arXiv subjects

Sheila J. Kannappan

Publications and source records attributed to Sheila J. Kannappan.

41 records · Page 3Linked to original sources

arXiv2004

Tools for Identifying Spurious Luminosity Offsets in Tully-Fisher Studies: Application at Low Redshift and Implications for High Redshift

Studies of high-redshift galaxies usually interpret offsets from the Tully- Fisher (TF) relation as luminosity evolution. However, apparent luminosity offsets may actually reflect rotation curve anomalies such as strong asymmetries or radial truncation, due to galaxy interactions or low S/N data. Here we analyze emission-line rotation curves from two low-z samples: the Close Pairs Survey, which contains many interacting galaxies, and the Nearby Field Galaxy Survey (NFGS), which represents the general galaxy population. In the spirit of high-z samples, these surveys reflect the natural diversity of emission-line galaxies, including peculiar and early-type morphologies. We adopt objective criteria to identify severe kinematic anomalies. In the Close Pairs Survey, TF offsets associated with such anomalies collectively resemble the ``differential luminosity evolution'' claimed in some high-z studies, with larger offsets at lower luminosities. With anomalous galaxies rejected, however, the TF relations for the two surveys are quite similar. Furthermore, the two surveys follow similar relations between color and TF residuals, with the Pairs Survey relation extending to bluer colors and brighter TF residuals. Strong outliers from this relation are nearly always kinematically anomalous. Thus the color-TF residual relation can serve as a powerful tool for separating reliable luminosity offsets from offsets associated with kinematic anomalies. We reanalyze high-z TF data from the FORS Deep Field and find: (1) overall luminosity evolution of ~0.3 mag; (2) strong slope evolution driven by kinematically anomalous galaxies, which show TF offsets of up to ~2 mag at low luminosities; and (3) an additional zero-point offset of ~0.2 mag linked to anomalous galaxies.

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arXiv2003

Forming Young Bulges within Existing Disks: Statistical Evidence for External Drivers

Contrary to traditional models of galaxy formation, recent observations suggest that some bulges form within preexisting disk galaxies. Such late-epoch bulge formation within disks seems to be linked to disk gas inflow and central star formation, caused by either internal secular processes or galaxy mergers and interactions. We identify a population of galaxies likely to be experiencing active bulge growth within disks, using the criterion that the color within the half-light radius is bluer than the outer disk color. Such blue-centered galaxies make up >10% of star-forming disk galaxies within the Nearby Field Galaxy Survey, a broad survey designed to represent the natural diversity of the low-z galaxy population over a wide range of luminosities and environments. Blue-centered galaxies correlate at 99% confidence with morphological peculiarities suggestive of minor mergers and interactions. From this and other evidence, we argue that external drivers rather than internal secular processes probably account for the majority of blue-centered galaxies. We go on to discuss quantitative plausibility arguments indicating that blue-centered evolutionary phases may represent an important mode of bulge growth for most disk galaxies, leading to significant changes in bulge-to-disk ratio without destroying disks. If this view is correct, bulge growth within disks may be a natural consequence of the repeated galaxy mergers and interactions inherent in hierarchical galaxy formation.

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arXiv2002

Building a CCD Spectrograph for Educational or Amateur Astronomy

We discuss the design of an inexpensive, high-throughput CCD spectrograph for a small telescope. By using optical fibers to carry the light from the telescope focus to a table-top spectrograph, one can minimize the weight carried by the telescope and simplify the spectrograph design. We recently employed this approach in the construction of IntroSpec, an instrument built for the 16-inch Knowles Telescope on the Harvard College campus.

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arXiv2002

Physical Sources of Scatter in the Tully-Fisher Relation

We analyze residuals from the Tully-Fisher relation for the emission-line galaxies in the Nearby Field Galaxy Survey, a broadly representative survey designed to fairly sample the variety of galaxy morphologies and environments in the local universe. For spirals brighter than M_R^i=-18, we find strong correlations between Tully-Fisher residuals and both B-R color and EW(Halpha). The extremes of the correlations are populated by Sa galaxies, which show consistently red colors, and spirals with morphological peculiarities, which are often blue. If we apply an EW(Halpha)-dependent or B-R color-dependent correction term to the Tully-Fisher relation, the scatter in the relation no longer increases from R to B to U but instead drops to a nearly constant level close to the scatter we expect from measurement errors. We argue that these results probably reflect correlated offsets in luminosity and color as a function of star formation history. Broadening the sample in morphology and luminosity, we find that most non-spirals brighter than M_R^i=-18 follow the same correlations as do spirals, albeit with greater scatter. However, the color and EW(Halpha) correlations do not apply to galaxies fainter than M_R^i=-18 or to emission-line S0 galaxies with anomalous gas kinematics. For the dwarf galaxy population, the parameters controlling Tully-Fisher residuals are instead related to the degree of recent disturbance: overluminous dwarfs have higher rotation curve asymmetries, brighter U-band effective surface brightnesses, and shorter gas consumption timescales than their underluminous counterparts. As a result, sample selection strongly affects the measured faint-end slope of the Tully-Fisher relation. Passively evolving, rotationally supported galaxies display a break toward steeper slope at low luminosities.

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arXiv2000

Rotation Curve Measurement using Cross-Correlation

Longslit spectroscopy is entering an era of increased spatial and spectral resolution and increased sample size. Improved instruments reveal complex velocity structure that cannot be described with a one-dimensional rotation curve, yet samples are too numerous to examine each galaxy in detail. Therefore, one goal of rotation curve measurement techniques is to flag cases in which the kinematic structure of the galaxy is more complex than a single-valued curve. We examine cross-correlation as a technique that is easily automated and works for low signal-to-noise spectra. We show that the technique yields well-defined errors which increase when the simple spectral model (template) is a poor match to the data, flagging those cases for later inspection. We compare the technique to the more traditional, parametric technique of simultaneous emission line fitting. When the line profile at a single slit position is non-Gaussian, the techniques disagree. For our model spectra with two well-separated velocity components, assigned velocities from the two techniques differ by up to ~52% of the velocity separation of the model components. However, careful use of the error statistics for either technique allows one to flag these non-Gaussian spectra.

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