arXiv subjects

A. Aragon-Salamanca

Publications and source records attributed to A. Aragon-Salamanca.

49 records · Page 3Linked to original sources

arXiv1998

Near-Infrared Fundamental Plane of Elliptical Galaxies

Near-infrared (2.2 $μm$) observations of a sample of 48 elliptical galaxies in the Coma cluster have been carried out and used to study the near-ir fundamental plane (FP) of ellipticals in this cluster. An rms scatter of 0.072 dex was found for this relation, similar to that of its optical counterpart, using the same sample of galaxies. This corresponds to an uncertainty of 18% in distances to individual galaxies derived from this relation. The sensitivity of the near-infrared FP to the star formation or changes in metallicity and stellar population among the ellipticals was explored and found to be small. Although, a likely source of scatter in this relation is contributions from the Asymptotic Giant Branch (AGB) population to their near-infrared light. Allowing for observational uncertainties, we find an intrinsic scatter of 0.060 dex in the near-ir FP. The cluster galaxies presented here, provide the zero-point for the peculiar velocity studies, using the near-infrared FP. Changes in the slopes of the $D-σ$ and $L-σ$ relations of ellipticals between the optical and near-infrared wavelengths were investigated and found to be due to variations in metallicity or age (or a combination of them). However, it was not possible to disentangle the effects of age and metallicity in these relations. Using evolutionary population synthesis models, we find that the effects of age and metallicity decouple on the $(M/L)_K$ vs. Mg$_2$ and $(M/L)_K$ vs. (V$-$K) diagrams.

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arXiv1998

Principal Component Analysis Of Synthetic Galaxy Spectra

We analyse synthetic galaxy spectra from the evolutionary models of Bruzual&Charlot and Fioc&Rocca-Volmerange using the method of Principal Component Analysis (PCA). We explore synthetic spectra with different ages, star formation histories and metalicities, and identify the Principal Components (PCs) of variance in the spectra due to these different model parameters. The PCA provides a more objective and informative alternative to diagnostics by individual spectral lines. We discuss how the PCs can be used to estimate the input model parameters and explore the impact of noise in this inverse problem. We also discuss how changing the sampling of the ages and other model parameters affects the resulting PCs. Our first two synthetic PCs agree with a similar analysis on observed spectra obtained by Kennicutt and the 2dF redshift survey. We conclude that with a good enough signal-to-noise (S/N>> 10) it is possible to derive age, star formation history and metallicity from observed galaxy spectra using PCA.

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arXiv1998

Spectral gradients in central cluster galaxies: further evidence of star formation in cooling flows

We have obtained radial gradients in the spectral features D4000 and Mg2 for a sample of 11 central cluster galaxies (CCGs). The new data strongly confirm the correlations between line-strength indices and the cooling flow phenomenon found in our earlier study. We find that such correlations depend on the presence and characteristics of emission lines in the inner regions of the CCGs. CCGs in cooling flow clusters exhibit a clear sequence in the D4000-Mg2 plane, with a neat segregation depending on emission-line types and blue morphology. This sequence can be modelled, using stellar population models with a normal IMF, by a recent burst of star formation. In CCGs with emission lines, the gradients in the spectral indices are flat or positive inside the emission-line regions, suggesting the presence of young stars. Outside the emission-line regions, and in cooling flow galaxies without emission lines, gradients are negative and consistent with those measured in CCGs in clusters without cooling flows and giant elliptical galaxies. Index gradients measured exclusively in the emission-line region correlate with mass deposition rate. We have also estimated the radial profiles of the mass transformed into new stars which are remarkably parallel to the radial behaviour of the mass deposition rate. A large fraction (probably most) of the cooling flow gas accreted into the emission-line region is converted into stars. We discuss the evolutionary sequence suggested by McNamara (1997), in which radio triggered star formation bursts take place several times during the lifetime of the cooling flow. This scenario is consistent with the available observations.

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arXiv1998

Evolution of the colour-magnitude relation of early-type galaxies in distant clusters

We present a thorough quantitative analysis of the evolution of the colour-magnitude relation for early-type galaxies in 17 distant clusters with redshifts 0.31 < z < 1.27 using the Kodama & Arimoto (1997) evolutionary model for elliptical galaxies. The model is calibrated to reproduce the colour- magnitude relation for Coma ellipticals at z ~ 0 and gives the evolution of the slope and zero-point as a function of redshift. We find no significant differences between the colour-magnitude relations of the clusters in our sample. The slopes can be reproduced by a single model sequence in which all elliptical galaxies are assumed to be equally old (the maximum age difference allowed for the brightest 3 magnitudes is only 1 Gyr) and to have mean stellar metallicities which vary as a function of galaxy luminosity. The zero-points of the colour-magnitude relations constrain the epoch of major star formation in early-type galaxies to z_f>2-4. This study provides two important constraints for any model of the formation of rich clusters: the uniformity of the ages of the stellar populations in the early-type galaxies and the universality of the metallicity sequence of these galaxies as a function of galaxy mass.

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arXiv1998

The K-band Hubble diagram for the brightest cluster galaxies: a test for galaxy formation and evolution models

The K-band Hubble diagram for a sample of brightest cluster galaxies (BCGs) in the redshift range 0<z<1 shows a very small scatter (0.3 magnitudes r.m.s). The BCGs exhibit very little luminosity evolution in this redshift range: if q_0=0.0 we detect no luminosity evolution; for q_0=0.5 we measure a small negative evolution. If the mass in stars of these galaxies had remained constant over this period of time, substantial positive luminosity evolution would be expected: BCGs should have been brighter in the past since their stars were younger. This suggests that the stellar mass of the BCGs has been assembled over time through merging and accretion. We estimate that the stellar mass in a typical BCG has grown by a factor ~ 2 since z=1 if q_0=0.0 or by factor ~ 4 if q_0=0.5. These results are in remarkably good agreement with the predictions of semi-analytic models of galaxy formation and evolution set in the context of a hierarchical scenario for structure formation.

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arXiv1998

The K-band Hubble diagram for the brightest cluster galaxies: a test of hierarchical galaxy formation models

We analyse the K-band Hubble diagram for a sample of brightest cluster galaxies (BCGs) in the redshift range 0 2) that is evolving passively. We can thus use evolutionary population synthesis models to estimate the rate of growth in stellar mass for these systems. We find that the stellar mass in a typical BCG has grown by a factor ~ 2 since z=1 if q_0=0.0 or by factor ~ 4 if q_0=0.5. These results are in good agreement with the predictions of semi-analytic models of galaxy formation and evolution set in the context of a hierarchical scenario for structure formation. The models predict a scatter in the luminosities of the BCGs that is somewhat larger than the observed one, but that depends on the criterion used to select the model clusters.

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arXiv1998

New Constraints on the Luminosity Evolution of Spheroidal Galaxies in Distant Clusters

We investigate various probes of luminosity evolution in the rich cluster environment, concentrating in particular on the spheroidal (E/S0) galaxies, using a newly-constructed catalog of morphologically-classified Hubble Space Telescope images of galaxies observed in the cores of 13 clusters with redshifts 0.17<z<0.56. An important distinction of this study compared to earlier work is the availability of new near-infrared ground-based photometry for a substantial subset of our HST fields, which we have used to select and study the various populations. We find no significant change in the characteristic luminosity of the spheroidal populations at redshifts of 0.31 and 0.56. As a more sensitive probe of luminosity evolution, deriving effective metric radii and mean effective surface brightnesses. At the standard condition corresponding to R_e = 1 kpc, we find convincing evidence of evolutionary brightening in both rest-frame B and K light, consistent with model predictions based on the passive evolution of stellar populations.

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arXiv1996

Line-Strength Gradients in Cooling Flow Galaxies

We present new results on line-strength gradients in a sample of 18 brightest cluster galaxies (13 in clusters with cooling flows and 5 in clusters without). Here we focus on the study of the Mg2 index and the 4000A break. We find that line-strength gradients vary markedly from galaxy to galaxy, depending both on the mass deposition rate and the presence or not of emission lines in the nuclear regions. Gradients are found to be flat, and even positive (i.e. bluer when going inwards), in the emission-line region of the cooling flow galaxies with emission lines. However, outside the region where emission lines are present, mean spectral gradients of brightest cluster galaxies, in clusters with and without cooling flows, are consistent with those observed in giant elliptical galaxies. In addition, and in agreement with previous studies, we confirm a correlation between central spectral indices and the mass deposition rates, although we find that cooling flow galaxies without emission lines do not follow this trend.

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arXiv1996

Faint galaxies close to QSOs with damped Lyman-alpha absorption systems

We have obtained very deep near-infrared images in the fields of 10 QSOs whose spectra contain damped Lyman-alpha absorption (DLA) systems with 1.7<z_abs <2.5. The main aim of our investigation is to provide new constraints on the properties of distant galaxies responsible for the DLA absorption. After subtracting the point spread function associated with the QSO light, we have detected galaxies very close to the QSO line of sight (projected distance 1.2-1.3arcsec) in two fields. These sources therefore represent promising candidate galaxies responsible for the DLA absorption. Placed at the absorber's redshift, the impact parameter is 10h_50^-1kpc and the luminosity is close to L_K^*. Such parameters are consistent with the hypothesis, verified for metallic systems at lower redshift, that slowly-evolving massive galaxies produce at least some of the absorption systems of high column density in QSO spectra out to beyond z=2. In addition to detecting these candidate DLA galaxies, the radio-loud QSOs in our sample show a significant excess of sources on larger scales (theta=7arcsec); this excess is not present in the radio-quiet QSO sightlines. We calculate that such an excess could be produced by luminous galaxies in the cores of clusters associated with radio-loud QSOs. Both results confirm that deep imaging of selected QSOs can be a powerful method of finding samples of likely z~2 galaxies. Follow-up near-infrared spectroscopy is required to secure galaxy redshifts and star formation rates, while deep HST imaging can determine sizes and morphologies, providing valuable information on galaxy properties at large look-back times.

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arXiv1995

The Current Star Formation Rate of the Local Universe

The Universidad Complutense de Madrid (UCM) Survey is a long-term project aiming at finding and analysing star-forming galaxies detected by their H$α$ emission in Schmidt objective-prism plates. The instrumental set-up limits the volume of the Universe surveyed to a redshift $z\lesssim0.045$. So far we have discovered several hundred emission-line galaxies (ELGs) undergoing star formation at a wide range of levels. Analyzing a complete sample of ELGs from the UCM survey we have computed the H$α$ luminosity function for the star-forming galaxies in the surveyed volume of the Universe. A Schechter function provides a good fit to the H$α$ luminosity function with the following parameters: $α= -1.3\pm0.2$; $L^*({\rm H}α) = 10^{42.15\pm0.08} $erg$ $s$^{-1}$ and $ϕ^*=10^{-3.2\pm0.2} $Mpc$^{-3}$ for $H_0 = 50 $km$ $s$^{-1} $Mpc$^{-1}$. Integrating over the full range of luminosities, we obtain an H$α$ luminosity density of $10^{39.1\pm0.2} $erg$ $s$^{-1} $Mpc$^{-3}$. Using the H$α$ emission as a star formation rate (SFR) estimator, this translates into a SFR density for the Local Universe of $0.013^{+0.007}_{-0.005} M_{\odot} $yr$^{-1} $Mpc$^{-3}$ in star-forming galaxies with $EW($H$α+$[NII]$)>10$Åand $z\lesssim0.045$, for a Scalo Initial Mass Function. This is the first observational determination of this quantity, which will provide a direct test of current galaxy formation and evolution models.

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arXiv1995

The life-cycle of star formation in distant clusters

We analyse the detailed distribution of star-forming and post-starburst members in three distant (z = 0.31) galaxy clusters in terms of evolutionary sequences that incorporate secondary bursts of star formation on pre-existing stellar populations. Using the number density of spectroscopically-confirmed members on the EW(Hδ) versus B-R plane from existing data, and for a larger K'-limited sample on the U-I versus I-K' plane from newly-acquired infrared images, we demonstrate that the proportion of cluster members undergoing secondary bursts of star formation during the last ~2 Gyr prior to the epoch of observation is probably as high as 30 per cent of the member galaxies. A key observation leading to this conclusion is the high proportion of Hδstrong galaxies in all three clusters. The evolutionary modelling, whilst necessarily approximate, returns the correct proportions of galaxies in various stages of the star formation cycle both in terms of spectral and colour properties. HST images for the three clusters indicate a high proportion of the active members show signs of interaction, whereas the Hδstrong galaxies appear mainly to be regular spheroidals. We examine results from recent merger simulations in the context of the populations in these clusters and confirm that the merging of individual galaxies, triggered perhaps by the hierarchical assembly of rich clusters at this epoch, is consistent with the star formation cycle identified in our data. The implications of such a high fraction of active objects in cluster cores is briefly discussed.

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arXiv1995

The Deficit of Distant Galaxy Clusters in the RIXOS X-ray Survey

Clusters of galaxies are the largest gravitationally bound systems and therefore provide an important way of studying the formation and evolution of the large scale structure of the Universe. Cluster evolution can be inferred from observations of the X-ray emission of the gas in distant clusters, but interpreting these data is not straightforward. In a simplified view, clusters grow from perturbations in the matter distribution: their intracluster gas is compressed and shock-heated by the gravitational collapse$^{1}$. The resulting X-ray emission is determined by the hydrostatic equilibrium of the gas in the changing gravitational potential. However, if processes such as radiative cooling or pre-collapse heating of the gas are important, then the X-ray evolution will be strongly influenced by the thermal history of the gas. Here we present the first results from a faint flux-limited sample of X-ray selected clusters compiled as part of the ROSAT International X-ray and Optical Survey (RIXOS). Very few distant clusters have been identified. Most importantly, their redshift distribution appears to be inconsistent with simple models based on the evolution of the gravitational potential. Our results suggest that radiative cooling or non-gravitational heating of the intracluster gas must play an important role in the evolution of clusters.

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arXiv1995

Spectral Indices in Cooling Flow Galaxies: Evidence for Star Formation

Through the study of two absorption spectral features in the optical range (Mg2 and the 4000 A break), we find evidence for star formation in the inner regions of cooling-flow galaxies. The application of simple stellar population models reveals that the measured indices are explained if a relatively small fraction of the total mass flow (5-17%) is forming new stars with a normal IMF. However, we argue that this is only a lower limit, and conclude that a large fraction of the gas accreted inside the galaxy could be forming stars. In addition, the analysis of line-strength gradients in the inner galaxy regions reveals that, in the mean, they are lower than those of normal ellipticals, and exhibit a hint of correlation with the mass accretion rate. Simultaneously, the spectral indices in the outer regions of some galaxies, with and without cooling flow, attain extremely low values, suggesting that they could be hosting star formation with an origin not related to the cooling flows.

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