arXiv subjects

Jim Schombert

Publications and source records attributed to Jim Schombert.

10 recordsLinked to original sources

arXiv2026

The Baryonic Mass-Halo Mass Relation of Extragalactic Systems

We combine data for extragalactic systems to quantify a relation between the observed baryonic mass $M_b$ and the enclosed dynamical mass $M_{200}$ inferred from kinematics or gravitational lensing. Our sample covers nine orders of magnitude in baryonic mass, including galaxies with kinematic or weak gravitational lensing data and groups and clusters of galaxies with new gravitational lensing data. For rich clusters with $M_b > 10^{14}\;\mathrm{M}_{\odot}$, the observed baryon fraction is consistent with the cosmic value, $f_b = 0.157$. For lower masses, the baryon fraction decreases systematically with mass. The variation is well described by $M_b/M_{200} = f_b \tanh(M_b/M_0)^{1/4}$ with $M_0 \approx 5 \times 10^{13}\;\mathrm{M}_{\odot}$. This relation is qualitatively similar to stellar mass-halo mass relations derived from abundance matching, but exhibits less scatter.

astro-ph.GA↗

arXiv2021

The Baryonic Tully-Fisher Relation in the Local Group and the Equivalent Circular Velocity of Pressure Supported Dwarfs

We explore the Baryonic Tully-Fisher Relation in the Local Group. Rotationally supported Local Group galaxies adhere precisely to the relation defined by more distant galaxies. For pressure supported dwarf galaxies, we determine the scaling factor $β_c$ that relates their observed velocity dispersion to the equivalent circular velocity of rotationally supported galaxies of the same mass such that $V_o = β_c σ_*$. For a typical mass-to-light ratio $Υ_* = 2\;\mathrm{M}_{\odot}/\mathrm{L}_{\odot}$ in the $V$-band, we find that $β_c = 2$. More generally, $\log β_c = 0.25 \log Υ_* +0.226$. This provides a common kinematic scale relating pressure and rotationally supported dwarf galaxies.

astro-ph.GA↗

arXiv2019

Dynamical Regularities in Galaxies

Galaxies are observed to obey a strict set of dynamical scaling relations. We review these relations for rotationally supported disk galaxies spanning many decades in mass, surface brightness, and gas content. The behavior of these widely varied systems can be summarized with a handful of empirical laws connected by a common acceleration scale.

astro-ph.GA↗

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.

astro-ph.GA↗

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.

astro-ph.GA↗

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.

astro-ph.GA↗

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 $3.6μ$.

astro-ph.GA↗

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.

astro-ph.CO↗

arXiv2000

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 < Vc < 300 km/s. We find a clear break in the optical Tully-Fisher relation: field galaxies with Vc < 90 km/s fall below the relation defined by brighter galaxies. These faint galaxies are however very gas rich; adding in the gas mass and plotting baryonic disk mass Md = M* + Mg in place of luminosity restores a 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 = A Vc^4.

astro-ph↗

arXiv1995

The Morphology of Low Surface Brightness Disk Galaxies

We present $UBVI$ and H$α$ 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 which appear merely as fuzzy blobs on photographic plates. This causes the stages of the Hubble sequence to be nonlinear in the sense that large morphological type distinctions are made between high surface brightness spirals when only small physical differences exist, and small morphological distinctions are made between low surface brightness galaxies even when large physical differences exist. Many LSB galaxies lack the old red disk conspicuous in higher surface brightness spirals. Their morphology is strikingly similar in all bands from $U$ to $I$, suggesting farily homogeneous stellar populations lacking a well developed giant branch. These properties, together with their very blue colors, suggest that LSB galaxies are relatively younger than their high surface brightness counterparts. A few of these LSB galaxies appear to be very young ($\simlt 1$~Gyr), and as such may represent local examples of protogalaxies.

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