arXiv subjects

J. Cepa

Publications and source records attributed to J. Cepa.

At least 109 records · Page 6Linked to original sources

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.

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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.

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arXiv2008

Star-forming galaxies in SDSS: signs of metallicity evolution

Evolution of galaxies through cosmic time has been widely studied at high redshift, but there are a few studies in this field at lower redshifts. However, low-redshifts studies will provide important clues to the evolution of galaxies, furnishing the required link between local and high-redshift universe. In this work we focus on the metallicity of the gas in spiral galaxies at low redshift looking for signs of chemical evolution. We analyze the metallicity contents of star forming galaxies of similar luminosities at different redshifts, we studied the metallicity of star forming galaxies from SDSS-DR5 (Sloan Digital Sky Survey-Data Release 5), using different redshift intervals from 0.1 to 0.4. We used the public data of SDSS-DR5 processed with the STARLIGHT spectral synthesis code, correcting the fluxes for dust extinction, estimating metallicities using the R23 method, and analyzing the samples with respect to the [NII]6583/[OII]3727 line ratio. From a final sample of 207 galaxies, we find a decrement in 12+log(O/H) corresponding to the redshift interval 0.3 < z < 0.4 of ~0.1 dex with respect to the rest of the sample, which can be interpreted as evidence of the metallicity evolution in low-z galaxies.

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arXiv2008

The ALHAMBRA Project: A large area multi medium-band optical and NIR photometric survey

(ABRIDGED) We describe the first results of the ALHAMBRA survey which provides cosmic tomography of the evolution of the contents of the Universe over most of Cosmic history. Our approach employs 20 contiguous, equal-width, medium-band filters covering from 3500 to 9700 A, plus the JHKs bands, to observe an area of 4 sqdeg on the sky. The optical photometric system has been designed to maximize the number of objects with accurate classification by SED and redshift, and to be sensitive to relatively faint emission lines. The observations are being carried out with the Calar Alto 3.5m telescope using the cameras LAICA and O-2000. The first data confirm that we are reaching the expected magnitude limits of AB<~25 mag in the optical filters from the blue to 8300 A, and from AB=24.7 to 23.4 for the redder ones. The limit in the NIR is (Vega) K_s~20, H~21, J~22. We expect to obtain accurate redshift values, Delta z/(1+z) <~ 0.03 for about 5x10^5 galaxies with I<~25 (60% complete), and z_med=0.74. This accuracy, together with the homogeneity of the selection function, will allow for the study of the redshift evolution of the large scale structure, the galaxy population and its evolution with redshift, the identification of clusters of galaxies, and many other studies, without the need for any further follow-up. It will also provide targets for detailed studies with 10m-class telescopes. Given its area, spectral coverage and its depth, apart from those main goals, the ALHAMBRA-Survey will also produce valuable data for galactic studies.

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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.

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arXiv2007

Redshift-distance relations from type Ia supernova observations. New constraints on grey dust models

Extinction due to intergalactic grey dust has been proposed as an alternative to accelerated expansion to account for the dimming of \s fluxes beyond $z \simeq 0.5$. The ``replenishing'' grey dust model, although fitting the observational data, does not seem to be based on physical assumptions. For this reason, in this paper we propose a new grey dust model whose dust distribution follows the comoving SFR density evolution, a reasonably established phenomenon. This new model is compared with the updated photometric data sample from the High Z Supernova Search Team (HZT). Also, using pairs of supernovae at different redshifts, the possibility of any ``patchy'' distribution proposed to explain the discrepancies between the ``high $z$'' dust model and the \s observations at very high redshift ($z \ge 0.9$) is ruled out. Finally, the data are compared with different models with and without dark energy and a best fit to a universe with adimensional parameters $Ω_{\mathrm{m0}} = 0.31$ and $Ω_{\mathrm{Λ0}} = 0.69$ is obtained, from which an age of the universe of $t_{0} \simeq 14.6 \times 10^{9}$ years is derived. This age is compatible with the age of the globular clusters using an equation of state $ω= -1$ and it obviates the need to resort to any kind of phantom energy.

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arXiv2006

X-ray Flashes or soft Gamma-ray Bursts? The case of the likely distant XRF 040912

In this work, we present a multi-wavelength study of XRF 040912, aimed at measuring its distance scale and the intrinsic burst properties. We performed a detailed spectral and temporal analysis of both the prompt and the afterglow emission and we estimated the distance scale of the likely host galaxy. We then used the currently available sample of XRFs with known distance to discuss the connection between XRFs and classical Gamma-ray Bursts (GRBs). We found that the prompt emission properties unambiguously identify this burst as an XRF, with an observed peak energy of E_p=17+/-13 keV and a burst fluence ratio S(2-30keV)/S(30-400keV)>1. A non-fading optical source with R~24 mag and with an apparently extended morphology is spatially consistent with the X-ray afterglow, likely the host galaxy. XRF 040912 is a very dark burst since no afterglow optical counterpart is detected down to R>25 mag (3 sigma limiting magnitude) at 13.6 hours after the burst. The host galaxy spectrum detected from 3800A to 10000A, shows a single emission line at 9552A. The lack of any other strong emission lines blue-ward of the detected one and the absence of the Ly alpha cut-off down to 3800A are consistent with the hypothesis of the [OII] line at redshift z=1.563+/-0.001. The intrinsic spectral properties rank this XRF among the soft GRBs in the E_peak-E_iso diagram. Similar results were obtained for most XRFs at known redshift. Only XRF 060218 and XRF 020903 represent a good example of instrinsic XRF(i-XRF) and are possibly associated with a different progenitor population. This scenario may calls for a new definition of XRFs.

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arXiv2006

Detection of optical linear polarization in the SN2006aj/XRF060218 non-spherical expansion

We have performed optical polarimetric observations of the SN2006aj associated to the X-ray flash (XRF) of February 18, 2006, XRF060218 that provide information on its expansion geometry. The data were acquired in the R-band with the 0.7m telescope of Crimea, 2.5m Nordic Optical Telescope and the 2.2m of Calar Alto. We report the detection of linear polarization between 3 and 39 days after the gamma-ray event (t-t_0). This represents the first polarization detection of a Ic supernova (SN) associated to an XRF. Our data exhibit a degree of linear polarization (P) around P~4% at t-t_0 ~ 3-5 days, followed by a constant polarization phase with P~1.4% at 13.7 < t-t_0 < 39 days. Our data suggest a decay in P, and more interestingly, show a position angle (θ) rotation of ~100 degrees comparing data taken before and after the R-band lightcurve peak. The reported polarization measurements can be explained by the evolution of an asymmetric SN expansion. We discuss on several ingredients that could account for the observed θrotation.

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arXiv2006

OSIRIS Software: The Mask Designer Tool

OSIRIS is a Day One instrument that will be available at the 10m GTC telescope which is being built at La Palma observatory in the Canary Islands. This optical instrument is designed to obtain wide-field narrow-band images using tunable filters and to do low-resolution spectroscopy in both long-slit and multislit modes. For the multislit spectroscopy mode, we have developed a software to assist the observers to design focal plane masks. In this paper we describe the characteristics of this Mask Designer tool. We discuss the main design concepts, the functionality and particular features of the software.

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arXiv2004

The oxygen abundance gradient in M101: the reliability of the P method

We present the oxygen abundance determination for 90 HII regions in the inner parts of the grand design galaxy M101. The abundances were derived employing the P method (Pilyugin 2001a). A comparison is made with previous determinations using another calibration and direct measurements of electron temperature to derive the oxygen abundance. The results show agreement with the abundances derived from the electron temperature method and also show that the older calibration is not as accurate as the P method.

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arXiv2004

Constraints on the cosmic equation of state: age conflict versus phantom energy. Age-redshift relations in an accelerated universe

The currently available values and confidence limits for Omega_m0, H_0 and globular cluster ages still indicate that the dark energy that dominates the Universe could also be a form of quintessence or phantom energy. In fact, current cosmological values favor phantom energy. To increase the likelihood of a cosmological constant as dark energy instead of phantom energy, the possibilities seem to lie in reducing globular cluster ages, the Hubble constant, or both, and possibly advancing the epoch of globular cluster formation. For a set of possible dark energy equations of state that includes the cosmological constant, quintessence or phantom energy, age-redshift analytical expressions for null curvature universes that include ordinary matter are derived together with the corresponding ages for these universes.

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arXiv2004

High Resolution HI Observations of the Galaxy NGC404: a dwarf S0 with abundant Interstellar gas

(Abridged) We present HI maps of the S0 galaxy NGC404, at a spatial resolution of 15.2" x 14.4" and a velocity resolution of 2.6km/s. The HI has been traced out to a radius R~8R_25 or 48 disc scale-lengths, making it one of the largest HI extents reported (800" or 12.6kpc at the assumed distance of 3.3Mpc). Approximately 75% of the HI resides in a doughnut which is seen close to face-on and the optical galaxy fits snugly within the hole of the doughnut. The remaining 25% of the neutral gas is found in an annulus concentric with the doughnut and with a somewhat larger ellipticity. A total HI mass of 1.52 x 10^8 Msol is found, which implies an M(HI)/L_B = 0.22 in solar units. We argue that most if not all of this gas is of external origin, most likely due to the merger of a dwarf irregular galaxy with M_B ~ -15.5mag. The velocity field shows a steeply declining observed rotation curve, compatible with Keplerian decline. However, because the galaxy is close to face-on there is a degeneracy in the determination of the intrinsic rotation curve and inclination. We therefore analysed two extreme cases, producing tilted-ring model fits forcing either a Keplerian or a Flat rotation curve through the observations; both approaches result in equally plausible fits. Assuming a distance of 3.3Mpc, a total mass of 3 x 10^10 Msol is found on the basis of the Keplerian rotation curve. This implies a global M/L_B ratio of ~44 in solar units which is high and likely a reflection of the low blue luminosity of the galaxy (~15 times lower than the average S0 luminosity). Values for a Flat rotation curve are a factor of 4 higher.

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arXiv2002

A Correlation between Light Concentration and Cluster Local Density for Elliptical Galaxies

Using photometric and redshift data for the Virgo and Coma clusters, we present evidence for a correlation between the light concentration of elliptical galaxies (including dwarf ellipticals) and the local 3-D (i.e. non-projected) density of the clusters: more concentrated ellipticals are located in denser regions. The null hypothesis (i.e. the absence of any relation) is rejected at a significance level better than 99.9%. In order to explain the observed relation, a power law relating the galaxy light concentration and the cluster 3-D density is proposed. We study how the projection effects affect the form and dispersion of the data-points in the light concentration-projected density diagram. The agreement between our model and the observed data suggests that there is a paucity of dwarf elliptical galaxies in the cluster central regions.

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arXiv2002

Triaxial stellar systems following the r^{1/n} luminosity law: an analytical mass-density expression, gravitational torques and the bulge/disc interplay

We have investigated the structural and dynamical properties of triaxial stellar systems whose surface brightness profiles follow the r^{1/n} luminosity law - extending the analysis of Ciotti (1991) who explored the properties of spherical r^{1/n} systems. A new `analytical' expression that accurately reproduces the spatial (i.e. deprojected) luminosity density profiles (error < 0.1%) is presented for detailed modelling of the Sersic family of luminosity profiles. We evaluate both the symmetric and the non--axisymmetric components of the gravitational potential and force, and compute the torques as a function of position. For a given triaxiality, stellar systems with smaller values of n have a greater non-axisymmetric gravitational field component. We also explore the strength of the non-axisymmetric forces produced by bulges with differing n and triaxiality on systems having a range of bulge-to-disc ratios. The increasing disc-to-bulge ratio with increasing galaxy type (decreasing n) is found to heavily reduce the amplitude of the non-axisymmetric terms, and therefore reduce the possibility that triaxial bulges in late-type systems may be the mechanism or perturbation for nonsymmetric structures in the disc. Using seeing-convolved r^{1/n}-bulge plus exponential-disc fits to the K-band data from a sample of 80 nearby disc galaxies, we probe the relations between galaxy type, Sersic index n and the bulge-to-disc luminosity ratio. These relations are shown to be primarily a consequence of the relation between n and the total bulge luminosity. In the K-band, the trend of decreasing bulge-to-disc luminosity ratio along the spiral Hubble sequence is predominantly, although not entirely, a consequence of the change in the total bulge luminosity; the trend between the total disc luminosity and Hubble type is much weaker.

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arXiv2001

The effects of seeing on Sersic profiles. II. The Moffat PSF

The effects of seeing on Sersic r^{1/n} profile parameters are extensively studied using a Moffat function. This analytical approximation to the point spread function (PSF) is shown to provide the best fit to the PSF predicted from atmospheric turbulence theory when beta=4.765. The Moffat PSF is additionally shown to contain the Gaussian PSF as a limiting case beta->infinity. The Moffat function is also shown to be numerically well behaved when modelling narrow PSFs in HST images. Seeing effects are computed for elliptically symmetric surface brightness distributions. The widely used assumption of circular symmetry when studying the effects of seeing on intrinsically elliptical sources is shown to produce significant discrepancies with respect to the true effects of seeing on these sources. A prescription to correct raw (observed) central intensities, effective radii, index n and mean effective surface brightness is given.

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arXiv2001

Quantitative Morphology of the Intermediate-Redshift Galaxy Cluster Abell 2443 from Ground-Based Imaging -- Evidence for a galaxy concentration index correlation with cluster density

We present broad-band photometry and provide a quantitative analysis of the structure of galaxies in the inner region of the Abell Cluster 2443 (z~0.1). The galaxy parameters have been derived by fitting a two-component model (Sersic r^{1/n} bulge and exponential disk) to a magnitude-limited sample. Using a new method of analysis which takes into account the effects of seeing on the structural parameters and considers the ellipticity as an active parameter, we avoid systematic errors arising from assumptions of circular symmetry. 76% of the sample galaxies were classified with these models, the rest were morphologically peculiar. For the spiral galaxies, the relation between n and B/D is consistent with the trend observed in nearby field galaxy samples. The Sersic index n (which can be considered as a concentration index) of the elliptical galaxies is correlated with the local surface density of the cluster. Monte Carlo simulations were used to check the reliability of the method and determine the magnitude selection criteria.

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arXiv2000

The effects of seeing on Sersic profiles

The effects of seeing on the parameters of the Sèrsic profile are studied in an analytical form using a Gaussian point spread function. The surface brightness of Sèrsic profiles is proportional (in magnitudes) to $r^{1/n}$. The parameter $n$ serves to classify the type of profile and is related to the central luminosity concentration. It is the parameter most affected by seeing; furthermore, the value of $n$ that can be measured is always smaller than the real one. It is shown that the luminosity density of the Sèrsic profile with $n$ less than 0.5 has a central depression, which is physically unlikely. Also, the intrinsic ellipticity of the sources has been taken into account and we show that the parameters are dependent when the effects of seeing are non-negligible. Finally, a prescription for correcting raw effective radii, central intensities and $n$ parameters is given.

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arXiv1997

Star formation rates, efficiencies and initial mass functions in spiral galaxies (I Method)

A new method of evaluating relative (arm with respect to the interarm disk) star formation rates and relative star formation efficiencies, together with spiral arm-amplitudes, as a function of the galactocentric radius, using broad-band photometry is derived. The classical method for obtaining star formation rates from H_alpha photometry is discussed, and a new method is derived for diagnosing the possible presence of biased star formation due to different initial mass functions in the arms and in the interarm disk. As an example, these methods are applied to the spiral galaxies NGC 4321 and NGC 4254, obtaining their arm amplitudes, relative arm/interarm star formation efficiencies, and relative arm/interarm initial mass functions for each arm, as a function of the galactocentric radius. Both objects present evidence of massive star formation triggering in the spiral arms consistent with the spiral density-wave theory, and a different initial mass function in the arms from that in the rest of the disk, in the sense of favoring a larger fraction of massive stars in the arms. This biased star formation is present in the zones of the arms where there is triggering of massive star formation, and is then related to, and probably caused, by the density-wave system. However, due to this biased star formation, the spiral arms of the galaxies studied do not trigger the formation of a larger total mass of stars (of all spectral types) with respect to the interarm disk.

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