14525131
Far-UV Emission from Elliptical Galaxies at z=0.55
The restframe UV-to-optical flux ratio, characterizing the ``UV upturn'' phenomenon, is potentially the most sensitive tracer of age in elliptical galaxies; models predict that it may change by orders of magnitude over the course of a few Gyr. In order to trace the evolution of the UV upturn as a function of redshift, we have used the far-UV camera on the Space Telescope Imaging Spectrograph to image the galaxy cluster CL0016+16 at z=0.55. Our 25''x25'' field includes four bright elliptical galaxies, spectroscopically confirmed to be passively evolving cluster members. The weak UV emission from the galaxies in our image demonstrates that the UV upturn is weaker at a lookback time 5.6 Gyr earlier than our own, as compared to measurements of the UV upturn in cluster E and S0 galaxies at z=0 and z=0.375. These images are the first with sufficient depth to demonstrate the fading of the UV upturn expected at moderate redshifts. We discuss these observations and the implications for the formation history of galaxies.
35491181
A SPECTROSCOPIC SEARCH FOR LEAKING LYMAN CONTINUUM AT z ∼ 0.7*
We present the results of rest-frame, UV slitless spectroscopic observations of a sample of 32 z ~ 0.7 Lyman break galaxy (LBG) analogs in the COSMOS field. The spectroscopic search was performed with the Solar Blind Channel on the Hubble Space Telescope. We report the detection of leaking Lyman continuum (LyC) radiation from an active galactic nucleus-starburst composite. While we find no direct detections of LyC emission in the remainder of our sample, we achieve individual lower limits (3σ) of the observed non-ionizing UV-to-LyC flux density ratios, f -ν (1500 A)/f _ν(830 A) of 20 to 204 (median of 73.5) and 378.7 for the stack. Assuming an intrinsic Lyman break of 3.4 and an intergalactic medium transmission of LyC photons along the line of sight to the galaxy of 85%, we report an upper limit for the relative escape fraction in individual galaxies of 0.02-0.19 and a stacked 3σ upper limit of 0.01. We find no indication of a relative escape fraction near unity as seen in some LBGs at z ~ 3. Our UV spectra achieve the deepest limits to date at any redshift for the escape fraction in individual sources. The contrast between these z ~ 0.7 low escape fraction LBG analogs with z ~ 3 LBGs suggests that either the processes conducive to high f esc are not being selected for in the z 1 samples or the average escape fraction is decreasing from z ~ 3 to z ~ 1. We discuss possible mechanisms that could affect the escape of LyC photons
18506635
THE WFC3 GALACTIC BULGE TREASURY PROGRAM: A FIRST LOOK AT RESOLVED STELLAR POPULATION TOOLS
When the Wide Field Camera 3 (WFC3) is installed on the Hubble Space Telescope (HST), the astronomical community will have access to powerful new capabilities for investigating resolved stellar populations. The WFC3 Galactic Bulge Treasury program will obtain deep imaging in five photometric bands on four low-extinction fields. These data will have no proprietary period, and will enable a variety of science investigations not possible with previous data sets. To aid in planning for the use of these data and for future observing proposals, we provide an introduction to the Treasury program, its photometric system, and the associated calibration effort. The observing strategy is based upon a new photometric system employing five WFC3 bands spanning the UV, optical, and near-infrared: F390W, F555W, F814W, F110W, and F160W (analogous but not identical to the ground-based filters Washington C, V, I, J, and H). With these bands, one can construct reddening-free indices of temperature and metallicity. Using this photometric system, the program will target six fields in well studied star clusters, spanning a wide range of metallicity, and four fields in low-extinction windows of the Galactic bulge. The cluster data serve to calibrate the reddening-free indices, provide empirical population templates, and correct the transformation of theoretical isochrone libraries into the WFC3 photometric system. The bulge data will shed light on the bulge formation history, and will also serve as empirical population templates for other studies. One of the fields includes 12 candidate hosts of extrasolar planets. Color-magnitude diagrams (CMDs) are the most popular tool for analyzing resolved stellar populations. However, due to degeneracies among temperature, metallicity, and reddening in traditional CMDs, it can be difficult to draw robust conclusions from the data. The five-band system used for the bulge Treasury observations will provide reddening-free indices that are roughly orthogonal in temperature and metallicity, and we argue that model fitting in an index-index diagram will make better use of the information than fitting separate CMDs. We provide some results from simulations to show the expected quality of the data and their potential for differentiating between different star formation histories.
67759178
THE FIRST DETECTION OF BLUE STRAGGLER STARS IN THE MILKY WAY BULGE
We report the first detections of Blue Straggler Stars (BSS) in the bulge of the Milky Way. Proper motions from extensive space-based observations along a single sight line allow us to separate a sufficiently clean and well-characterized bulge sample such that we are able to detect a small population of bulge objects in the region of the color-magnitude diagram commonly occupied by young objects and blue stragglers. Variability measurements of these objects clearly establish that a fraction of them are blue stragglers. Out of the 42 objects found in this region of the color-magnitude diagram, we estimate that at least 18 are genuine BSS. We normalize the BSS population by our estimate of the number of horizontal branch stars in the bulge in order to compare the bulge to other stellar systems. The BSS fraction is clearly discrepant from that found in stellar clusters. The blue straggler population of dwarf spheroidals remains a subject of debate; some authors claim an anticorrelation between the normalized blue straggler fraction and integrated light. If this trend is real, then the bulge may extend it by three orders of magnitude in mass. Conversely, we find that the genuinely young (<5 Gyr) population in the bulge, must be at most 3.4% under the most conservative scenario for the BSS population.
119330474
100 Times Faster and 3 Times Sharper: Background-Dominated Observations of Stellar Populations with an 8-meter Optical-UV Space Telescope
An 8 m successor to the Hubble Space Telescope (HST) would make incredible gains in the study of stellar populations, especially in the Local Group. If diffraction-limited at 0.5 microns, the "Next HST" could produce high-resolution imaging of faint sources over a wide field in 1 percent of the time needed with the HST. With these capabilities, photometry of the ancient main sequence could be obtained for many sight-lines through Local Group galaxies, thus determining directly the ages of their structures and providing a formation history for the Local Group populations.
14120815
The Star Formation History in the Andromeda Halo
I present the preliminary results of a program to measure the star formation history in the halo of the Andromeda galaxy. Using the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope, we obtained the deepest optical images of the sky to date, in a field on the southeast minor axis of Andromeda, 51 arcmin (11 kpc) from the nucleus. The resulting color-magnitude diagram (CMD) contains approximately 300,000 stars and extends more than 1.5 mag below the main sequence turnoff, with 50% completeness at V=30.7 mag. We interpret this CMD using comparisons to ACS observations of five Galactic globular clusters through the same filters, and through chi-squared fitting to a finely-spaced grid of calibrated stellar population models. We find evidence for a major (approximately 30%) intermediate-age (6-8 Gyr) metal-rich ([Fe/H] > -0.5) population in the Andromeda halo, along with a significant old metal-poor population akin to that in the Milky Way halo. The large spread in ages suggests that the Andromeda halo formed as a result of a more violent merging history than that in our own Milky Way.
54816069
The white dwarf cooling sequence of the Galactic bulge
We collected F606W- and F814W-band time-series data of the Sagittarius low-reddening window in the Galactic bulge with the Advanced Camera far Surveys mounted on the Hubble Space Telescope. We sampled the region approximately every two weeks for one year, with the principal aim to detect a hidden population of isolated black holes and neutron stars in the Galactic disk through astrometric microlensing. We present preliminary results here based on a photometric catalog including ~3*10^5 stars down to F606W ~ 31 mag. Proper motions were also measured, with an accuracy of better than ~ 0.5 mas/yr at F606W ~ 28 mag in both coordinates. We were then able to separate disk and bulge stars and to obtain a clean bulge color-magnitude diagram. Together with a dozen candidate extreme horizontal branch stars we were able to identify for the first time a clearly defined white dwarf (WD) cooling sequence in the bulge. The comparison between theory and observations shows that a substantial fraction of the WDs (~40%) is systematically redder than the canonical cooling tracks for CO-core DA WDs. This evidence would suggest the presence of a significant number of He-core WDs in the bulge, formed in close binaries, as has been found in some Galactic globular and open clusters. The presence of close binaries in the bulge population is further supported by the finding of a candidate dwarf nova in outburst and a few candidate cataclysmic variables in quiescence in the same field.
11496647
A Far-Ultraviolet Analysis of the Stellar Populations in Six Elliptical and S0 Galaxies
We have analyzed the far-ultraviolet (FUV) spectra of six elliptical and S0 galaxies in order to characterize the stellar population that produces the ultraviolet flux in these galaxies. The spectra were obtained using the Hopkins Ultraviolet Telescope (HUT) during the Astro-2 mission aboard the Space Shuttle Endeavour in 1995 March and cover the spectral range from 820 to 1840 A with a resolution of 3 A. These data, together with the spectra of two galaxies observed with HUT on the Astro-1 mission, represent the only FUV spectra of early-type galaxies that extend to the Lyman limit at 912 A and therefore include the "turnover" in the spectral energy distribution below Lyα. Using an extensive new grid of LTE and non-LTE synthetic spectra that match the HUT resolution and cover the relevant parameter space of temperature and gravity, we have constructed synthetic spectral energy distributions by integrating over various predicted stellar evolutionary tracks for horizontal-branch stars and their progeny. When the computed models are compared with the HUT data, we find that models with supersolar metal abundances and helium best reproduce the flux across the entire HUT wavelength range, while those with subsolar Z and Y fit less well, partly because of a significant flux deficit shortward of 970 A in the models. High-Z models are preferred because the contribution from the later, hotter, post-HB evolutionary stages makes up a higher fraction of the sub-Lyα flux in these tracks. We find that AGB-Manque evolution is required in all of the fits to the HUT spectra, which suggests that all of the galaxies have some subdwarf B-star population. At any Z and Y, the model spectra that best match the HUT flux are dominated by stars evolving from a narrow range of envelope mass on the blue end of the horizontal branch. The Astro-1 and Astro-2 data are also the first with the resolution and signal-to-noise ratio needed to detect and measure absorption lines in the FUV spectra of elliptical galaxies, which allows a direct estimate of the abundances in the atmospheres of the stars that produce the UV flux. We find that most absorption features in the spectra are consistent with Z = 0.1 Z☉, significantly lower than the abundances implied by the best-fitting spectral energy distributions. However, given the strong observational and theoretical evidence for diffusion processes in the atmospheres of evolved stars, the observed atmospheric abundances may not reflect the interior abundances in the population producing the ultraviolet flux in elliptical galaxies.
118464623
Automated Classification of Variable Stars in the Asteroseismology Program of the Kepler Space Mission
We present the first results of the application of supervised classification methods to the Kepler Q1 long-cadence light curves of a subsample of 2288 stars measured in the asteroseismology program of the mission. The methods, originally developed in the framework of the CoRoT and Gaia space missions, are capable of identifying the most common types of stellar variability in a reliable way. Many new variables have been discovered, among which a large fraction are eclipsing/ellipsoidal binaries unknown prior to launch. A comparison is made between our classification from the Kepler data and the pre-launch class based on data from the ground, showing that the latter needs significant improvement. The noise properties of the Kepler data are compared to those of the exoplanet program of the CoRoT satellite. We find that Kepler improves on CoRoT by a factor of 2-2.3 in point-to-point scatter.
122947911
Reaction rate sensitivity of 44Ti production in massive stars and implications of a thick target yield measurement of 40Ca(alpha,gamma)44Ti
We evaluate two dominant nuclear reaction rates and their uncertainties that affect 44Ti production in explosive nucleosynthesis. Experimentally we develop thick target yields for the 40Ca(α, γ)44Ti reaction at E α = 4.13, 4.54, and 5.36 MeV using γ-ray spectroscopy. At the highest beam energy, we also performed an activation measurement which agrees with the thick target result. From the measured yields a stellar reaction rate was developed that is smaller than current statistical-model calculations and recent experimental results, which would suggest lower 44Ti production in scenarios for the α-rich freezeout. Special attention has been paid to assessing realistic uncertainties of stellar reaction rates produced from a combination of experimental and theoretical cross sections. With such methods, we also develop a re-evaluation of the 44Ti(α, p)47V reaction rate. Using these two rates we carry out a sensitivity survey of 44Ti synthesis in eight expansions representing peak temperature and density conditions drawn from a suite of recent supernova explosion models. Our results suggest that the current uncertainty in these two reaction rates could lead to as large an uncertainty in 44Ti synthesis as that produced by different treatments of stellar physics.
15724697
The Hubble Deep Field South - STIS imaging
We present the imaging observations made with the Space Telescope Imaging Spectrograph (STIS) of the Hubble Deep Field South. The field was imaged in four bandpasses: a clear CCD bandpass for 156 ks, a long-pass filter for 22–25 ks pixel-1 typical exposure, a near-UV bandpass for 23 ks, and a far-UV bandpass for 52 ks. The clear, visible image is the deepest observation ever made in the UV-optical wavelength region, reaching a 10 σ AB magnitude of 29.4 for an object of area 0.2 arcsec2. The field contains QSO J2233-606, the target of the STIS spectroscopy, and extends 50'' × 50'' for the visible images, and 25'' × 25'' for the ultraviolet images. We present the images, catalog of objects, and galaxy counts obtained in the field.