212 research outputs found
On the Evolutionary History of Stars and their Fossil Mass and Light
The total extragalactic background radiation can be an important test of the
global star formation history (SFH). Using direct observational estimates of
the SFH, along with standard assumptions about the initial mass function (IMF),
we calculate the total extragalactic background radiation and the observed
stellar density today. We show that plausible SFHs allow a significant range in
each quantity, but that their ratio is very tightly constrained. Current
estimates of the stellar mass and extragalactic background are difficult to
reconcile, as long as the IMF is fixed to the Salpeter slope above 1 Msun. The
joint confidence interval of these two quantities only agrees with that
determined from the allowed range of SFH fits at the 3-sigma level, and for our
best-fit values the discrepancy is about a factor of two. Alternative energy
sources that contribute to the background, such as active galactic nuclei
(AGN), Population III stars, or decaying particles, appear unlikely to resolve
the discrepancy. However, changes to the IMF allow plausible solutions to the
background problem. The simplest is an average IMF with an increased
contribution from stars around 1.5--4 Msun. A ``paunchy'' IMF of this sort
could emerge as a global average if low mass star formation is suppressed in
galaxies experiencing rapid starbursts. Such an IMF is consistent with
observations of star-forming regions, and would help to reconcile the fossil
record of star formation with the directly observed SFH.Comment: 21 pages, 7 figures, 3 tables; submitted to Monthly Notice
The PAndAS view of the Andromeda satellite system - I. A Bayesian search for dwarf galaxies using spatial and color-magnitude information
We present a generic algorithm to search for dwarf galaxies in photometric
catalogs and apply it to the Pan-Andromeda Archaeological Survey (PAndAS). The
algorithm is developed in a Bayesian framework and, contrary to most
dwarf-galaxy-search codes, makes use of both the spatial and color-magnitude
information of sources in a probabilistic approach. Accounting for the
significant contamination from the Milky Way foreground and from the structured
stellar halo of the Andromeda galaxy, we recover all known dwarf galaxies in
the PAndAS footprint with high significance, even for the least luminous ones.
Some Andromeda globular clusters are also recovered and, in one case,
discovered. We publish a list of the 143 most significant detections yielded by
the algorithm. The combined properties of the 39 most significant isolated
detections show hints that at least some of these trace genuine dwarf galaxies,
too faint to be individually detected. Follow-up observations by the community
are mandatory to establish which are real members of the Andromeda satellite
system. The search technique presented here will be used in an upcoming
contribution to determine the PAndAS completeness limits for dwarf galaxies.
Although here tuned to the search of dwarf galaxies in the PAndAS data, the
algorithm can easily be adapted to the search for any localised overdensity
whose properties can be modeled reliably in the parameter space of any catalog.Comment: 20 pages, 16 figures, 1 table; accepted for publication in ApJ. High
res pdf available at
https://www.dropbox.com/s/7zk7pme2wunwkjv/PAndAS_dwarf_galaxies.pd
Stellar Kinematics in the Complicated Inner Spheroid of M31: Discovery of Substructure Along the Southeastern Minor Axis and its Relationship to the Giant Southern Stream
We present the discovery of a kinematically-cold stellar population along the
SE minor axis of the Andromeda galaxy (M31) that is likely the forward
continuation of M31's giant southern stream. This discovery was made in the
course of an on-going spectroscopic survey of red giant branch (RGB) stars in
M31 using the DEIMOS instrument on the Keck II 10-m telescope. Stellar
kinematics are investigated in eight fields located 9-30 kpc from M31's center
(in projection). A likelihood method based on photometric and spectroscopic
diagnostics is used to isolate confirmed M31 RGB stars from foreground Milky
Way dwarf stars: for the first time, this is done without using radial velocity
as a selection criterion, allowing an unbiased study of M31's stellar
kinematics. The radial velocity distribution of the 1013 M31 RGB stars shows
evidence for the presence of two components. The broad (hot) component has a
velocity dispersion of 129 km/s and presumably represents M31's virialized
spheroid. A significant fraction (19%) of the population is in a narrow (cold)
component centered near M31's systemic velocity with a velocity dispersion that
decreases with increasing radial distance, from 55.5 km/s at R_proj=12 kpc to
10.6 km/s at R_proj=18 kpc. The spatial and velocity distribution of the cold
component matches that of the "Southeast shelf" predicted by the Fardal et al.
(2007) orbital model of the progenitor of the giant southern stream. The
metallicity distribution of the cold component matches that of the giant
southern stream, but is about 0.2 dex more metal rich on average than that of
the hot spheroidal component. We discuss the implications of our discovery on
the interpretation of the intermediate-age spheroid population found in this
region in recent ultra-deep HST imaging studies.Comment: 23 pages, 16 figures, 2 tables, accepted for publication in the
Astrophysical Journal. Changes from previous version: expanded discussion in
sections 4.2 and 7.2, removal of section 7.1.4 and associated figure
(discussion moved to section 7.1.2
A Bayesian Approach to Locating the Red Giant Branch Tip Magnitude (Part I)
We present a new approach for identifying the Tip of the Red Giant Branch
(TRGB) which, as we show, works robustly even on sparsely populated targets.
Moreover, the approach is highly adaptable to the available data for the
stellar population under study, with prior information readily incorporable
into the algorithm. The uncertainty in the derived distances is also made
tangible and easily calculable from posterior probability distributions. We
provide an outline of the development of the algorithm and present the results
of tests designed to characterize its capabilities and limitations. We then
apply the new algorithm to three M31 satellites: Andromeda I, Andromeda II and
the fainter Andromeda XXIII, using data from the Pan-Andromeda Archaeological
Survey (PAndAS), and derive their distances as
kpc, kpc and
kpc respectively, where the errors appearing in parentheses are the components
intrinsic to the method, while the larger values give the errors after
accounting for additional sources of error. These results agree well with the
best distance determinations in the literature and provide the smallest
uncertainties to date. This paper is an introduction to the workings and
capabilities of our new approach in its basic form, while a follow-up paper
shall make full use of the method's ability to incorporate priors and use the
resulting algorithm to systematically obtain distances to all of M31's
satellites identifiable in the PAndAS survey area.Comment: 11 pages, 18 figure
Hydrodynamic Simulation of the Cosmological X-ray Background
(Abridged) We use a hydrodynamic simulation of a LambdaCDM model to predict
the extragalactic X-ray background (XRB), focussing on emission from the
intergalactic medium (IGM). We also include X-rays from point sources
associated with galaxies in the simulation, and make maps of the angular
distribution of the emission. We find that filaments in the maps are not
evident, being diluted by projection. In the soft (0.5-2 keV) band, the mean
intensity of radiation from intergalactic and cluster gas is 2.3*10^-12
ergdeg^-2cm^-2s^-1, 35% of the total soft band emission. This is compatible at
the ~1 sigma level with estimates of the unresolved soft background from ROSAT
and {\it Chandra}. Only 4% of the hard (2-10 keV) emission is associated with
the IGM. Relative to AGN flux, the IGM component peaks at a lower redshift
(median z~0.45) so its clustering makes an important contribution to that of
the total XRB. The angular correlations on 0.1-10 arcmin scales are
significant, with an amplitude roughly consistent with an extrapolation of
recent ROSAT results to small scales. A cross-correlation of the XRB against
nearby galaxies taken from a simulated redshift survey also yields a strong
signal from the IGM. Although some recent papers have argued that the expected
soft band intensity from gas in galaxy, group, and cluster halos would exceed
XRB limits unless much of the gas is expelled by supernova feedback, we obtain
reasonable compatibility with current observations in a simulation that
incorporates cooling, star formation, and only modest feedback. A prediction of
our model is that the unresolved portion of the soft XRB will remain mostly
unresolved.Comment: Improved referencing of related papers. Submitted to ApJ, 19 pages,
17 postscript figures, most reduced in resolution, emulateapj.sty, for full
resolution version, see http://cfa-www.harvard.edu/~rcroft/xray.ps.g
Cooling Radiation and the Lyman-alpha Luminosity of Forming Galaxies
We examine the cooling radiation from forming galaxies in hydrodynamic
simulations of the LCDM model (cold dark matter with a cosmological constant),
focusing on the Ly-alpha line luminosities of high-redshift systems. Primordial
composition gas condenses within dark matter potential wells, forming objects
with masses and sizes comparable to the luminous regions of observed galaxies.
As expected, the energy radiated in this process is comparable to the
gravitational binding energy of the baryons, and the total cooling luminosity
of the galaxy population peaks at z ~= 2. However, in contrast to the classical
picture of gas cooling from the \sim 10^6 K virial temperature of a typical
dark matter halo, we find that most of the cooling radiation is emitted by gas
with T < 20,000 K. As a consequence, roughly 50% of this cooling radiation
emerges in the Ly-alpha line. While a galaxy's cooling luminosity is usually
smaller than the ionizing continuum luminosity of its young stars, the two are
comparable in the most massive systems, and the cooling radiation is produced
at larger radii, where the Ly-alpha photons are less likely to be extinguished
by dust. We suggest, in particular, that cooling radiation could explain the
two large (\sim 100 kpc), luminous (L_{Ly-alpha} \sim 10^{44} erg s^{-1})
``blobs'' of Ly-alpha emission found in Steidel et al.'s (1999) narrow band
survey of a z = 3 proto-cluster. Our simulations predict objects of the
observed luminosity at about the right space density, and radiative transfer
effects can account for the observed sizes and line widths. We discuss
observable tests of this hypothesis for the nature of the Ly-alpha blobs, and
we present predictions for the contribution of cooling radiation to the
Ly-alpha luminosity function of galaxies as a function of redshift.Comment: Submitted to ApJ. 28 pages including 9 PS figures. Version with color
figures available at
http://donald.astro.umass.edu/~fardal/papers/cooling/cooling.htm
Tracing Galaxy Formation with Stellar Halos II: Relating Substructure in Phase- and Abundance-Space to Accretion Histories
This paper explores the mapping between the observable properties of a
stellar halo in phase- and abundance-space and the parent galaxy's accretion
history in terms of the characteristic epoch of accretion and mass and orbits
of progenitor objects. The study utilizes a suite of eleven stellar halo models
constructed within the context of a standard LCDM cosmology. The results
demonstrate that coordinate-space studies are sensitive to the recent (0-8
Gyears ago) merger histories of galaxies (this timescale corresponds to the
last few to tens of percent of mass accretion for a Milky-Way-type galaxy).
Specifically, the {\it frequency, sky coverage} and {\it fraction of stars} in
substructures in the stellar halo as a function of surface brightness are
indicators of the importance of recent merging and of the luminosity function
of infalling dwarfs. The {\it morphology} of features serves as a guide to the
orbital distribution of those dwarfs. Constraints on the earlier merger history
(> 8 Gyears ago) can be gleaned from the abundance patterns in halo stars:
within our models, dramatic differences in the dominant epoch of accretion or
luminosity function of progenitor objects leave clear signatures in the
[alpha/Fe] and [Fe/H] distributions of the stellar halo - halos dominated by
very early accretion have higher average [alpha/Fe], while those dominated by
high luminosity satellites have higher [Fe/H]. This intuition can be applied to
reconstruct much about the merger histories of nearby galaxies from current and
future data sets.Comment: 21 pages, 20 figures. To appear in the Astrophysical Journa
Major Substructure in the M31 Outer Halo: the South-West Cloud
We undertake the first detailed analysis of the stellar population and
spatial properties of a diffuse substructure in the outer halo of M31. The
South-West Cloud lies at a projected distance of ~100 kpc from the centre of
M31, and extends for at least ~50 kpc in projection. We use Pan-Andromeda
Archaeological Survey photometry of red giant branch stars to determine a
distance to the South-West Cloud of 793 +/- 45 kpc. The metallicity of the
cloud is found to be [Fe/H] = -1.3 +/- 0.1. This is consistent with the
coincident globular clusters PAndAS-7 and PAndAS-8, which have metallicities
determined using an independent technique of [Fe/H] = -1.35 +/- 0.15. We
measure a brightness for the Cloud of M_V = -12.1 mag; this is ~75 per cent of
the luminosity implied by the luminosity-metallicity relation. Under the
assumption that the South-West Cloud is the visible remnant of an accreted
dwarf satellite, this suggests that the progenitor object was amongst M31's
brightest dwarf galaxies prior to disruption.Comment: 13 pages, 9 figures, accepted for publication in MNRA
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