55 research outputs found
Brightest Cluster Galaxies at the Present Epoch
We have observed 433 z<=0.08 brightest cluster galaxies (BCGs) in a full-sky
survey of Abell clusters. The BCG Hubble diagram is consistent to within 2% of
a Omega_m=0.3, Lambda=0.7 Hubble relation. The L_m-alpha relation for BCGs,
which uses alpha, the log-slope of the BCG photometric curve of growth, to
predict metric luminosity, L_m, has 0.27 mag residuals. We measure central
stellar velocity dispersions, sigma, of the BCGs, finding the Faber-Jackson
relation to flatten as the metric aperture grows to include an increasing
fraction of the total BCG luminosity. A 3-parameter "metric plane" relation
using alpha and sigma together gives the best prediction of L_m, with 0.21 mag
residuals. The projected spatial offset, r_x, of BCGs from the X-ray-defined
cluster center is a gamma=-2.33 power-law over 1<r_x<10^3 kpc. The median
offset is ~10 kpc, but ~15% of the BCGs have r_x>100 kpc. The absolute
cluster-dispersion normalized BCG peculiar velocity |Delta V_1|/sigma_c follows
an exponential distribution with scale length 0.39+/-0.03. Both L_m and alpha
increase with sigma_c. The alpha parameter is further moderated by both the
spatial and velocity offset from the cluster center, with larger alpha
correlated with the proximity of the BCG to the cluster mean velocity or
potential center. At the same time, position in the cluster has little effect
on L_m. The luminosity difference between the BCG and second-ranked galaxy, M2,
increases as the peculiar velocity of the BCG within the cluster decreases.
Further, when M2 is a close luminosity "rival" of the BCG, the galaxy that is
closest to either the velocity or X-ray center of the cluster is most likely to
have the larger alpha. We conclude that the inner portions of the BCGs are
formed outside the cluster, but interactions in the heart of the galaxy cluster
grow and extend the envelopes of the BCGs.Comment: Accepted for publication in the Astrophysical Journa
The Stellar Halos of Massive Elliptical Galaxies II: Detailed Abundance Ratios at Large Radius
We study the radial dependence in stellar populations of 33 nearby early-type
galaxies with central stellar velocity dispersions sigma* > 150 km/s. We
measure stellar population properties in composite spectra, and use ratios of
these composites to highlight the largest spectral changes as a function of
radius. Based on stellar population modeling, the typical star at 2 R_e is old
(~10 Gyr), relatively metal poor ([Fe/H] -0.5), and alpha-enhanced
([Mg/Fe]~0.3). The stars were made rapidly at z~1.5-2 in shallow potential
wells. Declining radial gradients in [C/Fe], which follow [Fe/H], also arise
from rapid star formation timescales due to declining carbon yields from
low-metallicity massive stars. In contrast, [N/Fe] remains high at large
radius. Stars at large radius have different abundance ratio patterns from
stars in the center of any present-day galaxy, but are similar to Milky Way
thick disk stars. Our observations are thus consistent with a picture in which
the stellar outskirts are built up through minor mergers with disky galaxies
whose star formation is truncated early (z~1.5-2).Comment: ApJ in press, 12 pages, 6 figure
A Correction to the Standard Galactic Reddening Map: Passive Galaxies as Standard Crayons
We present corrections to the Schlegel, Finkbeiner, Davis (SFD98) reddening
maps over the Sloan Digital Sky Survey northern Galactic cap area. To find
these corrections, we employ what we dub the "standard crayon" method, in which
we use passively evolving galaxies as color standards by which to measure
deviations from the reddening map. We select these passively evolving galaxies
spectroscopically, using limits on the H alpha and O II equivalent widths to
remove all star-forming galaxies from the SDSS main galaxy catalog. We find
that by correcting for known reddening, redshift, color-magnitude relation, and
variation of color with environmental density, we can reduce the scatter in
color to below 3% in the bulk of the 151,637 galaxies we select. Using these
galaxies we construct maps of the deviation from the SFD98 reddening map at 4.5
degree resolution, with 1-sigma error of ~ 1.5 millimagnitudes E(B-V). We find
that the SFD98 maps are largely accurate with most of the map having deviations
below 3 millimagnitudes E(B-V), though some regions do deviate from SFD98 by as
much as 50%. The maximum deviation found is 45 millimagnitudes in E(B-V), and
spatial structure of the deviation is strongly correlated with the observed
dust temperature, such that SFD98 underpredicts reddening in regions of low
dust temperature. Our maps of these deviations, as well as their errors, are
made available to the scientific community as supplemental correction to SFD98
at http://www.peekandgraves2010.com.Comment: 12 pages, 7 figures. Accepted to the ApJ. Reddening correction maps
and associated software can be found at http://www.peekandgraves2010.co
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