arXiv subjects

J. Perea

Publications and source records attributed to J. Perea.

53 records · Page 3Linked to original sources

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

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

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.

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↗

arXiv2007

Seyfert's Sextet: A Slowly Dissolving Stephan's Quintet?

We present a multiwavelength study of the highly evolved compact galaxy group known as Seyfert's Sextet (HCG79: SS). We interpret SS as a 2-3 Gyr more evolved analog of Stephan's Quintet (HCG92: SQ). We postulate that SS formed by sequential acquisition of 4-5 primarily late-type field galaxies. Four of the five galaxies show an early-type morphology which is likely the result of secular evolution driven by gas stripping. Stellar stripping has produced a massive/luminous halo and embedded galaxies that are overluminous for their size. These are interpreted as remnant bulges of the accreted spirals. H79d could be interpreted as the most recent intruder being the only galaxy with an intact ISM and uncertain evidence for tidal perturbation. In addition to stripping activity we find evidence for past accretion events. H79b (NGC6027) shows a strong counter-rotating emission line component interpreted as an accreted dwarf spiral. H79a shows evidence for an infalling component of gas representing feedback or possible cross fueling by H79d. The biggest challenge to this scenario involves the low gas fraction in the group. If SS formed from normal field spirals then much of the gas is missing. Finally, despite its advanced stage of evolution, we find no evidence for major mergers and infer that SS (and SQ) are telling us that such groups coalesce via slow dissolution.

astro-ph↗

arXiv2006

Nuclear Activity in UZC Compact Groups of Galaxies

We analyse the level of nuclear activity in galaxies belonging to UZC-CGs. Spectra are available for 868 galaxies (90% of the whole catalog); 67 % of them show nuclear activity (AGN or Star formation). To carry out a detailed study about the nuclear activity and its relationship with properties of the host galaxy and parent group we select a sample of 215 groups with spectra available for all their members. From the analysis of this sample and using diagnostic diagrams to do the nuclear classification we found that 37% of emission galaxies host an HII nuclear region, 43% an AGN and 20% a Transition Object. AGNs are located mainly in bright early type galaxies meanwhile HII are in fainter and later types. Groups dominated by HII show significant lower velocity dispersions and larger sizes than groups dominated by AGNs or TO.

astro-ph↗

arXiv2006

Low Luminosity Activity in Hickson Compact Groups

With the aim of studying the influence of environment on the nuclear activity of galaxies, we have selected a well defined sample of 65 Compact Groups of galaxies with concordant redshift in the Hickson Catalog. In this proceeding, we present the results of the classification of nuclear activity for 42 galaxies, based on newly obtained spectral observations. In this subsample, 71% of the galaxies turned out to have emission lines in their nuclei. 73% of these emission-line galaxies were found to have characteristics consistent with low luminosity AGN (LLAGN), which makes compact groups extremely rich in such objects.

astro-ph↗

arXiv2002

Ripples and tails in the compact group of galaxies Hickson 54

HCG 54 has been classified as a compact galaxy group by Hickson, but its nature is uncertain because of its unusual properties. We present here a study of HCG~54 based on deep optical images and spectroscopy as well as high spatial and spectral resolution HI observations obtained at the VLA. Both optical and radio data indicate clearly that HCG~54 is a product of a recent merger involving at least two galaxies. Our optical images have revealed a blue elongated structure associated with HCG 54a embedded in a rounder and redder stellar envelope. Several ripples or shells whose center is located near HCG 54a and b are also detected. These observed features are similar to those found in the numerical simulations of tidal interactions involving two equal mass disk galaxies. This system is embedded in an HI cloud 12 kpc in diameter, and a 20 kpc HI tidal tail emerges from its western edge. Both the HI distribution and kinematics are consistent with a recent history of a deeply penetrating interaction or a merger. Based on the analysis of our new data we propose that HCG 54 is the remnant of a merger in an advanced stage, whose main body is what has been called HCG 54a, while HCG 54b marks the location of a strong starburst induced by the interaction, as evidenced by the Wolf Rayet stellar population that we detect. Optical spectra of HCG 54c and d show HII region features and are interpreted as tidal debris of the collision undergoing active star formation.

astro-ph↗

arXiv2001

Where is the Neutral Atomic Gas in Hickson Groups?

We have analyzed the total HI contents of 72 Hickson compact groups of galaxies (HCGs) and the detailed spatial distributions and kinematics of HI within a subset of 16 groups using the high angular resolution observations obtained with the VLA in order to investigate a possible evolutionary scenario for these densest systems in the present day galaxy hierarchy. For the more homogeneous subsample of 48 groups, we found a mean HI deficiency of DefHI = 40 (40% of the expected HI for the LBs and morphological types of the member galaxies). The individual galaxies show larger degrees of deficiency than the groups globally, DefHI = 0.62 (24% of the expected HI), due in most cases to efficient gas stripping from individual galaxies into the group environment visible in the VLA maps. The degree of deficiency is found to be similar to the central galaxies of Virgo and Coma cluster, and Coma I group, in spite of the significantly different characteristics of these environments. Phase transformation of the HI should explain the deficiency. The detection rate of HCGs at X-ray wavelengths is larger for HI deficient groups. In the evolutionary scenario we propose, the amount of detected HI would decrease further with evolution, by continuous tidal stripping and/or heating. The H2 content also tends to be lower than expected for the galaxies in HI deficient groups, this may suggest that the HI stripping by frequent tidal interaction breaks the balance between the disruption of molecular clouds by star formation and the replenishment from the ambient HI.

astro-ph↗

arXiv1999

Mass Distribution in Compact Groups

New redshift surveys of galaxies in the field of compact groups have discovered a population of faint galaxies which act as satellites orbiting in the potential well of the bright group. Here we analyze the mass distribution of the groups by comparing the mass derived from the bright members and the mass obtained from the satellite galaxies. Our analysis indicates the presence of a dark halo around the main group with a mass roughly four times that measured for the dominant galaxies of the compact group. We found that heavier halos are ruled out by the observations when comparing the distribution of positions and redshifts of the satellite galaxies with the distribution of satellites surrounding isolated spiral galaxies. The results agree with a picture where compact groups may form a stable system with galaxies moving in a common dark halo.

astro-ph↗

arXiv1999

Tracing interactions in HCGs through the HI

We present a global study of HI spectral line mapping for 16 Hickson Compact Groups (HCGs) combining new and unpublished VLA data, plus the analysis of the HI content of individual galaxies. Sixty percent of the groups show morphological and kinematical signs of perturbations (from multiple tidal features to concentration of the HI in a single enveloping cloud) and sixty five of the resolved galaxies are found to be HI deficient with respect to a sample of isolated galaxies. In total, 77% of the groups suffer interactions among all its members which provides strong evidence of their reality. We find that dynamical evolution does not always produce HI deficiency, but when this deficiency is observed, it appears to correlate with a high group velocity dispersion and in some cases with the presence of a first-ranked elliptical. The X-ray data available for our sample are not sensitive enough for a comparison with the HI mass; however this study does suggest a correlation between HI deficiency and hot gas since velocity dispersions are known from the literature to correlate with X-ray luminosity.

astro-ph↗

arXiv1999

On two mass estimators for clusters of galaxies

The two most common mass estimators that use velocity dispersions and positions of objects in a stellar system, the virial mass estimator (VME) and the projected mass estimator (PME), are revisited and tested using N-body experiments. We consider here only spherical, isolated, and isotropic velocity dispersion systems. We have found that the PME can overestimate masses by ~ 20%, for realistic cluster mass profiles, if applied only to regions around the total system's "effective" radius. The VME can yield a correct mass at different radii provided that the total potential energy term is correctly taken into account and the system is completely sampled; otherwise, it may lead to similar errors as the PME. A surface pressure (3PV) term recently alluded to be usually neglected when using the VME and therefore required as a correction term is here found not necessarily required, although it can be used to yield a reasonable correction term. The preferred method here, however, is the virial theorem due to its simplicity and better agreement with N-body experiments. The possible reasons for the mass discrepancies found when using the PME and the VME in some N-body simulations are also briefly discussed.

astro-ph↗

arXiv1997

Effects of Interaction Induced Activities in Hickson Compact Groups: CO and FIR Study

A study of CO and FIR emission in a distance limited (z<0.03) complete sample of Hickson Compact Group (HCG) galaxies was conducted in order to examine the effects of their unique environment on the ISM and star formation activity. Ubiquitous tidal interactions in these dense groups would predict enhanced activities among the HCG galaxies compared to isolated galaxies. Instead their CO and FIR properties are surprisingly similar to isolated spirals. The CO data for 80 HCG galaxies presented here indicate that the spirals globally show the same H2 content as the isolated comparison sample, although 20% are deficient in CO emission. The CO deficiency appears linked with the group evolution, and gas exhaustion through past star formation and removal of external gas reserve by tidal stripping of the outer HI disk offer a possible explanation. The IRAS data for the entire sample of 161 HCG galaxies were re-analyzed using ADDSCAN/SCANPI, improving the sensitivity by a factor of 3-5 over the existing Point Source Catalog (PSC) and better resolving the contribution from individual galaxies. We find that FIR emission in HCG galaxies is similar to isolated, Virgo cluster, and weakly interacting galaxies. A factor two enhancement in the 25 micr/100 micr flux ratio among the HCG spirals is found, which suggests intense, localized nuclear starburst activity similar to HII galaxies. A number of early-type galaxies in HCGs are detected in CO and FIR, lending further support to the idea that tidal interactions and tidally induced evolution of the groups and member galaxies are important in our sample.

astro-ph↗

arXiv1997

Non-linear Dependence of L(B) on L(FIR) and M(H2) among Spiral Galaxies and Effects of Tidal Interaction

Through the study of a carefully selected sample of isolated spiral galaxies, we have established that two important global physical quantities for tracing star forming activities, L(FIR) and M(H2), have non-linear dependence on another commonly cited global quantity L(B). Furthermore we show that simple power law relations can effectively describe these non-linear relations for spiral galaxies spanning four orders of magnitude in FIR and M(H2) and nearly three orders of magnitude in L(B). While the existence of non-linear dependence of M(H2) (assuming a constant CO-to-H2 conversion) and L(FIR) on optical luminosity L(B) has been previously noted in the literature, an improper normalization of simple scaling by L(B) has been commonly used in many previous studies to claim enhanced molecular gas content and induced activities among tidally interacting and other types of galaxies. We remove these non-linear effects using the template relations derived from the isolated galaxy sample and conclude that strongly interacting galaxies do not have enhanced molecular gas content, contrary to previous claims. With these non-linear relations among L(B), L(FIR) and M(H2) properly taken into account, we confirm again that the FIR emission and the star formation efficiency L(FIR)/M(H2) are indeed enhanced by tidal interactions. Virgo galaxies show the same level of M(H2) and L(FIR) as isolated galaxies. We do not find any evidence for enhanced star forming activity among barred galaxies.

astro-ph↗

arXiv1997

Hickson 96: a physical compact group

We analyze the morphology and dynamics of the galaxies of the Hickson 96 compact group by means of deep CCD images in the B, V and R photometric bands and long-slit spectroscopy. The four galaxies of this spiral rich group show signs of gravitational interaction. Two long tails come out from the contact region of the close pair H96ac. It is also there where both galaxies, bi-symmetrical in their inner parts, loose one of their arms. Moreover, both the photometric and kinematical center of H96a are displaced relative to the center of the disk. H96b, a giant elliptical galaxy, shows significant deviations from a $r^{1/4}$ law at the inner parts and small rotation along its major axis. We find at the centre of the galaxy an elongated component which is kinematically decoupled. A wide faint plume seems to emerge from this galaxy. H96d, the smallest galaxy of the group, seems to be influenced by the bigger members, as indicated by three prominent knots of recent bursts of star formation in its blue disk, and by hints of optical bridges joining H96d with H96a and b. Finally the group has a low velocity dispersion (160 km/s) and is well isolated - no galaxies with comparable magnitude to H96a or H96b are found in its neighborhood. All these results lead us to conclude that Hickson 96 constitutes a real physical system.

astro-ph↗

arXiv1996

The ESO Nearby Abell Cluster Survey. III. Distribution and Kinematics of Emission-Line Galaxies

We have studied the frequency of occurrence, the kinematics and the spatial distribution of Emission-Line Galaxies (ELG) in clusters in the ESO Nearby Abell Cluster Survey (ENACS). More than 90% of the ELG are spirals, but the ELG represent only 30% of all spirals. The true ELG fraction in clusters is 0.10; this is consistent with that in the field (0.21) as a result of differences in morphological mix. The average ELG fraction decreases with increasing velocity dispersion of the cluster. Only 1/5 of the clusters shows a significant difference in the average velocities of the ELG and the other (non-ELG) galaxies; in a similar fraction of the clusters significant differences in velocity dispersion are seen. Yet, the velocity dispersion of the ELG is, on average, 20% larger than that of the non-ELG. This larger velocity dispersion is largely intrinsic, i.e. it is not likely to be the result of velocity offsets of the ELG. The surface density of the ELG is significantly less peaked towards the centre than that of the non-ELG. The ELG are distributed fairly smoothly within the cluster with at most 25% in compact substructures. The distribution and kinematics of the ELG yield virial mass estimates that, on average, are 50% higher than those based on the non-ELG. This probably means that the ELG are on more radial orbits than the non-ELG. All observed properties of the ELG are consistent with most of them being on their first approach of the high-density cluster core.

astro-ph↗

arXiv1995

The ESO Nearby Abell Cluster Survey. I. Description of the Dataset and Definition of Physical Systems

Summary - We describe the results of the ESO Key-programme on `Structure and Dynamics of Rich Galaxy Clusters' (or ENACS). The final database contains a total of 5634 galaxies in the directions of 107 clusters from the ACO catalogue with richness $R\ge 1$ and mean redshifts $z \le 0.1$. Of the 5634 redshifts, 4465 are based on absorption lines only, 583 on emission lines only and 586 on both. The overall reliability of the 5634 accepted redshifts is 0.98. For 5615 galaxies an $R_{25}$ magnitude was obtained. We discuss various methods for defining the compact, physically relevant systems in the 107 `pencil-beam' surveys. We have chosen to apply a fixed velocity gap of 1000 km/s to separate galaxies that do not form part of the same system. With this gap size, systems are not broken up into sub-systems, and field galaxies are hardly linked to the systems. We present the mean redshifts of the 220 systems (with at least 4 members) identified in the 107 pencil beam surveys. On average, about 75~\% of the 5634 galaxies are in the largest system found in the direction of the rich Abell cluster candidate. Half of the 25~\% of galaxies outside the largest system belong to a secondary system along the same line of sight. For about 90~\% of the nearby rich Abell cluster candidates studied here we find a redshift system that either contains more than half of the total number of redshifts, or that has at least two times as many redshifts as the next largest system.

astro-ph↗