71 research outputs found
Normal and unusual transient events in IRAC images
The Spitzer Space Telescope Infrared Array Camera (IRAC) is a four-channel camera that uses two pairs of 256 x 256 pixel InSb and Si:As IBC detectors to provide simultaneous images at 3.6, 4.5, 5.8, and 8 microns. IRAC experiences a flux of cosmic rays that produce transient events in images from each of the arrays, with 5-7 pixels per second being affected in an IRAC integration. The vast majority of these transient events can be adequately characterized so they can be effectively detected and flagged by a pipeline software module. However, because of the nature of the arrays and their arrangement in the camera structure, a small fraction of the cosmic ray hits on IRAC produce transients with unusual morphologies which cannot be characterized in a general way. We present nominal cosmic ray rates observed for IRAC on-orbit and rates observed during a period of elevated solar proton flux following a series of X-class solar flares in late 2003. We also present a guide for observers to help identify unusual transient events in their data. We comment on the physical nature of the production of many o9f these unusual transients and how this mechanism differs from the production of "normal" transient events
Globular Cluster Populations in Four Early-Type Poststarburst Galaxies
We present a study of the globular cluster systems of four early-type
poststarburst galaxies using deep g and I-band images from the ACS camera
aboard the Hubble Space Telescope (HST). All the galaxies feature shells
distributed around their main bodies and are thus likely merger remnants. The
color distribution of the globular clusters in all four galaxies shows a broad
peak centered on g-I ~ 1.4, while PGC 6240 and PGC 42871 show a significant
number of globular clusters with g-I ~ 1.0. The latter globular clusters are
interpreted as being of age ~ 500 Myr and likely having been formed in the
merger. The color of the redder peak is consistent with that expected for an
old metal-poor population that is very commonly found around normal galaxies.
However, all galaxies except PGC 10922 contain several globular clusters that
are significantly brighter than the maximum luminosity expected of a single old
metal-poor population. To test for multiple-age populations of overlapping g-I
color, we model the luminosity functions of the globular clusters as composites
of an old metal-poor subpopulation with a range of plausible specific
frequencies and an intermediate-age subpopulation of solar metallicity. We find
that three of the four sample galaxies show evidence for the presence of an
intermediate-age (~ 1 Gyr) globular cluster population, in addition to the old
metal-poor GC population seen in normal early-type galaxies. None of the
galaxies show a significant population of clusters consistent with an old,
metal-rich red cluster population that is typically seen in early-type
galaxies.Comment: 41 pages, 13 figures, accepted for publication in AJ. Some figues
have been downgraded to reduce their size
The Red Nova-like Variable in M31 - A Blue Candidate in Quiescence
M31-RV was an extraordinarily luminous (~10^6 Lsun) eruptive variable,
displaying very cool temperatures (roughly 1000 Kelvins) as it faded. The
photometric behavior of M31-RV (and several other very red novae, i.e. luminous
eruptive red variables) has led to several models of this apparently new class
of astrophysical object. One of the most detailed models is that of
"mergebursts": hypothetical mergers of close binary stars. These are predicted
to rival or exceed the brightest classical novae in luminosity, but to be much
cooler and redder than classical novae, and to become slowly hotter and bluer
as they age. This prediction suggests two stringent and definitive tests of the
mergeburst hypothesis. First, there should always be a cool red remnant, and
NOT a hot blue remnant at the site of such an outburst. Second, the inflated
envelope of a mergeburst event should be slowly contracting, hence it must
display a slowly rising effective temperature. We have located a luminous,
UV-bright object within 0.4 arcsec (1.5 sigma of the astrometric position) of
M31-RV in archival WFPC2 images taken 10 years after the outburst: it resembles
an old nova. Twenty years after the outburst, the object remains much too hot
to be a mergeburst. Its behavior remains consistent with that of theoretical
nova models which erupt on a low mass white dwarf. Future Hubble UV and visible
images could determine if the M31-RV analogs (in M85 and in M99) are also
behaving like old novae.Comment: Accepted for publication in ApJ, comments welcom
Normal and unusual transient events in IRAC images
The Spitzer Space Telescope Infrared Array Camera (IRAC) is a four-channel camera that uses two pairs of 256 x 256 pixel InSb and Si:As IBC detectors to provide simultaneous images at 3.6, 4.5, 5.8, and 8 microns. IRAC experiences a flux of cosmic rays that produce transient events in images from each of the arrays, with 5-7 pixels per second being affected in an IRAC integration. The vast majority of these transient events can be adequately characterized so they can be effectively detected and flagged by a pipeline software module. However, because of the nature of the arrays and their arrangement in the camera structure, a small fraction of the cosmic ray hits on IRAC produce transients with unusual morphologies which cannot be characterized in a general way. We present nominal cosmic ray rates observed for IRAC on-orbit and rates observed during a period of elevated solar proton flux following a series of X-class solar flares in late 2003. We also present a guide for observers to help identify unusual transient events in their data. We comment on the physical nature of the production of many o9f these unusual transients and how this mechanism differs from the production of "normal" transient events
Ten Simple Rules for Digital Data Storage
Data is the central currency of science, but the nature of scientific data has changed dramatically with the rapid pace of technology. This change has led to the development of a wide variety of data formats, dataset sizes, data complexity, data use cases, and data sharing practices. Improvements in high throughput DNA sequencing, sustained institutional support for large sensor networks, and sky surveys with large-format digital cameras have created massive quantities of data. At the same time, the combination of increasingly diverse research teams and data aggregation in portals (e.g. for biodiversity data, GBIF or iDigBio) necessitates increased coordination among data collectors and institutions. As a consequence, “data” can now mean anything from petabytes of information stored in professionally-maintained databases, through spreadsheets on a single computer, to hand-written tables in lab notebooks on shelves. All remain important, but data curation practices must continue to keep pace with the changes brought about by new forms and practices of data collection and storage.</jats:p
Normal and unusual transient events in IRAC images
The Spitzer Space Telescope Infrared Array Camera (IRAC) is a four-channel camera that uses two pairs of 256 x 256 pixel InSb and Si:As IBC detectors to provide simultaneous images at 3.6, 4.5, 5.8, and 8 microns. IRAC experiences a flux of cosmic rays that produce transient events in images from each of the arrays, with 5-7 pixels per second being affected in an IRAC integration. The vast majority of these transient events can be adequately characterized so they can be effectively detected and flagged by a pipeline software module. However, because of the nature of the arrays and their arrangement in the camera structure, a small fraction of the cosmic ray hits on IRAC produce transients with unusual morphologies which cannot be characterized in a general way. We present nominal cosmic ray rates observed for IRAC on-orbit and rates observed during a period of elevated solar proton flux following a series of X-class solar flares in late 2003. We also present a guide for observers to help identify unusual transient events in their data. We comment on the physical nature of the production of many o9f these unusual transients and how this mechanism differs from the production of "normal" transient events
An exponential decline at the bright end of the z=6 galaxy luminosity function
We present the results of a search for the most luminous star-forming
galaxies at redshifts z~6 based on CFHT Legacy Survey data. We identify a
sample of 40 Lyman break galaxies brighter than magnitude z'=25.3 across an
area of almost 4 square degrees. Sensitive spectroscopic observations of seven
galaxies provide redshifts for four, of which only two have moderate to strong
Lyman alpha emission lines. All four have clear continuum breaks in their
spectra. Approximately half of the Lyman break galaxies are spatially resolved
in 0.7 arcsec seeing images, indicating larger sizes than lower luminosity
galaxies discovered with the Hubble Space Telescope, possibly due to on-going
mergers. The stacked optical and infrared photometry is consistent with a
galaxy model with stellar mass ~ 10^{10} solar masses. There is strong evidence
for substantial dust reddening with a best-fit A_V=0.7 and A_V>0.48 at 2 sigma
confidence, in contrast to the typical dust-free galaxies of lower luminosity
at this epoch. The spatial extent and spectral energy distribution suggest that
the most luminous z~6 galaxies are undergoing merger-induced starbursts. The
luminosity function of z=5.9 star-forming galaxies is derived. This agrees well
with previous work and shows strong evidence for an exponential decline at the
bright end, indicating that the feedback processes which govern the shape of
the bright end are occurring effectively at this epoch.Comment: 14 pages, 11 figures, AJ in press, revised to address referee
comment
Spitzer IRS 16 micron Observations of the GOODS Fields
We present Spitzer 16 micron imaging of the Great Observatories Origins Deep
Survey (GOODS) fields. We survey 150 square arcminutes in each of the two GOODS
fields (North and South), to an average 3 sigma depth of 40 and 65 micro-Jy
respectively. We detect about 1300 sources in both fields combined. We validate
the photometry using the 3-24 micron spectral energy distribution of stars in
the fields compared to Spitzer spectroscopic templates. Comparison with ISOCAM
and AKARI observations in the same fields show reasonable agreement, though the
uncertainties are large. We provide a catalog of photometry, with sources cross
correlated with available Spitzer, Chandra, and HST data. Galaxy number counts
show good agreement with previous results from ISOCAM and AKARI, with improved
uncertainties. We examine the 16 to 24 micron flux ratio and find that for most
sources it lies within the expected locus for starbursts and infrared luminous
galaxies. A color cut of S_{16}/S_{24}>1.4 selects mostly sources which lie at
1.1<z<1.6, where the 24 micron passband contains both the redshifted 9.7 micron
silicate absorption and the minimum between PAH emission peaks. We measure the
integrated galaxy light of 16 micron sources, and find a lower limit on the
galaxy contribution to the extragalactic background light at this wavelength to
be 2.2\pm 0.2$ nW m^{-2} sr^{-1}.Comment: Accepted for Publication in the AJ. 53 preprint pages, including 15
figures and 8 tables. Table 1-4 are truncated in the ms.tex but are included
in full in the tar file (and will be available in the online version of the
AJ
The SPLASH Survey: A Spectroscopic Analysis of the Metal-Poor, Low-Luminosity M31 dSph Satellite Andromeda X
Andromeda X (And X) is a newly discovered low-luminosity M31 dwarf spheroidal
galaxy (dSph) found by Zucker et al. (2007) in the Sloan Digital Sky Survey
(SDSS - York et al. 2000). In this paper, we present the first spectroscopic
study of individual red giant branch stars in And X, as a part of the SPLASH
Survey (Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo).
Using the Keck II telescope and multiobject DEIMOS spectrograph, we target two
spectroscopic masks over the face of the galaxy and measure radial velocities
for ~100 stars with a median accuracy of sigma_v ~ 3 km/s. The velocity
histogram for this field confirms three populations of stars along the sight
line: foreground Milky Way dwarfs at small negative velocities, M31 halo red
giants over a broad range of velocities, and a very cold velocity ``spike''
consisting of 22 stars belonging to And X with v_rad = -163.8 +/- 1.2 km/s. By
carefully considering both the random and systematic velocity errors of these
stars (e.g., through duplicate star measurements), we derive an intrinsic
velocity dispersion of just sigma_v = 3.9 +/- 1.2 km/s for And X, which for its
size, implies a minimum mass-to-light ratio of M/L =37^{+26}_{-19} assuming the
mass traces the light. Based on the clean sample of member stars, we measure
the median metallicity of And X to be [Fe/H] = -1.93 +/- 0.11, with a slight
radial metallicity gradient. The dispersion in metallicity is large,
sigma([Fe/H]) = 0.48, possibly hinting that the galaxy retained much of its
chemical enrichment products. We discuss the potential for better understanding
the formation and evolution mechanisms for M31's system of dSphs through
(current) kinematic and chemical abundance studies, especially in relation to
the Milky Way sample. (abridged version)Comment: Accepted for Publication in Astrophys. J. 14 pages including 7
figures and 2 tables (journal format
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