Showing papers by "European Southern Observatory published in 2002"

Theoretical isochrones in several photometric systems I. Johnson-Cousins-Glass, HST/WFPC2, HST/NICMOS, Washington, and ESO Imaging Survey filter sets

Abstract: We provide tables of theoretical isochrones in several photometric systems. To this aim, the following steps are followed: (1) first, we re-write the formalism for converting synthetic stellar spectra into tables of bolometric corrections. The resulting formulas can be applied to any photometric system, provided that the zero-points are specified by means of either ABmag, STmag, VEGAmag, or a standard star system that includes well-known spectrophotometric standards. Interstellar ab- sorption can be considered in a self-consistent way. (2) We assemble an extended and updated library of stellar intrinsic spectra. It is mostly based on "non-overshooting" ATLAS9 models, suitably extended to both low and high eective temperatures. This oers an excellent coverage of the parameter space of Te ,l ogg ,a nd (M/H). We briefly discuss the main uncertainties and points still deserving more improvement. (3) From the spectral library, we derive tables of bolometric corrections for Johnson- Cousins-Glass, HST/WFPC2, HST/NICMOS, Washington, and ESO Imaging Survey systems (this latter consisting on the WFI, EMMI, and SOFI filter sets). (4) These tables are used to convert several sets of Padova isochrones into the corresponding absolute magnitudes and colours, thus providing a useful database for several astrophysical applications. All data files are made available in electronic form.

A star in a 15.2-year orbit around the supermassive black hole at the centre of the Milky Way

Abstract: A star in a 15.2-year orbit around the supermassive black hole at the centre of the Milky Way

Drizzle: A Method for the Linear Reconstruction of Undersampled Images

Abstract: We have developed a method for the linear reconstruction of an image from undersampled, dithered data. The algorithm, known as Variable-Pixel Linear Reconstruction, or informally as “Drizzle”, preserves photometry and resolution, can weight input images according to the statistical significance of each pixel, and removes the effects of geometric distortion both on image shape and photometry. This paper presents the method and its implementation. The photometric and astrometric accuracy and image fidelity of the algorithm as well as the noise characteristics of output images are discussed. In addition, we describe the use of drizzling to combine dithered images in the presence of cosmic rays.

Master of the cosmic microwave background anisotropy power spectrum: a fast method for statistical analysis of large and complex cosmic microwave background data sets

Abstract: We describe a fast and accurate method for estimation of the cosmic microwave background (CMB) anisotropy angular power spectrum—Monte Carlo Apodized Spherical Transform Estimator (MASTER). Originally devised for use in the interpretation of the BOOMERANG experimental data, MASTER is both a computationally efficient method suitable for use with the currently available CMB data sets (already large in size, despite covering small fractions of the sky, and affected by inhomogeneous and correlated noise) and a very promising application for the analysis of very large future CMB satellite mission products.

Chandra Deep Field South: The 1 Ms Catalog

Abstract: In this paper we present the source catalog obtained from a 942 ks exposure of the Chandra Deep Field South (CDFS), using the Advanced CCD Imaging Spectrometer (ACIS-I) on the Chandra X-ray Observatory. Eleven individual pointings made between 1999 October and 2000 December were combined to generate the final image used for object detection. Catalog generation proceeded simultaneously using two different methods; a method of our own design using a modified version of the SExtractor algorithm, and a wavelet transform technique developed specifically for Chandra observations. The detection threshold has been set in order to have less than 10 spurious sources, as assessed by extensive simulations. We subdivided the catalog into four sections. The primary list consists of objects common to the two detection methods. Two secondary lists contain sources which were detected by (1) the SExtractor algorithm alone and (2) the wavelet technique alone. The fourth list consists of possible diffuse or extended sources. The flux limits at the aimpoint for the soft (0.5-2 keV) and hard (2-10 keV) bands are 5.5 × 10-17 erg s-1 cm-2 and 4.5 × 10-16 erg s-1 cm-2, respectively. The total number of sources is 346; out of them, 307 were detected in the 0.5-2 keV band, and 251 in the 2-10 keV band. We also present optical identifications for the cataloged sources. Our primary optical data are R band imaging from VLT/FORS1 to a depth of R ~ 26.5 (Vega). In regions of the field not covered by the VLT/FORS1 deep imaging, we use R-band data obtained with the Wide Field Imager (WFI) on the ESO-MPI 2.2 m telescope, as part of the ESO Imaging Survey (EIS), which cover the entire X-ray survey. We found that the FORS1/Chandra offsets are small, ~1''. Coordinate cross-correlation finds 85% of the Chandra sources covered by FORS1 R to have counterparts within the 3 σ error box (15 depending on off-axis angle and X-ray signal-to-noise). The unidentified fraction of sources, approximately ~10%-15%, is close to the limit expected from the observed X-ray flux to R-band ratio distribution for the identified sample.

The evolution of x-ray clusters of galaxies

Abstract: ■ Abstract Considerable progress has been made over the past decade in the study of the evolutionary trends of the population of galaxy clusters in the Universe. In this review we focus on observations in the X-ray band. X-ray surveys with the ROSAT satellite, supplemented by follow-up studies with ASCA and Beppo-SAX, have allowed an assessment of the evolution of the space density of clusters out to z 1 and the evolution of the physical properties of the intracluster medium out to z 0.5. With the advent of Chandra and Newton-XMM and their unprecedented sensitivity and angular resolution, these studies have been extended beyond redshift unity and have revealed the complexity of the thermodynamical structure of clusters. The properties of the intracluster gas are significantly affected by nongravitational processes including star formation and Active Galactic Nuclei (AGN) activity. Convincing evidence has emerged for modest evolution of both the bulk of the X-ray cluster population and their thermodynamical properties since redshift unity. Such an observational scenario is consistent with hierarchical models of structure formation in a flat low-density

First stars. I. The extreme r-element rich, iron-poor halo giant CS 31082-001 - Implications for the r-process site(s) and radioactive cosmochronology

Abstract: We present a high-resolution (R = 75 000, S/N 500) spectroscopic analysis of the bright (V = 11.7), extreme halo giant CS 31082-001 ((Fe/H) = 2.9), obtained in an ESO-VLT Large Programme dedicated to very metal-poor stars. We nd CS 31082-001 to be extremely rich in r-process elements, comparable in this respect only to the similarly metal-poor, but carbon-enriched, giant CS 22892-052. As a result of the extreme overabundance of the heaviest r-process elements, and negligible blending from CH and CN molecular lines, a reliable measurement is obtained of the U II line at 386 nm, for the rst time in a halo star, along with numerous lines of Th II, as well as lines of 25 other r-process elements. Abundance estimates for a total of 43 elements (44 counting Hydrogen) are reported in CS 31082-001, almost half of the entire periodic table. The main atmospheric parameters of CS 31082- 001 are as follows: Te = 4825 50 K, logg =1 :5 0: 3( cgs), (Fe/H) = 2.9 0:1 (in LTE), and microturbulence 1.8 0.2 km s 1 . Carbon and nitrogen are not signicantly enhanced relative to iron. As usual in giant stars, Li is depleted by dilution (log(Li/H) = 0.85). The -elements show the usual enhancements with respect to iron, with (O/Fe) = 0:6 0:2 (from (O I) 6300 A), (Mg/Fe) = 0:45 0:16, (Si/Fe) = 0:24 0:1, and (Ca/Fe) = 0:41 0:08, while (Al/Fe) is near 0.5. The r-process elements show unusual patterns: among the lightest elements (Z 40), Sr and Zr follow the Solar r-element distribution, but Ag is down by 0.8 dex. All elements with 56 Z 72 follow the Solar r-element pattern, reduced by about 1.25 dex. Accordingly, the (r/Fe) enhancement is about +1.7 dex (a factor of 50), very similar to that of CS 22892-052. Pb, in contrast, seems to be below the shifted Solar r-process distribution, possibly indicating an error in the latter, while thorium is more enhanced than the lighter nuclides. In CS 31082-001, log(Th/Eu) is 0:22 0:07, higher than in the Solar System ( 0.46) or in CS 22892-052 ( 0.66). If CS 31082-001 and CS 22892-052 have similar ages, as expected for two extreme halo stars, this implies that the production ratios were dierent by about 0.4 dex for the two objects. Conversely, if the Th/Eu production ratio were universal, an age of 15 Gyr for CS 22892-052 would imply a negative age for CS 31082-001. Thus, while a universal production ratio for the r-process elements seems to hold in the interval 56 Z 72, it breaks down in the actinide region. When available, the U/Th is thus preferable to Th/Eu for radioactive dating, for two reasons: (i) because of its faster decay rate and smaller sensitivity to observational errors, and (ii) because the inital production ratio of the neighboring nuclides 238 Ua nd 232 Th is more robustly predicted than the 151 Eu/ 232 Th ratio. Our current best estimate for the age of CS 31082-001 is 14:0 2: 4G yr. However, the computed actinide production ratios should be veried by observations of daughter elements such as Pb and Bi in the same star, which are independent of the subsequent history of star formation and nucelosynthesis in the Galaxy.

The Luminosity and Mass Function of the Trapezium Cluster: From B Stars to the Deuterium-burning Limit

Abstract: We use the results of a new multiepoch, multiwavelength, near-infrared census of the Trapezium cluster in Orion to construct and analyze the structure of its infrared (K-band) luminosity function. Specifically, we employ an improved set of model luminosity functions to derive this cluster's underlying initial mass function (IMF) across the entire range of mass from OB stars to substellar objects down to near the deuterium-burning limit. We derive an IMF for the Trapezium cluster that rises with decreasing mass, having a Salpeter-like IMF slope until near ~0.6 M☉ where the IMF flattens and forms a broad peak extending to the hydrogen-burning limit, below which the IMF declines into the substellar regime. Independent of the details, we find that substellar objects account for no more than ~22% of the total number of likely cluster members. Further, the substellar Trapezium IMF breaks from a steady power-law decline and forms a significant secondary peak at the lowest masses (10-20 times the mass of Jupiter). This secondary peak may contain as many as ~30% of the substellar objects in the cluster. Below this substellar IMF peak, our K-band luminosity function (KLF) modeling requires a subsequent sharp decline toward the planetary mass regime. Lastly, we investigate the robustness of pre-main-sequence luminosity evolution as predicted by current evolutionary models, and we discuss possible origins for the IMF of brown dwarfs.

N2H+ and C18O Depletion in a Cold Dark Cloud

Abstract: We present sensitive, high angular resolution molecular line observations of C18O and N2H+ toward the dark globule B68. We directly compare these data with the near-infrared extinction measurements of Alves, Lada, & Lada to derive the first evidence for the depletion of N2H+, and by inference N2, in a prestellar dark cloud. We also find widespread C18O depletion throughout the centrally condensed core of the B68 cloud. Specifically, we find the N2H+ emission to peak in a shell partially surrounding the peak of dust extinction. Moreover, N2H+ peaks inside the much larger C18O depletion hole and has a smaller depletion zone, confirming theoretical predictions. These data are analyzed through a direct coupling of time-dependent chemical models to a radiation transfer code. This analysis highlights the importance of photodissociation at cloud edges and suggests that the CO abundance declines by 2 orders of magnitude from edge to center. In contrast, N2H+ declines in abundance, at minimum, by at least a factor of 2. Indeed, it is entirely possible that both N2H+ and N2 are completely absent from the central regions of the B68 core. The depletion of N2H+, and its parent molecule N2, opens up the possibility that the centers of dense cores, prior to the formation of a star, may evade detection by conventional methods of probing cores using molecular emission. Under these conditions, H2D+ may be the sole viable molecular probe of the innermost regions of star-forming cores.

The Luminosity & Mass Function of the Trapezium Cluster: From B stars to the Deuterium Burning Limit

Abstract: We use the results of a new, multi-epoch, multi-wavelength, near-infrared census of the Trapezium Cluster in Orion to construct and to analyze the structure of its infrared (K band) luminosity function. Specifically, we employ an improved set of model luminosity functions to derive this cluster's underlying Initial Mass Function (IMF) across the entire range of mass from OB stars to sub-stellar objects down to near the deuterium burning limit. We derive an IMF for the Trapezium Cluster that rises with decreasing mass, having a Salpeter-like IMF slope until near ~0.6 M_sun where the IMF flattens and forms a broad peak extending to the hydrogen burning limit, below which the IMF declines into the sub-stellar regime. Independent of the details, we find that sub-stellar objects account for no more than ~22% of the total number of likely cluster members. Further, the sub-stellar Trapezium IMF breaks from a steady power-law decline and forms a significant secondary peak at the lowest masses (10-20 times the mass of Jupiter). This secondary peak may contain as many as \~30% of the sub-stellar objects in the cluster. Below this sub-stellar IMF peak, our KLF modeling requires a subsequent sharp decline toward the planetary mass regime. Lastly, we investigate the robustness of pre-main sequence luminosity evolution as predicted by current evolutionary models, and we discuss possible origins for the IMF of brown dwarfs.

The Bulk of the Cosmic Infrared Background Resolved by ISOCAM

Abstract: Deep extragalactic surveys with ISOCAM revealed the presence of a large density of faint mid-infrared (MIR) sources. We have computed the 15m integrated galaxy light produced by these galaxies above a sensitivity limit of 50 Jy. It sets a lower limit to the 15m extragalactic background light of (2:4 0:5) nW m 2 sr 1 .T he redshift distribution of the ISOCAM galaxies is inferred from the spectroscopically complete sample of galaxies in the Hubble Deep Field North (HDFN). It peaks around z 0.8 in agreement with studies in other elds. The rest-frame 15m and bolometric infrared (8{1000m) luminosities of ISOCAM galaxies are computed using the correlations that we establish between the 6.75, 12, 15m and infrared (IR) luminosities of local galaxies. The resulting IR luminosities were double-checked using radio (1.4 GHz) flux densities from the ultra-deep VLA and WSRT surveys of the HDFN on a sample of 24 galaxies as well as on a sample of 109 local galaxies in common between ISOCAM and the NRAO VLA Sky Survey (NVSS). This comparison shows for the rst time that MIR and radio luminosities correlate up to z 1. This result validates the bolometric IR luminosities derived from MIR luminosities unless both the radio-far infrared (FIR) and the MIR-FIR correlations become invalid around z 1. The fraction of IR light produced by active nuclei was computed from the cross-correlation with the deepest X-ray surveys from the Chandra and XMM-Newton observatories in the HDFN and Lockman Hole respectively. We nd that at most 20% of the 15m integrated galaxy light is due to active galactic nuclei (AGNs) unless a large population of AGNs was missed by Chandra and XMM-Newton. About 75% of the ISOCAM galaxies are found to belong to the class of luminous infrared galaxies (LIR 10 11 L). They exhibit star formation rates of the order of100 M yr 1 . The comoving density of infrared light due to these luminous IR galaxies was more than 40 times larger at z 1 than today. The contribution of ISOCAM galaxies to the peak of the cosmic infrared background (CIRB) at 140m was computed from the MIR-FIR correlations for star forming galaxies and from the spectral energy distribution of the Seyfert 2, NGC 1068, for AGNs. We nd that the galaxies unveiled by ISOCAM surveys are responsible for the bulk of the CIRB, i.e. (16 5) nW m 2 sr 1 as compared to the (25 7) nW m 2 sr 1 measured with the COBE satellite, with less than 10% due to AGNs. Since the CIRB contains most of the light radiated over the history of star formation in the universe, this means that a large fraction of present-day stars must have formed during a dusty starburst event similar to those revealed by ISOCAM.

The Chandra Deep Field-South: The 1 Million Second Exposure*

Abstract: We present the main results from our 940 ks observation of the Chandra Deep Field-South using the source catalog described in an accompanying paper by Giacconi et al. We extend the measurement of source number counts to 5.5 × 10-17 ergs cm-2 s-1 in the soft 0.5-2 keV band and 4.5 × 10-16 ergs cm-2 s-1 in the hard 2-10 keV band. The hard-band log N-log S shows a significant flattening (slope 0.6) below ≈10-14 ergs cm-2 s-1, leaving at most 10%-15% of the X-ray background to be resolved, the main uncertainty lying in the measurement of the total flux of the X-ray background (XRB). On the other hand, the analysis in the very hard 5-10 keV band reveals a relatively steep log N-log S (slope 1.3) down to 10-15 ergs cm-2 s-1. Together with the evidence of a progressive flattening of the average X-ray spectrum near the flux limit, this indicates that there is still a nonnegligible population of faint hard sources to be discovered at energies not well probed by Chandra, which possibly contributes to the 30 keV bump in the spectrum of the XRB. We use optical redshifts and identifications, obtained with the Very Large Telescope, for one-quarter of the sample to characterize the combined optical and X-ray properties of the Chandra Deep Field-South sample. Different source types are well separated in a parameter space that includes X-ray luminosity, hardness ratio, and R-K color. Type II objects, while redder on average than the field population, have colors that are consistent with being hosted by a range of galaxy types. Type II active galactic nuclei are mostly found at z 1, in contrast with predictions based on active galactic nucleus population synthesis models, thus suggesting a revision of their evolutionary parameters.

HST color-magnitude diagrams of 74 galactic globular clusters in the HST F439W and F555W bands ⋆

Abstract: We present the complete photometric database and the color-magnitude diagrams for 74 Galactic globular clusters observed with the HST/WFPC2 camera in the F439W and F555W bands. A detailed discussion of the various reduction steps is also presented, and of the procedures to transform instrumental magnitudes into both the HST F439W and F555W flight system and the standard Johnson B and V systems. We also describe the artificial star experiments which have been performed to derive the star count completeness in all the relevant branches of the color magnitude diagram. The entire photometric database and the completeness function will be made available on the Web immediately after the publication of the present paper.

The Chemical Composition and Age of the Metal-poor Halo Star BD +17°3248*

Abstract: We have combined new high-resolution spectra obtained with the Hubble Space Telescope (HST )a nd ground-based facilities to make a comprehensive new abundance analysis of the metal-poor, halo star BD +17 � 3248. We have detected the third r-process peak elements osmium, platinum, and (for the first time in a metal-poor star) gold, elements whose abundances can only be reliably determined using HST. Our observations illustrate a pattern seen in other similar halo stars with the abundances of the heavier neutron capture elements, including the third r-process peak elements, consistent with a scaled solar system r-process distribution. The abundances of the lighter neutron capture elements, including germanium and silver, fall below that same scaled solar r-process curve, a result similar to that seen in the ultra–metal-poor star CS 22892-052. A single site with two regimes or sets of conditions, or perhaps two different sites for the lighter and heavier neutron capture elements, might explain the abundance pattern seen in this star. In addition, we have derived a

The Bulk of the Cosmic Infrared Background Resolved by ISOCAM

Abstract: We have computed the 15 microns integrated galaxy light produced by ISOCAM galaxies above a sensitivity limit of 50 microJy. It sets a lower limit to the 15 microns extragalactic background light of (2.4+/-0.5) nW/m^2/Hz. The redshift distribution of the ISOCAM galaxies is inferred from the spectroscopically complete sample of galaxies in the Hubble Deep Field North (HDFN). We demonstrate that mid (MIR) and far (FIR) infrared luminosities correlate for local star forming galaxies and that this correlation is consistent with the radio-FIR one up to z ~ 1. From these correlations, about 75 % of the ISOCAM galaxies are found to belong to the class of luminous infrared galaxies (L[IR] >= 10^11 Lsol), with star formation rates of the order of ~ 100 Msol/yr. The cross-correlation with the deepest X-ray surveys from the Chandra and XMM-Newton observatories in the HDFN and Lockman Hole respectively, indicates that at most 20 % of the 15 microns integrated galaxy light is due to active galactic nuclei (AGNs) unless a large population of AGNs was missed by Chandra and XMM-Newton. The contribution of ISOCAM galaxies to the peak of the cosmic infrared background (CIRB) at 140 microns was computed from the MIR-FIR correlations for star forming galaxies and from the spectral energy distribution of the Seyfert 2, NGC 1068, for AGNs. We find that the galaxies unveiled by ISOCAM surveys are responsible for the bulk of the CIRB, i.e (16+/-5) nW/m^2/Hz as compared to the (25+/-7) nW/m^2/Hz measured with the COBE satellite, with less than 10 % due to AGNs. Since the CIRB contains most of the light radiated over the history of star formation in the universe, this means that a large fraction of present-day stars must have formed during a dusty starburst event similar to those revealed by ISOCAM.

The CORALIE survey for southern extra-solar planets - IX. A 1.3-day period brown dwarf disguised as a planet

Abstract: In this article we present the case of HD 41004 AB, a system composed of a K0V star and a 3.7-mag fainter M-dwarf companion. We have obtained 86 CORALIE spectra of this system with the goal of obtaining precise radial-velocity measure- ments. Since HD 41004 A and B are separated by only 0.5 00 , in every spectrum taken for the radial-velocity measurement, we are observing the blended spectra of the two stars. An analysis of the measurements has revealed a velocity variation with an amplitude of about 50 m s 1 and a periodicity of 1.3 days. This radial-velocity signal is consistent with the expected variation induced by the presence of a companion to either HD 41004 A or HD 41004 B, or to some other eect due to e.g. activity related phenomena. In particular, such a small velocity amplitude could be the signature of the presence of a very low mass giant plane- tary companion to HD 41004 A, whose light dominates the spectra. The radial-velocity measurements were then complemented with a photometric campaign and with the analysis of the bisector of the CORALIE Cross-Correlation Function (CCF). While the former revealed no significant variations within the observational precision of0.003-0.004 mag (except for an observed flare event), the bisector analysis showed that the line profiles are varying in phase with the radial-velocity. This latter result, complemented with a series of simulations, has shown that we can explain the observations by considering that HD 41004 B has a brown-dwarf companion orbiting with the observed 1.3-day period. As the spectrum of the fainter HD 41004 B "moves" relative to the one of HD 41004 A (with an amplitude of a few km s 1 ), the relative position of the spectral lines of the two spectra changes, thus changing the blended line-profiles. This variation is large enough to explain the observed radial-velocity and bisector variations, and is compatible with the absence of any photometric signal. If confirmed, this detection represents the first discovery of a brown dwarf in a very short period (1.3-day) orbit around an M dwarf. Finally, this case should be taken as a serious warning about the importance of analyzing the bisector when looking for planets using radial-velocity techniques.

Explosive Nucleosynthesis in Aspherical Hypernova Explosions and Late-Time Spectra of SN 1998bw

Abstract: Aspherical explosion models for the hypernova (hyperenergetic supernova) SN 1998bw are presented. Nucleosynthesis in aspherical explosions is examined with a two-dimensional hydrodynamical code and a detailed nuclear reaction network. Aspherical explosions lead to a strong α-rich freezeout, thus enhancing the abundance ratios [44Ca, 48Ti, 64Zn/Fe] in the ejecta. The nebular line profiles of the Fe-dominated blend near 5200 A and of [O I] 6300, 6363 A are calculated and compared with the observed late-time spectra of SN 1998bw. Compared with the spherical model, the unusual features of the observed nebular spectra can be better explained if SN 1998bw is a strongly aspherical explosion with a kinetic energy of ~1052 ergs viewed from near the jet direction.

The K20 survey - I. Disentangling old and dusty star-forming galaxies in the ERO population

Abstract: We present the results of VLT optical spectroscopy of a complete sample of 78 EROs with over a field of 52 arcmin2 . About 70% of the 45 EROs with have been spectroscopically identified with old passively evolving and dusty star-forming galaxies at . The two classes are about equally populated and for each of them we present and discuss the average spectrum. From the old ERO average spectrum and for we derive a minimum age of ~3 Gyr, corresponding to a formation redshift of z f ≳ 2.4. PLE models with such formation redshifts well reproduce the density of old EROs (consistent with being passively evolving ellipticals), whereas the predictions of the current hierarchical merging models are lower than the observed densities by large factors (up to an order of magnitude). From the average spectrum of the star-forming EROs we estimate a substantial dust extinction with ≳ 0.5. The star formation rates, corrected for the average reddening, suggest a significant contribution from EROs to the cosmic star-formation density at .

A classic type 2 QSO

Abstract: In the Chandra Deep Field-South 1 Ms exposure, we have found, at redshift 3.700 ± 0.005, the most distant type 2 active galactic nucleus ever detected. It is the source with the hardest X-ray spectrum with redshift z > 3. The optical spectrum has no detected continuum emission to a 3 σ detection limit of ~3 × 10-19 ergs s-1 cm-2 A-1 and shows narrow lines of Lyα, C IV, N V, He II, O VI, [O III], and C III]. Their FWHM line widths have a range of ~700-2300 km s-1 with an average of approximately ~1500 km s-1. The emitting gas is metal-rich (Z 2.5-3 Z☉). In the X-ray spectrum of 130 counts in the 0.5-7 keV band, there is evidence for intrinsic absorption with NH 1024 cm-2. An iron Kα line with rest-frame energy and equivalent width of ~6.4 keV and ~1 keV, respectively, in agreement with the obscuration scenario, is detected at a 2 σ level. If confirmed by our forthcoming XMM-Newton observations, this would be the highest redshift detection of Fe Kα. Depending on the assumed cosmology and the X-ray transfer model, the 2-10 keV rest frame luminosity corrected for absorption is ~1045 ± 0.5 ergs cm-2 s-1, which makes our source a classic example of the long-sought type 2 QSO. From standard population synthesis models, these sources are expected to account for a relevant fraction of the black hole-powered QSO distribution at high redshift.

A New Spectral Classification System for the Earliest O Stars: Definition of Type O2

Abstract: High-quality, blue-violet spectroscopic data are collected for 24 stars that have been classified as type O3 and that display the hallmark N IV and N V lines. A new member of the class is presented; it is the second known in the Cyg OB2 association, and only the second in the northern hemisphere. New digital data are also presented for several of the other stars. Although the data are inhomogeneous, the uniform plots by subcategory reveal some interesting new relationships. Several issues concerning the classification of the hottest O-type spectra are discussed, and new digital data are presented for the five original O3 dwarfs in the Carina Nebula, in which the N IV, N V features are very weak or absent. New spectral types O2 and O3.5 are introduced here as steps toward resolving these issues. The relationship between the derived absolute visual magnitudes and the spectroscopic luminosity classes of the O2–O3 stars shows more scatter than at later O types, at least partly because some overluminous dwarfs are unresolved multiple systems, and some close binary systems of relatively low luminosity and mass emulate O3 supergiant spectra. However, it also appears that the behavior of He II λ4686, the primary luminosity criterion at later O types, responds to other phenomena in addition to luminosity at spectral types O2–O3. There is evidence that these spectral types may correspond to an immediate pre-WN phase, with a correspondingly large range of luminosities and masses. A complete census of spectra classified into the original O3 subcategories considered here (not including intermediate O3/WN types or O3 dwarfs without N IV, N V features) totals 45 stars; 34 of them belong to the Large Magellanic Cloud and 20 of the latter to 30 Doradus.

Mid-Infrared line diagnostics of active galaxies - A spectroscopic AGN survey with ISO-SWS

Abstract: We present medium resolution (R 1500) ISO-SWS 2.4-45m spectra of a sample of 29 galaxies with active nuclei. This data set is rich in fine structure emission lines tracing the narrow line regions and (circum-)nuclear star formation regions, and it provides a coherent spectroscopic reference for future extragalactic studies in the mid-infrared. We use the data set to briefly discuss the physical conditions in the narrow line regions (density, temperature, excitation, line profiles) and to test for possible dierences between AGN sub-types. Our main focus is on new tools for determining the properties of dusty galaxies and on the AGN-starburst connection. We present mid-IR line ratio diagrams which can be used to identify composite (starburst+ AGN) sources and to distinguish between emission excited by active nuclei and emission from (circum-nuclear) star forming regions. For instance, line ratios of high to low excitation lines like (O IV)25.9m/(Ne II)12.8m, that have been used to probe for AGNs in dusty objects, can be examined in more detail and with better statistics now. In addition, we present two- dimensional diagnostic diagrams that are fully analogous to classical optical diagnostic diagrams, but better suited for objects with high extinction. Finally, we discuss correlations of mid-infrared line fluxes to the mid- and far-infrared continuum. We compare these relations to similar relations in starburst galaxies in order to examine the contribution of AGNs to the bolometric luminosities of their host galaxies. The spectra are available in electronic form from the authors.

O/Fe in metal-poor main sequence and subgiant stars ?

Abstract: A study of the O/Fe ratio in metal-poor main sequence and subgiant stars is presented using the (Oi) 6300 A line, the Oi 7774 A triplet, and a selection of weak Feii lines observed on high-resolution spectra acquired with the VLT UVES spectrograph. The (Oi) line is detected in the spectra of 18 stars with 2:4< (Fe/H)< 0:5, and the triplet is observed for 15 stars with (Fe/H) ranging from 1: 0t o 2:7. The abundance analysis was made first using standard model atmospheres taking into account non-LTE eects on the triplet: the (Oi) line and the triplet give consistent results with (O/Fe) increasing quasi-linearly with decreasing (Fe/H) reaching (O/Fe)'+0: 7a t (Fe/H)= 2:5. This trend is in reasonable agreement with other results for (O/Fe) in metal-poor dwarfs obtained using standard atmospheres and both ultraviolet and infrared OH lines. There is also broad agreement with published results for (O/Fe) for giants obtained using standard model atmospheres and the (Oi) line, and the OH infrared lines, but the Oi lines give higher (O/Fe) values which may, however, fall into place when non-LTE eects are considered. When hydrodynamical model atmospheres representing stellar granulation in dwarf and subgiant stars replace standard models, the (O/Fe) from the (Oi )a nd Feii lines is decreased by an amount which increases with decreasing (Fe/H). These 3D eects on (O/Fe) is compounded by the opposite behaviour of the (Oi) (continuous opacity eect) and Feii lines (excitation eect). The (O/Fe) vs. (Fe/H) relation remains quasi-linear extending to (O/Fe)'+0.5 at (Fe/H)= 2:5, but with a tendency of a plateau with (O/Fe)'+0.3 for 2:0< (Fe/H)< 1:0, and a hint of cosmic scatter in (O/Fe) at (Fe/H)' 1:0. Use of the hydrodynamical models disturbs the broad agreement between the oxygen abundances from the (Oi), Oi and OH lines, but 3D non-LTE eects may serve to erase these dierences. The (O/Fe) values from the (Oi) line and the hydrodynamical model atmospheres for dwarfs and subgiant stars are lower than the values for giants using standard model atmospheres and the (Oi )a nd Oi lines.

XMM-Newton observation of the Lockman Hole - II. Spectral analysis

Abstract: We present the results of the X-ray spectral analysis of the first deep X-ray survey with the XMM-Newton observa- tory during Performance Verification. The X-ray data of the Lockman Hole field and the derived cumulative source counts were reported by Hasinger et al. (2001). We restrict the analysis to the sample of 98 sources with more than 70 net counts (flux limit in the (0.5-7) keV band of 1:6 10 15 erg cm 2 s 1 ) of which 61 have redshift identification. We find no correlation between the spectral index and the intrinsic absorption column density NH and, for both the Type-1 and Type-2 AGN populations, we obtainh i' 2. The progressive hardening of the mean X-ray source spectrum with decreasing flux is essentially due to an increase in intrinsic absorption. The marked separation between the two AGN populations in several diagnostics diagrams, involving X-ray colour, X-ray flux, optical/near IR colour and optical brightness, is also a consequence of dierent absorption column densities and enables the classification of optically faint obscured AGN. The Type-2 and obscured AGN have weaker soft X-ray and optical fluxes and redder R K 0 colours. They follow the evolutionary tracks of their host galaxies in a color- redshift diagram. About 27% of the subsample with R K 0 colour are EROs (R K 0 5) and most of these 18 X-ray selected EROs contain an obscured AGN as revealed by their high X-ray-to-optical/near IR flux ratios. There are six sources in our sample with LX(0:5 10)> 10 44 erg s 1 and log(NH)> 10 22 cm 2 : which are likely Type-2 QSOs and we thus derive a density of69 objects of this class per square degree.

Chandra observations of NGC 253. II: On the origin of diffuse X-ray emission in the halos of starburst galaxies

Abstract: We present a detailed case study of the diffuse X-ray and Hemission in the halo of NGC 253, a nearby edge-on starburst galaxy driving a galactic superwind. The arcsecond spatial resolution of the ACIS imaging spectroscope on the Chandra X-ray Observatory allows us to study the spatial and spectral properties of the diffuse X-ray emitting plasma, at a height of between 3 and 9 kpc above the disk in the northern halo of NGC 253, with greatly superior spatial and spectral resolution compared to previous X-ray instruments. We find statistically significant structure within the diffuse emission on angular scales down to � 10 '' (� 130 pc), and place limits on the luminosity of any X-ray-emitting "clouds" on smaller scales. There is no statistically significant evidence for any spatial variation in the spectral properties of the diffuse emission over scales from several � 400 pc to � 3 kpc. The spectrum of the diffuse X-ray emission is clearly thermal, although with the higher spectral resolution and sensitivity of Chandra it is clear that current simple spectral models do not provide a physically meaningful description of the spectrum. In particular, the fitted metal abundances are unphysically low. There is no convincing evidence for diffuse X-ray emission at energies above 2 keV in the halo. We show that the X-shaped soft X-ray morphology of the superwind previously revealed by ROSAT is matched by very similar X-shaped Hemission, extending at least 8 kpc above the plane of the galaxy. In the northern halo the X-ray emission appears to lie slightly interior to the boundary marked by the Hemission. The total 0.3 - 2.0 keV energy band X-ray luminosity of the northern halo LX � 5 × 10 38 ergs −1 , is very similar to the halo Hluminosity of LH� � 4×10 38 ergs −1 , both of which are a small fraction of the estimated wind energy injection rate of � 10 42 ergs −1 from supernovae in the starburst. We show that there are a variety of models that can simultaneously explain spatially-correlated X-ray and Hemission in the halos of starburst galaxies, although the physical origin of the various emission components can be very different in different models. These findings indicate that the physical origin of the X-ray-emitting million- degree plasma in superwinds is closely linked to the presence of much cooler and denser T � 10 4 gas, not only within the central kpc regions of starbursts, but also on � 10 kpc-scales within the halos of these galaxies.

VLT/UVES Abundances in Four Nearby Dwarf Spheroidal Galaxies: II. Implications for Understanding Galaxy Evolution

Abstract: We have used UVES on VLT-UT2 to take spectra of 15 individual red giant stars in the centers of four nearby dwarf spheroidal galaxies: Sculptor, Fornax, Carina and Leo I. We measure the abundance variations of numerous elements in these low mass stars with a range of ages (1-15Gyr old). This means that we can effectively measure the chemical evolution of these galaxies WITH TIME. Our results show a significant spread in metallicity with age, but an overall trend consistent with what might be expected from a closed (or perhaps leaky) box chemical evolution scenario over the last 10-15Gyr. We notice that each of these galaxies show broadly similar abundance patterns for all elements measured. This suggests a fairly uniform progression of chemical evolution with time, despite quite a large range of star formation histories. It seems likely that these galaxies had similar initial conditions, and evolve in a similar manner with star formation occurring at a uniformly low rate, even if at different times. With our accurate measurements we find evidence for small variations in abundances which are correlated to variations in star formation histories. The alpha-elements suggest that dSph chemical evolution has not been affected by very high mass stars (>15-20 Msun). The abundance patterns we measure for stars in dwarf spheroidal galaxies are significantly different from those typically observed in the disk, bulge and inner-halo of our Galaxy. This suggests that it is NOT possible to construct a significant fraction of our Galaxy from STARS formed in these dwarf spheroidal galaxies which subsequently merged into our own. Any merger scenario involving dSph has to occur in the very early Universe whilst they are still gas rich, so the majority of mass transfer is gas, and few stars.

Measuring the Redshift Evolution of Clustering: the Hubble Deep Field South

Abstract: ABSTRA C T We present an analysis of the evolution of galaxy clustering in the redshift interval 0 # z # 4:5 in the Hubble Deep Field South (HDF-South). The HST optical data are combined with infrared ISAAC/VLT observations, and photometric redshifts are used for all the galaxies brighter than IAB # 27:5. The clustering signal is obtained in different redshift bins using two different approaches: a standard one, which uses the best redshift estimate of each object, and a second one, which takes into account the redshift probability function of each object. This second method makes it possible to improve the information in the redshift intervals where contamination from objects with insecure redshifts is important. With both methods, we find that the clustering strength up to z . 3:5 in the HDF-South is consistent with the previous results in the HDF-North. Whereas at redshift lower than z , 1 the HDF galaxy population is un/anti-biasedOb # 1U with respect to the underlying dark matter, at high redshift the bias increases up to bOz , 3U . 2‐3, depending on the cosmological model. These results support previous claims that, at high redshift, galaxies are preferentially located in massive haloes, as predicted by the biased galaxy formation scenario. In order to quantify the impact of cosmic errors on our analyses, we have used analytical expressions from Bernstein. Once the behaviour of higher-order moments is assumed, our results show that errors in the clustering measurements in the HDF surveys are indeed dominated by pure shot-noise in most regimes, as assumed in our analysis. We also show that future observations with instruments like the Advanced Camera on HST will improve the signal-to-noise ratio by at least a factor of 2, as a consequence, more detailed analyses of the errors will be required. In fact, pure shot-noise will give a smaller contribution with respect to other sources of errors, such as finite volume effects or non-Poissonian discreteness effects.

Constraints on reionization from the thermal history of the intergalactic medium

Abstract: The temperature of the diffuse, photoheated intergalactic medium (IGM) depends on its reionization history because the thermal timescales are long. The widths of the hydrogen Lyα absorption lines seen in the spectra of distant quasars that arise in the IGM can be used to determine its temperature. We use a wavelet analysis of the Lyα forest region of quasar spectra to demonstrate that there is a relatively sudden increase in the line widths between redshifts z ≈ 3.5 and 3.0, which we associate with entropy injection resulting from the reionization of He II. The subsequent falloff in temperature after z ≈ 3.5 is consistent with a thermal evolution dominated by adiabatic expansion. If, as expected, the temperature also drops rapidly after hydrogen reionization, then the high temperatures inferred from the line widths before He II reionization imply that hydrogen reionization occurred below redshift z = 9.

The Axisymmetric Ejecta of Supernova 1987A

Abstract: Extensive early observations proved that the ejecta of supernova 1987A (SN 1987A) are aspherical. The most important of these early observations include (1) the "Bochum event" that revealed small-scale spectroscopic structure indicating chemical inhomogeneities of the ejecta, (2) spectropolarimetry that showed deviations from spherical symmetry, and (3) speckle observations that revealed both the asymmetry of the ejecta and the "mystery spot" manifested as a secondary source off center from the bulk of the supernova ejecta. Fifteen years after the supernova explosion, the Hubble Space Telescope has resolved the rapidly expanding ejecta. The late-time images and spectroscopy provide a geometrical picture that is consistent with early observations and suggests a highly structured, axially symmetric geometry. We present here a new synthesis of the old and new data. We show that the Bochum event, presumably a clump of 56Ni, and the late-time image, the locus of excitation by 44Ti, are most naturally accounted for by sharing a common position angle of about 14°, the same as the mystery spot and early speckle data on the ejecta, and that they are both oriented along the axis of the inner circumstellar ring at 45° to the plane of the sky. We also demonstrate that the polarization represents a prolate geometry with the same position angle and axis as the early speckle data and the late-time image, and hence that the geometry has been fixed in time and throughout the ejecta. The Bochum event and the Doppler kinematics of the [Ca II]/[O II] emission in spatially resolved Hubble Space Telescope spectra of the ejecta can be consistently integrated into this geometry. The radioactive clump is deduced to fall approximately along the axis of the inner circumstellar ring and therefore to be redshifted in the north, whereas the [Ca II]/[O II] 7300 A emission is redshifted in the south. We present a jet-induced model for the explosion and argue that such a model can account for many of the observed asymmetries. In the jet models, the oxygen and calcium are not expected to be distributed along the jet but primarily in an expanding torus that shares the plane and northern blue shift of the inner circumstellar ring.

First Stars. II. Elemental abundances in the extremely metal-poor star CS 22949--037. A diagnostic of early massive supernovae

Abstract: CS 22949-037 is one of the most metal-poor giants known ((Fe=H) 4:0), and it exhibits large overabundances of carbon and nitrogen (Norris et al.). Using VLT-UVES spectra of unprecedented quality, regarding resolution and S=N ratio, covering a wide wavelength range (from= 350 to 900 nm), we have determined abundances for 21 elements in this star over a wide range of atomic mass. The major new discovery is an exceptionally large oxygen enhancement, (O=Fe)= 1:97 0:1, as measured from the (O I) line at 630.0 nm. We find an enhancement of (N/Fe) of 2:56 0:2, and a milder one of (C=Fe)= 1:17 0:1, similar to those already reported in the literature. This implies Z? = 0:01 Z. We also find carbon isotopic ratios 12 C/ 13 C= 42: 0a nd 13 C/ 14 N= 0:03 +0:035 0:015 , close to the equilibrium value of the CN cycle. Lithium is not detected. Na is strongly enhanced ((Na=Fe)=+2:1 0:2), while S and K are not detected. The silicon-burning elements Cr and Mn are underabundant, while Co and Zn are overabundant ((Zn=Fe)=+0:7). Zn is measured for the first time in such an extremely metal-poor star. The abundances of the neutron-capture elements Sr, Y, and Ba are strongly decreasing with the atomic number of the element: (Sr=Fe) +0:3, (Y=Fe) 0:1, and (Ba=Fe) 0:6. Among possible progenitors of CS 22949-037, we discuss the pair- instability supernovae. Such very massive objects indeed produce large amounts of oxygen, and have been found to be possible sources of primary nitrogen. However, the predicted odd/even eect is too large, and the predicted Zn abundance much too low. Other scenarios are also discussed. In particular, the yields of a recent model (Z35Z) from Heger and Woosley are shown to be in fair agreement with the observations. The only discrepant prediction is the very low abundance of nitrogen, possibly curable by taking into account other eects such as rotationally induced mixing. Alternatively, the absence of lithium in our star, and the values of the isotopic ratios 12 C/ 13 Ca nd 13 C/ 14 N close to the equilibrium value of the CN cycle, suggest that the CNO abundances now observed might have been altered by nuclear processing in the star itself. A 30-40 M supernova, with fallback, seems the most likely progenitor for CS 22949-037.

The K20 survey - III. Photometric and spectroscopic properties of the sample

Abstract: The K20 survey is an ESO VLT optical and near-infrared spectroscopic survey aimed at obtaining spectral infor- mation and redshifts of a complete sample of about 550 objects to Ks 20:0 over two independent fields with a total area of 52 arcmin 2 . In this paper we discuss the scientific motivation of such a survey, we describe the photometric and spectroscopic properties of the sample, and we release the Ks-band photometric catalog. Extensive simulations showed that the sample is photometrically highly complete to Ks= 20. The observed galaxy counts and the R Ks color distribution are consistent with literature results. We observed spectroscopically 94% of the sample, reaching a spectroscopic redshift identification complete- ness of 92% to Ks 20:0 for the observed targets, and of 87% for the whole sample (i.e. counting also the unobserved targets). Deep spectroscopy was complemented with multi-band deep imaging in order to derive tested and reliable photometric red- shifts for the galaxies lacking spectroscopic redshifts. The results show a very good agreement between the spectroscopic and the photometric redshifts with = 0:01 and with a dispersion ofz= 0:09. Using both the spectroscopic and the photometric redshifts, we reached an overall redshift completeness of about 98%. The size of the sample, the redshift complete- ness, the availability of high quality photometric redshifts and multicolor spectral energy distributions make the K20 survey database one of the most complete samples available to date for constraining the currently competing scenarios of galaxy formation and for a variety of other galaxy evolution studies.