arXiv subjects

S. F. Sanchez

Publications and source records attributed to S. F. Sanchez.

At least 91 records · Page 5Linked to original sources

arXiv2009

Measuring Baryon Acoustic Oscillations along the line of sight with photometric redshifs: the PAU survey

Baryon Acoustic Oscillations (BAO) provide a standard ruler of known physical length, making it a promising probe of the nature of dark energy. The detection of BAO requires measuring galaxy positions and redshifts. "Transversal" (angular distance) BAO measure the angular size of this scale, while "line-of-sight" (or "radial") BAO require precise redshifts, but provide a direct measurement of the Hubble parameter at different redshifts, a more sensitive probe of dark energy. The main goal of this paper is to show that a precision of sigma_z ~0.003(1 + z) is sufficient to measure BAO in the radial direction. This precision can be achieved for bright, red galaxies, by using a filter system comprising about 40 filters, each with a width of ~100 A, from ~ 4000 A to ~ 8000 A, supplemented by two broad-band filters. We describe a practical implementation, a new galaxy survey, PAU, to be carried out with a telescope/camera combination with an etendue of about 20 m^2deg^2, and covering 8000 sq. deg. in the sky in four years. We expect to measure positions and redshifts for over 14 million red, early-type galaxies with L > L* and i_AB < 22.5 in the interval 0.1 < z < 0.9, with sigma_z < 0.003(1 + z). This population has a number density n > 10^-3 Mpc^-3 h^3 within the 9 (Gpc/h)^3 volume of the survey, ensuring that the error in the determination of the BAO scale is not limited by shot-noise. By itself, such a survey will deliver precisions of order 5% in the dark-energy equation of state parameter w, if assumed constant, and can determine its time derivative when combined with future CMB measurements. In addition, PAU will yield high-quality redshift and low-resolution spectroscopy for hundreds of millions of other galaxies.

astro-ph↗

arXiv2009

Near-IR Galaxy Counts and Evolution from the Wide-Field ALHAMBRA survey

The ALHAMBRA survey aims to cover 4 square degrees using a system of 20 contiguous, equal width, medium-band filters spanning the range 3500 A to 9700 A plus the standard JHKs filters. Here we analyze deep near-IR number counts of one of our fields (ALH08) for which we have a relatively large area (0.5 square degrees) and faint photometry (J=22.4, H=21.3 and K=20.0 at the 50% of recovery efficiency for point-like sources). We find that the logarithmic gradient of the galaxy counts undergoes a distinct change to a flatter slope in each band: from 0.44 at [17.0, 18.5] to 0.34 at [19.5, 22.0] for the J band; for the H band 0.46 at [15.5, 18.0] to 0.36 at [19.0, 21.0], and in Ks the change is from 0.53 in the range [15.0, 17.0] to 0.33 in the interval [18.0, 20.0]. These observations together with faint optical counts are used to constrain models that include density and luminosity evolution of the local type-dependent luminosity functions. Our models imply a decline in the space density of evolved early-type galaxies with increasing redshift, such that only 30% - 50% of the bulk of the present day red-ellipticals was already in place at z~1.

astro-ph.CO↗

arXiv2008

Obscured star formation in intermediate-density environments: A Spitzer study of the Abell 901/902 supercluster

We explore the amount of obscured star-formation as a function of environment in the A901/902 supercluster at z=0.165 in conjunction with a field sample drawn from the A901 and CDFS fields, imaged with HST as part of the STAGES and GEMS surveys. We combine the COMBO-17 near-UV/optical SED with Spitzer 24um photometry to estimate both the unobscured and obscured star formation in galaxies with Mstar>10^{10}Msun. We find that the star formation activity in massive galaxies is suppressed in dense environments, in agreement with previous studies. Yet, nearly 40% of the star-forming galaxies have red optical colors at intermediate and high densities. These red systems are not starbursting; they have star formation rates per unit stellar mass similar to or lower than blue star-forming galaxies. More than half of the red star-forming galaxies have low IR-to-UV luminosity ratios, relatively high Sersic indices and they are equally abundant at all densities. They might be gradually quenching their star-formation, possibly but not necessarily under the influence of gas-removing environmental processes. The other >40% of the red star-forming galaxies have high IR-to-UV luminosity ratios, indicative of high dust obscuration. They have relatively high specific star formation rates and are more abundant at intermediate densities. Our results indicate that while there is an overall suppression in the star-forming galaxy fraction with density, the small amount of star formation surviving the cluster environment is to a large extent obscured, suggesting that environmental interactions trigger a phase of obscured star formation, before complete quenching.

astro-ph↗

arXiv2008

Optimal filter systems for photometric redshift estimation

In the next years, several cosmological surveys will rely on imaging data to estimate the redshift of galaxies, using traditional filter systems with 4-5 optical broad bands; narrower filters improve the spectral resolution, but strongly reduce the total system throughput. We explore how photometric redshift performance depends on the number of filters n_f, characterizing the survey depth through the fraction of galaxies with unambiguous redshift estimates. For a combination of total exposure time and telescope imaging area of 270 hrs m^2, 4-5 filter systems perform significantly worse, both in completeness depth and precision, than systems with n_f >= 8 filters. Our results suggest that for low n_f, the color-redshift degeneracies overwhelm the improvements in photometric depth, and that even at higher n_f, the effective photometric redshift depth decreases much more slowly with filter width than naively expected from the reduction in S/N. Adding near-IR observations improves the performance of low n_f systems, but still the system which maximizes the photometric redshift completeness is formed by 9 filters with logarithmically increasing bandwidth (constant resolution) and half-band overlap, reaching ~0.7 mag deeper, with 10% better redshift precision, than 4-5 filter systems. A system with 20 constant-width, non-overlapping filters reaches only ~0.1 mag shallower than 4-5 filter systems, but has a precision almost 3 times better, dz = 0.014(1+z) vs. dz = 0.042(1+z). We briefly discuss a practical implementation of such a photometric system: the ALHAMBRA survey.

astro-ph↗

arXiv2008

Integral Field Spectroscopy of two radio galaxies at z~2.3

In this article we study the morphology, kinematics and ionization properties of the giant ionized gas nebulae surrounding two high redshift radio galaxies, 4C40.36 (z=2.27) and 4C48.48 (z=2.34).}{Integral Field Spectroscopy observations were taken using the PPAK bundle of the PMAS spectrograph, mounted on the 3.5m on the Calar Alto Observatory, in order to cover a field-of-view of 64" X 72" centered in each radio galaxy. The observations spanned over 5 nights, using two different spectral resolutions (with FWHM~4 AA and ~8 AA respectively), covering the optical wavelength range from ~3700 AA to ~7100 AA, which corresponds to the rest-frame ultraviolet range from ~1100 AA to ~2000 AA >. Various emission lines are detected within this wavelength range, including Lyalpha (1216 AA), NV (1240 AA), CIV (1549 AA), HeII (1640 AA), OIII] (1663 AA) and CIII] (1909Å). The dataset was used to derive the spatial distribution of the flux intensity of each of these lines and the gas kinematics. The properties of the emission lines in the nuclear regions were studied in detail.In agreement with previous studies, we find that both objects are embedded in a large ionized gas nebula, where Ly alpha emission is extended across ~100 kpc or more. The CIV and HeII emission lines are also spatially extended. The nebulae are generally aligned with the radio axis, although we detect emission far from it. In 4C+48.48, there is a band of low Ly-alpha/CIV running perpendicular to the radio axis, at the location of the active nucleus. This feature might be the observational signature of an edge-on disk of neutral gas. The kinematics of both nebulae are inconsistent with stable rotation, although they are not inconsistent with infall or outflow.

astro-ph↗

arXiv2008

Two New Large Separation Gravitational Lenses from SDSS

We present discovery images, together with follow-up imaging and spectroscopy, of two large separation gravitational lenses found by our survey for wide arcs (the CASSOWARY). The survey exploits the multicolor photometry of the Sloan Digital Sky Survey to find multiple blue components around red galaxies. CASSOWARY~2 (or "the Cheshire Cat") is composed of two massive early-type galaxies at z = 0.426 and 0.432 respectively lensing two background sources, the first a star-forming galaxy at z = 0.97 and the second a high redshift galaxy (z> 1.4). There are at least three images of the former source and probably four or more of the latter, arranged in two giant arcs. The mass enclosed within the larger arc of radius 11 arcsecs is about 33 x 10^{12} solar masses. CASSOWARY~3 comprises an arc of three bright images of a z = 0.725 source, lensed by a foreground elliptical at z = 0.274. The radius of the arc is about 4 arcsecs and the enclosed mass is 2.5 x 10^{12} solar masses. Together with earlier discoveries like the Cosmic Horseshoe and the 8 O'Clock Arc, these new systems, with separations intermediate between the arcsecond separation lenses of typical strong galaxy lensing and the larger separation cluster lenses, probe the very high end of the galaxy mass function.

astro-ph↗

arXiv2008

The night-sky at the Calar Alto Observatory II: The sky at the near infrared

We present here the characterization of the night sky-brightness at the near-infrared, the telescope seeing, and the fraction of useful time at the Calar Alto observatory. For this study we have collected a large dataset comprising 7311 near-infrared images taken regularly along the last four years for the ALHAMBRA survey (J, H and Ks-bands), together with a more reduced dataset of additional near-infrared images taken for the current study. In addition we collected the information derived by the meteorological station at the observatory during the last 10 years, together with the results from the cloud sensor for the last ~2 years. We analyze the dependency of the near-infrared night sky-brightness with the airmass and the seasons, studying its origins and proposing a zenithal correction. A strong correlation is found between the night sky-brightness in the Ks-band and the air temperature, with a gradient of ~ -0.08 mag per 1 C degree. The typical (darkest) night sky-brightness in the J, H and Ks-band are 15.95 mag (16.95 mag), 13.99 mag (14.98 mag) and 12.39 mag (13.55 mag), respectively. These values show that Calar Alto is as dark in the near-infrared as most of the other astronomical astronomical sites in the world that we could compare with. Only Mauna Kea is clearly darker in the Ks-band. The typical telescope seeing at the 3.5m is ~1.0" when converted to the V-band, being only slightly larger than the atmospheric seeing measured at the same time by the seeing monitor, ~0.9". Finally we estimate the fraction of useful time based on the relative humidity, gust wind speed and presence of clouds. This fraction, ~72%, is very similar to the one derived in Paper I, based on the fraction of time when the extinction monitor is working.

astro-ph↗

arXiv2008

Integral Field Spectroscopy of HH 262: The Spectral Atlas

HH 262 is a group of emitting knots displaying an "hour-glass" morphology in the Halpha and [SII] lines, located 3.5' to the northeast of the young stellar object L1551-IRS5, in Taurus. We present new results of the kinematics and physical conditions of HH 262 based on Integral Field Spectroscopy covering a field of 1.5'x3', which includes all the bright knots in HH 262. These data show complex kinematics and significant variations in physical conditions over the mapped region of HH 262 on a spatial scale of <3". A new result derived from the IFS data is the weakness of the [NII] emission (below detection limit in most of the mapped region of HH 262), including the brightest central knots. Our data reinforce the association of HH 262 with the redshifted lobe of the evolved molecular outflow L1551-IRS5. The interaction of this outflow with a younger one, powered by L1551 NE, around the position of HH 262 could give rise to the complex morphology and kinematics of HH 262.

astro-ph↗

arXiv2008

Spatially extended absorption around the z=2.63 radio galaxy MRC 2025-218: outflow or infall?

We present an investigation into the absorber in front of the z=2.63 radio galaxy MRC 2025-218, using integral field spectroscopy obtained at the Very Large Telescope, and long slit spectroscopy obtained at the Keck II telescope. The properties of MRC 2025-218 are particularly conducive to study the nature of the absorbing gas, i.e., this galaxy shows bright and spatially extended Ly-alpha emission, along with bright continuum emission from the active nucleus. Ly-alpha absorption is detected across ~40x30 kpc^2, has a covering factor of ~1, and shows remarkably little variation in its properties across its entire spatial extent. This absorber is kinematically detached from the extended emission line region (EELR). Its properties suggest that the absorber is outside of the EELR. We derive lower limits to the HI, HII and H column densities for this absorber of 3x10^16, 7x10^17 and 2x10^18 cm^-2, respectively. Moreover, the relatively bright emission from the active nucleus has allowed us to measure a number of metal absorption lines: CI, CII, CIV, NV, OI, SiII, SiIV, AlII and AlIII. The column density ratios are most naturally explained using photoionization by a hard continuum, with an ionization parameter U~0.0005-0.005. Shocks or photoionization by young stars cannot reproduce satisfactorily the measured column ratios. Using the ratio between the SiII* and SiII column densities, we derive a lower limit of >10 cm^-3 for the electron density of the absorber. The data do not allow useful constraints to be placed on the metallicity of the absorber. We consider two possibilities for the nature of this absorber: the cosmological infall of gas, and an outflow driven by supernovae or the radio-jets.

astro-ph↗

arXiv2008

First results from the ALHAMBRA-Survey

We present the first results from the ALHAMBRA survey. ALHAMBRA will cover a relatively wide area (4 square degrees) using a purposely-designed set of 20 medium-band filters, down to an homogeneous magnitude limit AB~25 in most of them, adding also deep near-infrared imaging in JHK. To this aim we are using the Calar Alto 3.5m telescope. A small area of the ALHAMBRA survey has already been observed through our complete filter set, and this allows for the first time to check all the steps of the survey, including the pipelines that have been designed for the project, the fulfilment of the data quality expectations, the calibration procedures, and the photometric redshift machinery for which ALHAMBRA has been optimised. We present here the basic results regarding the properties of the galaxy sample selected in a 15x15 square arcmin area of the ALHAMBRA-8 field, which includes approximately 10000 galaxies with precise photometric redshift measurements. In a first estimate, approximately 500 of them must be galaxies with z>2.

astro-ph↗

arXiv2008

Galaxy Evolution in Dense Environments: Properties of Interacting Galaxies in the A901/02 Supercluster

We present a study of galaxies in the STAGES survey of the Abell 901/902 supercluster at z~0.165, based on HST ACS F606W, COMBO-17, Spitzer 24um, XMM-Newton X-ray, and gravitational lensing maps. We characterize galaxies with strong, externally-triggered morphological distortions and normal, relatively undisturbed galaxies, using visual classification and quantitative CAS parameters. We compare normal and distorted galaxies in terms of their frequency, distribution within the cluster, star formation properties, and relationship to dark matter (DM) or surface mass density, and intra-cluster medium (ICM) density. We report here our preliminary results.

astro-ph↗

arXiv2008

Morphological Transformations of Galaxies in the A901/02 Supercluster from STAGES

We present a study of galaxies in the Abell 901/902 Supercluster at z~0.165, based on HST ACS F606W, COMBO-17, Spitzer 24um, XMM-Newton X-ray, and gravitational lensing maps, as part of the STAGES survey. We characterize galaxies with strong externally-triggered morphological distortions and normal relatively undisturbed galaxies, using visual classification and quantitative CAS parameters. We compare normal and distorted galaxies in terms of their frequency, distribution within the cluster, star formation properties, and relationship to dark matter (DM) or surface mass density, and intra-cluster medium (ICM) density. We revisit the morphology density relation, which postulates a higher fraction of early type galaxies in dense environments, by considering separately galaxies with a low bulge-to-disk (B/D) ratio and a low gas content as these two parameters may not be correlated in clusters. We report here on our preliminary analysis.

astro-ph↗

arXiv2008

Characterizing Barred Galaxies in the Abell 901/902 Supercluster

In dense clusters, higher densities at early epochs as well as physical processes, such as ram pressure stripping and tidal interactions become important, and can have direct consequences for the evolution of bars and their host disks. To study bars and disks as a function of environment, we are using the STAGES ACS HST survey of the Abell 901/902 supercluster (z~0.165), along with earlier field studies based the SDSS and the Ohio State University Bright Spiral Galaxy Survey (OSUBSGS). We explore the limitations of traditional methods for characterizing the bar fraction, and in particular highlight uncertainties in disk galaxy selection in cluster environments. We present an alternative approach for exploring the proportion of bars, and investigate the properties of bars as a function of host galaxy color, Sersic index, stellar mass, star formation rate (SFR), specific SFR, and morphology.

astro-ph↗

arXiv2007

The interplay between ionized gas and massive stars in the HII galaxy IIZw70: integral field spectroscopy with PMAS

We performed an integral field spectroscopic study for the HII galaxy IIZw70 in order to investigate the interplay between its ionized interstellar medium (ISM) and the massive star formation (SF). Observations were taken in the optical spectral range (3700-6800 A) with the Potsdam Multi-Aperture Spectrophotometer (PMAS) attached to the 3.5 m telescope at CAHA. We created and analysed maps of spatially distributed emission-lines, continuum emission and properties of the ionized ISM (e.g. physical-chemical conditions, dust extinction, kinematics). We investigated the relation of these properties to the spatial distribution and evolutionary stage of the massive stars. For the first time we have detected the presence of Wolf-Rayet (WR) stars in this galaxy. The peak of the ionized gas emission coincides with the location of the WR bump. The region of the galaxy with lower dust extinction corresponds to the region that shows the lowest values of velocity dispersion and radial velocity. The overall picture suggests that the ISM of this region is being disrupted via photoionization and stellar winds, leading to a spatial decoupling between gas+stars and dust clouds. The bulk of dust appears to be located at the boundaries of the region occupied by the probable ionizing cluster. We also found that this region is associated to the nebular emission in HeII4686 and to the intensity maximum of most emission lines. This indicates that the hard ionizing radiation responsible for the HeII4686 nebular emission can be related to the youngest stars. Within $\sim$ 0.4 x 0.3 kpc^2 in the central burst, we derived O/H using direct determinations of Te[OIII]. We found abundances in the range 12+log(O/H)=7.65-8.05, yielding an error-weighted mean of 12+log(O/H)=7.86 $\pm$0.05.

astro-ph↗

arXiv2007

Non-LTE dust nucleation in sub-saturated vapors

We use the kinetic theory of nucleation to explore the properties of dust nucleation in sub-saturated vapors. Due to radiation losses, the sub-critical clusters have a smaller temperature compared to their vapor. This alters the dynamical balance between attachment and detachment of monomers, allowing for stable nucleation of grains in vapors that are sub-saturated for their temperature. We find this effect particularly important at low densities and in the absence of a strong background radiation field. We find new conditions for stable nucleation in the n-T phase diagram. The nucleation in the non-LTE regions is likely to be at much slower rate than in the super-saturated vapors. We evaluate the nucleation rate, warning the reader that it does depend on poorly substantiated properties of the macro-molecules assumed in the computation. On the other hand, the conditions for nucleation depend only on the properties of the large stable grains and are more robust. We finally point out that this mechanism may be relevant in the early universe as an initial dust pollution mechanism, since once the interstellar medium is polluted with dust, mantle growth is likely to be dominant over non-LTE nucleation in the diffuse medium.

astro-ph↗

arXiv2007

The Cosmic Horseshoe: Discovery of an Einstein Ring around a Giant Luminous Red Galaxy

We report the discovery of an almost complete Einstein ring of diameter 10" in Sloan Digital Sky Survey (SDSS) Data Release 5 (DR5). Spectroscopic data from the 6m telescope of the Special Astrophysical Observatory reveals that the deflecting galaxy has a line-of-sight velocity dispersion in excess of 400 km/s and a redshift of 0.444, whilst the source is a star-forming galaxy with a redshift of 2.379. From its color and luminosity, we conclude that the lens is an exceptionally massive Luminous Red Galaxy (LRG) with a mass within the Einstein radius of 5 x 10^12 solar masses. This remarkable system provides a laboratory for probing the dark matter distribution in LRGs at distances out to 3 effective radii, and studying the properties of high redshift star-forming galaxies.

astro-ph↗

arXiv2007

The night-sky at the Calar Alto Observatory

We present a characterization of the main properties of the night-sky at the Calar Alto observatory for the time period between 2004 and 2007. We use optical spectrophotometric data, photometric calibrated images taken in moonless observing periods, together with the observing conditions regularly monitored at the observatory, such as atmospheric extinction and seeing. We derive, for the first time, the typical moonless night-sky optical spectrum for the observatory. The spectrum shows a strong contamination by different pollution lines, in particular from Mercury lines, which contribution to the sky-brightness in the different bands is of the order of ~0.09 mag, ~0.16 mag and ~0.10 mag in B, V and R respectively. The zenith-corrected values of the moonless night-sky surface brightness are 22.39, 22.86, 22.01, 21.36 and 19.25 mag arcsec^-2 in U, B, V, R and I, which indicates that Calar Alto is a particularly dark site for optical observations up to the I-band. The fraction of astronomical useful nights at the observatory is ~70%, with a ~30% of photometric nights. The typical extinction at the observatory is k_V~0.15 mag in the Winter season, with little dispersion. In summer the extinction has a wider range of values, although it does not reach the extreme peaks observed at other sites. The median seeing for the last two years (2005-6) was ~0.90", being smaller in the Summer (~0.87") than in the Winter (~0.96"). We conclude in general that after 26 years of operations Calar Alto is still a good astronomical site, being a natural candidate for future large aperture optical telescopes.

astro-ph↗

arXiv2007

Deep GEMINI GMOS-IFU spectroscopy of BAL QSOs: I. Decoupling the BAL, QSO, starburst, NLR, supergiant bubbles and galactic wind in Mrk 231

In this work we present the first results of a study of BAL QSOs (at low and high redshift), based on very deep Gemini GMOS integral field spectroscopy. In particular, the results obtained for the nearest BAL IR QSO Mrk 231 are presented. Very deep three-dimensional spectra and maps clearly show that the BAL systems I and II are extended (reaching 1.4-1.6" = 1.2-1.3 kpc, from the nucleus) and clearly elongated at the position angle close to the radio jet PA. Which suggest that the BAL systems I and II are both associated with the radio jet, and supporting the bipolar jet-wind model for some BALs. For the nuclear region of Mrk 231, the QSO and host-galaxy components were modelled, using a new technique of decoupling 3D spectra. From this study, the following main results were found: (i) in the pure host galaxy spectrum an extreme nuclear starburst component was clearly observed, mainly as a very strong increase in the flux, at the blue wavelengths; (ii) the BAL system I is observed in the spectrum of the host galaxy; (iii) in the clean/pure QSO emission spectrum, only broad lines were detected. 3D GMOS individual spectra (specially in the IR Ca II triplet) and maps confirm the presence of an extreme and young nuclear starburst (8 < age < 15 Myr), which was detected mainly in a ring or toroid with a radius r = 0.3" = 200 pc, around the very nucleus. The physical properties of the four expanding nuclear bubbles were analysed, using the GMOS 3D spectra and maps. These results suggest that an important part of the nuclear NLR is generated by the OF process and the associated low velocity ionizing shocks.

astro-ph↗