Search

We present the detection of very extended stellar populations around the Large Magellanic Cloud (LMC) out to R~21 degrees, or ~18.5 kpc at the LMC distance of 50 kpc, as detected in the Survey of the MAgellanic Stellar History (SMASH) performed with the Dark Energy Camera on the NOAO Blanco 4m Telescope. The deep (g~24) SMASH color magnitude diagrams (CMDs) clearly reveal old (~9 Gyr), metal-poor ([Fe/H]=-0.8 dex) main-sequence stars at a distance of 50 kpc. The surface brightness of these detections is extremely low with our most distant detection having 34 mag per arcsec squared in g-band. The SMASH radial density profile breaks from the inner LMC exponential decline at ~13-15 degrees and a second component at larger radii has a shallower slope with power-law index of -2.2 that contributes ~0.4% of the LMC's total stellar mass. In addition, the SMASH densities exhibit large scatter around our best-fit model of ~70% indicating that the envelope of stellar material in the LMC periphery is highly disturbed. We also use data from the NOAO Source catalog to map the LMC main-sequence populations at intermediate radii and detect a steep dropoff in density on the eastern side of the LMC (at R~8 deg) as well as an extended structure to the far northeast. These combined results confirm the existence of a very extended, low-density envelope of stellar material with disturbed shape around the LMC. The exact origin of this structure remains unclear but the leading options include a classical accreted halo or tidally stripped outer disk material., Comment: 9 pages, 5 figues, 1 table, submitted to ApJ

We characterize the absolute magnitudes and colors of RR Lyrae stars in the globular cluster M5 in the ugriz filter system of the Dark Energy Camera (DECam). We provide empirical Period-Luminosity (P-L) relationships in all 5 bands based on 47 RR Lyrae stars of the type ab and 14 stars of the type c. The P-L relationships were found to be better constrained for the fundamental mode RR Lyrae stars in the riz passbands, with dispersion of 0.03, 0.02 and 0.02 magnitudes, respectively. The dispersion of the color at minimum light was found to be small, supporting the use of this parameter as a means to obtain accurate interstellar extinctions along the line of sight up to the distance of the RR Lyrae star. We found a trend of color at minimum light with pulsational period that, if taken into account, brings the dispersion in color at minimum light to < 0.016 magnitudes for the (r-i), (i-z), and (r-z) colors. These calibrations will be very useful for using RR Lyrae stars from DECam observations as both standard candles for distance determinations and color standards for reddening measurements., Accepted for publication in AJ

We report the discovery of RAVE J203843.2-002333, a bright (V = 12.73), very metal-poor ([Fe/H] = -2.91), r-process-enhanced ([Eu/Fe] = +1.64 and [Ba/Eu] = -0.81) star selected from the RAVE survey. This star was identified as a metal-poor candidate based on its medium-resolution (R ~ 1,600) spectrum obtained with the KPNO/Mayall Telescope, and followed-up with high-resolution (R ~ 66,000) spectroscopy with the Magellan/Clay Telescope, allowing for the determination of elemental abundances for 24 neutron-capture elements, including thorium and uranium. RAVE J2038-0023 is only the fourth metal-poor star with a clearly measured U abundance. The derived chemical-abundance pattern exhibits good agreement with those of other known highly r-process-enhanced stars, and evidence in hand suggests that it is not an actinide-boost star. Age estimates were calculated using Th/X and U/X abundance ratios, yielding a mean age of 13.0 +/- 1.1 Gyr., Comment: 25 pages, 6 figures; accepted for publication in ApJ

We report the discovery of one RR Lyrae star in the ultra--faint satellite galaxy Hydra II based on time series photometry in the g, r and i bands obtained with the Dark Energy Camera at Cerro Tololo Interamerican Observatory, Chile. The RR Lyrae star has a mean magnitude of $i = 21.30\pm 0.04$ which translates to a heliocentric distance of $151\pm 8$ kpc for Hydra II; this value is $\sim 13\%$ larger than the estimate from the discovery paper based on the average magnitude of several blue horizontal branch star candidates. The new distance implies a slightly larger half-light radius of $76^{+12}_{-10}$ pc and a brighter absolute magnitude of $M_V = -5.1 \pm 0.3$, which keeps this object within the realm of the dwarf galaxies. The pulsational properties of the RR Lyrae star ($P=0.645$ d, $\Delta g = 0.68$ mag) suggest Hydra II may be a member of the intermediate Oosterhoff or Oosterhoff II group. A comparison with other RR Lyrae stars in ultra--faint systems indicates similar pulsational properties among them, which are different to those found among halo field stars and those in the largest of the Milky Way satellites. We also report the discovery of 31 additional short period variables in the field of view (RR Lyrae, SX Phe, eclipsing binaries, and a likely anomalous cepheid). However, given their magnitudes and large angular separation from Hydra II, they must be field stars not related to Hydra II., Comment: Revised version after comments from the referee. Accepted for publication in AJ

We present the discovery of a very faint stellar system, SMASH 1, that is potentially a satellite of the Large Magellanic Cloud. Found within the Survey of the MAgellanic Stellar History (SMASH), SMASH 1 is a compact ($r_h = 9.1^{+5.9}_{-3.4}$ pc) and very low luminosity (M_V = -1.0 +/- 0.9, $L_V=10^{2.3 +/- 0.4}$ Lsun) stellar system that is revealed by its sparsely populated main sequence and a handful of red-giant-branch candidate member stars. The photometric properties of these stars are compatible with a metal-poor ([Fe/H]=-2.2) and old (13 Gyr) isochrone located at a distance modulus of ~18.8, i.e. a distance of ~57 kpc. Situated at 11.3$^\circ$ from the LMC in projection, its 3-dimensional distance from the Cloud is ~13 kpc, consistent with a connection to the LMC, whose tidal radius is at least 16 kpc. Although the nature of SMASH 1 remains uncertain, its compactness favors it being a stellar cluster and hence dark-matter free. If this is the case, its dynamical tidal radius is only, Comment: 7 pages, 4 figures, 1 table. Accepted for publication in ApJL. v2: corrected figure 4 in which rh for globular clusters were miscalculated. The updated figure reflect the published version of the paper

The newly installed Wide Field Camera 3 (WFC3) on the Hubble Space Telescope has been used to obtain multi-band images of the nearby spiral galaxy M83. These new observations are the deepest and highest resolution images ever taken of a grand-design spiral, particularly in the near ultraviolet, and allow us to better differentiate compact star clusters from individual stars and to measure the luminosities of even faint clusters in the U band. We find that the luminosity function for clusters outside of the very crowded starburst nucleus can be approximated by a power law, dN/dL \propto L^{alpha}, with alpha = -2.04 +/- 0.08, down to M_V ~ -5.5. We test the sensitivity of the luminosity function to different selection techniques, filters, binning, and aperture correction determinations, and find that none of these contribute significantly to uncertainties in alpha. We estimate ages and masses for the clusters by comparing their measured UBVI,Halpha colors with predictions from single stellar population models. The age distribution of the clusters can be approximated by a power-law, dN/dt propto t^{gamma}, with gamma=-0.9 +/- 0.2, for M > few x 10^3 Msun and t < 4x10^8 yr. This indicates that clusters are disrupted quickly, with ~80-90% disrupted each decade in age over this time. The mass function of clusters over the same M-t range is a power law, dN/dM propto M^{beta}, with beta=-1.94 +/- 0.16, and does not have bends or show curvature at either high or low masses. Therefore, we do not find evidence for a physical upper mass limit, M_C, or for the earlier disruption of lower mass clusters when compared with higher mass clusters, i.e. mass-dependent disruption. We briefly discuss these implications for the formation and disruption of the clusters.

We present results from a medium-resolution (R ~ 2, 000) spectroscopic follow-up campaign of 1,694 bright (V < 13.5), very metal-poor star candidates from the RAdial Velocity Experiment (RAVE). Initial selection of the low-metallicity targets was based on the stellar parameters published in RAVE Data Releases 4 and 5. Follow-up was accomplished with the Gemini-N and Gemini-S, the ESO/NTT, the KPNO/Mayall, and the SOAR telescopes. The wavelength coverage for most of the observed spectra allows for the determination of carbon and ��-element abundances, which are crucial for con- sidering the nature and frequency of the carbon-enhanced metal-poor (CEMP) stars in this sample. We find that 88% of the observed stars have [Fe/H], 134 pages, accepted for publication in AJ

We present the discovery of a new dwarf galaxy, Hydra II, found serendipitously within the data from the ongoing Survey of the MAgellanic Stellar History (SMASH) conducted with the Dark Energy Camera on the Blanco 4m Telescope. The new satellite is compact (r_h = 68 +/- 11 pc) and faint (M_V = -4.8 +/- 0.3), but well within the realm of dwarf galaxies. The stellar distribution of HydraII in the color-magnitude diagram is well-described by a metal-poor ([Fe/H] = -2.2) and old (13 Gyr) isochrone and shows a distinct blue horizontal branch, some possible red clump stars, and faint stars that are suggestive of blue stragglers. At a heliocentric distance of 134 +/- 10 kpc, Hydra II is located in a region of the Galactic halo that models have suggested may host material from the leading arm of the Magellanic Stream. A comparison with N-body simulations hints that the new dwarf galaxy could be or could have been a satellite of the Magellanic Clouds., Comment: 7 pages, 4 figures. Accepted for publication in ApJL. v2: minor corrections, conclusions unchanged

Using data acquired as part of a unique Hubble Heritage imaging program of broadband colors of the interacting spiral system M51/NGC 5195, we have conducted a photometric study of the stellar associations across the entire disk of the galaxy in order to assess trends in size, luminosity, and local environment associated with recent star formation activity in the system. Starting with a sample of over 900 potential associations, we have produced color-magnitude and color-color diagrams for the 120 associations that were deemed to be single-aged. It has been found that main sequence turnoffs are not evident for the vast majority of the stellar associations in our set, potentially due to the overlap of isochronal tracks at the high mass end of the main sequence, and the limited depth of our images at the distance of M51. In order to obtain ages for more of our sample, we produced model spectral energy distributions (SEDs) to fit to the data from the GALEXEV simple stellar population (SSP) models of Bruzual and Charlot (2003). These SEDs can be used to determine age, size, mass, metallicity, and dust content of each association via a simple chi-squared minimization to each association's B, V, and I-band fluxes. The derived association properties are mapped as a function of location, and recent trends in star formation history of the galaxy are explored in light of these results. This work is the first phase in a program that will compare these stellar systems with their environments using ultraviolet data from GALEX and infrared data from Spitzer, and ultimately we plan to apply the same stellar population mapping methodology to other nearby face-on spiral galaxies., 13 pages, 3 figures, 1 table. Accepted to The Astronomical Journal

We present a multi-wavelength photometric study of ~15,000 resolved stars in the nearby spiral galaxy M83 (NGC5236, D=4.61Mpc) based on Hubble Space Telescope Wide Field Camera 3 observations using four filters: F336W, F438W, F555W, and F814W. We select 50 regions (an average size of 260 pc by 280 pc) in the spiral arm and inter-arm areas of M83, and determine the age distribution of the luminous stellar populations in each region. This is accomplished by correcting for extinction towards each individual star by comparing its colors with predictions from stellar isochrones. We compare the resulting luminosity weighted mean ages of the luminous stars in the 50 regions with those determined from several independent methods, including the number ratio of red-to-blue supergiants, morphological appearance of the regions, surface brightness fluctuations, and the ages of clusters in the regions. We find reasonably good agreement between these methods. We also find that young stars are much more likely to be found in concentrated aggregates along spiral arms, while older stars are more dispersed. These results are consistent with the scenario that star formation is associated with the spiral arms, and stars form primarily in star clusters and then disperse on short timescales to form the field population. The locations of Wolf-Rayet stars are found to correlate with the positions of many of the youngest regions, providing additional support for our ability to accurately estimate ages. We address the effects of spatial resolution on the measured colors, magnitudes, and age estimates. While individual stars can occasionally show measurable differences in the colors and magnitudes, the age estimates for entire regions are only slightly affected., 28 pages, 17 figures, 2 tables. Accepted for publication in ApJ

We use new WFC3 observations of the nearby grand design spiral galaxy M83 to develop two independent methods for estimating the ages of young star clusters. The first method uses the physical extent and morphology of Halpha emission to estimate the ages of clusters younger than tau ~10 Myr. It is based on the simple premise that the gas in very young (tau < few Myr) clusters is largely coincident with the cluster stars, is in a small, ring-like structure surrounding the stars in slightly older clusters (e.g., tau ~5 Myr), and is in a larger ring-like bubble for still older clusters (i.e., ~5-10 Myr). The second method is based on an observed relation between pixel-to-pixel flux variations within clusters and their ages. This method relies on the fact that the brightest individual stars in a cluster are most prominent at ages around 10 Myr, and fall below the detection limit (i.e., M_V < -3.5) for ages older than about 100 Myr. These two methods are the basis for a new morphological classification system which can be used to estimate the ages of star clusters based on their appearance. We compare previous age estimates of clusters in M83 determined from fitting UBVI Halpha measurements using predictions from stellar evolutionary models with our new morphological categories and find good agreement at the ~95% level. The scatter within categories is ~0.1 dex in log tau for young clusters (10 Myr) clusters. A by-product of this study is the identification of 22 "single-star" HII regions in M83, with central stars having ages ~4 Myr., 33 pages, 10 figures, 3 tables; published in March ApJ

The Large and Small Magellanic Clouds (LMC and SMC) are unique local laboratories for studying the formation and evolution of small galaxies in exquisite detail. The Survey of the MAgellanic Stellar History (SMASH) is an NOAO community DECam survey of the Clouds mapping 480 square degrees (distributed over ~2400 square degrees at ~20% filling factor) to ~24th mag in ugriz with the goal of identifying broadly distributed, low surface brightness stellar populations associated with the stellar halos and tidal debris of the Clouds. SMASH will also derive spatially-resolved star formation histories covering all ages out to large radii from the MCs that will further complement our understanding of their formation. Here, we present a summary of the survey, its data reduction, and a description of the first public Data Release (DR1). The SMASH DECam data have been reduced with a combination of the NOAO Community Pipeline, PHOTRED, an automated PSF photometry pipeline based mainly on the DAOPHOT suite, and custom calibration software. The attained astrometric precision is ~15 mas and the accuracy is ~2 mas with respect to the Gaia DR1 astrometric reference frame. The photometric precision is ~0.5-0.7% in griz and ~1% in u with a calibration accuracy of ~1.3% in all bands. The median 5 sigma point source depths in ugriz bands are 23.9, 24.8, 24.5, 24.2, 23.5 mag. The SMASH data already have been used to discover the Hydra II Milky Way satellite, the SMASH 1 old globular cluster likely associated with the LMC, and very extended stellar populations around the LMC out to R~18.4 kpc. SMASH DR1 contains measurements of ~100 million objects distributed in 61 fields. A prototype version of the NOAO Data Lab provides data access, including a data discovery tool, SMASH database access, an image cutout service, and a Jupyter notebook server with example notebooks for exploratory analysis., 23 pages, 12 figures. Revised and slightly reorganized based on referee's comments. Accepted for publication in AJ