Search

We verified for photometric stability a set of DA white dwarfs with Hubble Space Telescope magnitudes from the near-ultraviolet to the near-infrared and ground-based spectroscopy by using time-spaced observations from the Las Cumbres Observatory network of telescopes. The initial list of 38 stars was whittled to 32 final ones which comprise a high quality set of spectrophotometric standards. These stars are homogeneously distributed around the sky and are all fainter than r ~ 16.5 mag. Their distribution is such that at least two of them would be available to be observed from any observatory on the ground at any time at airmass less than two. Light curves and different variability indices from the Las Cumbres Observatory data were used to determine the stability of the candidate standards. When available, Pan-STARRS1, Zwicky Transient Facility and TESS data were also used to confirm the star classification. Our analysis showed that four DA white dwarfs may exhibit evidence of photometric variability, while a fifth is cooler than our established lower temperature limit, and a sixth star might be a binary. In some instances, due to the presence of faint nearby red sources, care should be used when observing a few of the spectrophotometric standards with ground-based telescopes. Light curves and finding charts for all the stars are provided., Comment: 48 pages, 50 figures, accepted for publication on ApJ

We present observations of the dwarf galaxies GALFA Dw3 and GALFA Dw4 with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope (HST). These galaxies were initially discovered as optical counterparts to compact HI clouds in the GALFA survey. Both objects resolve into stellar populations which display an old red giant branch, younger helium burning, and massive main sequence stars. We use the tip of the red giant branch method to determine the distance to each galaxy, finding distances of 7.61$_{-0.29}^{+0.28}$ Mpc and 3.10$_{-0.17}^{+0.16}$ Mpc, respectively. With these distances we show that both galaxies are extremely isolated, with no other confirmed objects within ~1.5 Mpc of either dwarf. GALFA Dw4 is also found to be unusually compact for a galaxy of its luminosity. GALFA Dw3 and Dw4 contain HII regions with young star clusters and an overall irregular morphology; they show evidence of ongoing star formation through both ultraviolet and H$\alpha$ observations and are therefore classified as dwarf irregulars (dIrrs). The star formation histories of these two dwarfs show distinct differences: Dw3 shows signs of a recently ceased episode of active star formation across the entire dwarf, while Dw4 shows some evidence for current star formation in spatially limited HII regions. Compact HI sources offer a promising method for identifying isolated field dwarfs in the Local Volume, including GALFA Dw3 & Dw4, with the potential to shed light on the driving mechanisms of dwarf galaxy formation and evolution., Comment: 18 pages, 7 figures, 1 table, submitted to ApJ

With the conclusion of the third observing run for Advanced LIGO/Virgo (O3), we present a detailed analysis of both triggered and serendipitous observations of 17 gravitational wave (GW) events (7 triggered and 10 purely serendipitous) from the Searches After Gravitational-waves Using ARizona Observatories (SAGUARO) program. We searched a total of 4935 deg$^2$ down to a median 5$\sigma$ transient detection depth of 21.1 AB mag using the Mt Lemmon 1.5 m telescope, the discovery engine for SAGUARO. In addition to triggered events within 24~hours, our transient search encompassed a time interval following GW events of $<120$~hrs, providing observations on $\sim$ 1/2 of the events accessible to the Mt Lemmon 1.5 m telescope. We covered 2.1--86\% of the LVC total probability ($P_{\rm total}$) for individual events, with a median $P_{\rm total} \approx 8\%$ within $<120$~hours. Following improvements to our pipeline and the addition of serendipitous observations, we find a total of 7 new optical candidates across 5 GW events which we are unable to rule out after searching for additional information and comparing to kilonova models. Using both publicly available and our own late-time data, we investigated a total of 252 optical candidates for these 17 events, finding only 65\% were followed up in some capacity by the community. Of the total 252 candidates, we are able to rule out an additional 12 previously reported counterpart candidates. In light of these results, we discuss lessons learned from the SAGUARO GW counterpart search. We discuss how community coordination of observations and candidate follow-up, as well as the role of archival data, are crucial to improving the efficiency of follow-up efforts and preventing unnecessary duplication of effort with limited EM resources., Comment: 44 pages, 21 figures, 2 tables, submitted to ApJ

We present Searches After Gravitational-waves Using ARizona Observatories (SAGUARO), a comprehensive effort dedicated to the discovery and characterization of optical counterparts to gravitational wave (GW) events. SAGUARO utilizes ground-based facilities ranging from 1.5m to 10m in diameter, located primarily in the Northern Hemisphere. We provide an overview of SAGUARO's telescopic resources, pipeline for transient detection, and database for candidate visualization. We describe SAGUARO's discovery component, which utilizes the $5$~deg$^2$ field-of-view optical imager on the Mt. Lemmon 1.5m telescope, reaching limits of $\approx 21.3$~AB mag while rapidly tiling large areas. We also describe the follow-up component of SAGUARO, used for rapid vetting and monitoring of optical candidates. With the onset of Advanced LIGO/Virgo's third observing run, we present results from the first three SAGUARO searches following the GW events S190408an, S190425z and S190426c, which serve as a valuable proof-of-concept of SAGUARO. We triggered and searched 15, 60 and 60 deg$^{2}$ respectively, 17.6, 1.4 and 41.8 hrs after the initial GW alerts. We covered 7.8, 3.0 and 5.1\% of the total probability within the GW event localizations, reaching 3$\sigma$ limits of 19.8, 21.3 and 20.8 AB mag, respectively. Although no viable counterparts associated with these events were found, we recovered 6 known transients and ruled out 5 potential candidates. We also present Large Binocular Telescope spectroscopy of PS19eq/SN2019ebq, a promising kilonova candidate that was later determined to be a supernova. With the ability to tile large areas and conduct detailed follow-up, SAGUARO represents a significant addition to GW counterpart searches., Comment: 16 pages, 7 figures, 1 table. Accepted to ApJL

We present precise photometry and spectroscopy for 23 candidate spectrophotometric standard white dwarfs. The selected stars are distributed in the Northern hemisphere and around the celestial equators and are all fainter than r ~ 16.5 mag. This network of stars, when established as standards, together with the three Hubble Space Telescope primary CALSPEC white dwarfs, will provide a set of spectrophotometric standards to directly calibrate data products to better than 1%. These new faint standard white dwarfs will have enough signal-to-noise ratio in future deep photometric surveys and facilities to be measured accurately while still avoiding saturation in such surveys. They will also fall within the dynamic range of large telescopes and their instruments for the foreseeable future. This paper discusses the provenance of the observational data for our candidate standard stars. The comparison with models, reconciliation with reddening, and the consequent derivation of the full spectral energy density distributions for each of them is reserved for a subsequent paper., Comment: 31 pages, 17 figures, 10 tables, ApJ in press (accepted on December 23rd, 2018)

High-resolution spectroscopic observations were taken of 29 extended main sequence turn-off (eMSTO) stars in the young ($\sim$200 Myr) LMC cluster, NGC 1866 using the Michigan/Magellan Fiber System and MSpec spectrograph on the Magellan-Clay 6.5-m telescope. These spectra reveal the first direct detection of rapidly rotating stars whose presence has only been inferred from photometric studies. The eMSTO stars exhibit H-alpha emission (indicative of Be-star decretion disks), others have shallow broad H-alpha absorption (consistent with rotation $\gtrsim $150 km s$^{-1}$), or deep H-alpha core absorption signaling lower rotation velocities ($ \lesssim $150 km s$^{-1}$ ). The spectra appear consistent with two populations of stars - one rapidly rotating, and the other, younger and slowly rotating., Comment: 9 pages, 4 figures, Accepted for publication in ApJ Letters

The dwarf spheroidal galaxies (dSph) of the Milky Way are among the most attractive targets for indirect searches of dark matter. In this work, we reconstruct the dark matter annihilation (J-factor) and decay profiles for the newly discovered dSph Reticulum II. Using an optimized spherical Jeans analysis of kinematic data obtained from the Michigan/Magellan Fiber System (M2FS), we find Reticulum II's J-factor to be among the largest of any Milky Way dSph. We have checked the robustness of this result against several ingredients of the analysis. Unless it suffers from tidal disruption or significant inflation of its velocity dispersion from binary stars, Reticulum II may provide a unique window on dark matter particle properties., Comment: 5 pages, 4 figures. Match the ApJL accepted version

We report five Local Volume dwarf galaxies (two of which are presented here for the first time) uncovered during a comprehensive archival search for optical counterparts to ultra-compact high velocity clouds (UCHVCs). The UCHVC population of HI clouds are thought to be candidate gas-rich, low mass halos at the edge of the Local Group and beyond, but no comprehensive search for stellar counterparts to these systems has been presented. Careful visual inspection of all publicly available optical and ultraviolet imaging at the position of the UCHVCs revealed six blue, diffuse counterparts with a morphology consistent with a faint dwarf galaxy beyond the Local Group. Optical spectroscopy of all six candidate dwarf counterparts show that five have an H$\alpha$-derived velocity consistent with the coincident HI cloud, confirming their association, the sixth diffuse counterpart is likely a background object. The size and luminosity of the UCHVC dwarfs is consistent with other known Local Volume dwarf irregular galaxies. The gas fraction ($M_{HI}/M_{star}$) of the five dwarfs are generally consistent with that of dwarf irregular galaxies in the Local Volume, although ALFALFA-Dw1 (associated with ALFALFA UCHVC HVC274.68+74.70$-$123) has a very high $M_{HI}/M_{star}$$\sim$40. Despite the heterogenous nature of our search, we demonstrate that the current dwarf companions to UCHVCs are at the edge of detectability due to their low surface brightness, and that deeper searches are likely to find more stellar systems. If more sensitive searches do not reveal further stellar counterparts to UCHVCs, then the dearth of such systems around the Local Group may be in conflict with $\Lambda$CDM simulations., Comment: 18 pages, 4 tables, 4 figures, ApJ Accepted

We show that a combination of infrared photometry from WISE and 2MASS surveys can yield highly pure samples of M giant stars. We take advantage of the new WISE$\cap$2MASS M giant selection to trace the Sagittarius trailing tail behind the Galactic disk in the direction of the anti-centre. The M giant candidates selected via broad-band photometry are confirmed spectroscopically using AAOmega on the AAT in 3 fields around the extremity of the Sgr trailing tail in the Southern Galactic hemisphere. We demonstrate that at the Sgr longitude $\tilde \Lambda_{\odot} = 204^{\circ}$, the line-of-sight velocity of the trailing tail starts to deviate from the track of the Law & Majewski (2010) model, confirming the prediction of Belokurov et al. (2014). This discovery serves to substantiate the measurement of low differential orbital precession of the Sgr stream which in turn may imply diminished dark matter content within 100 kpc., Comment: 8 pages, 7 figures; submitted to MNRAS, first referee report received

We present deep photometry of the Carina dwarf Spheroidal galaxy in the B,V filters from CTIO/MOSAIC, out to and beyond the tidal radius. The accurately calibrated photometry is combined with spectroscopic metallicity distributions of Red Giant Branch stars to determine the detailed star formation and chemical evolution history. The star formation history confirms the episodic formation history of Carina and quantifies the duration and strength of each episode in great detail, as a function radius from the centre. Two main episodes of star formation occurred at old (>8 Gyr) and intermediate (2-8 Gyr) ages, both enriching stars starting from low metallicities ([Fe/H]<-2 dex). By dividing the SFH into two components, we determine that 60pm9 percent of the total number of stars formed within the intermediate age episode. Furthermore, within the tidal radius (0.48 degrees or 888 pc) a total mass in stars of 1.07pm0.08 x10^6 M_sun was formed, giving Carina a stellar mass-to-light ratio of 1.8pm0.8. Combining the detailed star formation history with spectroscopic observations of RGB stars, we are able to determine the detailed age-metallicity relation of each episode and the timescale of alpha-element evolution of Carina from individual stars. The oldest episode displays a tight age-metallicity relation over 6 Gyr with steadily declining alpha-element abundances and a possible alpha-element knee at [Fe/H]~ -2.5 dex. The intermediate age sequence displays a more complex age-metallicity relation starting from low metallicity and a sequence in alpha-element abundances with a slope much steeper than observed in the old episode, starting from [Fe/H]=-1.8 dex and [Mg/Fe]~0.4 dex and declining to Mg-poor values ([Mg/Fe]<-0.5 dex). This indicates clearly that both episodes of star formation formed from gas with different abundance patterns, inconsistent with simple evolution in an isolated system., Comment: accepted for publication in A&A. 14 pages, 17 figures

We present deep CMDs for two Subaru Suprime-Cam fields in the Virgo Stellar Stream(VSS)/Virgo Overdensity(VOD) and compare them to a field centred on the highest concentration of Sagittarius (Sgr) Tidal Stream stars in the leading arm, Branch A of the bifurcation. A prominent population of MS stars is detected in all three fields and can be traced as faint as g~24 mag. Using theoretical isochrone fitting we derive an age of 9.1(+1.0;-1.1)Gyr, a median abundance of [Fe/H]=-0.70 (+0.15; -0.20)dex and d_helio of 30.9+-3.0kpc for the MS of the Sgr Stream Branch A. The dominant main sequence populations in the two VSS/VOD fields (Lsun ~265 deg, Bsun ~13 deg) are located at a mean distance of 23.3+-1.6kpc and have an age ~8.2Gyr and an abundance [Fe/H]=-0.67(+0.16;-0.12)dex similar to the Sgr Stream stars. These parameters are also in good agreement with the age of the main population in the Sagittarius dwarf (8.0+-1.5Gyr). They also agree with the peak in the metallicity distribution of 2-3Gyr old M-giants, [Fe/H] ~ -0.6dex, in the Sgr leading arm north. We then compare the results from the VSS/VOD fields with the Sgr Tidal Stream model by Law & Majewski based on a triaxial Galactic halo that is calibrated with SDSS Sgr A-branch and 2MASS M-giant stars. We find that the most prominent feature in the CMDs, the MS population at 23kpc, is not explained by the model. Instead the model predicts in these directions a low density filamentary structure of Sgr debris stars at ~9kpc and a slightly higher concentration of Sgr stars spread from 42-53kpc. At best there is only marginal evidence for the presence of these populations in our data. Our findings then suggest that while there are probably some Sgr debris stars present, the dominant stellar population in the VOD originates from a different halo structure that has almost identical age and metallicity as some sections of the Sgr tidal stream., Comment: 11 pages, 8 figures, 5 tables, accepted for publication in ApJ

Using a variety of stellar tracers -- blue horizontal branch stars, main-sequence turn-off stars and red giants -- we follow the path of the Sagittarius (Sgr) stream across the sky in Sloan Digital Sky Survey data. Our study presents new Sgr debris detections, accurate distances and line-of-sight velocities that together help to shed new light on the puzzle of the Sgr tails. For both the leading and the trailing tail, we trace the points of their maximal extent, or apo-centric distances, and find that they lie at $R^L$ = 47.8 $\pm$ 0.5 kpc and $R^T$ = 102.5 $\pm$ 2.5 kpc respectively. The angular difference between the apo-centres is 93.2 $\pm$ 3.5 deg, which is smaller than predicted for logarithmic haloes. Such differential orbital precession can be made consistent with models of the Milky Way in which the dark matter density falls more quickly with radius. However, currently, no existing Sgr disruption simulation can explain the entirety of the observational data. Based on its position and radial velocity, we show that the unusually large globular cluster NGC 2419 can be associated with the Sgr trailing stream. We measure the precession of the orbital plane of the Sgr debris in the Milky Way potential and show that, surprisingly, Sgr debris in the primary (brighter) tails evolves differently to the secondary (fainter) tails, both in the North and the South., Comment: 17 pages, 13 figures. Accepted to MNRAS

Using deep V, B-V wide-field photometry, we have conducted a new study of stellar over-densities in the Fornax dwarf spheroidal galaxy by determining detailed Star Formation Histories from Colour-Magnitude Diagram analysis. We have concentrated on the relatively young stellar component (<4 Gyr old), and compared this to the underlying Fornax field population. We have studied in more detail the previously known inner shell-like structure and shown that it has a well-defined age-metallicity relation with a peak at ~1.5 Gyr, [Fe/H]=-0.6 dex. Comparison to the Fornax centre shows that the over-dense feature is consistent with the age-metallicity relation of young field stars, and likely formed from Fornax gas. This is consistent with a scenario in which the over-density was formed by the re-accretion of previously expelled gas. We have also discovered a new stellar over-density, located 0.3 degrees (0.7 kpc) from the centre, which is only 100 Myr old, with solar metallicity. This feature constitutes some of the youngest, most metal-rich stars stars observed in Fornax to date. It is unclear how the young over-density was formed, although the age and metallicity of stars suggest this feature may represent the last star formation activity of the Fornax dSph., Comment: Accepted for publication in Astronomy & Astrophysics

We present deep photometry in the B,V and I filters from CTIO/MOSAIC for about 270.000 stars in the Fornax dwarf Spheroidal galaxy, out to a radius of r_ell\sim0.8 degrees. By combining the accurately calibrated photometry with the spectroscopic metallicity distributions of individual Red Giant Branch stars we obtain the detailed star formation and chemical evolution history of Fornax. Fornax is dominated by intermediate age (1-10 Gyr) stellar populations, but also includes ancient (10-14 Gyr), and young (<1 Gyr) stars. We show that Fornax displays a radial age gradient, with younger, more metal-rich populations dominating the central region. This confirms results from previous works. Within an elliptical radius of 0.8 degrees, or 1.9 kpc from the centre, a total mass in stars of 4.3x10^7 Msun was formed, from the earliest times until 250 Myr ago. Using the detailed star formation history, age estimates are determined for individual stars on the upper RGB, for which spectroscopic abundances are available, giving an age-metallicity relation of the Fornax dSph from individual stars. This shows that the average metallicity of Fornax went up rapidly from [Fe/H]<-2.5 dex to [Fe/H]=-1.5 dex between 8-12 Gyr ago, after which a more gradual enrichment resulted in a narrow, well-defined sequence which reaches [Fe/H]\sim-0.8 dex, \sim3 Gyr ago. These ages also allow us to measure the build-up of chemical elements as a function of time, and thus determine detailed timescales for the evolution of individual chemical elements. A rapid decrease in [Mg/Fe] is seen for the stars with [Fe/H]>-1.5 dex, with a clear trend in age., Comment: 18 pages, 20 figures

We build a sample of distant (D > 80 kpc) stellar halo stars with measured radial velocities. Faint ($20 < g <22$) candidate blue horizontal branch (BHB) stars were selected using the deep, but wide, multi-epoch Sloan Digital Sky Survey photometry. Follow-up spectroscopy for these A-type stars was performed using the VLT-FORS2 instrument. We classify stars according to their Balmer line profiles, and find 7 are bona fide BHB stars and 31 are blue stragglers (BS). Owing to the magnitude range of our sample, even the intrinsically fainter BS stars can reach out to D ~ 90 kpc. We complement this sample of A-type stars with intrinsically brighter, intermediate-age, asymptotic giant branch stars. A set of 4 distant cool carbon stars is compiled from the literature and we perform spectroscopic follow-up on a further 4 N-type carbon stars using the WHT-ISIS instrument. Altogether, this provides us with the largest sample to date of individual star tracers out to r ~ 150 kpc. We find that the radial velocity dispersion of these tracers falls rapidly at large distances and is surprisingly cold (sigma_r ~ 50-60 km/s) between 100-150 kpc. Relating the measured radial velocities to the mass of the Milky Way requires knowledge of the (unknown) tracer density profile and anisotropy at these distances. Nonetheless, by assuming the stellar halo stars between 50-150 kpc have a moderate density fall-off (with power-law slope alpha < 5) and are on radial orbits (sigma^2_t/sigma^2_r < 1), we infer that the mass within 150 kpc is less than 10^12 M_solar and suggest it probably lies in the range (5-10) x 10^11 M_solar. We discuss the implications of such a low mass for the Milky Way., Comment: 15 pages, 12 figures. Accepted for publication in MNRAS

We present deep wide-field photometry of three recently discovered faint Milky Way satellites: Leo V, Pisces II, and Canes Venatici II. Our main goals are to study the structure and star formation history of these dwarfs; we also search for signs of tidal disturbance. The three satellites have similar half-light radii ($\sim 60-90$ pc) but a wide range of ellipticities. Both Leo V and CVn II show hints of stream-like overdensities at large radii. An analysis of the satellite color-magnitude diagrams shows that all three objects are old ($>$ 10 Gyr) and metal-poor ([Fe/H] $\sim -2$), though neither the models nor the data have sufficient precision to assess when the satellites formed with respect to cosmic reionization. The lack of an observed younger stellar population ($\la 10$ Gyr) possibly sets them apart from the other satellites at Galactocentric distances $\ga 150$ kpc. We present a new compilation of structural data for all Milky Way satellite galaxies and use it to compare the properties of classical dwarfs to the ultra-faints. The ellipticity distribution of the two groups is consistent at the $\sim$2-$\sigma$ level. However, the faintest satellites tend to be more aligned toward the Galactic center, and those satellites with the highest ellipticity ($\ga 0.4$) have orientations ($\Delta \theta_{GC}$) in the range $20^{\circ} \lesssim \Delta \theta_{GC} \lesssim 40^{\circ}$. This latter observation is in rough agreement with predictions from simulations of dwarf galaxies that have lost a significant fraction of their dark matter halos and are being tidally stripped., Comment: 25 pages, 7 figures, ApJ accepted; version updated to match ApJ accepted

With the NEOWISE portion of the \emph{Wide-field Infrared Survey Explorer} (WISE) project, we have carried out a highly uniform survey of the near-Earth object (NEO) population at thermal infrared wavelengths ranging from 3 to 22 $\mu$m, allowing us to refine estimates of their numbers, sizes, and albedos. The NEOWISE survey detected NEOs the same way whether they were previously known or not, subject to the availability of ground-based follow-up observations, resulting in the discovery of more than 130 new NEOs. The survey's uniformity in sensitivity, observing cadence, and image quality have permitted extrapolation of the 428 near-Earth asteroids (NEAs) detected by NEOWISE during the fully cryogenic portion of the WISE mission to the larger population. We find that there are 981$\pm$19 NEAs larger than 1 km and 20,500$\pm$3000 NEAs larger than 100 m. We show that the Spaceguard goal of detecting 90% of all 1 km NEAs has been met, and that the cumulative size distribution is best represented by a broken power law with a slope of 1.32$\pm$0.14 below 1.5 km. This power law slope produces $\sim13,200\pm$1,900 NEAs with $D>$140 m. Although previous studies predict another break in the cumulative size distribution below $D\sim$50-100 m, resulting in an increase in the number of NEOs in this size range and smaller, we did not detect enough objects to comment on this increase. The overall number for the NEA population between 100-1000 m are lower than previous estimates. The numbers of near-Earth comets will be the subject of future work., Comment: Accepted to ApJ

We report the discovery of two new Milky Way satellites in the neighboring constellations of Pisces and Pegasus identified in data from the Sloan Digital Sky Survey. Pisces II, an ultra-faint dwarf galaxy lies at the distance of ~180 kpc, some 15 degrees away from the recently detected Pisces I. Segue 3, an ultra-faint star cluster lies at the distance of 16 kpc. We use deep follow-up imaging obtained with the 4-m Mayall telescope at Kitt Peak National Observatory to derive their structural parameters. Pisces II has a half-light radius of ~60 pc, while Segue 3 is twenty times smaller at only 3pc., Comment: Accepted for publication in ApJL

We present MMT/Megacam imaging of the Leo~IV dwarf galaxy in order to investigate its structure and star formation history, and to search for signs of association with the recently discovered Leo~V satellite. Based on parameterized fits, we find that Leo~IV is round, with $\epsilon < 0.23$ (at the 68\% confidence limit) and a half-light radius of $r_{h} \simeq 130$ pc. Additionally, we perform a thorough search for extended structures in the plane of the sky and along the line of sight. We derive our surface brightness detection limit by implanting fake structures into our catalog with stellar populations identical to that of Leo~IV. We show that we are sensitive to stream-like structures with surface brightness $\mu_{r}\lesssim29.6$ mag arcsec$^{-2}$, and at this limit, we find no stellar bridge between Leo IV (out to a radius of $\sim$0.5 kpc) and the recently discovered, nearby satellite Leo V. Using the color magnitude fitting package StarFISH, we determine that Leo~IV is consistent with a single age ($\sim$14 Gyr), single metallicity ($[Fe/H]\sim-2.3$) stellar population, although we can not rule out a significant spread in these value. We derive a luminosity of $M_{V}=-5.5\pm0.3$. Studying both the spatial distribution and frequency of Leo~IV's 'blue plume' stars reveals evidence for a young ($\sim$2 Gyr) stellar population which makes up $\sim$2\% of its stellar mass. This sprinkling of star formation, only detectable in this deep study, highlights the need for further imaging of the new Milky Way satellites along with theoretical work on the expected, detailed properties of these possible 'reionization fossils'., Comment: 26 pages, 14 figures, emulateapj format, ApJ accepted version

We present imaging of the recently discovered Hercules Milky Way satellite and its surrounding regions to study its structure, star formation history and to thoroughly search for signs of disruption. We robustly determine the distance, luminosity, size and morphology of Hercules utilizing a bootstrap approach to characterize our uncertainties. We derive a distance to Hercules of $133 \pm 6$ kpc via a comparison to empirical and theoretical isochrones. As previous studies have found, Hercules is very elongated, with $\epsilon=0.67\pm0.03$ and a half light radius of $r_{h} \simeq 230$ pc. Using the color magnitude fitting package StarFISH, we determine that Hercules is old ($>12$ Gyr) and metal poor ($[Fe/H]\sim-2.0$), with a spread in metallicity, in agreement with previous spectroscopic work. We infer a total absolute magnitude of $M_V=-5.3\pm0.4$. Our innovative search for external Hercules structure both in the plane of the sky and along the line of sight yields some evidence that Hercules is embedded in a larger stream of stars. A clear stellar extension is seen to the Northwest with several additional candidate stellar overdensities along the position angle of Hercules out to $\sim$35' ($\sim$1.3 kpc). While the association of any of the individual stellar overdensities with Hercules is difficult to determine, we do show that the summed color magnitude diagram of all three is consistent with Hercules' stellar population. Finally, we estimate that any change in the distance to Hercules across its face is at most $\sim$6 kpc; and the data are consistent with Hercules being at the same distance throughout., Comment: 50 pages, 15 figures, submitted to the Astrophysical Journal

We announce the discovery of a new Milky Way satellite Segue 2 found in the data of the Sloan Extension for Galactic Understanding and Exploration (SEGUE). We followed this up with deeper imaging and spectroscopy on the Multiple Mirror Telescope. From this, we derive a luminosity of M_v = -2.5, a half-light radius of 34 pc and a systemic velocity of -40$ km/s. Our MMT data also provides evidence for a stream around Segue 2 at a similar heliocentric velocity, and the SEGUE data show that it is also present in neighboring fields. We resolve the velocity dispersion of Segue 2 as 3.4 km/s and the possible stream as about 7 km/s. This object shows points of comparison with other recent discoveries, Segue 1, Boo II and Coma. We speculate that all four objects may be representatives of a population of satellites of satellites -- survivors of accretion events that destroyed their larger but less dense parents. They are likely to have formed at redshifts z > 10 and are good candidates for fossils of the reionization epoch., Comment: MNRAS, Submitted

We report the discovery of a new Milky Way dwarf spheroidal galaxy in the constellation of Leo identified in data from the Sloan Digital Sky Survey. Leo V lies at a distance of about 180 kpc, and is separated by about 3 degrees from another recent discovery, Leo IV. We present follow-up imaging from the Isaac Newton Telescope and spectroscopy from the Hectochelle fiber spectrograph at the Multiple Mirror Telescope. Leo V's heliocentric velocity is 173.4 km/s, which is offset by about 40 km/s from that of Leo IV. A simple interpretation of the kinematic data is that both objects may lie on the same stream, though the implied orbit is only modestly eccentric (e = 0.2), Comment: Submitted to ApJ (Letters)

We use N-body simulations to explore the origin and a plausible orbit for the Orphan Stream, one of the faintest substructures discovered so far in the outer halo of our Galaxy. We are able to reproduce its position, velocity and distance measurements by appealing to a single wrap of a double-component satellite galaxy. We find that the progenitor of the Orphan Stream could have been an object similar to today's Milky Way dwarfs, such as Carina, Draco, Leo II or Sculptor; and unlikely to be connected to Complex A or Ursa Major II. Our models suggest that such progenitors, if accreted on orbits with apocenters smaller than ~35 kpc, are likely to give rise to very low surface brightness streams, which may be hiding in the outer halo and remain largely undetected with current techniques. The systematic discovery of these ghostly substructures may well require wide field spectroscopic surveys of the Milky Way's outer stellar halo., Comment: 9 pages, 5 figures, MNRAS in press. Accepted version with minor changes

We present deep, wide-field g and r photometry of the transition type dwarf galaxy Leo T, obtained with the blue arm of the Large Binocular Telescope. The data confirm the presence of both very young (<1 Gyr) as well as much older (>5 Gyr) stars. We study the structural properties of the old and young stellar populations by preferentially selecting either population based on their color and magnitude. The young population is significantly more concentrated than the old population, with half-light radii of 104+-8 and 148+-16 pc respectively, and their centers are slightly offset. Approximately 10% of the total stellar mass is estimated to be represented by the young stellar population. Comparison of the color-magnitude diagram (CMD) with theoretical isochrones as well as numerical CMD-fitting suggest that star formation began over 10 Gyr ago and continued in recent times until at least a few hundred Myr ago. The CMD-fitting results are indicative of two distinct star formation bursts, with a quiescent period around 3 Gyr ago, albeit at low significance. The results are consistent with no metallicity evolution and [Fe/H] ~ -1.5 over the entire age of the system. Finally, the data show little if any sign of tidal distortion of Leo T., Comment: 8 pages, 9 figures, some small textual changes, accepted for publication in the Astrophysical Journal

Kallivayalil et al. have used the \textit{Hubble Space Telescope} to measure proper motions of the LMC and SMC using images in 21 and five fields, respectively, all centered on known QSOs. These results are more precise than previous measurements, but have surprising and important physical implications: for example, the LMC and SMC may be approaching the Milky Way for the first time; they might not have been in a binary system; and the origin of the Magellanic Stream needs to be re-examined. Motivated by these implications, we have reanalyzed the original data in order to check the validity of these measurements. Our work has produced a proper motion for the LMC that is in excellent agreement with that of Kallivayalil et al., and for the SMC that is in acceptable agreement. We have detected a dependence between the brightness of stars and their mean measured motion in a majority of the fields in both our reduction and that of Kallivayalil et al. Correcting for this systematic error and for the errors caused by the decreasing charge transfer efficiency of the detector produces better agreement between the measurements from different fields. With our improved reduction, we do not need to exclude any fields from the final averages and, for the first time using proper motions, we are able to detect the rotation of the LMC. The best-fit amplitude of the rotation curve at a radius of 275 arcmin in the disk plane is $120 \pm 15$ km s$^{-1}$. This value is larger than the 60--70 km s$^{-1}$ derived from the radial velocities of HI and carbon stars, but in agreement with the value of 107 km s$^{-1}$ derived from the radial velocities of red supergiants., Comment: Accepted for publication in AJ; 38 manuscript pages, 14 figures (some in color)

We present optical and X-ray observations of the afterglow and host galaxy of the short-hard GRB 060121. The faint R-band afterglow is seen to decline as t^(-0.66) while the X-ray falls as t^-1.18, indicating the presence of the cooling break between the two frequencies. However, the R-band afterglow is very faint compared to the predicted extrapolation of the X-ray afterglow to the optical regime (specifically, beta_OX ~ 0.2), while the K-band is consistent with this extrapolation (beta_KX ~ 0.6), demonstrating suppression of the optical flux. Late time {\it HST} observations place stringent limits on the afterglow R-band flux implying a break in the R-band lightcurve. They also show that the burst occurred at the edge of a faint red galaxy which most likely lies at a significantly higher redshift than the previous optically identified short-duration bursts. Several neighboring galaxies also have very red colors that are similarly suggestive of higher redshift. We consider possible explanations for the faintness and color of the burst. Our preferred model is that the burst occurred at moderately high redshift and was significantly obscured; however, it is also possible that the burst lies at z > 4.5 in which case the faintness of the R-band afterglow could be attributed to the Lyman-break. We discuss the implications that either scenario would have for the nature of the progenitors of short bursts., Comment: 4 pages, 2 Figures - submitted to ApJ Letters

This article presents a measurement of the proper motion of the Sculptor dwarf spheroidal galaxy determined from images taken with the Hubble Space Telescope using the Space Telescope Imaging Spectrograph in the imaging mode., Comment: 38 pages, 15 figures, 6 tables, Accepted for publication in AJ, March 2006

We have compiled a new sample of 240 halo objects with accurate distance and radial velocity measurements, including globular clusters, satellite galaxies, field blue horizontal branch stars and red giant stars from the Spaghetti survey. The new data lead to a significant increase in the number of known objects for Galactocentric radii beyond 50 kpc, which allows a reliable determination of the radial velocity dispersion profile out to very large distances. The radial velocity dispersion shows an almost constant value of 120 km/s out to 30 kpc and then continuously declines down to 50 km/s at about 120 kpc. This fall-off puts important constraints on the density profile and total mass of the dark matter halo of the Milky Way. For a constant velocity anisotropy, the isothermal profile is ruled out, while both a dark halo following a truncated flat model of mass $1.2^{+1.8}_{-0.5}\times 10^{12}$ M_sun and an NFW profile of mass $0.8^{+1.2}_{-0.2}\times 10^{12}$ M_sun and c=18 are consistent with the data. The significant increase in the number of tracers combined with the large extent of the region probed by these has allowed a more precise determination of the Milky Way mass in comparison to previous works. We also show how different assumptions for the velocity anisotropy affect the performance of the mass models., Comment: 11 pages, 7 figures. Accepted for publication in MNRAS

This article presents a measurement of the proper motion of the Ursa Minor dwarf spheroidal galaxy determined from images taken with the Hubble Space Telescope in two distinct fields., Comment: AJ, accepted; 23 pages, 17 figures (Fig. 1 abridged), 5 tables

We report the findings of a new search for RR Lyraes in an M31 halo field located 40 arcminutes from the nucleus of the galaxy along the minor axis. We detected 37 variable stars, of which 24 are classified as RR Lyraes and the others are ambiguous. Estimating a completeness fraction of ~24%, we calculate that there are approximately 100 RR Lyraes in the field, which is consistent with what is expected from deep HST color-magnitude diagrams. We calculate a mean magnitude of g=25.15+/ 0.03, which we interpret to mean that the mean metallicity of RR Lyraes is significantly lower than that of the M31 halo as a whole. The presence of ancient, metal-poor stars opens the possibility that, initially, the M31 halo appeared much like the Milky Way halo., Comment: 43 pages, 9 figures. Accepted for publication in AJ

We present early observations of the afterglow of the Gamma-Ray Burst (GRB) 030329 and the spectroscopic discovery of its associated supernova SN 2003dh. We obtained spectra of the afterglow of GRB 030329 each night from March 30.12 (0.6 days after the burst) to April 8.13 (UT) (9.6 days after the burst). The spectra cover a wavelength range of 350 nm to 850 nm. The early spectra consist of a power-law continuum (F_nu ~ nu^{-0.9}) with narrow emission lines originating from HII regions in the host galaxy, indicating a low redshift of z=0.1687. However, our spectra taken after 2003 Apr. 5 show broad peaks in flux characteristic of a supernova. Correcting for the afterglow emission, we find the spectrum of the supernova is remarkably similar to the type Ic `hypernova' SN 1998bw. While the presence of supernovae have been inferred from the light curves and colors of GRB afterglows in the past, this is the first direct, spectroscopic confirmation that a subset of classical gamma-ray bursts originate from supernovae., Comment: published by ApJ Letters; additional material avilable at http://cfa-www.harvard.edu/cfa/oir/Research/GRB/

This article presents and discusses a method for measuring the proper motions of the Galactic dwarf spheroidal galaxies using images taken with the Hubble Space Telescope. The method involves fitting an effective point spread function to the image of a star or quasi-stellar object to determine its centroid with an accuracy of about 0.005 pixel (0.25 milliarcseconds) -- an accuracy sufficient to measure the proper motion of a dwarf spheroidal galaxy using images separated by just a few years. The data consist of images, dithered to reduce the effects of undersampling, taken at multiple epochs with the Space Telescope Imaging Spectrograph or the Wide Field Planetary Camera. The science fields are in the directions of the Carina, Fornax, Sculptor, and Ursa Minor dwarf spheroidal galaxies and each has at least one quasi-stellar object whose identity has been established by other studies. The rate of change with time of the centroids of the stars of the dwarf spheroidal with respect to the centroid of the quasi-stellar object is the proper motion. Four independent preliminary measurements of the proper motion of Fornax for three fields agree within their uncertainties. The weighted average of these measurements is mu_alpha = 49 +- 13 milliarcseconds/century and mu_delta = -59 +- 13 milliarcseconds/century. The Galactocentric velocity derived from the proper motion implies that Fornax is near perigalacticon, may not be bound to the Milky Way, and is not a member of any of the proposed streams of galaxies and globular clusters in the Galactic halo. If Fornax is bound, the Milky Way must have a mass of at least (1.6 +- 0.8) x 10^{12} solar masses., Comment: 4 tables, 16 figures (22 postscript files), 30+ pages. Accepted for publication in AJ. v2: Revised to remove a typo in the abstract and improve the formatting

The last 10-20 years has seen a profound shift in views of how the Galaxy's halo formed. The idea of a monolithic early collapse of a single system (Eggen, Lynden-Bell and Sandage 1962) has been challenged by observations at high redshift and by cosmological models of structure formation. These findings imply that we should see clear evidence of hierarchical formation processes in nearby galaxies. Recent studies of our Galaxy, made possible by large-scale CCD surveys such as the Sloan Digital Sky Survey (SDSS), have begun to reveal tantalizing evidence of substructure in the outer halo. We review evidence for tidal streams associated with known Milky Way satellites and for star streams whose progenitors are still unknown. This includes results from the SDSS and our own ongoing pencil-beam halo survey, the Spaghetti survey., Comment: 10 pages, invited talk at the meeting ``The Dynamics, Structure and History of Galaxies'', Dunk Island, Australia, July 2001

We present WFPC2 and ground-based VI photometry of NGC 121 and a nearby field in the outer SMC. For NGC 121, we measure a true distance modulus of (m-M)0 = 18.96 +/- 0.04 (distance of 61.9 +/- 1.1 kpc), age of 10.6 +/- 0.5 Gyr, metallicity of [Fe/H] = -1.03 +/- 0.06, and initial mass of 4.1 +/- 0.4 x 10^5 Msun, assuming a Salpeter IMF with lower cutoff at 0.1 Msun. In the outer SMC field, we find evidence of stars covering a wide range of ages -- from 2 Gyr old to at least 9-12 Gyr old. We have measured the distance, extinction, and star formation history (past star formation rates and enrichment history) using a CMD fitting algorithm. The distance modulus of the SMC is measured to be (m-M)0 = 18.88 +/- 0.08, corresponding to a distance of 59.7 +/- 2.2 kpc. The overall star formation rate appears to have been relatively constant over this period, although there may be small gaps in the star forming activity too small to be resolved. The lack of current star-forming activity is a selection effect, as the field was intentionally chosen to avoid recent activity. The mean metallicity of this field has increased from an average of [Fe/H] = -1.3 +/- 0.3 for stars older than 8 Gyr to [Fe/H] = -0.7 +/- 0.2 in the past 3 Gyr., Comment: 29 pages, 14 figures accepted for publication in ApJ

As part of the Spaghetti Project Survey (SPS) we have detected a concentration of giant stars well above expectations for a smooth halo model. The position (l~350, b~50) and distance (~50 kpc) of this concentration match those of the Northern over-density detected by SDSS (Yanny et al. 2000, Ivezic et al. 2000). We find additional evidence for structure at ~80 kpc in the same direction. We present radial velocities for many of these stars, including the first published results from the 6.5m Magellan telescope. The radial velocities for stars in these structures are in excellent agreement with models of the dynamical evolution of the Sgr dwarf tidal debris, whose center is 60 degrees away. The metallicity of stars in these streams is lower than that of the main body of the Sgr dwarf, which may indicate a radial metallicity gradient prior to disruption., Comment: 10 pages, 3 figures accepted in Astrophysical Journal Letters

We critically examine the use of the Washington photometric system (with the 51 filter) for identifying distant halo giants. While this is the most powerful photometric technique for isolating G and K giant stars, spectroscopic follow-up of giant candidates is vital. There are two situations in which interlopers outnumber genuine giants in the diagnostic M-51/M-T2 plot, and are indistinguishable photometrically from the giants. (1) In deep surveys covering tens of square degrees, very metal-poor halo dwarfs are a significant contaminant. An example is our survey of the outer halo (Morrison et al. 2000, Dohm-Palmer et al. 2000), where these metal-poor dwarfs dominate the number of photometric giant candidates at magnitudes fainter than V = 18 and cannot be isolated photometrically. (2) In deep surveys of smaller areas with low photometric precision, most objects in the giant region of the color-color plot are dwarfs whose photometric errors have moved them there. Color errors in M-51 and M-T2 need to be smaller than 0.03 mag to avoid this problem. An example of a survey whose photometric errors place the giant identifications under question is the survey for extra-tidal giants around the Carina dwarf spheroidal of Majewski et al. (2000a). Accurate photometry and spectroscopic follow-up of giant candidates are essential when using the Washington system to identify the rare outer-halo giants., Comment: Accepted for publication in Jan. 2001 Astronomical Journal

We present imaging results from a high Galactic latitude survey designed to examine the structure of the Galactic halo. The objective of the survey is to identify candidate halo stars which can be observed spectroscopically to obtain radial velocities and confirm halo membership. The Washington filter system is used for its ability to distinguish between dwarfs and giants, as well as provide a metallicity indicator. Our most successful imaging run used the BTC camera on the CTIO 4m telescope in April 1999. Photometric conditions during these observations provided superb photometry, with average errors for a star at $M=18.5$ of 0.009, 0.008, 0.011, and 0.009 for $C$, $M$, $DDO51$, and $T2$ respectively. These data are available with the electronic version of this paper, as well as through ADC (http://adc.gsfc.nasa.gov/). We use these data as a template to describe the details of our photometric reduction process. It is designed to perform CCD reductions and stellar photometry automatically during the observation run without the aid of external packages, such as IRAF and IDL. We describe necessary deviations from this procedure for other instruments used in the survey up to June 2000. Preliminary results from spectroscopic observations indicate a 97% efficiency in eliminating normal dwarfs from halo giant candidates for $M<18.5$. Unfortunately, low-metallicity subdwarfs cannot be photometrically distinguished from giants using the Washington filters. These major contaminates unavoidably reduced the overall giant identification efficiency to 66% for $M<18.5$. Our improved knowledge of these stars will increase this efficiency for future spectroscopic observations., Comment: Accepted for publication in the November, 2000 Astronomical Journal. 33 pages, plus 9 figures. The full data set will be available in the electronic version of the paper, as well as through ADC

We present deep HST F555W (V) and F814W (I) observations of a central field in the Local Group dwarf spheroidal (dSph) galaxy Leo I. The resulting color-magnitude diagram (CMD) reaches I \simeq 26 and reveals the oldest ~10-15 Gyr old turnoffs. Nevertheless, a horizontal branch is not obvious in the CMD. Given the low metallicity of the galaxy, this likely indicates that the first substantial star formation in the galaxy may have been somehow delayed in Leo I in comparison with the other dSph satellites of the Milky Way. The subgiant region is well and uniformly populated from the oldest turnoffs up to the 1 Gyr old turnoff, indicating that star formation has proceeded in a continuous way, with possible variations in intensity but no big gaps between successive bursts, over the galaxy's lifetime. The structure of the red-clump of core He-burning stars is consistent with the large amount of intermediate-age population inferred from the main sequence and the subgiant region. In spite of the lack of gas in Leo I, the CMD clearly shows star formation continuing until 1 Gyr ago and possibly until a few hundred Myrs ago in the central part of the galaxy., Comment: 26 pages with 8 figures (fig 2 not available electronically). To be published in ApJ, April 1 1999 (vol.514, #2)

We report on HST observations of six candidate old globular clusters in the Large Magellanic Cloud: NGC 1754, NGC 1835, NGC 1898, NGC 1916, NGC 2005 and NGC 2019. Deep exposures with the F555W and F814W filters provide us with colour-magnitude diagrams that reach to an apparent magnitude in V of ~25, well below the main sequence turnoff. These particular clusters are involved with significantly high LMC field star densities and care was taken to subtract the field stars from the cluster colour-magnitude diagrams accurately. In two cases there is significant variable reddening across at least part of the image, but only for NGC 1916 does the differential reddening preclude accurate measurements of the CMD characteristics. The morphologies of the colour- magnitude diagrams match well those of Galactic globular clusters of similar metallicity. All six have well-developed horizontal branches, while four clearly have stars on both sides of the RR Lyrae gap. The abundances obtained from measurements of the height of the red giant branch above the level of the horizontal branch are 0.3 dex higher, on average, than previously measured spectroscopic abundances. Detailed comparisons with Galactic globular cluster fiducials show that all six clusters are old objects, very similar in age to classical Galactic globulars such as M5, with little age spread among the clusters. This result is consistent with ages derived by measuring the magnitude difference between the horizontal branch and main sequence turnoff. We also find a similar chronology by comparing the horizontal branch morphologies and abundances with the horizontal branch evolutionary tracks of Lee, Demarque, & Zinn (1994). Our results imply that the LMC formed at the same time as the Milky Way Galaxy., Comment: 23 pages, 18 PostScript figures, LaTeX, accepted by MNRAS. Uses mn.sty and epsf.sty. Requires ols.sty (included)

We use a large set of radial velocities in the Ursa Minor and Draco dwarf spheroidal galaxies to search for binary stars and to infer the binary frequency. Of the 118 stars in our sample with multiple observations, six are velocity variables with $\chi^2$ probabilities below 0.001. We use Monte Carlo simulations that mimic our observations to determine the efficiency with which our observations find binary stars. Our best, though significantly uncertain, estimate of the binary frequency for stars near the turnoff in Draco and UMi is 0.2--0.3 per decade of period in the vicinity of periods of one year, which is 3--5$\times$ that found for the solar neighborhood. This frequency is high enough that binary stars might significantly affect the measured velocity dispersions of some dwarf spheroidal galaxies according to some previous numerical experiments. However, in the course of performing our own experiments, we discovered that this previous work had inadvertently overestimated binary orbital velocities. Our first set of simulations of the effects of binaries is based on the observed scatter in the individual velocity measurements for the multiply-observed Draco and Ursa Minor stars. This scatter is small compared to measured velocity dispersions and, so, the effect of binaries on the dispersions is slight. This result is supported by our second set of experiments, which are based on a model binary population normalized by the observed binary frequency in Draco and Ursa Minor. We conclude that binary stars have had no significant effect on the measured velocity dispersion and inferred mass-to-light ratio of any dwarf spheroidal galaxy., Comment: 33 pages, 95kb uuencoded, gzipped postscript; Accepted by Astronomical Journal; gzipped, tarred postscript of text, tables, figures available at ftp://as.arizona.edu/pub/edo (binaries_in_dsph.tar.gz)