Person:
Gorgas García, Francisco Javier

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First Name
Francisco Javier
Last Name
Gorgas García
Affiliation
Universidad Complutense de Madrid
Faculty / Institute
Ciencias Físicas
Department
Física de la Tierra y Astrofísica
Area
Astronomía y Astrofísica
Identifiers
UCM identifierORCIDScopus Author IDWeb of Science ResearcherIDDialnet ID

Search Results

Now showing 1 - 9 of 9
  • Publication
    Line strengths and line strength gradients in bulges along the Hubble sequence
    (Springer, 1999) Goudfrooij, Paul; Gorgas García, Francisco Javier; Jablonka, Pascale
    We present first results of a comprehensive survey of deep long-slit spectra along the minor axis of bulges of edge-on spiral galaxies. Our results indicate that stellar populations in bulges are fairly old and encompass a range of metallicities. The luminosity-weighted ages of bulges range from those found for cluster ellipticals to slightly 'younger' (by up to only a few Gyr, however). Their alpha/Fe element ratio is typically supersolar, consistent with those found in giant ellipticals. The radial line-strength gradients in bulges correlate with bulge luminosity. Generally, these findings are more compatible with predictions of the 'dissipative collapse' model than with those of the 'secular evolution' model.
  • Publication
    Spectral indexes in cooling flow galaxies: evidence of star formation
    (Wiley, 1995-11-15) Cardiel López, Nicolás; Gorgas García, Francisco Javier; Aragón Salamanca, A.
    Spectroscopic observations of central dominant cluster galaxies, with and without cooling flows, are presented. Through the analysis of absorption spectral features, namely the strength of the magnesium absorption at lambda 5175 Angstrom and the lambda 4000-Angstrom break, both in the galaxy centres and as a function of radius, we have been able to estimate the ongoing star formation induced by the large amounts of gas accreted on to cooling now galaxies. A correlation between the central spectral indices and the mass accretion rate is found in the sense that galaxies located in clusters with large cooling flows exhibit lower Mg-2 and D-4000 indices. A similar correlation with D-4000 was previously reported by Johnstone, Fabian & Nulsen. Our work, with the inclusion of the correlation in Mg-2, adds further weight to the conclusion that these spectral anomalies are caused by recent star formation. The application of simple stellar population models reveals that the measured indices are explained if a relatively small fraction of the total mass now (5-17 per cent) is forming new stars with a normal initial mass function. 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. We find that spectral gradients in some cooling now galaxies flatten in the internal regions (r less than or similar to r(e)), where emission lines are usually detected. Gradients measured in the inner galaxy regions are, in the mean, lower than those of normal ellipticals, and exhibit a hint of a correlation with M. Application of the same population models to the observed spectral gradients allows us to conclude that the ongoing star formation is concentrated towards the inner parts of the cooling now galaxies and, therefore, the star formation does not follow the X-ray derived mass accretion profiles. 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 that is not related to the cooling flows.
  • Publication
    Line-strength indices in bright spheroidal galaxies: Evidence for a stellar population dichotomy between spheroidal and elliptical galaxies
    (IOP Publishing, 1997-05-20) Gorgas García, Francisco Javier; Pedraz, Santos; Guzmán, Rafael; Cardiel López, Nicolás; González, J. J.
    We present new measurements of central line-strength indices (namely, Mg-2, (Fe), and H beta) and gradients for a sample of six bright spheroidal (Sph) galaxies in the Virgo Cluster. Comparison with similar measurements for elliptical (E) galaxies, galactic globular clusters (GGCs), and stellar population models yield the following results: (1) in contrast with bright E galaxies, bright Sph galaxies are consistent with solar abundance [Mg/Fe] ratios; (2) bright Sph galaxies exhibit metallicities ranging from values typical for metal-rich GGCs to those for E galaxies; (3) although absolute mean ages are quite model dependent, we find evidence that the stellar populations of some (if not all) Sph galaxies look significantly younger than GCCs; and (4) Mg-2 gradients of bright Sph galaxies are significantly shallower than those of E galaxies. We conclude that the dichotomy found in the structural properties of Sph and E galaxies is also observed in their stellar populations. A tentative interpretation in terms of differences in star formation histories is suggested.
  • Publication
    Spectral gradients in central cluster galaxies: further evidence of star formation in cooling flows
    (Wiley, 1998-08-21) Cardiel López, Nicolás; Gorgas García, Francisco Javier; Aragón Salamanca, A.
    We have obtained radial gradients in the spectral features of the lambda 4000-Angstrom break (D(4000)) and Mg(2) for a sample of 11 central cluster galaxies (CCGs): eight in clusters with cooling flows and three in clusters without. After careful removal of the emission lines found within the D(4000) and Mg(2) bandpasses for some objects, 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. The nuclear indices are correlated with the mass deposition rate ((M) over dot) only when emission lines are found in the central regions of the galaxies. The central D(4000) and Mg(2) indices in cooling flow galaxies without emission lines are completely consistent with the indices measured in CCGs in clusters without cooling flows. CCGs in cooling flow clusters exhibit a clear sequence in the D(4000)-Mg(2) plane, with a neat segregation depending on emission-line type and blue morphology. This sequence can be modelled, using stellar population models with a normal initial mass function (IMF), by a recent (similar to 0.1 Gyr old) burst of star formation, although model uncertainties do not allow us to completely discard continuous star formation or a series of bursts over the last few Gyr. 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 hows and giant elliptical galaxies. Index gradients measured exclusively in the emission-line region correlate with hi. Using the same population models we have estimated the radial profiles of the mass transformed into new stars. The derived profiles are remarkably parallel to the expected radial behaviour of the mass deposition rate derived from X-ray observations. Moreover, a large fraction (probably most) of the cooling flow gas accreted into the emission-line region is converted into stars. In the Light of these new data, we discuss the evolutionary sequence suggested by McNamara, in which radio-triggered star formation bursts take place several times during the lifetime of the cooling flow. We conclude that this scenario is consistent with the available observations.
  • Publication
    Empirical calibration of the λ4000 Å break
    (EDP Sciences, 1999-10) Gorgas García, Francisco Javier; Cardiel López, Nicolás; Pedraz, S.; González, J. J.
    Empirical fitting functions, describing the behaviour of the λ4000 Å break, D_4000, in terms of effective temperature, metallicity and surface gravity, are presented. For this purpose, the break has been measured in 392 stars from the Lick/IDS Library. We have followed a very detailed error treatment in the reduction and fitting procedures, allowing for a reliable estimation of the break uncertainties. This calibration can be easily incorporated into stellar population models to provide accurate predictions of the break amplitude for, relatively old, composite systems.
  • Publication
    Reliable random error estimation in the measurement of line-strength indices
    (EDP Sciences, 1998-02) Cardiel López, Nicolás; Gorgas García, Francisco Javier; Cenarro, J.; González, J. J.
    We present a new set of accurate formulae for the computation of random errors in the measurement of atomic and molecular line-strength indices. The new expressions are in excellent agreement with numerical simulations. We have found that, in some cases, the use of approximated equations can give misleading line-strength index errors. It is important to note that accurate errors can only be achieved after a full control of the error propagation throughout the data reduction with a parallel processing of data and error images. Finally, simple recipes for the estimation of the required signal-to-noise ratio to achieve a fixed index error are presented.
  • Publication
    Line-strengths in early-type galaxies
    (Wiley, 1990-07-15) Gorgas García, Francisco Javier; Efstathiou, G.; Aragón Salamanca, A.
    We have analysed Mg, Hβ and Fe line-strengths in a sample of elliptical, SO and brightest cluster galaxies. For 15 galaxies, our spectra extend to approximately the half-light radius(r_e), and we are able to measure radial line-strength gradients. The metallic line-strength gradients vary markedly from object to object, and do not correlate strongly with parameters such as total luminosity and rotation, though we find sorne evidence that gradients in the Mg_2 index correlate with central velocity dispersion and central line-strength. The highly variable line-strength gradients in early-type galaxies shows that they have experienced different star formation histories. We suggest that this may be explained if they formed by the mergers of subunits in which star formation had proceeded to varying degrees of completion. We find that the line-strengths at r~r_e in elliptical galaxies are slightly larger than those of metal­ rich galactic globulars, suggesting that typical elliptical galaxies have roughly solar abundance at r~r_e and therefore that most ellipticals have relatively weak abundance gradients. The relative line-strengths in the outer parts of ellipticals differ from those in the nuclei of low-luminosity ellipticals, indicating that these stellar populations do not represent a simple one-parameter family governed by mean metal abundance. We find no significant differences in the central Mg and Fe line-strengths of the brightest cluster galaxies and normal ellipticals with the same central velocity dispersion. How­ ever, we find that two cD galaxies show Hβ in emission and are also at the centres of the prodigious cooling flows with mass-deposition rates of ≥ 100 M_⨀ yr^-1. Galaxies with cooling flows have identical Mg and Fe line-strengths to galaxies without cooling flows. We show that this implies that only a small fraction of the total luminosity of cooling flow galaxies could come from ongoing star formation with a normal stellar initial mass function.
  • Publication
    Old stellar populations .4. Empirical fitting functions for features in the Spectra of G and K stars
    (University Chicago Press, 1993-05) Gorgas García, Francisco Javier; Faber, S. M.; Burstein, David; González, J. Jesús; Courteau, Stéphane; Prosser, Charles
    Empirical fitting functions are presented that model the behavior of 11 strong atomic and molecular features in the spectra of Galactic G and K stars. The functions express the strength of spectral features as a function of V - K, surface gravity, and metallicity. The gravity calibration rests on stars in Galactic globular and open clusters, for which gravities have been derived by fitting to stellar evolutionary isochrones. The range of application of the fitting functions is set by the calibrating stars. There is good metallicity coverage from [Fe/H] = -0.75 to +0.50 dex, and cluster ages span 3.15 x 10^9 yr. However, the range in age is large only near solar metallicity, and the gravity partial derivatives may therefore be uncertain at metallicities much higher or lower than this value. In addition, the fitting functions build in whatever abundance ratio trajectories of elements relative to iron are present in local Galactic stars; these might differ from the element ratios present in external galaxies. In general, the behavior of the fitting functions matches expectations as to how these spectral features should behave as a function of basic stellar atmospheric parameters. The exception is CN (4150 Å), for which we find strong evidence for rapid onset of giant-branch mixing of CNO products to the stellar surface, setting in at V - K = 2.10. This confirms other recent studies of this index. The paper also presents new Lick line strength data on 112 field dwarfs and 92 cluster stars, plus colors, metallicities, and surface gravities for all Lick G and K stars published previously. The fitting functions are inverted to produce new output gravity and metallicity estimates for these stars and new average metallicities for all clusters, with errors of ±0.15 dex in [Fe/H] and ±0.23 dex in log g per star.
  • Publication
    Spectral indexes in cooling flow galaxies: evidence of star-formation
    (Wiley-Blackwell, 1995-11-15) Cardiel López, Nicolás; Gorgas García, Francisco Javier; Aragón Salamanca, A.
    Spectroscopic observations of central dominant cluster galaxies, with and without cooling flows, are presented. Through the analysis of absorption spectral features, namely the strength of the magnesium absorption at 5175 Å and the 4000 Å break, both in the galaxy centres and as a function of radius, we have been able to estimate the ongoing star formation induced by the large amounts of gas accreted onto cooling- flow galaxies. A correlation between the central spectral indices and the mass accretion rate is found in the sense that galaxies located in clusters with large cooling flows exhibit lower Mg_(2) and D_(4000) indices. A similar correlation with D_(4000) was previously reported by Johnstone, Fabian & Nulsen (1987). Our work, with the inclusion of the correlation in Mg_(2), adds further weight to the conclusion that these spectral anomalies are caused by recent star formation. The application of simple stellar population models reveals that the measured indices are explained if a relatively small fraction of the total mass ow (5-17%) is forming new stars with a normal initial mass function. 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. We find that spectral gradients in some cooling ow galaxies flatten in the internal regions ( r <̃ r_(e)), where emission lines are usually detected. Gradients measured in the inner galaxy regions are, in the mean, lower than those of normal ellipticals, and exhibit a hint of a correlation with M. Application of the same population models to the observed spectral gradients allows us to conclude that the ongoing star formation is concentrated towards the inner parts of the cooling ow galaxies and, therefore, the star formation does not follow the X-ray derived mass accretion profiles. Simultaneously, the spectral indices in the outer regions of some galaxies with and without cooling ow attain extremely low values, suggesting that they could be hosting star formation with an origin not related to the cooling flows.