209 research outputs found
Full-depth Coadds of the WISE and First-year NEOWISE-Reactivation Images
The Near Earth Object Wide-field Infrared Survey Explorer (NEOWISE)
Reactivation mission released data from its first full year of observations in
2015. This data set includes ~2.5 million exposures in each of W1 and W2,
effectively doubling the amount of WISE imaging available at 3.4 and 4.6
microns relative to the AllWISE release. We have created the first ever
full-sky set of coadds combining all publicly available W1 and W2 exposures
from both the AllWISE and NEOWISE-Reactivation (NEOWISER) mission phases. We
employ an adaptation of the unWISE image coaddition framework (Lang 2014),
which preserves the native WISE angular resolution and is optimized for forced
photometry. By incorporating two additional scans of the entire sky, we not
only improve the W1/W2 depths, but also largely eliminate time-dependent
artifacts such as off-axis scattered moonlight. We anticipate that our new
coadds will have a broad range of applications, including target selection for
upcoming spectroscopic cosmology surveys, identification of distant/massive
galaxy clusters, and discovery of high-redshift quasars. In particular, our
full-depth AllWISE+NEOWISER coadds will be an important input for the Dark
Energy Spectroscopic Instrument (DESI) selection of luminous red galaxy and
quasar targets. Our full-depth W1/W2 coadds are already in use within the DECam
Legacy Survey (DECaLS) and Mayall z-band Legacy Survey (MzLS) reduction
pipelines. Much more work still remains in order to fully leverage NEOWISER
imaging for astrophysical applications beyond the solar system.Comment: coadds available at http://unwise.me, zoomable full-sky rendering at
http://legacysurvey.org/viewe
The Metallicity of the Monoceros Stream
We present low-resolution MMT Hectospec spectroscopy of 594 candidate
Monoceros stream member stars. Based on strong color-magnitude diagram
overdensities, we targeted three fields within the stream's footprint, with 178
deg < l < 203 deg and -25 deg < b < 25 deg. By comparing the measured iron
abundances with those expected from smooth Galactic components alone, we
measure, for the first time, the spectroscopic metallicity distribution
function for Monoceros. We find the stream to be chemically distinct from both
the thick disk and halo, with [Fe/H] = -1, and do not detect a trend in the
stream's metallicity with Galactic longitude. Passing from b = +25 deg to b =
-25 deg the median Monoceros metallicity trends upward by 0.1 dex, though
uncertainties in modeling sample contamination by the disk and halo make this a
marginal detection. In each field, we find Monoceros to have an intrinsic
[Fe/H] dispersion of 0.10-0.22 dex. From the CaII K line, we measure [Ca/Fe]
for a subsample of metal poor program stars with -1.1 < [Fe/H] < -0.5. In two
of three fields, we find calcium deficiencies qualitatively similar to
previously reported [Ti/Fe] underabundances in Monoceros and the Sagittarius
tidal stream. Further, using 90 spectra of thick disk stars in the Monoceros
pointings with b ~ +/-25 deg, we detect a 0.22 dex north/south metallicity
asymmetry coincident with known stellar density asymmetry at R_GC ~ 12 kpc and
|Z| ~ 1.7 kpc. Our median Monoceros [Fe/H] = -1.0 and its relatively low
dispersion naturally fit the expectation for an appropriately luminous M_V ~
-13 dwarf galaxy progenitor.Comment: accepted for publication in Ap
Imaging Redshift Estimates for BL Lacertae Objects
We have obtained high dynamic range, good natural seeing i' images of BL
Lacertae objects (BL Lacs) to search for the AGN host and thus constrain the
source redshift. These objects are drawn from a sample of bright flat-spectrum
radio sources that are either known (via recent Fermi LAT observations)
gamma-ray emitters or similar sources that might be detected in continuing
gamma-ray observations. All had spectroscopic confirmation as BL Lac sources,
but no redshift solution. We detected hosts for 25/49 objects. As these
galaxies have been argued to be standard candles, our measured host magnitudes
provide redshift estimates (ranging from 0.2--1.0). Lower bounds are
established on the redshifts of non-detections. The mean of the fit redshifts
(and lower limits) is higher than those of spectroscopic solutions in the
radio- and gamma-ray- loud parent samples, suggesting corrections may be needed
for the luminosity function and evolution of these sources.Comment: 15 pages, to appear in the Astrophysical Journa
A new physical interpretation of optical and infrared variability in quasars
Changing-look quasars are a recently identified class of active galaxies in
which the strong UV continuum and/or broad optical hydrogen emission lines
associated with unobscured quasars either appear or disappear on timescales of
months to years. The physical processes responsible for this behaviour are
still debated, but changes in the black hole accretion rate or accretion disk
structure appear more likely than changes in obscuration. Here we report on
four epochs of spectroscopy of SDSS J110057.70-005304.5, a quasar at a redshift
of whose UV continuum and broad hydrogen emission lines have faded,
and then returned over the past 20 years. The change in this quasar
was initially identified in the infrared, and an archival spectrum from 2010
shows an intermediate phase of the transition during which the flux below
rest-frame 3400\AA\ has decreased by close to an order of magnitude.
This combination is unique compared to previously published examples of
changing-look quasars, and is best explained by dramatic changes in the
innermost regions of the accretion disk. The optical continuum has been rising
since mid-2016, leading to a prediction of a rise in hydrogen emission line
flux in the next year. Increases in the infrared flux are beginning to follow,
delayed by a 3 year observed timescale. If our model is confirmed, the
physics of changing-look quasars are governed by processes at the innermost
stable circular orbit (ISCO) around the black hole, and the structure of the
innermost disk. The easily identifiable and monitored changing-look quasars
would then provide a new probe and laboratory of the nuclear central engine.Comment: 13 pages, 4 figures, 3 tables. Published in MNRAS. All code and data
links on GitHub, https://github.com/d80b2t/WISE_L
The First Brown Dwarf Discovered by the Backyard Worlds: Planet 9 Citizen Science Project
The Wide-field Infrared Survey Explorer (WISE) is a powerful tool for finding
nearby brown dwarfs and searching for new planets in the outer solar system,
especially with the incorporation of NEOWISE and NEOWISE-Reactivation data. So
far, searches for brown dwarfs in WISE data have yet to take advantage of the
full depth of the WISE images. To efficiently search this unexplored space via
visual inspection, we have launched a new citizen science project, called
"Backyard Worlds: Planet 9," which asks volunteers to examine short animations
composed of difference images constructed from time-resolved WISE coadds. We
report the discovery of the first new substellar object found by this project,
WISEA J110125.95+540052.8, a T5.5 brown dwarf located approximately 34 pc from
the Sun with a total proper motion of 0.7 as yr. WISEA
J110125.95+540052.8 has a WISE magnitude of , this
discovery demonstrates the ability of citizen scientists to identify moving
objects via visual inspection that are 0.9 magnitudes fainter than the
single-exposure sensitivity, a threshold that has limited prior motion-based
brown dwarf searches with WISE.Comment: 9 pages, 4 figures, 1 table. Accepted for publication in the
Astrophysical Journal Letter
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