2,607 research outputs found
The Coexistence of Classical Bulges, Pseudobulges, and Supermassive Black Holes
Some S0 and early-type spiral galaxies possess "composite bulges"; in these
galaxies, the photometric bulge -- the central stellar light in excess of the
disk light -- is composed of both a "(disky) pseudobulge", with a flattened,
disklike morphology and relatively cool stellar kinematics, and a rounder,
kinematically hot "classical bulge" embedded within. I speculate that
supermassive black holes (SMBH) in such galaxies may correlate with the
classical-bulge component only, and not with the pseudobulge component;
preliminary comparisons with SMBH masses appear to support this hypothesis.Comment: LaTeX, 4 pages, 2 PDF figures. To appear in the proceedings of "The
Monster's Fiery Breath: Feedback in Galaxies, Groups, and Clusters", eds.
Sebastian Heinz and Eric Wilcots (AIP conference series
Imfit: A Fast, Flexible New Program for Astronomical Image Fitting
I describe a new, open-source astronomical image-fitting program called
Imfit, specialized for galaxies but potentially useful for other sources, which
is fast, flexible, and highly extensible. A key characteristic of the program
is an object-oriented design which allows new types of image components (2D
surface-brightness functions) to be easily written and added to the program.
Image functions provided with Imfit include the usual suspects for galaxy
decompositions (Sersic, exponential, Gaussian), along with Core-Sersic and
broken-exponential profiles, elliptical rings, and three components which
perform line-of-sight integration through 3D luminosity-density models of disks
and rings seen at arbitrary inclinations.
Available minimization algorithms include Levenberg-Marquardt, Nelder-Mead
simplex, and Differential Evolution, allowing trade-offs between speed and
decreased sensitivity to local minima in the fit landscape. Minimization can be
done using the standard chi^2 statistic (using either data or model values to
estimate per-pixel Gaussian errors, or else user-supplied error images) or
Poisson-based maximum-likelihood statistics; the latter approach is
particularly appropriate for cases of Poisson data in the low-count regime. I
show that fitting low-S/N galaxy images using chi^2 minimization and
individual-pixel Gaussian uncertainties can lead to significant biases in
fitted parameter values, which are avoided if a Poisson-based statistic is
used; this is true even when Gaussian read noise is present.Comment: pdflatex, 27 pages, 19 figures. Revised version, accepted by ApJ.
Programs, source code, and documentation available at:
http://www.mpe.mpg.de/~erwin/code/imfit
The Dependence of Bar Frequency on Galaxy Mass, Colour, and Gas Content -- and Angular Resolution -- in the Local Universe
I use distance- and mass-limited subsamples of the Spitzer Survey of Stellar
Structure in Galaxies (S4G) to investigate how the presence of bars in spiral
galaxies depends on mass, colour, and gas content and whether large, SDSS-based
investigations of bar frequencies agree with local data. Bar frequency reaches
a maximum of at , declining
to both lower and higher masses. It is roughly constant over a wide range of
colours (-0.8) and atomic gas fractions ( to 1). Bars are thus as common in blue, gas-rich
galaxies are they are in red, gas-poor galaxies. This is in sharp contrast to
many SDSS-based studies of -0.1 galaxies, which report f_bar
increasing strongly to higher masses (from to ), redder colours, and lower gas fractions. The contradiction can be
explained if SDSS-based studies preferentially miss bars in, and underestimate
the bar fraction for, lower-mass (bluer, gas-rich) galaxies due to poor spatial
resolution and the correlation between bar size and stellar mass. Simulations
of SDSS-style observations using the S4G galaxies as a parent sample, and
assuming that bars below a threshold angular size of twice the PSF FWHM cannot
be identified, successfully reproduce typical SDSS f_bar trends for stellar
mass and gas mass ratio. Similar considerations may affect high-redshift
studies, especially if bars grow in length over cosmic time; simulations
suggest that high-redshift bar fractions may thus be systematically
underestimated.Comment: pdflatex, 19 pages (+ appendix), 16 figures; revised and corrected
version, to appear in Monthly Notices of the Royal Astronomical Society.
Data, code, and Jupyter notebooks available at
https://github.com/perwin/s4g_barfraction
Asymptotic behavior and zero distribution of Carleman orthogonal polynomials
Let be an analytic Jordan curve and let be the
sequence of polynomials that are orthonormal with respect to the area measure
over the interior of . A well-known result of Carleman states that
\label{eq12}
\lim_{n\to\infty}\frac{p_n(z)}{\sqrt{(n+1)/\pi} [\phi(z)]^{n}}= \phi'(z)
locally uniformly on certain open neighborhood of the closed exterior of ,
where is the canonical conformal map of the exterior of onto the
exterior of the unit circle. In this paper we extend the validity of
(\ref{eq12}) to a maximal open set, every boundary point of which is an
accumulation point of the zeros of the 's. Some consequences on the
limiting distribution of the zeros are discussed, and the results are
illustrated with two concrete examples and numerical computations.Comment: 23 pages, 4 figure
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