196 research outputs found
The SAGA Survey: I. Satellite Galaxy Populations Around Eight Milky Way Analogs
We present the survey strategy and early results of the "Satellites Around
Galactic Analogs" (SAGA) Survey. The SAGA Survey's goal is to measure the
distribution of satellite galaxies around 100 systems analogous to the Milky
Way down to the luminosity of the Leo I dwarf galaxy (). We
define a Milky Way analog based on -band luminosity and local environment.
Here, we present satellite luminosity functions for 8 Milky Way analog galaxies
between 20 to 40 Mpc. These systems have nearly complete spectroscopic coverage
of candidate satellites within the projected host virial radius down to using low redshift color criteria. We have discovered a total of
25 new satellite galaxies: 14 new satellite galaxies meet our formal criteria
around our complete host systems, plus 11 additional satellites in either
incompletely surveyed hosts or below our formal magnitude limit. Combined with
13 previously known satellites, there are a total of 27 satellites around 8
complete Milky Way analog hosts. We find a wide distribution in the number of
satellites per host, from 1 to 9, in the luminosity range for which there are
five Milky Way satellites. Standard abundance matching extrapolated from higher
luminosities predicts less scatter between hosts and a steeper luminosity
function slope than observed. We find that the majority of satellites (26 of
27) are star-forming. These early results indicate that the Milky Way has a
different satellite population than typical in our sample, potentially changing
the physical interpretation of measurements based only on the Milky Way's
satellite galaxies.Comment: 22 pages, 19 figures, 2 tables. Updated to published version. Survey
website: http://sagasurvey.org
The Mass Distribution and Assembly of the Milky Way from the Properties of the Magellanic Clouds
We present a new measurement of the mass of the Milky Way (MW) based on
observed properties of its largest satellite galaxies, the Magellanic Clouds
(MCs), and an assumed prior of a {\Lambda}CDM universe. The large,
high-resolution Bolshoi cosmological simulation of this universe provides a
means to statistically sample the dynamical properties of bright satellite
galaxies in a large population of dark matter halos. The observed properties of
the MCs, including their circular velocity, distance from the center of the MW,
and velocity within the MW halo, are used to evaluate the likelihood that a
given halo would have each or all of these properties; the posterior PDF for
any property of the MW system can thus be constructed. This method provides a
constraint on the MW virial mass, 1.2 +0.7 -0.3(stat.) +0.3 -0.4 (sys.) x 10^12
M\odot (68% confidence), which is consistent with recent determinations that
involve very different assumptions. In addition, we calculate the posterior PDF
for the density profile of the MW and its satellite accretion history. Although
typical satellites of 10^12 M\odot halos are accreted over a wide range of
epochs over the last 10 Gyr, we find a \sim72% probability that the Magellanic
Clouds were accreted within the last Gyr, and a 50% probability that they were
accreted together.Comment: 9 pages, replaced with version published in ApJ. Animations available
at http://risa.stanford.edu/milkyway
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
SAGAbg I: A Near-Unity Mass Loading Factor in Low-Mass Galaxies via their Low-Redshift Evolution in Stellar Mass, Oxygen Abundance, and Star Formation Rate
Measuring the relation between star formation and galactic winds is
observationally difficult. In this work we make an indirect measurement of the
mass loading factor (the ratio between mass outflow rate and star formation
rate) in low-mass galaxies using a differential approach to modeling the
low-redshift evolution of the star-forming main sequence and mass-metallicity
relation. We use the SAGA (Satellites Around Galactic Analogs) background
galaxies, those spectra observed by the SAGA survey that are not associated
with the main SAGA host galaxies, to construct a sample of 11925
spectroscopically confirmed low-mass galaxies from
and measure a auroral line metallicity for 120 galaxies. The crux of the method
is to use the lowest redshift galaxies as the boundary condition of our model,
and to infer a mass-loading factor for the sample by comparing the expected
evolution of the low redshift reference sample in stellar mass, gas-phase
metallicity, and star formation rate against the observed properties of the
sample at higher redshift. We infer a mass-loading factor of , which is in line with direct measurements of the
mass-loading factor from the literature despite the drastically different set
of assumptions needed for each approach. While our estimate of the mass-loading
factor is in good agreement with recent galaxy simulations that focus on
resolving the dynamics of the interstellar medium, it is smaller by over an
order of magnitude than the mass-loading factor produced by many contemporary
cosmological simulations.Comment: 37 pages, 19 figures, accepted to Ap
Investigating interoperability of the LSST Data Management software stack with Astropy
The Large Synoptic Survey Telescope (LSST) will be an 8.4m optical survey telescope sited in Chile and capable of imaging the entire sky twice a week. The data rate of approximately 15TB per night and the requirements to both issue alerts on transient sources within 60 seconds of observing and create annual data releases means that automated data management systems and data processing pipelines are a key deliverable of the LSST construction project. The LSST data management software has been in development since 2004 and is based on a C++ core with a Python control layer. The software consists of nearly a quarter of a million lines of code covering the system from fundamental WCS and table libraries to pipeline environments and distributed process execution. The Astropy project began in 2011 as an attempt to bring together disparate open source Python projects and build a core standard infrastructure that can be used and built upon by the astronomy community. This project has been phenomenally successful in the years since it has begun and has grown to be the de facto standard for Python software in astronomy. Astropy brings with it considerable expectations from the community on how astronomy Python software should be developed and it is clear that by the time LSST is fully operational in the 2020s many of the prospective users of the LSST software stack will expect it to be fully interoperable with Astropy. In this paper we describe the overlap between the LSST science pipeline software and Astropy software and investigate areas where the LSST software provides new functionality. We also discuss the possibilities of re-engineering the LSST science pipeline software to build upon Astropy, including the option of contributing affliated packages
Orbiting Circum-galactic Gas as a Signature of Cosmological Accretion
We use cosmological SPH simulations to study the kinematic signatures of cool
gas accretion onto a pair of well-resolved galaxy halos. Cold-flow streams and
gas-rich mergers produce a circum-galactic component of cool gas that generally
orbits with high angular momentum about the galaxy halo before falling in to
build the disk. This signature of cosmological accretion should be observable
using background-object absorption line studies as features that are offset
from the galaxy's systemic velocity by ~100 km/s. Accreted gas typically
co-rotates with the central disk in the form of a warped, extended cold flow
disk, such that the observed velocity offset is in the same direction as galaxy
rotation, appearing in sight lines that avoid the galactic poles. This
prediction provides a means to observationally distinguish accreted gas from
outflow gas: the accreted gas will show large one-sided velocity offsets in
absorption line studies while radial/bi-conical outflows will not (except
possibly in special polar projections). This rotation signature has already
been seen in studies of intermediate redshift galaxy-absorber pairs; we suggest
that these observations may be among the first to provide indirect
observational evidence for cold accretion onto galactic halos. Cold mode halo
gas typically has ~3-5 times more specific angular momentum than the dark
matter. The associated cold mode disk configurations are likely related to
extended HI/XUV disks seen around galaxies in the local universe. The fraction
of galaxies with extended cold flow disks and associated offset absorption-line
gas should decrease around bright galaxies at low redshift, as cold mode
accretion dies out.Comment: 15 pages, 9 figures, edited to match published version. Includes
expanded discussion, with primary results unchange
The Lick AGN Monitoring Project 2011: Spectroscopic Campaign and Emission-Line Light Curves
In the Spring of 2011 we carried out a 2.5 month reverberation mapping
campaign using the 3 m Shane telescope at Lick Observatory, monitoring 15
low-redshift Seyfert 1 galaxies. This paper describes the observations,
reductions and measurements, and data products from the spectroscopic campaign.
The reduced spectra were fitted with a multicomponent model in order to isolate
the contributions of various continuum and emission-line components. We present
light curves of broad emission lines and the AGN continuum, and measurements of
the broad H-beta line widths in mean and root-mean square (rms) spectra. For
the most highly variable AGNs we also measured broad H-beta line widths and
velocity centroids from the nightly spectra. In four AGNs exhibiting the
highest variability amplitudes, we detect anticorrelations between broad H-beta
width and luminosity, demonstrating that the broad-line region "breathes" on
short timescales of days to weeks in response to continuum variations. We also
find that broad H-beta velocity centroids can undergo substantial changes in
response to continuum variations; in NGC 4593 the broad H-beta velocity shifted
by ~250 km/s over a one-month duration. This reverberation-induced velocity
shift effect is likely to contribute a significant source of confusion noise to
binary black hole searches that use multi-epoch quasar spectroscopy to detect
binary orbital motion. We also present results from simulations that examine
biases that can occur in measurement of broad-line widths from rms spectra due
to the contributions of continuum variations and photon-counting noise.Comment: 33 pages, 28 figures, accepted for publication in ApJ Supplement
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