Toward the General Red Giant Branch Slope-Metallicity-Age Calibration. I. Metallicities, Ages, and Kinematics for Eight Large Magellanic Cloud Clusters (original) (raw)
Related papers
Astronomy & Astrophysics, 2013
Aims. To enlarge our growing sample of well-studied star clusters in the Large Magellanic Cloud (LMC), we present CCD Washington CT 1 photometry to T 1 ∼ 23 in the fields of twenty-three mostly unstudied clusters located in the inner disc and outer regions of the LMC. Methods. We estimated cluster radii from star counts. Using the cluster Washington (T 1 , C − T 1) colour-magnitude diagrams, statistically cleaned from field star contamination, we derived cluster ages and metallicities from a comparison with theoretical isochrones of the Padova group. Whenever possible, we also derived ages using δT 1-the magnitude difference between the red giant clump and the main sequence turn off-and estimated metallicities from the standard giant branch procedure. We enlarged our sample by adding clusters with published ages and metallicities determined on a similar scale by applying the same methods. We examined relationships between their positions in the LMC, ages and metallicities. Results. We find that the two methods for age and metallicity determination agree well with each other. Fourteen clusters are found to be intermediate-age clusters (1-2 Gyr), with [Fe/H] values ranging from-0.4 to-0.7. The remaining nine clusters turn out to be younger than 1 Gyr, with metallicities between 0.0 and-0.4. Conclusions. Our 23 clusters represent an increase of ∼ 30% in the current total amount number of well-studied LMC clusters using Washington photometry. In agreement with previous studies, we find no evidence for a metallicity gradient. We also find that the younger clusters were formed closer to the LMC centre than the older ones.
Fundamental parameters of the LMC clusters NGC 1836, NGC 1860, NGC 1865, SL 444, LW 224 and SL 548
Monthly Notices of the Royal Astronomical Society, 2003
Complementing our recent Washington photometric studies on intermediate age and young Large Magellanic Cloud (LMC) clusters, we now turn our attention to six previously unstudied star clusters in the transition range 200-700 Myr. We study NGC 1836, 1860 and 1865, which are projected on the LMC bar; SL 444, also located in the central disc but outside the bar; and LW 224 and SL 548, both located in the outer disc. We derive ages and metallicities from extracted T 1 versus C-T 1 colour-magnitude diagrams (CMDs), using theoretical isochrones recently computed for the Washington photometric system. For the metallicity determinations, these CMDs are particularly sensitive. We also estimate ages and metallicities of the surrounding fields of NGC 1860 and 1865 by employing the δT 1 index defined in Geisler et al. (1997, AJ, 114, 1920) and theoretical isochrones. By adding the present cluster sample to those of our previous studies, we now gather 37 LMC clusters with homogeneous parameter determinations, which are employed to probe the chemical enrichment of the LMC and its spatial distribution. On average, inner disc clusters turned out to be not only younger than the outer ones, but also more metal-rich; some have solar metal content. Furthermore, inner clusters located to the west of the LMC centre are younger and more metal-rich than their eastern counterparts. We propose that a bursting formation mechanism, with an important formation event centred at ∼2.0 Gyr, provides a better description of the cluster age-metallicity relation than a closed-box chemical evolution model. In the outer disc, the field star formation seems to have lasted until 2 Gyr ago while it continued in the inner disc for almost 1 Gyr longer.
Astronomical Journal, 2011
The outer disk of the Large Magellanic Cloud (LMC) is studied in order to unveil clues about its formation and evolution. Complementing our previous studies in innermost fields (3 kpc lsim R <~ 7 kpc), we obtained deep color-magnitude diagrams in six fields with galactocentric distances from 5.2 kpc to 9.2 kpc and different azimuths. The comparison with isochrones shows that while the oldest population is approximately coeval in all fields, the age of the youngest populations increases with increasing radius. This agrees with the results obtained in the innermost fields. Low-resolution spectroscopy in the infrared Ca II triplet region has been obtained for about 150 stars near the tip of the red giant branch in the same fields. Radial velocities and stellar metallicities have been obtained from these spectra. The metallicity distribution of each field has been analyzed together with those previously studied. The metal content of the most metal-poor objects, which are also the oldest according to the derived age-metallicity relationships, is similar in all fields independently of the galactocentric distance. However, while the metallicity of the most metal-rich objects measured, which are the youngest ones, remains constant in the inner 6 kpc, it decreases with increasing radius from there on. The same is true for the mean metallicity. According to the derived age-metallicity relationships, which are consistent with being the same in all fields, this result may be interpreted as an outside-in formation scheme in opposition with the inside-out scenario predicted by ΛCDM cosmology for a galaxy like the LMC. The analysis of the radial velocities of our sample of giants shows that they follow a rotational cold disk kinematics. The velocity dispersion increases as metallicity decreases indicating that the most metal-poor/oldest objects are distributed in a thicker disk than the most metal-rich/youngest ones in agreement with the findings in other disks such as that of the Milky Way. They do not seem to be part of a hot halo, if one exists in the LMC. Based on observations collected at the European Southern Observatory, Chile, within the observing programs 074.B-0474 and 082.B-0900.
Age – metallicity relation in the Magellanic Clouds clusters
Astronomy & Astrophysics, 2013
Aims. We study small open star clusters, using Strömgren photometry to investigate a possible dependence between age and metallicity in the Magellanic Clouds (MCs). Our goals are to trace evidence of an age metallicity relation (AMR) and correlate it with the mutual interactions of the two MCs and to correlate the AMR with the spatial distribution of the clusters. In the Large Magellanic Cloud (LMC), the majority of the selected clusters are young (up to 1 Gyr), and we search for an AMR at this epoch, which has not been much studied. Methods. We report results for 15 LMC and 8 Small Magellanic Cloud (SMC) clusters, scattered all over the area of these galaxies, to cover a wide spatial distribution and metallicity range. The selected LMC clusters were observed with the 1.54 m Danish Telescope in Chile, using the Danish Faint Object Spectrograph and Camera (DFOSC) with a single 2k × 2k CCD. The SMC clusters were observed with the ESO 3.6 m Telescope, also in Chile, using the ESO Faint Object Spectrograph and Camera (EFOSC). The obtained frames were analysed with the conventional DAOPHOT and IRAF software. We used Strömgren filters in order to achieve reliable metallicities from photometry. Isochrone fitting was used to determine the ages and metallicities. Results. The AMR for the LMC displays a metallicity gradient, with higher metallicities for the younger ages. The AMR for LMC-SMC star clusters shows a possible jump in metallicity and a considerable increase at about 6 × 10 8 yr. It is possible that this is connected to the latest LMC-SMC interaction. The AMR for the LMC also displays a metallicity gradient with distance from the centre. The metallicities in SMC are lower, as expected for a metal-poor host galaxy.
The horizontal branch luminosity vs. metallicity in M 31 globular clusters
Astronomy & Astrophysics, 2012
Context. Thanks to the outstanding capabilites of the HS T , our current knowledge about the M31 globular clusters (GCs) is similar to our knowledge of the Milky Way GCs in the 1960s-1970s, which set the basis for studying the halo and galaxy formation using these objects as tracers, and established their importance in defining the cosmic distance scale. Aims. We intend to derive a new calibration of the M V (HB)-[Fe/H] relation by exploiting the large photometric database of old GCs in M31 in the HS T archive. Methods. We collected the BVI data for 48 old GCs in M31 and analysed them by applying the same methods and procedures to all objects. We obtained a set of homogeneous colour-magnitude diagrams (CMDs) that were best-fitted with the fiducial CMD ridge lines of selected Milky Way template GCs. Reddening, metallicity, Horizontal Branch (HB) luminosity and distance were determined self-consistently for each cluster.
2009
We present a new homogeneous set of metallicity estimates based on Lick indices for 245 old globular clusters of the M31 galaxy comprised in the Revised Bologna Catalog. The metallicity distribution of the M31 globular clusters is briefly discussed and compared with that of the Milky Way. Simple parametric statistics suggests that the [Fe/H] distribution is likely not unimodal. The strong correlation between metallicity and kinematics found in previous studies is confirmed. The most metal-rich GCs tend to be packed at the center of the system and share the galactic rotation as traced by the HI disk. Although the velocity dispersion around the curve increases with decreasing metallicity, also clusters with [Fe/H]<-1.0 display a clear rotational pattern, at odds with their Milky Way counterparts.
Metallicity and kinematics of a large sample of LMC and SMC clusters
Proceedings of The International Astronomical Union, 2009
We have carried out a large-scale investigation of the metallicity and kinematics for a number of LMC and SMC star clusters using Ca ii triplet spectra obtained at the VLT. Our sample includes 28 LMC and 16 SMC clusters, covering a wide range of ages and spatial extent of the host galaxy. We determine mean cluster velocities to about 2 km s-1 and metallicities to 0.05 dex (random error), from about 7 members per cluster. Herein we present the main results for this study for the cluster metallicity distributions, metallicity gradients, age-metallicity relations and kinematics.
Monthly Notices of …, 2001
Colour-magnitude diagrams are presented for the first time for L 32, L 38, K 28 (L 43), K 44 (L 68) and L 116, which are clusters projected onto the outer parts of the Small Magellanic Cloud (SMC). The photometry was carried out in the Washington system C and T 1 filters allowing the determination of ages by means of the magnitude difference between the red giant clump and the main sequence turnoff, and metallicities from the red giant branch locus. The clusters have ages in the range 2-6 Gyr, and metallicities between −1.65 <[Fe/H]< −1.10, increasing the sample of intermediateage clusters in the SMC. L 116, the outermost cluster projected onto the SMC, is a foreground cluster, and somewhat closer to us than the Large Magellanic Cloud. Our results, combined with those for other clusters in the literature, show epochs of sudden chemical enrichment in the age-metallicity plane, which favour a bursting star formation history as opposede to a continuous one for the SMC.