arXiv subjects

N. Cardiel

Publications and source records attributed to N. Cardiel.

64 records · Page 4Linked to original sources

arXiv2002

A new approach in data reduction: proper handling of random errors and image distortions

Data reduction procedures are aimed to minimize the impact of data acquisition imperfections on the measurement of data properties with a scientific meaning for the astronomer. To achieve this purpose, appropriate arithmetic manipulations with data and calibration frames must be performed. Furthermore, a full understanding of all the possible measurements relies on a solid constraint of their associated errors. We discuss different strategies for obtaining realistic determinations of final random errors. In particular, we highlight the benefits of considering the data reduction process as the full characterization of the raw-data frames, but avoiding, as far as possible, the arithmetic manipulation of that data until the final measure and analysis of the image properties. This philosophy will be used in the pipeline data reduction for ELMER and EMIR.

astro-ph↗

arXiv2002

Evidence of fast rotation in dwarf elliptical galaxies

In this Paper we investigate the kinematical properties of early-type dwarfs by significantly enlarging the scarce observational sample so far available. We present rotation curves and mean velocity dispersions for four bright dwarf ellipticals and two dwarf lenticular galaxies in the Virgo cluster. Most of these galaxies exhibit conspicuous rotation curves. In particular, five out of the six new galaxies are found to be close to the predictions for oblate spheroids flattened by rotation. Therefore, and contrary to the previous observational hints, the present data suggest that an important fraction of dwarf early-type galaxies may be rotationally supported.

astro-ph↗

arXiv2001

Empirical calibration of the near-IR Ca II triplet - III. Fitting functions

Using a near-IR stellar library of 706 stars with a wide coverage of atmospheric parameters, we study the behaviour of the Ca II triplet strength in terms of effective temperature, surface gravity and metallicity. Empirical fitting functions for recently defined line-strength indices, namely CaT*, CaT and PaT, are provided. These functions can be easily implemented into stellar populations models to provide accurate predictions for integrated Ca II strengths. We also present a thorough study of the various error sources and their relation to the residuals of the derived fitting functions. Finally, the derived functional forms and the behaviour of the predicted Ca II are compared with those of previous works in the field.

astro-ph↗

arXiv2001

Empirical calibration of the near-IR Ca II triplet - I. The stellar library and index definition

A new stellar library at the near-IR spectral region developed for the empirical calibration of the Ca II triplet and stellar population synthesis modeling is presented. The library covers the range 8348-9020 Angstrom at 1.5 Angstrom (FWHM) spectral resolution, and consists of 706 stars spanning a wide range in atmospheric parameters. We have defined a new set of near-IR indices, CaT*, CaT and PaT, which mostly overcome the limitations of previous definitions, the former being specially suited for the measurement of the Ca II triplet strength corrected for the contamination from Paschen lines. We also present a comparative study of the new and the previous calcium indices, as well as the corresponding transformations between the different systems. A thorough analysis of the sources of index errors and the procedure to calculate them is given. Finally, index and error measurements for the whole stellar library are provided together with the final spectra.

astro-ph↗

arXiv2001

Empirical calibration of the near-IR Ca II triplet - II. The stellar atmospheric parameters

We present an homogeneous set of stellar atmospheric parameters (Teff, log g, [Fe/H]) for a sample of about 700 field and cluster stars which constitute a new stellar library in the near-infrared developed for stellar population synthesis in this spectral region (8350-9020 Angstrom). Having compiled the available atmospheric data in the literature for field stars, we have found systematic deviations between the atmospheric parameters from different bibliographic references. The Soubiran, Katz & Cayrel (1998) sample of stars with very well determined fundamental parameters has been taken as our standard reference system, and other papers have been calibrated and bootstrapped against it. The obtained transformations are provided in this paper. Once most of the datasets were on the same system, final parameters were derived by performing error weighted means. Atmospheric parameters for cluster stars have also been revised and updated according to recent metallicity scales and colour-temperature relations.

astro-ph↗

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.

astro-ph↗

arXiv1997

Reliable random error estimation in the measurement of line-strength indices

We present a new set of accurate formulae for the computation of random errors in the measurement of atomic and molecular 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 frames. Finally, simple recipes for the estimation of the required signal-to-noise ratio to achieve a fixed index error are presented.

astro-ph↗

arXiv1997

Line-Strength Indices in Bright Spheroidals: Evidence for a Stellar Population Dichotomy between Spheroidal and Elliptical Galaxies

We present new measurements of central line-strength indices (namely Mg2, , and Hbeta and gradients for a sample of 6 bright spheroidal galaxies (Sph's) in the Virgo cluster. Comparison with similar measurements for elliptical galaxies (E's), galactic globular clusters (GGC's), and stellar population models yield the following results: (1) In contrast with bright E's, bright Sph's are consistent with solar abundance [Mg/Fe] ratios; (2) Bright Sph's exhibit metallicities ranging from values typical for metal-rich GGC's to those for E's; (3) Although absolute mean ages are quite model dependent, we find evidence that the stellar populations of some (if not all) Sph's look significantly younger than GGC's; and (4) Mg2 gradients of bright Sph's 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.

astro-ph↗

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.

astro-ph↗

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.

astro-ph↗