The horizontal branch luminosity vs. metallicity in M 31 globular clusters (original) (raw)
Related papers
The Astronomical Journal, 1996
Color Magnitude Diagrams (CMDs) of individual stars in 8 Globular Clusters (GCs) in M31 down to about 1 mag fainter (V∼ 26.5) than the Horizontal Branch (HB) have been obtained with the Hubble Space Telescope-HST. In particular, we observed G280 and G351 with the FOC (f /96 + F 430W and f /96 + F 480LP) while the WFPC2 (F555W, F814W) frames for G1, G58, G105, G108, G219+Bo468 were retrieved from the HST archive. The cluster metallicities-[Fe/H]-range from-1.8 to-0.4. Coupled with sufficiently accurate (to ∼ ±0.1 mag) measures of the mean brightness of the HB-V HB-, appropriate estimates of reddening for each cluster, and the adoption of a distance modulus to M31 of (m-M) o = 24.43, this has allowed us to yield a direct calibration for the mean absolute magnitude of the HB at the instability strip-M HB V
HST/ACS colour–magnitude diagrams of M 31 globular clusters
Astronomy and Astrophysics, 2009
Aims. With the aim of increasing the sample of M31 clusters for which a colour-magnitude diagram is available, we searched the HST archive for ACS images containing objects included in the Revised Bologna Catalogue of M31 globular clusters ⋆⋆ . Methods. Sixty-three such objects were found. We used the ACS images to confirm or revise their classification and were able to obtain useful CMDs for 11 old globular clusters and 6 luminous young clusters. We obtained simultaneous estimates of the distance, reddening, and metallicity of old clusters by comparing their observed field-decontaminated CMDs with a grid of template clusters of the Milky Way. We estimated the age of the young clusters by fitting with theoretical isochrones. Results. For the old clusters, we found metallicities in the range −0.4 ≤[Fe/H]≤ −1.9. The individual estimates generally agree with existing spectroscopic estimates. At least four of them display a clear blue horizontal branch, indicating ages > ∼ 10 Gyr. All six candidate young clusters are found to have ages < 1 Gyr. The photometry of the clusters is made publicly available through a dedicated web page. Conclusions. With the present work the total number of M31 GCs with reliable optical CMD increases from 35 to 44 for the old clusters, and from 7 to 11 for the young ones. The old clusters show similar characteristics to those of the MW. We discuss the case of the cluster B407, with a metallicity [Fe/H]≃ −0.6 and located at a large projected distance from the centre of M31 (R p = 19.8 kpc) and from the major axis of the galaxy (Y= 11.3 kpc). Metal-rich globulars at large galactocentric distances are rare both in M31 and in the Milky Way. B407, in addition, has a velocity in stark contrast with the rotation pattern shared by the bulk of M31 clusters of similar metallicity. This, along with other empirical evidence, supports the hypothesis that the cluster (together with B403) is physically associated with a substructure in the halo of M31 that has been interpreted as the relic of a merging event.
The Astronomical Journal, 2010
In this paper, we discuss the properties of CMDs, age, metallicity and radial velocities of eight massive LMC clusters using data taken from FORS2 multiobject spectrograph at the 8.2-meter VLT/UT1. The strong near-infrared Ca II triplet (CaT) lines of RGB stars obtained from the high S/N spectra are used to determine the metallicity and radial velocity of cluster members. We report for the first time spectroscopically determined metallicity values for four clusters based on the mean [Fe/H] value of ∼10 cluster members each. We found two concentrations in the distribution of ages of the target clusters. Six have ages between 0.8-2.2 Gyr and the other two, NGC 1754 and NGC 1786, are very old. The metallicity of the six intermediate age clusters, with a mean age of 1.5 Gyr, is −0.49 with a scatter of only 0.04. This tight distribution suggests that a close encounter between the LMC and SMC may have caused not only the restart of cluster formation in the LMC but the generation of the central bar. The metallicity for the two old clusters is similar to that of the other old, metal-poor LMC clusters. We find that the LMC cluster system exhibits disk-like rotation with no clusters appearing to have halo kinematics and there is no evidence of a metallicity gradient in the LMC, in contrast with the stellar population of the MW and M33, where the metallicity decreases as galactocentric distance increases. The LMC's stellar bar may be the factor responsible for the dilution of any kind of gradient in the LMC.
The distance of M�33 and the stellar population in its outskirts
Astronomy and Astrophysics, 2004
We present deep V,I photometry of two 9.4 ′ × 9.4 ′ field in the outer regions of the M 33 galaxy. We obtain a robust detection of the luminosity of the Red Giant Branch Tip (I T RGB = 20.72 ± 0.08) from which we derived a new estimate of the distance modulus of M 33, (m − M) 0 = 24.64 ± 0.15, corresponding to a distance D = 847 ± 60 Kpc. By comparison of the color and magnitude of the observed Red Giant Branch stars with ridge lines of template globular clusters we obtained the photometric metallicity distribution of the considered fields in three different metallicity scales. The derived metallicity distributions are very similar over a range of distances from the galactic center 10 ′ ≤ R ≤ 33 ′ , and are characterized by a well defined peak at [M/H] ≃ −0.7 ([Fe/H] ≃ −1.0, in the Zinn & West scale) and a weak metal-poor tail reaching [M/H] < ∼ − 2.0. Our observations demonstrate that Red Giant Branch and Asymptotic Giant Branch stars have a radial distribution that is much more extended than the young MS stars associated with the star-forming disc.
RR Lyrae-based calibration of the Globular Cluster Luminosity Function
Monthly Notices of the Royal Astronomical Society, 2005
We test whether the peak absolute magnitude M V (TO) of the Globular Cluster Luminosity Function (GCLF) can be used for reliable extragalactic distance determination. Starting with the luminosity function of the Galactic Globular Clusters listed in Harris catalogue, we determine M V (TO) either using current calibrations of the absolute magnitude M V (RR) of RR Lyrae stars as a function of the cluster metal content [Fe/H] and adopting selected cluster samples. We show that the peak magnitude is slightly affected by the adopted M V (RR)-[Fe/H] relation, with the exception of that based on the revised Baade-Wesselink method, while it depends on the criteria to select the cluster sample. Moreover, grouping the Galactic Globular Clusters by metallicity, we find that the metal-poor (MP) ([Fe/H]< −1.0, [Fe/H] ∼ −1.6) sample shows peak magnitudes systematically brighter by about 0.36 mag than those of the metal-rich (MR) ([Fe/H]> −1.0, ([Fe/H] ∼ −0.6) one, in substantial agreement with the theoretical metallicity effect suggested by synthetic Globular Cluster populations with constant age and mass function. Moving outside the Milky Way, we show that the peak magnitude of the MP clusters in M31 appears to be consistent with that of Galactic clusters with similar metallicity, once the same M V (RR)-[Fe/H] relation is used for distance determination. As for the GCLFs in other external galaxies, using Surface Brightness Fluctuations (SBF) measurements we give evidence that the luminosity functions of the blue (MP) Globular Clusters peak at the same luminosity within ∼0.2 mag, whereas for the red (MR) samples the agreement is within ∼0.5 mag even accounting for the theoretical metallicity correction expected for clusters with similar ages and mass distributions. Then, using the SBF absolute magnitudes provided by a Cepheid distance scale calibrated on a fiducial distance to Large Magellanic Cloud (LMC), we show that the M V (TO) value of the MP clusters in external galaxies is in excellent agreement with the value of both Galactic and M31 ones, as inferred by an RR Lyrae distance scale referenced to the same LMC fiducial distance. Eventually, adopting µ 0 (LMC) = 18.50 mag, we derive that the luminosity function of MP clusters in the Milky Way, M31, and external galaxies peak at M V (TO) = −7.66 ± 0.11, − 7.65 ± 0.19 and −7.67 ± 0.23 mag, respectively. This would suggest a value of −7.66 ± 0.09 mag (weighted mean), with any modification of the LMC distance modulus producing a similar variation of the GCLF peak luminosity.
Research in Astronomy and Astrophysics, 2012
In our Paper I, we performed the spectroscopic observations of 11 confirmed GCs in M31 with the Xinglong 2.16m telescope and we mainly focus on the fits method and the metallicity gradient for the M31 GC sample. In this paper, we analyzed and discussed more about the dynamics, metallicity and age, and their distributions as well as the relationships between these parameters. In our work, eight more confirmed GCs in the halo of M31 were observed, most of which lack the spectroscopic information before. These star clusters are located far from the galactic center at a projected radius of ∼ 14 to ∼ 117 kpc, which are more spatially extended than that in the previous work. The Lick absorption-line indices and the radial velocities have been measured primarily. Then the ages, metallicities [Fe/H] and [α/Fe] have been fitted by comparing the observed spectral feature indices and the SSP model of Thomas et al. in the Cassisi and Padova stellar evolutionary tracks, respectively. Our results show that most of the star clusters of our sample are older than 10 Gyr except B290∼ 5.5 Gyr, and most of them are metal-poor with the metallicity [Fe/H] < −1, suggesting that these clusters were born at the early stage of the galaxy's formation. We find that the metallicity gradient for the outer halo clusters with r p > 25 kpc may not exist with a slope of −0.005 ± 0.005 dex kpc −1 and if the outliers G001 and H11 are excluded, the slope dose not change significantly with a value of −0.002 ± 0.003 dex kpc −1. We also find that the metallicity is not a function of age for the GCs with age < 7 Gyr while for the old GCs with age > 7 Gyr there seems to be a trend that the older ones have lower metallicity. Besides, We plot metallicity distributions with the largest sample of M31 GCs so far and it shows the bimodality is not significant and the number of the metal-poor and metal-rich groups becomes comparable. The spatial distributions shows that the metal-rich group is more centrally concentrated while the metal-poor group is occupy a more extended halo and the young population is centrally concentrated while the old populaiton is more extended spatially to the outer halo.
Full spectral fitting of Milky Way and M 31 globular clusters: ages and metallicities
Astronomy & Astrophysics, 2012
Context. The formation and evolution of disk galaxies are long standing questions in astronomy. Understanding the properties of globular cluster systems can lead to important insights on the evolution of its host galaxy. Aims. We aim to obtain the stellar population parameters-age and metallicity-of a sample of M 31 and Galactic globular clusters. Studying their globular cluster systems is an important step towards understanding their formation and evolution in a complete way. Methods. Our analysis employs a modern pixel-to-pixel spectral fitting technique to fit observed integrated spectra to updated stellar population models. By comparing observations to models we obtain the ages and metallicities of their stellar populations. We apply this technique to a sample of 38 globular clusters in M 31 and to 41 Galactic globular clusters, used as a control sample. Results. Our sample of M 31 globular clusters spans ages from 150 Myr to the age of the Universe. Metallicities [Fe/H] range from-2.2 dex to the solar value. The age-metallicity relation obtained can be described as having two components: an old population with a flat age-[Fe/H] relation, possibly associated with the halo and/or bulge, and a second one with a roughly linear relation between age and metallicity, higher metallicities corresponding to younger ages, possibly associated with the M 31 disk. While we recover the very well known Galactic GC metallicity bimodality, our own analysis of M 31's metallicity distribution function (MDF) suggests that both GC systems cover basically the same [Fe/H] range yet M 31's MDF is not clearly bimodal. These results suggest that both galaxies experienced different star formation and accretion histories.
The Distance to the M31 Globular Cluster System
The Astronomical Journal, 1998
The distance to the centroid of the M31 globular cluster system is determined by fitting theoretical isochrones to the observed red-giant branches of fourteen globular clusters in M31. The mean true distance modulus of the M31 globular clusters is found to be µ 0 = 24.47 ± 0.07 mag. This is consistent with distance moduli for M31 that have been obtained using other distance indicators.
CCD Photometry of the Globular Cluster M5. I. The Color-Magnitude Diagram and Luminosity Functions
The Astrophysical Journal, 1996
We present new BV I photometry for the halo globular cluster M5 (NGC 5904 = C1516+022), and examine the Band I-band luminosity functions (LFs), based on over 20,000 stars-one of the largest samples ever gathered for a cluster luminosity function. Extensive artificial star tests have been conducted to quantify incompleteness as a function of magnitude and cluster radius. We do not see evidence in the LF of a "subgiant excess" or of a discrepancy in the relative numbers of stars on the red-giant branch and main sequence, both of which have been claimed in more metal-poor clusters. Enhancements of α-element have been taken into account in our analysis. This improves the agreement between the observed and predicted positions of the "red-giant bump". Depending on the average α-element enhancement among globular clusters and the distance calibration, the observed discrepancy between the theoretical and observed position for a large number of clusters (Fusi Pecci et al. 1990) can be almost completely removed. The helium abundance of M5, as determined by the population ratio R, is found to be Y = 0.19 ± 0.02. However, there is no other indication that the helium abundance is different from other clusters of similar metallicity, and values calculated for other helium indicators are consistent with Y ≈ 0.23. The relative ages of M5, Palomar 5, M4, NGC 288, NGC 362, NGC 1261, NGC 1851 and NGC 2808 are derived via the ∆V HB T O method using M5's horizontal branch (HB) as a bridge to compare clusters with very different HB morphology. We conclude that at the level of ∼ 1.5 Gyr these clusters of comparable metallicity are the same age with the possible exception of NGC 288 (older by 3.5 ± 1.5 if the reddest NGC 288 HB stars are on the zero-age horizontal branch) and Palomar 5 (which may be marginally younger). Even with NGC 288 set aside, there is a large range in HB morphology between the remaining clusters which appears to eliminate age as the sole second parameter determining HB morphology in the case of constant mass loss between RGB and HB (although a Reimers' mass-loss relation weakens this statement considerably). We are unable to chose between the two competing values for M5's (absolute) metallicity: [Fe/H] = −1.40 (Zinn & West 1984) and −1.17 (Sneden et al. 1992) based on recent high-dispersion spectroscopy. This level of discrepancy has a signifcant effect on the derivation of the distance modulus and absolute age of M5. From subdwarf fitting to the main sequence of the cluster, we find an apparent distance modulus (m − M) V = 14.41 ± 0.