spitzer Surface Photometry of LSB Galaxies

Surface photometry at 3.6$μ$m is presented for 61 low surface brightness (LSB) galaxies ($μ_o < 19$ 3.6$μ$m mag arcsecs$^{-2}$). The sample covers a range of luminosity from $-$11 to $-$22 in $M_{3.6}$ and size from 1 to 25 kpc. The morphologies in the mid-IR are comparable to those in the optical with 3.6$μ$m imaging reaches similar surface brightness depth as ground-based optical imaging. A majority of the resulting surface brightness profiles are single exponential in shape with very few displaying upward or downward breaks. The mean $V-3.6$ color of LSB is 2.3 with a standard deviation of 0.5. Color-magnitude and two color diagrams are well matched to models of constant star formation, where the spread in color is due to small changes in the star formation rate (SFR) over the last 0.5 Gyrs as also suggested by the specific star formation rate measured by H$α$.

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arXiv2013

The MOND phenomenology

The Lambda-CDM cosmological model is succesful at reproducing various independent sets of observations concerning the large-scale Universe. This model is however currently, and actually in principle, unable to predict the gravitational field of a galaxy from it observed baryons alone. Indeed the gravitational field should depend on the relative contribution of the particle dark matter distribution to the baryonic one, itself depending on the individual assembly history and environment of the galaxy, including a lot of complex feedback mechanisms. However, for the last thirty years, Milgrom's formula, at the heart of the MOND paradigm, has been consistently succesful at predicting rotation curves from baryons alone, and has been resilient to all sorts of observational tests on galaxy scales. We show that the few individual galaxy rotation curves that have been claimed to be highly problematic for the predictions of Milgrom's formula, such as Holmberg II or NGC 3109, are actually false alarms. We argue that the fact that it is actually possible to predict the gravitational field of galaxies from baryons alone presents a challenge to the current Lambda-CDM model, and may indicate a breakdown of our understanding of gravitation and dynamics, and/or that the actual lagrangian of the dark sector is very different and richer than currently assumed. On the other hand, it is obvious that any alternative must also, in fine, reproduce the successes of the Lambda-CDM model on large scales, where this model is so well-tested that it presents by itself a challenge to any such alternative.

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arXiv2013

Andromeda Dwarfs in Light of MOND. II. Testing Prior Predictions

We employ recently published measurements of the velocity dispersions in the newly discovered dwarf satellite galaxies of Andromeda to test our previously published predictions of this quantity. The data are in good agreement with our specific predictions for each dwarf made a priori with MOND, with reasonable stellar mass-to-light ratios, and no dark matter, while Newtonian dynamics point to quite large mass discrepancies in these systems. MOND distinguishes between regimes where the internal field of the dwarf, or the external field of the host, dominates. The data appear to recognize this distinction, which is a unique feature of MOND not explicable in LCDM.

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arXiv2013

Stellar Populations and the Star Formation Histories of LSB Galaxies: II. HII Regions

The luminosities, colors and Hα emission for 429 HII regions in 54 LSB galaxies are presented. While the number of HII regions per galaxy is lower in LSB galaxies compared to star-forming irregulars and spirals, there is no indication that the size or luminosity function of HII regions differs from other galaxy types. The lower number of HII regions per galaxy is consistent with their lower total star formation rates. The fraction of total $L_{Hα}$ contributed by HII regions varies from 10 to 90% in LSB galaxies (the rest of the H$α$ emission being associated with a diffuse component) with no correlation with galaxy stellar or gas mass. Bright HII regions have bluer colors, similar to the trend in spirals; their number and luminosities are consistent with the hypothesis that they are produced by the same HII luminosity function as spirals. Comparison with stellar population models indicates that the brightest HII regions in LSB galaxies range in cluster mass from a few $10^3 M_{\sun}$ (e.g., $ρ$ Oph) to globular cluster sized systems (e.g., 30 Dor) and that their ages are consistent with clusters from 2 to 15 Myrs old. The faintest HII regions are comparable to those in the LMC powered by a single O or B star. Thus, star formation in LSB galaxies covers the full range of stellar cluster mass.

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arXiv2013

Galaxy Cluster Bulk Flows and Collision Velocities in QUMOND

We examine the formation of clusters of galaxies in numerical simulations of a QUMOND cosmogony with massive sterile neutrinos. Clusters formed in these exploratory simulations develop higher velocities than those found in ΛCDM simulations. The bulk motions of clusters attain about 1000 km/s by low redshift, comparable to observations whereas ΛCDM simulated clusters tend to fall short. Similarly, high pairwise velocities are common in cluster-cluster collisions like the Bullet cluster. There is also a propensity for the most massive clusters to be larger in QUMOND and to appear earlier than in ΛCDM, potentially providing an explanation for 'pink elephants' like El Gordo. However, it is not obvious that the cluster mass function can be recovered.

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arXiv2013

Andromeda Dwarfs in Light of MOND

We compare the recently published velocity dispersions for seventeen Andromeda dwarf spheroidals with estimates of the MOND predictions, based on the luminosities of these dwarfs, with reasonable stellar M/L values, and no dark matter. We find that the two are consistent within the uncertainties. We further predict the velocity dispersions of another ten dwarfs for which only photometric data are currently available.

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