arXiv subjects

James Schombert

Publications and source records attributed to James Schombert.

At least 37 records · Page 2Linked to original sources

arXiv2019

Tracing the Dynamical Mass in Galaxy Disks Using HI Velocity Dispersion and its Implications for the Dark Matter Distribution in Galaxies

We present a method to derive the dynamical mass of face-on galaxy disks using their neutral hydrogen (HI) velocity dispersion. We have applied the method to nearby, gas rich galaxies that have extended HI gas disks and have low inclinations. The galaxy sample includes 4 large disk galaxies; NGC628, NGC6496, NGC3184, NGC4214 and 3 dwarf galaxies DDO46, DDO63 and DDO187. We have used archival data from the THINGS and LITTLE THINGS surveys to derive the HI gas distributions and SPITZER mid-infrared images to determine the stellar disk mass distributions. We examine the disk dynamical and baryonic mass ratios in the extreme outer disks where there is HI gas but no visible stellar disk. We find that for the large galaxies the disk dynamical and Hi gas mass surface densities are comparable in the outer disks. But in the smaller dwarf galaxies, for which the total HI gas mass dominates the baryonic mass i.e. M(HI)>M(stars), the disk dynamical mass is much larger than the baryonic mass. For these galaxies there must either be a very low luminosity stellar disk which provides the vertical support for the HI gas disk or there is halo dark matter associated with their disks, which is possible if the halo has an oblate shape so that the inner part of the dark matter halo is concentrated around the disk. Our results are important for explaining the equilibrium of HI disks in the absence of stellar disks, and is especially important for gas rich, dwarf galaxies that appear to have significant dark matter masses associated with their disks.

astro-ph.GA↗

arXiv2019

The halo mass function of late-type galaxies from HI kinematics

We present an empirical method to measure the halo mass function (HMF) of galaxies. We determine the relation between the \hi\ line-width from single-dish observations and the dark matter halo mass ($M_{200}$) inferred from rotation curve fits in the SPARC database, then we apply this relation to galaxies from the \hi\ Parkes All Sky Survey (HIPASS) to derive the HMF. This empirical HMF is well fit by a Schecther function, and matches that expected in $Λ$CDM over the range $10^{10.5} < M_{200} < 10^{12}\;\mathrm{M}_{\odot}$. More massive halos must be poor in neutral gas to maintain consistency with the power law predicted by $Λ$CDM. We detect no discrepancy at low masses. The lowest halo mass probed by HIPASS, however, is just greater than the mass scale where the Local Group missing satellite problem sets in. The integrated mass density associated with the dark matter halos of \hi-detected galaxies sums to $Ω_{\rm m,gal} \approx 0.03$ over the probed mass range.

astro-ph.GA↗

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.

astro-ph.GA↗

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.

astro-ph.GA↗

arXiv2017

Colors of Dwarf Ellipticals from GALEX to WISE

Multi-color photometry is presented for a sample of 60 dwarf ellipticals (dE) selected by morphology. The sample uses data from GALEX, SDSS and WISE to investigate the colors in filters NUV, ugri and W1 (3.4mum). We confirm the blueward shift in the color-magnitude relation for dwarf ellipticals, compared to CMR for bright ellipticals, as seen in previous studies. However, we find the deviation in color across the UV to near-IR for dE's is a strong signal of a younger age for dwarf ellipticals, one that indicates decreasing mean age with lower stellar mass. Lower mass dE's are found to have mean ages of 4 Gyrs and mean [Fe/H] values of -1.2. Age and metallicity increase to the most massive dE's with mean ages similar to normal ellipticals (12 Gyrs) and their lowest metallicities ([Fe/H] = -0.3). Deduced initial star formation rates for dE's, combined with their current metallicities and central stellar densities, suggests a connection between field LSB dwarfs and cluster dE's, where the cluster environment halts star formation for dE's triggering a separate evolutionary path.

astro-ph.GA↗

arXiv2014

Super-massive black hole mass scaling relations

Using black hole masses which span 10^5 to 10^(10) solar masses, the distribution of galaxies in the (host spheroid stellar mass)-(black hole mass) diagram is shown to be strongly bent. While the core-Sersic galaxies follow a near-linear relation, having a mean M_(bh)/M_(sph) mass ratio of ~0.5%, the Sersic galaxies follow a near-quadratic relation: M_bh~M_sph^(2.22+\-0.58). This is not due to offset pseudobulges, but is instead an expected result arising from the long-known bend in the M_(sph)-sigma relation and the log-linear M_(bh)-sigma relation.

astro-ph.GA↗

arXiv2014

The Supermassive Black Hole Mass - Spheroid Stellar Mass Relation for Sérsic and Core-Sérsic Galaxies

We have examined the relationship between supermassive black hole mass (M$_{BH}$) and the stellar mass of the host spheroid (M$_{sph,*}$) for a sample of 75 nearby galaxies. To derive the spheroid stellar masses we used improved 2MASS K$_s$-band photometry from the ARCHANGEL photometry pipeline. Dividing our sample into core-Sérsic and Sérsic galaxies, we find that they are described by very different M$_{BH}$-M$_{sph,*}$ relations. For core-Sérsic galaxies - which are typically massive and luminous, with M$_{BH} \gtrsim 2x10^8 M_\odot$ - we find M$_{BH} \propto M_{sph,*}^{(0.97 \pm 0.14)}$, consistent with other literature relations. However, for the Sérsic galaxies - with typically lower masses, M$_{sph,*} \lesssim 3x10^10 M_\odot$ - we find M$_BH \propto M_{sph,*}^{(2.22 \pm 0.58)}$, a dramatically steeper slope that differs by more than 2 standard deviations. This relation confirms that, for Sérsic galaxies, M$_{BH}$ is not a constant fraction of M$_{sph,*}$. Sérsic galaxies can grow via the accretion of gas which fuels both star formation and the central black hole, as well as through merging. Their black hole grows significantly more rapidly than their host spheroid, prior to growth by dry merging events that produce core-Sérsic galaxies, where the black hole and spheroid grow in lock step. We have additionally compared our Sérsic M$_{BH}$-M$_{sph,*}$ relation with the corresponding relation for nuclear star clusters, confirming that the two classes of central massive object follow significantly different scaling relations.

astro-ph.CO↗

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.

astro-ph.GA↗

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$α$.

astro-ph.GA↗

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.

astro-ph.CO↗

arXiv2013

The Structure of Galaxies: II. Fitting Functions and Scaling Relations for Ellipticals

Surface photometry of 311 ellipticals from the 2MASS imaging database is analyzed with respect to the two most common fitting functions; the r^1/4 law and the Sersic r^1/n model. The advantages and disadvantages of each fitting function are examined. In particular, the r^1/4 law performs well in the middle regions, but is inadequate for the core (inner 5 kpcs) and the outer regions (beyond the half-light radius) which do not have r^1/4 shapes. It is found that the Sersic r^1/n model produce good fits to the core regions of ellipticals (r < r_half), but is an inadequate function for the entire profile of an elliptical from core to halo due to competing effects on the Sersic n index and the fact that the interior shape of an elliptical is only weakly correlated with its halo shape. In addition, there are a wide range of Sersic parameters that will equally describe the shape of the outer profile, degrading the Sersic models usefulness as a describer of the entire profile. Empirically determined parameters, such as half-light radius and total luminosity, have less scatter than fitting function variables. The scaling relations for ellipticals are often non-linear, but for ellipticals brighter than M_J < -23 the following structural relations are found: L propto r^0.8 \pm 0.1, L propto Sigma^-0.5 \pm 0.1 and Sigma propto r^-1.5 \pm 0.1.

astro-ph.CO↗

arXiv2012

The Structure of Galaxies: I. Surface Photometry Techniques

This project uses the 2MASS all-sky image database to study the structure of galaxies over a range of luminosities, sizes and morphological types. This first paper in this series will outline the techniques, reliability and data products to our surface photometry program. Our program will analyze all acceptable galaxies (meeting our criteria for isolation from companions and bright stars) from the Revised Shapley-Ames and Uppsala galaxy catalogs. Resulting photometry and surface brightness profiles are released using a transparent scheme of data storage which includes not only all the processed data but knowledge of the processing steps and calibrating parameters.

astro-ph.IM↗

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

Systematic Bias in 2MASS Galaxy Photometry

We report the discovery of a serious bias in galaxy photometry reported in the 2MASS Extended Source Catalog (Jarrett et al. 2000). Due to an undetermined flaw in the 2MASS surface photometry routines, isophotal and total magnitudes calculated by their methods underestimate the luminosity of galaxies from 10% to 40%. This is found to be due to incorrectly determined scalelengths and isophotal radii, which are used to define the aperture sizes for Kron and total fluxes. While 2MASS metric aperture luminosities are correct (and, thus, colors based on those apertures), comparison to other filters (e.g. optical) based on total magnitudes will produce erroneous results. We use our own galaxy photometry package (ARCHANGEL) to determine correct total magnitudes and colors using the same 2MASS images, but with a more refined surface brightness reduction scheme. Our resulting colors, and color-magnitude relation, are more in line with model expectations and previous pointed observations.

astro-ph.IM↗

arXiv2010

Nonlinear Psychometric Thresholds for Physics and Mathematics

We analyze 5 years of student records at the University of Oregon to estimate the probability of success (as defined by superior undergraduate record; sufficient for admission to graduate school) in Physics and Mathematics as a function of SAT-M score. We find evidence of a nonlinear threshold: below SAT-M score of roughly 600, the probability of success is very low. Interestingly, no similar threshold exists in other majors, such as Sociology, History, English or Biology, whether on SAT combined, SAT-R or SAT-M. Our findings have significant implications for the demographic makeup of graduate populations in mathematically intensive subjects, given the current distribution of SAT-M scores.

physics.ed-ph↗

arXiv2010

Introductory Astronomy as a Measure of Grade Inflation

We use four years of introductory astronomy scores to analyze the ability of the current population to perform college level work and measure the amount of grade inflation across various majors. Using an objective grading scale, one that is independent of grading curves, we find that 29% of intro astronomy students fail to meet minimal standards for college level work. Of the remaining students, 41% achieve satisfactory work, 30% achieve mastery of the topics. Intro astronomy scores correlate with SAT and college GPA. Sequential mapping of the objective grade scheme onto GPA finds that college grades are inflated by 0.2 for natural sciences majors, 0.3 for social sciences, professional schools and undeclared majors), 0.5 for humanities majors. It is unclear from the data whether grade inflation is due to easier grading curves or depression of course material. Experiments with student motivation tools indicates that poor student performance is due to deficiency in student abilities rather than social factors (such as study time or decreased interest in academics), i.e., more stringent admission standards would resolve grade inflation.

physics.ed-ph↗

arXiv2010

Data Mining the University: College GPA Predictions from SAT Scores

We analyze a data set comprised of academic records of undergraduates at the University of Oregon from 2000-2004. We find correlations of roughly 0.35 to 0.5 between SAT scores and upper division, in-major GPA (henceforth, GPA). Interestingly, low SAT scores do not preclude high performance in most majors. That is, the distribution of SAT scores after conditioning on high GPA (e.g., 3.5 or even 4.0) typically extends below 1000 (the average among test takers). We hypothesize that "overachievers" overcome cognitive deficits through hard work, and discuss to what extent they can be identified from high school records. Only a few majors seem to exhibit a "cognitive threshold" -- such that high GPA (mastery of the subject matter) is very unlikely below a certain SAT threshold (i.e., no matter how dedicated or hard working the student). Our results suggest that almost any student admitted to university can achieve academic success, if they work hard enough. In addition to our primary result, we find that the best predictor of GPA is a roughly equally weighted sum of SAT and high school GPA, measured in standard deviation units. Using a sub-population of honors college students, we can estimate how students at elite universities would fare at a typical state university, allowing us to comment on issues such as grade inflation. Finally, we observe that 1) SAT scores fluctuate little on retest (very high reliability), 2) SAT and GRE scores (where available) correlate at roughly 0.75 (consistent with the notion that both tests measure a stable general cognitive ability) and 3) the SAT distribution of students that obtained a degree does not differ substantially from that of the entering class.

physics.ed-ph↗

arXiv2009

Network Tools for Astronomical Data Retrieval

The first step in a science project is the acquisition and understanding of the relevant data. This paper outlines the results of a project to design and test network tools specifically oriented at retrieving astronomical data. The tools range from simple data transfer methods to more complex browser-emulating scripts. When integrated with a defined sample or catalog, these scripts provide seamless techniques to retrieve and store data of varying types. Examples are given on how these tools can be used to leapfrog from website to website to acquire multi-wavelength datasets. This project demonstrates the capability to use multiple data websites, in conjunction, to perform the type of calculations once reserved for on-site datasets.

astro-ph.IM↗