Distances, Ages, and Epoch of Formation of Globular Clusters

Abstract

We review the results on distances and absolute ages of Galactic globular clusters (GCs) obtained after the release of the Hipparcos catalog. Several methods aimed at the deÐnition of the Population II local distance scale are discussed, and their results compared, exploiting new results for RR Lyraes in the Large Magellanic Cloud (LMC). We Ðnd that the so-called short distance and long distance scales may be reconciled whether or not a consistent reddening scale is adopted for Cepheids and RR Lyrae variables in the LMC. Emphasis is given in the paper to the discussion of distances and ages of GCs derived using Hipparcos parallaxes of local subdwarfs. We Ðnd that the selection criteria adopted to choose the local subdwarfs, as well as the size of the corrections applied to existing systematic biases, are the main culprit for the di †erences found among the various independent studies that Ðrst used Hipparcos parallaxes and the subdwarf Ðtting technique. We also caution that the absolute age of M92 (usually considered one of the oldest clusters) still remains uncertain due to the lack of subdwarfs of comparable metallicity with accurate parallaxes. Distances and ages for the nine clusters discussed in a previous paper by Gratton et al. are rederived using an enlarged sample of local subdwarfs, which includes about 90% of the metal-poor dwarfs with accurate parallaxes (*n/n ¹ 0.12) in the whole Hipparcos catalog. On average, our revised distance moduli are decreased by 0.04 mag with respect to Gratton et al. The corresponding age of the GCs is t \ 11.5^2.6 Gyr, where the error bars refer to the 95% conÐdence range. The relation between the zero-age horizontal branch (ZAHB) absolute magnitude and metallicity for the nine program clusters turns out to be Thanks.12) to Hipparcos the major contribution to the total error budget associated with the subdwarf Ðtting technique has been moved from parallaxes to photometric calibrations, reddening, and metallicity scale. This total uncertainty still amounts to about^0.12 mag.We then compare the corresponding (true) LMC distance modulus mag with k LMC \ 18.64^0.12 other existing determinations. We conclude that at present the best estimate for the distance of the LMC is suggesting that distances from the subdwarf Ðtting method are D1 p too k LMC \ 18.54^0.03^0.06, long. Consequently, our best estimate for the age of the GCs is revised to Age \ 12.9^2.9 Gyr (95% conÐdence range). The best relation between ZAHB absolute magnitude and metallicity is M V (ZAHB) \ (0.18^0.09)([Fe/H] ] 1.5) ] (0.63^0.07). Finally, we compare the ages of the GCs with the cosmic star formation rate recently determined by studies of the Hubble Deep Field (HDF), exploiting the determinations of and provided by Type Ia supernovae surveys. We Ðnd that the epoch ) M \ 0.3 ) " \ 0.7 of formation of the GCs (at z D 3) matches well the maximum of the star formation rate for elliptical galaxies in the HDF as determined by Franceschini et al.

Observational Constraints with Recent Data on the DGP Modified Gravity

“…Finally we do the consistency test, comparing the age of universe derived from this model with the age of old stars and Old High Redshift Galaxies (OHRG) in various redshifts. [84]. Here we take three OHRG for comparison with the DGP model, namely the LBDS 53W091, a 3.5-Gyr old radio galaxy at z = 1.55 [87], the LBDS 53W069 a 4.0-Gyr old radio galaxy at z = 1.43 [88] and a quasar, APM 08279 + 5255 at z = 3.91 with an age of t = 2.1 +0.9 −0.1 Gyr [89].…”

Observations of globular clusters with FLAMES

“…Gratton et al 2003). Furthermore, within the framework of a standard ΛCDM model, where the age of the Universe is accurately fixed at 13.7 ± 0.2 Gyr by the WMAP results (Spergel et al 2003), the age of GCs can be used to constrain the epoch of formation of the Galaxy, linking the local Universe to the distant one (see Carretta et al 2000;and Gratton et al 2003). Relative ages are fundamental to describe the early history of our Galaxy.…”

Redshift-Independent Distances in the Nasa/Ipac Extragalactic Database: Methodology, Content, and Use of Ned-D

“…Applied to the LMC by Carretta et al (2000), to obtain a distance modulus of (m-M)=18.64, for a linear distance of 53.5 kpc, with a statistical error of 0.12 mag or 3.0 kpc (6%).…”