arXiv subjects

Basilio X. Santiago

Publications and source records attributed to Basilio X. Santiago.

24 records · Page 2Linked to original sources

arXiv1997

HST Observations of the Field Star Population in the Large Magellanic Cloud

We present $V$ and $I$ photometry, obtained with the Hubble Space Telescope, for $\sim 15,800$ stars in a field in the inner disk of the Large Magellanic Cloud. We confirm previous results indicating that an intense star formation event, probably corresponding to the formation of the LMC disk, occurred a few times $10^9$ years ago. We find a small but real difference between our field and one further out in the disk observed by Gallagher et al (1996): either star formation in the inner disk commenced slightly earlier, or the stars are slightly more metal rich. We also find evidence for a later burst, around 1 Gyr ago, which may correspond to the formation of the LMC bar. About 5% of the stars in our field are substantially older than either burst, and are probably members of an old disk or halo population with age $\sim 12$ Gyr.

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arXiv1996

The two-point correlation function and morphological segregation in the Optical Redshift Survey

We study the clustering of galaxies in real and redshift space using the Optical Redshift Survey (ORS). We estimate the two point correlation function in redshift space, $ξ(s)$, for several subsamples of ORS, spanning nearly a factor of 30 in volume and detect significant variations in $ξ(s)$ among the subsamples covering small volumes. For volumes \gtsima $(75 h^{-1} {\rm Mpc})^{3}$ the ORS subsamples present very similar clustering patterns. Powerlaw fits to $ξ(s)$ give best-fit values in the range $1.5 \leq γ_{s} \leq 1.7 $ and $6.5 \leq s_{0} \leq 8.8 h^{-1}$ Mpc for several samples extending to redshifts of 8000 km s$^{-1}$. We find that $ξ(s)$ is larger for the magnitude-limited sample than for diameter-limited one within a radius of 4000 km s$^{-1}$. We interpret this as an indirect result of morphological segregation coupled with differences in morphological mix. We split ORS into morphological subsamples and confirm the existence of morphological segregation of galaxies out to scales of $s \sim 10 h^{-1}$ Mpc. Our results indicate that the relative bias factor between early type galaxies and late-types may be weakly dependent on scale. If real, this would suggest non-linear biasing. We also compute correlations as a function of radial and projected separations, $ξ(r_p, π)$ and derive the real space correlation function, $ξ(r)$. The results obtained in real space confirm those found using $ξ(s)$.

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arXiv1996

HST Photometry of 47 Tuc and Analysis of the Stellar Luminosity Function in Milky-Way Clusters

We present V and I photometry for a field 5' from the center of 47 Tuc. The field was imaged with the HST WFPC2 as part of the Medium Deep Survey project. The luminosity function (LF) continually rises in the domain $5 < M_I < 9$ with a slope $Δlog Φ(M) / ΔM \sim 0.15$ and then drops off sharply. We compare our LF with that derived by De Marchi \& Paresce for a neighbouring HST field. The two independent LFs are very similar in the entire range of luminosities probed ($M_I < 10$). Comparisons are also made to other HST LFs derived by several authors for different clusters in the Galaxy. We use the KS test to assess the significance of the differences found. The LFs obtained with HST are consistent with being derived from the same population down to $M_I \sim 9.0$. Beyond that, statistically significant variations arise. Globular cluster LFs also differ according to the prominence of a plateau in the bright end ($5 < M_I < 6.5$). Mass functions are sensitive to the mass-luminosity relation. Different approaches to deriving the 47 Tuc mass function from its LF lead to markedly different results at the low mass end. For $M > 0.4 M_\odot$, the 47 Tuc mass function is significantly different from that of \wcen. The calibrated HST $M_V(V-I)$ colour-magnitude diagrams (CMDs) show a trend with metallicity in the sense of metal richer systems having redder CMDs for a fixed absolute magnitude. The main-sequence becomes shallower with increasing metallicity. For metal-rich systems, the CMDs derived from HST are in general agreement with previous ground-based studies. However, the CMDs of metal poor subdwarfs observed from the ground are shallower than those of globular clusters observed with HST.

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arXiv1995

The Optical Redshift Survey II: Derivation of the Luminosity and Diameter Functions and of the Density Field

We quantify the effects of Galactic extinction on the derived luminosity and diameter functions and on the density field of a redshift sample. Galaxy magnitudes are more affected by extinction than are diameters, although the effect on the latter is more variable from galaxy to galaxy, making it more difficult to quantify. We develop a maximum-likelihood approach to correct the luminosity function, the diameter function and the density field for extinction effects. The effects of random and systematic photometric errors are also investigated. The derived density field is robust to both random and systematic magnitude errors as long as these are uncorrelated with position on the sky, since biases in the derived selection function and number counts tend to cancel one another. Extinction-corrected luminosity and diameter functions are derived for several subsamples of the {\it Optical Redshift Survey} (ORS). Extinction corrections for the diameter-limited subsamples are found to be unreliable, possibly due to the superposition of random and systematic errors. The ORS subsamples are combined using overall density scaling factors from a full-sky redshift survey of \iras\ galaxies, allowing the reconstruction of the optical galaxy density field over most of the sky to 8000 \kms.

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arXiv1995

The Globular Clusters in M87: A Bimodal Colour Distribution

We present $V$ and $I$ photometry for $\sim 150$ globular clusters with $20 < V < 27$ $\sim 2.5$ arcmin from the centre of M87, the cD galaxy in the Virgo cluster. The data were acquired with the Hubble Space Telescope which, with effective resolution 0.1 arcsec, allows us for the first time to distinguish between globular clusters and most background galaxies on the basis of morphology, and to obtain accurate photometry even at faint magnitudes. The $(V-I)$ distribution of the clusters is clearly bimodal, implying a corresponding bimodality in the metallicity distribution, with peaks at [Fe/H]$\approx -1.5$ and $-0.5$. We also find that the brightest clusters are predominantly metal poor, while the fainter clusters are divided roughly eaually between metal poor and metal rich. Our sample is essentially complete to $ V\approx 25$, a full magnitude past the expected peak of a ``universal" luminosity function, and the observed luminosity function is well represented by the standard gaussian distribution with $ =-7.2$ and $σ= 1.4$.

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arXiv1995

HST Star Counts at High Galactic Latitudes

We use star counts from 13 deep HST fields imaged with the {\it Wide Field Camera - 2} in order to constrain the amount of dark matter in the Galaxy that can be in the form of low-mass main sequence stars or white-dwarfs. Based on the number of red stars found in our fields, we exclude the possibility that more than 25\% of the massive dark halo is made up of M dwarfs or subdwarfs; fairly massive ($M \sim 0.2 M_\odot$) and yet extremely faint ($M_I$ \gtsima $14.5$) stellar candidates would have to be invoked in order to make the observed number of stars compatible with that predicted by a stellar dark halo. White dwarfs must also be intrinsically very faint ($M_I$ \gtsima $14$) in order to be consistent with the observed number of faint stars in the HST fields. We also rule out an increasing or flat stellar luminosity function beyond $M_V \sim 13$. The inferred slope of the disk luminosity function is intermediary between local, volume-limited surveys and ground-based photometric ones. Finally, the magnitude counts are well fitted by existing models for the structure of the Galaxy, with only small changes in the fiducial values of the model parameters. The colour distribution, however, is not well reproduced by the models. It is unclear at present if this reflects inadequacies of the available models or uncertainties in the colour-magnitude diagrams for low metallicity stars and in the photometric calibration.

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