140 research outputs found
unWISE tomography of Planck CMB lensing
MB lensing tomography, or the cross-correlation between CMB lensing maps and
large-scale structure tracers over a well-defined redshift range, has the
potential to map the amplitude and growth of structure over cosmic time,
provide some of the most stringent tests of gravity, and break important
degeneracies between cosmological parameters. In this work, we use the unWISE
galaxy catalog to provide three samples at median redshifts
and 1.5, fully spanning the Dark Energy dominated era, together with the most
recent Planck CMB lensing maps. We obtain a combined cross-correlation
significance over the range of scales . We
measure the redshift distribution of unWISE sources by a combination of
cross-matching with the COSMOS photometric catalog and cross-correlation with
BOSS galaxies and quasars and eBOSS quasars. We also show that magnification
bias must be included in our analysis and perform a number of null tests. In a
companion paper, we explore the derived cosmological parameters by modeling the
non-linearities and propagating the redshift distribution uncertainties.Comment: 51 pages, 22 figures. Comments welcome! Revisions reflect version
accepted by JCA
On Galactic density modeling in the presence of dust extinction
Inferences about the spatial density or phase-space structure of stellar
populations in the Milky Way require a precise determination of the effective
survey volume. The volume observed by surveys such as Gaia or near-infrared
spectroscopic surveys, which have good coverage of the Galactic mid-plane
region, is highly complex because of the abundant small-scale structure in the
three-dimensional interstellar dust extinction. We introduce a novel framework
for analyzing the importance of small-scale structure in the extinction. This
formalism demonstrates that the spatially-complex effect of extinction on the
selection function of a pencil-beam or contiguous sky survey is equivalent to a
low-pass filtering of the extinction-affected selection function with the
smooth density field. We find that the angular resolution of current 3D
extinction maps is sufficient for analyzing Gaia sub-samples of millions of
stars. However, the current distance resolution is inadequate and needs to be
improved by an order of magnitude, especially in the inner Galaxy. We also
present a practical and efficient method for properly taking the effect of
extinction into account in analyses of Galactic structure through an effective
selection function. We illustrate its use with the selection function of
red-clump stars in APOGEE using and comparing a variety of current 3D
extinction maps.Comment: Code available at https://github.com/jobovy/mwdust and at
https://github.com/jobovy/apogee-map
The stellar population structure of the Galactic disk
The spatial structure of stellar populations with different chemical
abundances in the Milky Way contains a wealth of information on Galactic
evolution over cosmic time. We use data on 14,699 red-clump stars from the
APOGEE survey, covering 4 kpc <~ R <~ 15 kpc, to determine the structure of
mono-abundance populations (MAPs)---stars in narrow bins in [a/Fe] and
[Fe/H]---accounting for the complex effects of the APOGEE selection function
and the spatially-variable dust obscuration. We determine that all MAPs with
enhanced [a/Fe] are centrally concentrated and are well-described as
exponentials with a scale length of 2.2+/-0.2 kpc over the whole radial range
of the disk. We discover that the surface-density profiles of low-[a/Fe] MAPs
are complex: they do not monotonically decrease outwards, but rather display a
peak radius ranging from ~5 kpc to ~13 kpc at low [Fe/H]. The extensive radial
coverage of the data allows us to measure radial trends in the thickness of
each MAP. While high-[a/Fe] MAPs have constant scale heights, low-[a/Fe] MAPs
flare. We confirm, now with high-precision abundances, previous results that
each MAP contains only a single vertical scale height and that low-[Fe/H],
low-[a/Fe] and high-[Fe/H], high-[a/Fe] MAPs have intermediate (h_Z~300 to 600
pc) scale heights that smoothly bridge the traditional thin- and thick-disk
divide. That the high-[a/Fe], thick disk components do not flare is strong
evidence against their thickness being caused by radial migration. The
correspondence between the radial structure and chemical-enrichment age of
stellar populations is clear confirmation of the inside-out growth of galactic
disks. The details of these relations will constrain the variety of physical
conditions under which stars form throughout the MW disk.Comment: Code available at https://github.com/jobovy/apogee-map
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Measuring reddening with Sloan Digital Sky Survey stellar spectra and recalibrating SFD
We present measurements of dust reddening using the colors of stars with spectra in the Sloan Digital Sky Survey. We measure reddening as the difference between the measured and predicted colors of a star, as derived from stellar parameters from the Sloan Extension for Galactic Understanding and Exploration Stellar Parameter Pipeline. We achieve uncertainties of 56, 34, 25, and 29 mmag in the colors u – g, g – r, r – i, and i – z, per star, though the uncertainty varies depending on the stellar type and the magnitude of the star. The spectrum-based reddening measurements confirm our earlier "blue tip" reddening measurements, finding reddening coefficients different by –3%, 1%, 1%, and 2% in u – g, g – r, r – i, and i – z from those found by the blue tip method, after removing a 4% normalization difference. These results prefer an RV = 3.1 Fitzpatrick reddening law to O'Donnell or Cardelli et al. reddening laws. We provide a table of conversion coefficients from the Schlegel et al. (SFD) maps of E(B – V) to extinction in 88 bandpasses for four values of RV , using this reddening law and the 14% recalibration of SFD first reported by Schlafly et al. and confirmed in this work.Astronom
Low Surface Brightness Imaging of the Magellanic System: Imprints of Tidal Interactions between the Clouds in the Stellar Periphery
We present deep optical images of the Large and Small Magellanic Clouds (LMC
and SMC) using a low cost telephoto lens with a wide field of view to explore
stellar substructure in the outskirts of the stellar disk of the LMC (r < 10
degrees from the center). These data have higher resolution than existing star
count maps, and highlight the existence of stellar arcs and multiple spiral
arms in the northern periphery, with no comparable counterparts in the South.
We compare these data to detailed simulations of the LMC disk outskirts,
following interactions with its low mass companion, the SMC. We consider
interaction in isolation and with the inclusion of the Milky Way tidal field.
The simulations are used to assess the origin of the northern structures,
including also the low density stellar arc recently identified in the DES data
by Mackey et al. 2015 at ~ 15 degrees. We conclude that repeated close
interactions with the SMC are primarily responsible for the asymmetric stellar
structures seen in the periphery of the LMC. The orientation and density of
these arcs can be used to constrain the LMC's interaction history with and
impact parameter of the SMC. More generally, we find that such asymmetric
structures should be ubiquitous about pairs of dwarfs and can persist for 1-2
Gyr even after the secondary merges entirely with the primary. As such, the
lack of a companion around a Magellanic Irregular does not disprove the
hypothesis that their asymmetric structures are driven by dwarf-dwarf
interactions.Comment: Submitted to ApJ. Comments are welcome
CGRaBS: An All-Sky Survey of Gamma-Ray Blazar Candidates
We describe a uniform all-sky survey of bright blazars, selected primarily by
their flat radio spectra, that is designed to provide a large catalog of likely
gamma-ray AGN. The defined sample has 1625 targets with radio and X-ray
properties similar to those of the EGRET blazars, spread uniformly across the
|b| > 10 deg sky. We also report progress toward optical characterization of
the sample; of objects with known R < 23, 85% have been classified and 81% have
measured redshifts. One goal of this program is to focus attention on the most
interesting (e.g., high redshift, high luminosity, ...) sources for intensive
multiwavelength study during the observations by the Large Area Telescope (LAT)
on GLAST.Comment: 18 pages, 6 figures, 1 machine-readable table available at
http://astro.stanford.edu/CGRaBS/ ; accepted for publication in ApJ
Gravitational Microlensing Event Statistics for the Zwicky Transient Facility
Microlensing surveys have discovered thousands of events with almost all
events discovered within the Galactic bulge or toward the Magellanic clouds.
The Zwicky Transient Facility (ZTF), while not designed to be a microlensing
campaign, is an optical time-domain survey that observes the entire northern
sky every few nights including the Galactic plane. ZTF observes
stars in g-band and r-band and can significantly contribute to the observed
microlensing population. We predict that ZTF will observe 1100
microlensing events in three years of observing within degrees
latitude of the Galactic plane, with 500 events in the outer Galaxy
(). This yield increases to 1400 (800) events
by combining every three ZTF exposures, 1800 (900) events if ZTF
observes for a total of five years, and 2400 (1300) events for a
five year survey with post-processing image stacking. Using the microlensing
modeling software PopSyCLE, we compare the microlensing populations in the
Galactic bulge and the outer Galaxy. We also present an analysis of the
microlensing event ZTF18abhxjmj to demonstrate how to leverage these population
statistics in event modeling. ZTF will constrain Galactic structure, stellar
populations, and primordial black holes through photometric microlensing.Comment: 19 pages, 13 figures, 5 tables, accepted to ApJ (6/4/2020),
microlensing simulation catalogs available at
https://portal.nersc.gov/project/uLens/Galactic_Microlensing_Distribution
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