arXiv subjects

James Schombert

Publications and source records attributed to James Schombert.

43 records · Page 3Linked to original sources

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↗

arXiv2009

The Age of Ellipticals and the Color-Magnitude Relation

Using new narrowband color observations of early-type galaxies in clusters, we reconstruct the color-magnitude relation (CMR) with a higher degree of accuracy than previous work. We then use the spectroscopically determined ages and metallicities from three samples (Trager et al 2008, Thomas et al 2005, Gallazzi et al 2006), combined with multi-metallicity SED models, to compare predicted colors for galaxies with young ages (less than 8 Gyr) with the known CMR. We find that the CMR cannot by reproduced by the spectroscopically determined ages and metallicities in any of the samples despite the high internal accuracies to the spectroscopic indices. In contrast, using only the $<$Fe$>$ index to determine [Fe/H], and assuming a mean age of 12 Gyr for a galaxy's stellar population, we derive colors that exactly match not only the color zeropoint of the CMR but also its slope. We consider the source of young age estimates, the H$β$ index, and examine the conflict between red continuum colors and large H$β$ values in galaxy spectra. We conclude that our current understanding of stellar populations is insufficient to correctly interpret H$β$ values and that the sum of our galaxy observations supports an old and monolithic scenario of galaxy formation. This result has a devastating impact on every study that has used the H$β$ index to calculate galaxy age, as the use of the H$β$ versus MgFe diagram will result in incorrectly deduced young ages.

astro-ph.CO↗

arXiv2000

Gas Rich Dwarfs from the PSS-II III. HI Profiles and Dynamical Masses

We present Arecibo neutral hydrogen data on a sample of optically selected dwarf galaxies. The sample ranges in HI mass from 10^6 M_sun to 5x10^9 M_sun, with a mean of 7.9x10^8 M_sun. Using estimated HI radii, the HI surface densities range from 0.6 to 20 M_sun pc^-2, all well below the critical threshold for star formation (Kennicutt 1998). M_HI/L values of the LSB dwarfs range from 0.3 to 12 with a mean value of 2.0. Dynamical masses, calculated from the HI profile widths, range from 10^8 M_sun to 10^11 M_sun. There is a strong correlation between optical luminosity and dynamical mass for LSB dwarfs implying that the dark matter (whether baryonic or non-baryonic) follows the detectable baryonic matter.

astro-ph↗

arXiv1998

AGN Activity in Giant LSB Galaxies

A search of large, HI-rich disk galaxies finds a significantly higher fraction of low luminosity AGN signatures compared to other late-type galaxies. Approximately half of the galaxies selected in this sample have AGN-like behavior in their cores, the rest have HII nuclei resulting from simple star formation. Since AGN behavior is not evident in all the sample galaxies, which where selected by high gas mass, we speculate that it is the fuel flow rate that is the common feature between late-type LSB disks and other active nuclear galaxies.

astro-ph↗

arXiv1996

Gas Rich Dwarfs from the PSS-II I. Catalog and Characteristics

This project is a visual search for field dwarf galaxies using Second Palomar Sky Survey photographic plates. A morphologically selected sample of 310 objects yielded 145 detections of true dwarfs within a redshift search window of 0 to 10,000 km sec^-1. We confirm the low mass, dwarf nature of the same by comparison of luminosity, isophotal size, HI mass and HI profile width distribution of other dwarf samples. The goal of this project is to use these newly discovered dwarf galaxies to map large scale structure as a test of biased galaxy formation. Initial indicators are that the large scale distribution of dwarf galaxies is identical to bright, high mass galaxies, in contradiction to theory using biasing. The full analysis of the sample will be reported in the final paper of our series.

astro-ph↗

arXiv1996

Color Evolution from z=0 to z=1

Rest frame Stromgren photometry (3500A, 4100A, 4750A and 5500A) is presented for 509 galaxies in 17 rich clusters between z=0 and z=1 as a test of color evolution. Our observations confirm a strong, rest frame, Butcher-Oemler effect where the fraction of blue galaxies increases from 20% at z=0.4 to 80% at z=0.9. We also find that a majority of these blue cluster galaxies are composed of normal disk or post-starbursts systems based on color criteria. When comparing our colors to the morphological results from HST imaging, we propose that the blue cluster galaxies are a population of late-type, LSB objects who fade and are then destroyed by the cluster tidal field. After isolating the red objects from Butcher-Oemler objects, we have compared the mean color of these old, non-star forming objects with SED models in the literature as a test for passive galaxy evolution in ellipticals. We find good agreement with single burst models which predict a mean epoch of galaxy formation at z=5. Tracing the red envelope for ellipticals places the earliest epoch of galaxy formation at z=10.

astro-ph↗

arXiv1996

Photometric Signatures of Starbursts in Interacting Galaxies and the Butcher-Oemler Effect

This paper presents new and synthetic narrow band photometry of ellipticals, spirals, Seyferts and interacting galaxies in an attempt to identify the cause of the unusually high fraction of blue cluster galaxies in distant clusters (Butcher-Oemler effect). The properties and distribution of the low redshift sample specifically points to starbursts as the origin of the blue narrow band colors in interacting Arp galaxies. Comparison between theoretical models and multicolor diagrams, particularly 4000A break colors, indicates a photometric signature which differs from both normal disk galaxy star formation and nonthermal components. This photometric signature is absent for the Butcher-Oemler galaxies whose general color distribution, compared to present-day clusters, is consistent with a majority of the blue population involved in normal star formation rates (spiral-like) with the addition of a small fraction of bright, blue interacting/merger systems. This photometric picture of the Butcher- Oemler galaxies is in agreement with the morophological evidence from HST imaging.

astro-ph↗