2,136 research outputs found
How Common are the Magellanic Clouds?
We introduce a probabilistic approach to the problem of counting dwarf
satellites around host galaxies in databases with limited redshift information.
This technique is used to investigate the occurrence of satellites with
luminosities similar to the Magellanic Clouds around hosts with properties
similar to the Milky Way in the object catalog of the Sloan Digital Sky Survey.
Our analysis uses data from SDSS Data Release 7, selecting candidate
Milky-Way-like hosts from the spectroscopic catalog and candidate analogs of
the Magellanic Clouds from the photometric catalog. Our principal result is the
probability for a Milky-Way-like galaxy to host N_{sat} close satellites with
luminosities similar to the Magellanic Clouds. We find that 81 percent of
galaxies like the Milky Way are have no such satellites within a radius of 150
kpc, 11 percent have one, and only 3.5 percent of hosts have two. The
probabilities are robust to changes in host and satellite selection criteria,
background-estimation technique, and survey depth. These results demonstrate
that the Milky Way has significantly more satellites than a typical galaxy of
its luminosity; this fact is useful for understanding the larger cosmological
context of our home galaxy.Comment: Updated to match published version. Added referenc
Progress report on the relativistic three-particle quantization condition
We describe recent work on the relativistic three-particle quantization
condition, generalizing and applying the original formalism of Hansen and
Sharpe, and of Brice\~no, Hansen and Sharpe. In particular, we sketch three
recent developments: the generalization of the formalism to include K-matrix
poles; the numerical implementation of the quantization condition in the
isotropic approximation; and ongoing work extending the description of the
three-particle divergence-free K matrix beyond the isotropic approximation.Comment: 7 pages, 1 figure, Proceedings of Lattice 201
Active galactic nucleus feedback in clusters of galaxies
Observations made during the last ten years with the Chandra X-ray
Observatory have shed much light on the cooling gas in the centers of clusters
of galaxies and the role of active galactic nucleus (AGN) heating. Cooling of
the hot intracluster medium in cluster centers can feed the supermassive black
holes found in the nuclei of the dominant cluster galaxies leading to AGN
outbursts which can reheat the gas, suppressing cooling and large amounts of
star formation. AGN heating can come in the form of shocks, buoyantly rising
bubbles that have been inflated by radio lobes, and the dissipation of sound
waves.Comment: Refereed review article published in Chandra's First Decade of
Discovery Special Feature edition of the Proceedings of the National Academy
of Science
A multiscale approach to environment and its influence on the colour distribution of galaxies
We present a multiscale approach to measurements of galaxy density, applied
to a volume-limited sample constructed from SDSS DR5. We populate a rich
parameter space by obtaining independent measurements of density on different
scales for each galaxy, avoiding the implicit assumptions involved, e.g., in
the construction of group catalogues. As the first application of this method,
we study how the bimodality in galaxy colour distribution (u-r) depends on
multiscale density. The u-r galaxy colour distribution is described as the sum
of two gaussians (red and blue) with five parameters: the fraction of red
galaxies (f_r) and the position and width of the red and blue peaks (mu_r,
mu_b, sigma_r and sigma_b). Galaxies mostly react to their smallest scale (<
0.5 Mpc) environments: in denser environments red galaxies are more common
(larger f_r), redder (larger mu_r) and with a narrower distribution (smaller
sigma_r), while blue galaxies are redder (larger mu_b) but with a broader
distribution (larger sigma_b). There are residual correlations of f_r and mu_b
with 0.5 - 1 Mpc scale density, which imply that total or partial truncation of
star formation can relate to a galaxy's environment on these scales. Beyond 1
Mpc (0.5 Mpc for mu_r) there are no positive correlations with density. However
f_r (mu_r) anti-correlates with density on >2 (1) Mpc scales at fixed density
on smaller scales. We examine these trends qualitatively in the context of the
halo model, utilizing the properties of haloes within which the galaxies are
embedded, derived by Yang et al, 2007 and applied to a group catalogue. This
yields an excellent description of the trends with multiscale density,
including the anti-correlations on large scales, which map the region of
accretion onto massive haloes. Thus we conclude that galaxies become red only
once they have been accreted onto haloes of a certain mass.Comment: 22 pages, 14 figures. Accepted for publication in MNRAS
On the Populations of Radio Galaxies with Extended Morphology at z<0.3
Extended extragalactic radio sources have traditionally been classified into
FR I and II types, based on the ratio r of the separation S between the
brightest regions on either sides of the host galaxy and the total size T of
the radio source. Here we examine the distribution of various physical
properties as a function of r of 1040 luminous radio galaxies (RGs) at z<0.3
from the SDSS, NVSS, and FIRST. About 2/3 of the RGs are lobe-dominated (LD),
and 1/3 have prominent jets. If we follow the original definition of the FR
types (a division based solely on r), FR I and FR II RGs overlap in their host
galaxy properties. However, the rare, LD sources with r>0.8 AND OIII5007 line
luminosity >10^6 Lsun are markedly different on average from the rest of the
RGs, in the sense that they are hosted in lower mass galaxies, live in
relatively sparse environments, and have higher accretion rates onto the
central SMBH. Thus these objects and the rest of RGs form a well-defined
dichotomy. Motivated by the stark differences in the nuclear emission line
properties of the RG subsamples, we suggest that the accretion rate onto the
SMBH may play the primary role in creating the different morphologies. At
relatively high accretion rates, the accretion system may produce powerful jets
that create the "classical double" morphology (roughly corresponding to the LD
sources with r>0.8 and emission lines); at lower accretion rates the jets from
a radiatively inefficient accretion flow generate radio lobes without apparent
"hot spots" at the edge (corresponding to the majority of LD sources). At
slightly lower accretion rates AND in galaxies with dense galactic structure,
sources with prominent jets result. It is possible that while the high
accretion rate systems could affect sub-Mpc scale environments, the jets from
lower accretion rate systems may efficiently suppress activity within the host
galaxies.Comment: ApJ, accepted. 20 pages, 16 figures, 4 tables. Image quality of
Figures 1 & 2 degraded due to size limits. Table 4 is available and kept
up-to-date at http://member.ipmu.jp/yen-ting.lin/RG/index.html. Higher
resolution version available at
http://member.ipmu.jp/yen-ting.lin/Data/ytlin.pd
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