ESA Science & Technology - Publication Archive

E-print arXiv:astro-ph/0612384, for ApJS COSMOS Special Issue, 2007 in press

We present the first identification of large-scale structures (LSS) at z < 1.1 in the Cosmic Evolution Survey (COSMOS). The structures are identified from adaptive smoothing of galaxy counts in the pseudo-3d space (R.A., Dec., z) using the COSMOS photometric redshift catalog. The technique is tested on a simulation including galaxies distributed in model clusters and a field galaxy population - recovering structures on all scales from 1 to 202 without a priori assumptions for the structure size or density profile. Our procedure makes no a priori selection on galaxy spectral energy distribution (SED, for example the Red Sequence), enabling an unbiased investigation of environmental effects on galaxy evolution. The COSMOS photometric redshift catalog yields a sample of 1.5 × 10⁵ galaxies with redshift accuracy, delta z_FWHM/(1 + z) d 0.1 at z < 1.1 down to I_AB d 25 mag. Using this sample of galaxies, we identify 42 large-scale structures and clusters. Projected surface-density maps for the structures indicate multiple peaks and internal structure in many of the most massive LSS. The stellar masses (determined from the galactic SEDs) for the LSS range from M_* ~ 10¹¹ up to ~ 3 × 10¹³ Msun. Five LSS have total stellar masses exceeding 10¹³ Msun. (Total masses including non-stellar baryons and dark matter are expected to be ~ 50 - 100 times greater.) The derived mass function for the LSS is consistent (within the expected Poisson and cosmic variances) with those derived from optical and X-ray studies at lower redshift.

We present a deep image of the radio galaxy MRC 1138-262 taken with the Hubble Space Telescope (HST) at a redshift of z = 2.2. The galaxy is known to have properties of a cD galaxy progenitor and be surrounded by a 3 Mpc-sized structure, identified with a protocluster. The morphology shown on the new deep HST ACS image is reminiscent of a spider's web. More than 10 individual clumpy features are observed, apparently star-forming satellite galaxies in the process of merging with the progenitor of a dominant cluster galaxy 11 Gyr ago. There is an extended emission component, implying that star formation was occurring over a 50 × 40 kpc region at a rate of more than 100 M_solar yr^-1. A striking feature of the newly named "Spiderweb galaxy" is the presence of several faint linear galaxies within the merging structure. The dense environments and fast galaxy motions at the centers of protoclusters may stimulate the formation of these structures, which dominate the faint resolved galaxy populations in the Hubble Ultra Deep Field. The new image provides a unique testbed for simulations of forming dominant cluster galaxies.

We have used HST imaging of the central regions of the globular cluster 47 Tucanae (=NGC 104), taken with the WFPC2 and ACS instruments between 1995 and 2002, to derive proper motions and U- and V-band magnitudes for 14 366 stars within 100" (about 5 core radii) of the cluster center. This represents the largest set of member velocities collected for any globular cluster. The stars involved range in brightness from just fainter than the horizontal branch of the cluster to more than 2.5 mag below the main-sequence turnoff. In the course of obtaining these kinematic data, we also use a recent set of ACS images to define a list of astrometrically calibrated positions (and F475W magnitudes) for nearly 130 000 stars in a larger, ~3'×3' central area. We describe our data reduction procedures in some detail and provide the full position, photometric, and velocity data in the form of downloadable electronic tables. We have used the star counts to obtain a new estimate for the position of the cluster center and to define the density profile of main-sequence turnoff and giant branch stars in to essentially zero radius, thus constraining the global spatial structure of the cluster better than before. A single-mass, isotropic King model fit to it is then used as a rough point of reference against which to compare the gross characteristics of our proper-motion data. We search in particular for any evidence of very fast-moving stars, in significantly greater numbers than expected for the extreme tails of the velocity distribution in a sample of our size. We find that likely fewer than 0.1%, and no more than about 0.3%, of stars with measured proper motions have total speeds above the nominal central escape velocity of the cluster. At lower speeds, the proper-motion velocity distribution very closely matches that of a regular King model (which is itself nearly Gaussian given the high stellar density) at all observed radii.

We study the nuclear star clusters in spiral galaxies of various Hubble types using spectra obtained with STIS on-board HST. We observed the nuclear clusters in 40 galaxies, selected from two previous HST/WFPC2 imaging surveys. At a spatial resolution of about 0.2", the spectra provide a better separation of cluster light from underlying galaxy light than is possible with ground-based spectra. Approximately half of the spectra have sufficient signal-to-noise ratio for detailed stellar population analysis. For the other half we only measure the continuum slope, as quantified by the B-V color. To infer the star formation history, metallicity and dust extinction, we fit weighted superpositions of single-age stellar population templates to the high signal-to-noise spectra. We use the results to determine the luminosity-weighted age, mass-to-light ratio, and masses of the clusters. The models provide excellent fits to the data and generally require a mixture of populations of different ages. Approximately half of the sample clusters contain a population younger than 1 Gyr.

Contents: Hubble News Update; Staff Changes; The AstroAsciiData Python Module; Creating a Legacy Archive for Hubble; The New ST-ECF Web; Scisoft VI

We present a follow-up imaging study (HST + ground based observations) of the complex gravitational lens system RXS J113155.4-123155. The latter consists of a quadruply imaged QSO and of an Einstein ring. Thanks to the MCS deconvolution technique (Magain, Courbin, Sohy 1998), we retrieve accurate relative positions for the lensed QSO images and photometry in the Bessel B, V, R, I filters and in the J-SOFI, F160W and Ks filters. The HST frames unveil structures in the Einstein ring as well as an unidentified object X in the vicinity of the lensing galaxy. We discuss the lightcurves and the chromatic flux ratio variations observed in this system and deduce that both intrinsic variability and microlensing took place during a span longer than one year. We also demonstrate that microlensing may easily account for the so called anomalous flux ratios presented in the discovery paper. However, the actual flux ratios are still poorly reproduced when modelling the lens potential with a Singular Isothermal Ellipsoid+shear. We argue that this disagreement can hardly be explained by milli-lensing due to substructures in the lensing galaxy. A solution proposed elsewhere (Claeskens et al. 2005) consists in a more complex lens model including an octupole term to the lens gravitational potential.

We report on the analysis of the FUSE spectrum of the most metal-poor blue compact dwarf (BCD) galaxy I Zw 18. From a simultaneous fitting of HI and metal absorption lines arising from the neutral interstellar medium (ISM), we infer abundances of heavy elements (Fe, O, Si, Ar, and N) which are several times lower than in the HII regions. The only exception is Fe, whose abundance is the same. The abundance pattern of the ISM suggests ancient star formation (SF) activity with an age of at least a Gyr that enriched the H i phase. A more recent episode that started 10 to several 100 Myr ago is responsible for the additional enrichment of alpha-elements and N in the HII regions.

The determination of the metal abundances in the neutral interstellar medium (ISM) of dwarf star-forming galaxies is a key step in understand- ing their physical and chemical evolution. This type of investigation has been possible in the last 5 years thanks to FUSE. We will give a flavor of the issues involved by presenting the work that we are performing in this astrophysical field.

Contents: Ageing of Spectral Lamps in Space; Extracting Spectra with Optimal Weights in aXe1.5; ESA-ESO Topical Science Working Groups; NEON Observing Schools

Sirius B is the nearest and brightest of all white dwarfs, but it is very difficult to observe at visible wavelengths due to the overwhelming scattered light contribution from Sirius A. However, from space we can take advantage of the superb spatial resolution of the Hubble Space Telescope (HST) to resolve the A and B components. Since the closest approach in 1993, the separation between the two stars has become increasingly favourable and we have recently been able to obtain a spectrum of the complete Balmer line series for Sirius B using the HST Space Telescope Imaging Spectrograph (STIS). The quality of the STIS spectra greatly exceeds that of previous ground-based spectra, and can be used to provide an important determination of the stellar temperature (T_eff= 25193 K) and gravity (log g= 8.556). In addition, we have obtained a new, more accurate, gravitational redshift of 80.42 +/- 4.83 km s^-1 for Sirius B. Combining these results with the photometric data and the Hipparcos parallax, we obtain new determinations of the stellar mass for comparison with the theoretical mass-radius relation. However, there are some disparities between the results obtained independently from log g and the gravitational redshift which may arise from flux losses in the narrow 50 × 0.2 arcsec² slit. Combining our measurements of T_eff and log g with the Wood evolutionary mass-radius relation, we obtain a best estimate for the white dwarf mass of 0.978 M_solar. Within the overall uncertainties, this is in agreement with a mass of 1.02 M_solar obtained by matching our new gravitational redshift to the theoretical mass-radius relation.

We present results from an HST/ACS imaging study of the metal-poor blue compact dwarf galaxy SBS 1415+437. It has been argued previously that this is a very young galaxy that started to form stars only ~100 Myr ago. However, we find that the optical color-magnitude diagram prominently reveals asymptotic giant branch and red giant branch (RGB) stars. The brightness of the RGB tip yields a distance D H 13.6 Mpc. The color of the RGB implies that its stars must be older than <1.3 Gyr, with the exact age depending on the assumed metallicity and dust extinction. The number of RGB stars implies that most of the stellar mass resides in this evolved population. In view of these and other HST results for metal-poor galaxies, it seems that the local universe simply may not contain any galaxies that are currently undergoing their first burst of star formation.

We have identified three multiply imaged galaxies in Hubble Space Telescope images of the redshift z=0.68 cluster responsible for the large-separation quadruply lensed quasar, SDSS J1004+4112. Spectroscopic redshifts have been secured for two of these systems using the Keck I 10 m telescope. The most distant lensed galaxy, at z=3.332, forms at least four images, and an Einstein ring encompassing 3.1 times more area than the Einstein ring of the lensed QSO images at z=1.74, due to the greater source distance. For a second multiply imaged galaxy, we identify Ly_alpha emission at a redshift of z=2.74. The cluster mass profile can be constrained from near the center of the brightest cluster galaxy, where we observe both a radial arc and the fifth image of the lensed quasar, to the Einstein radius of the highest redshift galaxy, ~110 kpc. Our preliminary modeling indicates that the mass approximates an elliptical body, with an average projected logarithmic gradient of ~=-0.5. The system is potentially useful for a direct measurement of world models in a previously untested redshift range. Based on observations made with the NASA/ESA Hubble Space Telescope, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.

We investigate the nature of the diffuse intracluster ultraviolet light seen in 12 local starburst galaxies, using longslit ultraviolet spectroscopy obtained with the Space Telescope Imaging Spectrograph (STIS) aboard the Hubble Space Telescope (HST ). We take this faint intracluster light to be the field in each galaxy and compare its spectroscopic signature with Starburst99 evolutionary synthesis models and with neighboring star clusters. Our main result is that the diffuse ultraviolet light in 11 of the 12 starbursts lacks the strong O star wind features that are clearly visible in spectra of luminous clusters in the same galaxies. The difference in stellar features dominating cluster and field spectra indicates that the field light comes primarily from a different stellar population and not from scattering of UV photons originating in the massive clusters.

Contents: HST News and Status; ST-ECF Update; Hubble's 15th Anniversary; New Development in aXe; STIS CTE Correction Science Case; MultiDrizzle in the Archive Pipelines.

A light echo around SN 1993J was observed 8.2 yr after explosion by a Hubble Space Telescope Wide Field Planetary Camera 2 observation, adding to the small family of supernovae with light echoes. The light echo was formed by supernova light scattered from a dust sheet that lies 220 pc away from the supernova, 50 pc thick along the line of sight, as inferred from the radius and width of the light echo. The dust inferred from the light echo surface brightness is 1000 times denser than the intercloud dust. The graphite-to-silicate fraction cannot be determined by our BVI photometric measurements. However, a pure graphite model can be excluded on the basis of comparison with the data. With future observations, it will be possible to measure the expansion rate of the light echo, from which an independent distance to M81 can be obtained.

The massive star that underwent a collapse of its core to produce supernova (SN)1993J was subsequently identified as a non-variable red supergiant star in images of the galaxy M81 taken before explosion. It showed an excess in ultraviolet and B-band colours, suggesting either the presence of a hot, massive companion star or that it was embedded in an unresolved young stellar association. The spectra of SN1993J underwent a remarkable transformation from the signature of a hydrogen-rich type II supernova to one of a helium-rich (hydrogen-deficient) type Ib. The spectral and photometric peculiarities were best explained by models in which the 13-20 solar mass supergiant had lost almost its entire hydrogen envelope to a close binary companion, producing a 'type IIb' supernova, but the hypothetical massive companion stars for this class of supernovae have so far eluded discovery. Here we report photometric and spectroscopic observations of SN1993J ten years after the explosion. At the position of the fading supernova we detect the unambiguous signature of a massive star: the binary companion to the progenitor.

We show that one can obtain a good fit to the measured main sequence mass function (MF) of a large sample of Galactic clusters (young and old) with a tapered Salpeter power law distribution function with an exponential truncation. The average value of the power law index is very close to Salpeter (~2.3), whereas the characteristic mass is in the range 0.1 - 0.5 M_solar and does not seem to vary in a systematic way with the present cluster parameters such as metal abundance and central concentration. However, a remarkable correlation with age is seen, in that the peak mass of young clusters increases with it. This trend does not extend to globular clusters, whose peak mass is firmly at ~0.35 M_solar. This correlation is due to the onset of mass segregation following early dynamical interactions in the loose cluster cores. Differences between globular and younger clusters may depend on the initial environment of star formation, which in turn affects their total mass.

I have developed an automated slitless-spectrum extraction software package that allows for the simultaneous obtention of hundreds of point-source spectra within a field. MULTISPEC is an spectroscopic analog to crowded-field PSF-fitting photometry packages such as DAOPHOT or HSTPHOT in the sense that it fits spatial profiles to the spectrum of each star simultaneously. The code has been written in IDL, and implemented and tested for the STIS NUV-MAMA objective-prism. It could be rather easily adapted to observations with other spectral elements with slitless capabilities (objective prisms, grisms, and gratings) in HST or in other telescopes. I discuss the techniques employed and I show the first results obtained with MULTISPEC.

Thanks to their proximity, local starbursts are perfectly suited for high-resolution and sensitivity multiwavelength observations aimed to test our ideas about star formation, evolution of massive stars, physics and chemical evolution of the interstellar medium (ISM). High-resolution UV spectroscopy with FUSE and STIS has recently given the possibility to characterize in great detail the neutral ISM in local starbursts thanks to the presence in this spectral range of many absorption lines from ions of the most common heavy elements. Here we present the results for two nearby starburst galaxies, I Zw 18 and NGC 1705, and show how these results relate to the star-formation (SF) history and evolutionary state of these stellar systems.

We present new results on the star formation history of the dwarf irregular galaxy NGC 1569. The data were obtained with the Hubble Space TelescopeNICMOS/NIC2 in the F110W (J ) and F160W(H) near-infrared (NIR) filters and interpreted with the synthetic color-magnitude diagram method. The galaxy has experienced complex star formation (SF) activity. The best fit to the data is found by assuming three episodes of activity in the last 1-2 Gyr. The most recent and strong episode constrained by these NIR data started ~3.7x10⁷ yr ago and ended ~1:3 x 10⁷ yr ago, although we cannot exclude the possibility that up to three SF episodes occurred in this time interval. The average star formation rate (SFR) of the episode is ~3.2 M_Sun yr^-1 kpc^-2, in agreement with literature data. A previous episode produced stars between ~1.5x10⁸ and ~4x10⁷ yr ago, with a mean SFR about two-thirds lower than the mean SFR of the youngest episode. An older SF episode occurred about 1x10⁹ yr ago. All these SFRs are 2-3 orders of magnitude higher than those derived for late-type dwarfs of the Local Group. In all cases an initial mass function similar to Salpeter's allows for a good reproduction of the data, but we cannot exclude flatter mass functions. These results have been obtained adopting a distance of 2.2 Mpc and a reddening E(B - V )=0.56. A larger distance would require younger episodes and higher SFRs.We have explored some possible scenarios using the astrated mass in the best-fit model, in order to constrain the past star formation history. We cannot rule out a low past SFR, but we can safely conclude that the last 1-2 Gyr have been peculiar.