ABSTRACT Using a shallow, two-color survey carried out with the Dark Energy Camera, we detect the southern, possibly trailing arm of the Orphan Stream. The stream is reliably detected to a decl. of ...−38°, bringing the total known length of the Orphan Stream to 108°. We find a slight offset or "S" shape in the stream at δ −14° that would be consistent with the transition from leading to trailing arms. This coincides with a moderate concentration of 137 25 stars (to g = 21.6) that we consider a possible remnant of the Orphan progenitor. The position of this feature is in agreement with previous predictions.
We report the first hypervelocity star (HVS) discovered from the LAMOST spectroscopic survey. It is a B-type star with a heliocentric radial velocity of about 620 km s super(-1), which projects to a ...Galactocentric radial velocity component of ~477km s super(-1). With a heliocentric distance of ~13 kpc and an apparent magnitude of ~13 mag, it is the closest bright HVS currently known. With a mass of ~9 M sub(middot in circle), it is one of the three most massive HVSs discovered so far. The star is clustered on the sky with many other known HVSs, and its position suggests a possible connection to Galactic center structures. With the current poorly determined proper motion, a Galactic center origin of this HVS remains consistent with the data at the 1 sub(sigma) level, while a disk runaway origin cannot be excluded. We discuss the potential of the LAMOST survey to discover a large statistical sample of HVSs of different types.
We describe the near real-time transient-source discovery engine for the intermediate Palomar Transient Factory (iPTF), currently in operations at the Infrared Processing and Analysis Center (IPAC), ...Caltech. We coin this system the IPAC/iPTF Discovery Engine (or IDE). We review the algorithms used for PSF-matching, image subtraction, detection, photometry, and machine-learned (ML) vetting of extracted transient candidates. We also review the performance of our ML classifier. For a limiting signal-to-noise ratio of 4 in relatively unconfused regions, bogus candidates from processing artifacts and imperfect image subtractions outnumber real transients by ≃10:1. This can be considerably higher for image data with inaccurate astrometric and/or PSF-matching solutions. Despite this occasionally high contamination rate, the ML classifier is able to identify real transients with an efficiency (or completeness) of ≃97% for a maximum tolerable false-positive rate of 1% when classifying raw candidates. All subtraction-image metrics, source features, ML probability-based real-bogus scores, contextual metadata from other surveys, and possible associations with known Solar System objects are stored in a relational database for retrieval by the various science working groups. We review our efforts in mitigating false-positives and our experience in optimizing the overall system in response to the multitude of science projects underway with iPTF.
We describe a methodology to classify periodic variable stars identified using photometric time-series measurements constructed from the Wide-field Infrared Survey Explorer (WISE) full-mission ...single-exposure Source Databases. This will assist in the future construction of a WISE Variable Source Database that assigns variables to specific science classes as constrained by the WISE observing cadence with statistically meaningful classification probabilities. We have analyzed the WISE light curves of 8273 variable stars identified in previous optical variability surveys (MACHO, GCVS, and ASAS) and show that Fourier decomposition techniques can be extended into the mid-IR to assist with their classification. Combined with other periodic light-curve features, this sample is then used to train a machine-learned classifier based on the random forest (RF) method. Consistent with previous classification studies of variable stars in general, the RF machine-learned classifier is superior to other methods in terms of accuracy, robustness against outliers, and relative immunity to features that carry little or redundant class information. For the three most common classes identified by WISE: Algols, RR Lyrae, and W Ursae Majoris type variables, we obtain classification efficiencies of 80.7%, 82.7%, and 84.5% respectively using cross-validation analyses, with 95% confidence intervals of approximately + or -2%. These accuracies are achieved at purity (or reliability) levels of 88.5%, 96.2%, and 87.8% respectively, similar to that achieved in previous automated classification studies of periodic variable stars.
Abstract
We fit the mass and radial profile of the Orphan–Chenab Stream’s (OCS) dwarf-galaxy progenitor by using turnoff stars in the Sloan Digital Sky Survey and the Dark Energy Camera to constrain
...N
-body simulations of the OCS progenitor falling into the Milky Way on the 1.5 PetaFLOPS MilkyWay@home distributed supercomputer. We infer the internal structure of the OCS’s progenitor under the assumption that it was a spherically symmetric dwarf galaxy composed of a stellar system embedded in an extended dark matter halo. We optimize the evolution time, the baryonic and dark matter scale radii, and the baryonic and dark matter masses of the progenitor using a differential evolution algorithm. The likelihood score for each set of parameters is determined by comparing the simulated tidal stream to the angular distribution of OCS stars observed in the sky. We fit the total mass of the OCS’s progenitor to (2.0 ± 0.3) × 10
7
M
⊙
with a mass-to-light ratio of
γ
= 73.5 ± 10.6 and (1.1 ± 0.2) × 10
6
M
⊙
within 300 pc of its center. Within the progenitor’s half-light radius, we estimate a total mass of (4.0 ± 1.0) × 10
5
M
⊙
. We also fit the current sky position of the progenitor’s remnant to be (
α
,
δ
) = ((166.0 ± 0.9)°, (−11.1 ± 2.5)°) and show that it is gravitationally unbound at the present time. The measured progenitor mass is on the low end of previous measurements and, if confirmed, lowers the mass range of ultrafaint dwarf galaxies. Our optimization assumes a fixed Milky Way potential, OCS orbit, and radial profile for the progenitor, ignoring the impact of the Large Magellanic Cloud.
Abstract
We analyze stellar proper motions in the COSMOS field to assess the presence of bulk motions. At bright magnitudes (
G
-band 18.5–20.76 AB), we use the proper motions of 1010 stars in the ...Gaia DR2 catalog. At the faint end, we compute proper motions of 11,519 pointlike objects at
i
-band magnitudes 19–25 AB using Hubble ACS and Subaru HSC data, which span two epochs about 11 yr apart. In order to measure these proper motions with unprecedented accuracy at faint magnitudes, we developed a foundational set of astrometric tools that will be required for joint survey processing of data from the next generation of optical/infrared surveys. The astrometric grids of Hubble ACS and Subaru HSC mosaics were corrected at the catalog level using proper motion–propagated and parallax-corrected Gaia DR2 sources. These astrometric corrections were verified using compact extragalactic sources. Upon comparison of our measured proper motions with Gaia DR2, we estimate the uncertainties in our measurements to be ∼2–3 mas yr
−1
axis
−1
, down to 25.5 AB mag. We correct proper motions for the mean motion of the Sun, and we find that late-type main-sequence stars predominantly in the thin disk in the COSMOS field have space velocities mainly toward the Galactic center. We detect candidate high-velocity (≥220 km s
−1
) stars, six of them at ∼0.4–6 kpc, from the Gaia sample, and five of them at ∼20 kpc, from the faint star HSC and ACS sample. The sources from the faint star sample may be candidate halo members of the Sangarius stream.
We present observations of 77 early-type galaxies imaged with the PC1 CCD of the Hubble Space Telescope (HST) WFPC2. 'Nuker-law' parametric fits to the surface brightness profiles are used to ...classify the central structure into 'core' or 'power-law' forms. Core galaxies are typically rounder than power-law galaxies. Nearly all power-law galaxies with central ellipticities >= 0.3 have stellar disks, implying that disks are present in power-law galaxies with < 0.3 but are not visible because of unfavorable geometry. A few low-luminosity flattened core galaxies also have disks; these may be transition forms from power-law galaxies to more luminous core galaxies, which lack disks. Several core galaxies have strong isophote twists interior to their break radii, although power-law galaxies have interior twists of similar physical significance when the photometric perturbations implied by the twists are evaluated. Central color gradients are typically consistent with the envelope gradients; core galaxies have somewhat weaker color gradients than power-law galaxies. Nuclei are found in 29% of the core galaxies and 60% of the power-law galaxies. Nuclei are typically bluer than the surrounding galaxy. While some nuclei are associated with active galactic nuclei (AGNs), just as many are not; conversely, not all galaxies known to have a low-level AGN exhibit detectable nuclei in the broadband filters. NGC 4073 and 4382 are found to have central minima in their intrinsic starlight distributions; NGC 4382 resembles the double nucleus of M31. In general, the peak brightness location is coincident with the photocenter of the core to a typical physical scale of <1 pc. Five galaxies, however, have centers significantly displaced from their surrounding cores; these may be unresolved asymmetric double nuclei. Finally, as noted by previous authors, central dust is visible in about half of the galaxies. The presence and strength of dust correlates with nuclear emission; thus, dust may outline gas that is falling into the central black hole. The prevalence of dust and its morphology suggest that dust clouds form, settle to the center, and disappear repeatedly on ~108 yr timescales. We discuss the hypothesis that cores are created by the decay of a massive black hole binary formed in a merger. Apart from their brightness profiles, there are no strong differences between core galaxies and power-law galaxies that demand this scenario; however, the rounder shapes of core, their lack of disks, and their reduced color gradients may be consistent with it.
Almost every known low-luminosity Milky Way dwarf spheroidal (dSph) satellite galaxy contains at least one RR Lyrae star. Assuming that a fraction of distant (60 < d sub(helio) < 100 kpc) Galactic ...halo RR Lyrae stars are members of yet to be discovered low-luminosity dSph galaxies, we perform a guided search for these low-luminosity dSph galaxies. In order to detect the presence of dSph galaxies, we combine stars selected from more than 123 sightlines centered on RR Lyrae stars identified by the Palomar Transient Factory. We find that this method is sensitive enough to detect the presence of Segue 1-like galaxies (MV = (ProQuest: Formulae and/or non-USASCII text omitted), rh = 30 pc) even if only ~20 sightlines were occupied by such dSph galaxies. Yet, when our method is applied to the Sloan Digital Sky Survey Data Release 10 imaging catalog, no signal is detected. An application of our method to sightlines occupied by pairs of close (<200 pc) horizontal branch stars, also did not yield a detection. Thus, we place upper limits on the number of low-luminosity dSph galaxies with half-light radii from 30 pc to 120 pc, and in the probed volume of the halo. Stronger constraints on the luminosity function may be obtained by applying our method to sightlines centered on RR Lyrae stars selected from the Pan-STARRS1 survey, and eventually, from the Large Synoptic Survey Telescope. In Appendix A, we present spectroscopic observations of an RRab star in the Bootes 3 dSph and a light curve of an RRab star near the Bootes 2 dSph.
ABSTRACT We investigate whether the recently discovered Phoenix stream may be part of a much longer stream that includes the previously discovered Hermus stream. Using a simple model of the Galaxy ...with a disk, bulge, and a spherical dark matter halo, we show that a nearly circular orbit, highly inclined with respect to the disk, can be found that fits the positions, orientations, and distances of both streams. While the two streams are somewhat misaligned in the sense that they do not occupy the same plane, nodal precession due to the Milky Way disk potential naturally brings the orbit into line with each stream in the course of half an orbit. We consequently consider a common origin for the two streams as plausible. Based on our best-fitting orbit, we make predictions for the positions, distances, radial velocities, and proper motions along each stream. If our hypothesis is borne out by measurements, then at 183° ( 235° with respect to the Galactic center) and 76 kpc in length, Phoenix-Hermus would become the longest cold stream yet found. This would make it a particularly valuable new probe of the shape and mass of the Galactic halo out to 20 kpc.
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