At a Crossroads: Stellar Streams in the South Galactic Cap

We examine the distribution of old, metal-poor stars in a portion of the recently released PanSTARRs survey. We find an interesting confluence of four new cold stellar stream candidates that appear to converge on or pass near the south Galactic pole. The stream candidates, which we designate Murrumbidgee, Molonglo, Orinoco, and Kwando, lie at a distance of approximately 20 kpc and range in length from 13 to 95 degrees, or about 5 to 33 kpc. The stream candidates are between 100 and 300 pc in width, and are estimated to contain between 3000 and 8000 stars each, suggesting progenitors similar to modern day globular clusters. The trajectories of the streams imply orbits that range from hyperbolic to nearly circular. The Molonglo stream is nearly parallel to, at the same distance as, and offset by only 2.5 degrees from the previously discovered ATLAS stream, suggesting a possible common origin. Orinoco and Kwando also have similarly shaped, moderately eccentric, obliquely viewed orbits that suggest distinct progenitors within a common, larger parent body.Comment: 9 pages, 4 figures, accepted for publication in the Astrophysical Journal, final versio

Finding Stellar Streams in Photometric Surveys

The advent of high‐quality, wide‐area photometric databases has enabled significant advances in our knowledge of the structure, constituents, and accretion history of our Galaxy. Matched‐filtering techniques allow us to map the spatial distributions of stellar populations with particular color‐magnitude characteristics. This has already led to the detection of several long tidal streams in the Sloan Digital Sky Survey footprint. These streams are variously attributed to disrupted globular clusters and dwarf galaxies and offer us a new window into the structure, dark matter content, and formation history of the Galaxy. Similar techniques applied to future photometric surveys will enable us to expand the search for stellar debris streams over much of the Local Group

Detection of a 50° long Trailing Tidal Tail for the Globular Cluster M5

Using photometry and proper motions from Gaia Data Release 2, we detect a 50° long stream of about 70 stars extending westward from the halo globular cluster M5. Based on the similarities in distance, proper motions, inferred color–magnitude distribution, and trajectory, we identify this stream as the trailing tidal tail of M5. While the surface density of stars is very low (≃1.5 star per square degree, or ≈35 mag per square arcsecond), selecting only stars with proper motions consistent with the orbit of the cluster yields a detection significance of ≈10σ. While we find a possible continuation of the stream to ≈85°, increasing foreground contamination combined with a greater predicted stream distance makes it difficult to detect with current data even if the stream continues unabated. The nonuniform distribution of stars in the stream appears to be consistent with episodic tidal stripping, with the most recently shed stars now trailing the cluster by tens of degrees. We provide a table of the highest-ranked candidate stream stars for ongoing and future spectroscopic surveys

A Matched Filter Analysis of SDSS DR8 Photometry in the Vicinity of the Cetus Polar Stream

We examine the region of sky in the vicinity of the Cetus Polar Stream and find indications of at least three narrow and metal poor streams at distances ranging from from 28 to 37 kpc and extending over 60 degrees of sky. We suggest that we may have resolved the substructure in this region into a number of relatively cold streams, and that the original Cetus Polar Stream detection may have sampled two or more of these streams. Similarities in distance and orientation suggest that these streams may be dynamically related and/or share a common origin

FOSSILs in the Galactic Halo

We use a matched filter to detect compact groups of old, metal-poor stars that we term FOSSILs (Fragments of Old Stellar Systems in Limbo). With size scales on the order of 10 arcminutes, distances ranging from 2 to 200 kpc, and memberships ranging from a handful to several dozen stars, these FOSSILs stand out from the surrounding field and are presumably signatures of, or debris from, ancient star clusters and dwarf galaxies. They may be localized concentrations of stars within more extensive tidal streams, and in some cases may be the signatures of extant but heretofore undetected ultrafaint galaxies. Using magnitudes and colors from the Pan-STARRs survey, we detect ∼70 such FOSSILs at 5σ or greater in a 2200 square degree region in the vicinity of the north Galactic pole. A subsample of more populous FOSSILs that could be candidate ultrafaint dwarf galaxies suggests a total population of 200 such objects within 200 kpc of the Galactic center. Spectroscopic and astrometric follow-up of these FOSSILs will be required to determine the nature of these structures, deepen our understanding of the make-up and accretion history of the Galactic halo, and perhaps alleviate the missing satellites problem

Globular Clusters in the Sombrero Galaxy (NGC 4594)

The Sombrero galaxy, NGC 4594, contains the most numerous globular cluster system of any nearby spiral. It is an ideal candidate in which to study the globular clusters and contrast them with those in Local Group spirals. Here we present B and I imaging from the CTIO Schmidt telescope which gives a field-of-view of 31' x 31'. Using DAOPHOT we have detected over 400 globular clusters and derived their magnitudes, B--I colors and photometric metallicities. We have attempted to separate our sample into disk and bulge/halo globular cluster populations, based on location in the galaxy. There is some evidence that the disk population is more metal--rich than the bulge/halo globular clusters, however contamination, dust reddening and small number statistics makes this result very tentative. We find that the median metallicity of the bulge/halo globular clusters is [Fe/H] = -0.8. This metallicity is consistent with previous estimates based on smaller samples. It is also similar to the metallicity predicted by the globular cluster metallicity -- galaxy luminosity relation. As with our Galaxy, there is no radial metallicity gradient in the halo globular clusters. This suggests that the spheriodal component of NGC 4594 did not form by a dissipational process.Comment: 7 pages, Latex. To be published in the Astronomical Journal. Full paper available at http://www.ucolick.org/~forbes/home.htm

Automated Classification of Periodic Variable Stars detected by the Wide-field Infrared Survey Explorer

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 +/-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.Comment: 48 pages, 17 figures, 1 table, accepted by A