Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userGALEX Ultraviolet Photometry of Globular Clusters in M31
62
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
We present ultraviolet photometry for globular clusters (GCs) in M31 from 15 square deg of imaging using the Galaxy Evolution Explorer (GALEX). We detect 200 and 94 GCs with certainty in the near-ultraviolet (NUV; 1750 - 2750 Angstroms) and far-ultraviolet (FUV; 1350 - 1750 Angstroms) bandpasses, respectively. Our rate of detection is about 50% in the NUV and 23% in the FUV, to an approximate limiting V magnitude of 19. Out of six clusters with [Fe/H]>-1 seen in the NUV, none is detected in the FUV bandpass. Furthermore, we find no candidate metal-rich clusters with significant FUV flux, because of the contribution of blue horizontal-branch (HB) stars, such as NGC 6388 and NGC 6441, which are metal-rich Galactic GCs with hot HB stars. We show that our GALEX photometry follows the general color trends established in previous UV studies of GCs in M31 and the Galaxy. Comparing our data with Galactic GCs in the UV and with population synthesis models, we suggest that the age range of M31 and Galactic halo GCs are similar.
rate research
Read More
We present ultraviolet (UV) photometry of M31 globular clusters (GCs) found in 23 Galaxy Evolution Explorer (GALEX) images covering the entirety of M31. We detect 485 and 273 GCs (and GC candidates) in the near-ultraviolet (NUV; 2267 A) and far-ultraviolet (FUV; 1516 A), respectively. Comparing M31 data with those of Galactic GCs in the UV with the aid of population models, we find that the age ranges of old GCs in M31 and the Galactic halo are similar. Three metal-rich ([Fe/H]>-1) GCs in M31 produce significant FUV flux making their FUV-V colors unusually blue for their metallicities. These are thought to be analogs of the two peculiar Galactic GCs NGC 6388 and NGC 6441 with extended blue HB stars. Based on the models incorporating helium enriched subpopulations in addition to the majority of the population that have a normal helium abundance, we suggest that even small fraction of super-helium-rich subpopulations in GCs can reproduce the observed UV bright metal-rich GCs. Young clusters in M31 show distinct UV and optical properties from GCs in Milky Way. Population models indicate that their typical age is less than ~ 2 Gyrs. A large fraction of young GCs have the kinematics of the thin, rapidly rotating disk component. However, a subset of the old GCs also shares the thin-disk kinematics of the younger clusters. The existence of young GCs on the outskirts of M31 disk suggests the occurrence of a significant recent star formation in the thin-disk of M31. Old thin-disk GCs may set constraints on the epoch of early formation of the M31 thin-disk. We detect 12 (10) intermediate-age GC candidates in NUV (FUV). We suggest that some of spectroscopically identified intermediate-age GCs may not be truly intermediate in age, but rather older GCs that possess developed HB.
We present 2MASS $JHK_{rm s}$ photometry for 913 star clusters and candidates in the field of M31, which are selected from the latest Revised Bologna Catalog of M31 globular clusters (GCs) and candidates. The photometric measurements in this paper supplement this catalog, and provide a most comprehensive and homogeneous photometric catalog for M31 GCs in the $JHK_{rm s}$ bandpasses. In general, our photometry is consistent with previous measurements. The globular cluster luminosity function (GCLF) peaks for the confirmed GCs derived by fitting a $t_5$ distribution using maximum likelihood method are: $J_0 = 15.348_{-0.208}^{+0.206}$, $H_0 = 14.703_{-0.180}^{+0.176}$, and ${K_{rm s}}_0 = 14.534_{-0.146}^{+0.142}$, all of which agree well with previous studies. The GCLFs are different between metal-rich (MR) and metal-poor (MP), inner and outer subpopulations, as that MP clusters are fainter than their MR counterparts, and the inner clusters are brighter than the outer ones, which confirm previous results. The NIR colors of the GC candidates are on average redder than those of the confirmed GCs, which lead to an obscure bimodal distribution of the color indices. The relation of $(V-K_{rm s})_0$ and metallicity shows a notable departure from linearity, with a shallower slope towards the redder end. The color-magnitude diagram (CMD) and color-color diagram show that many GC candidates are located out of the evolutionary tracks, suggesting that some of them may be false M31 GC candidates. The CMD also shows that the initial mass function of M31 GCs covers a large range, and the majority of the clusters have initial masses between $10^3$ and $10^6$ $M_{odot}$.
We present a comprehensive catalog of 700 confirmed star clusters in the field of M31 compiled from three major existing catalogs. We detect 418 and 257 star clusters in Galaxy Evolution Explorer (GALEX) near-ultraviolet (NUV) and far-ultraviolet (FUV) imaging, respectively. Our final catalog includes photometry of star clusters in up to 16 passbands ranging from FUV to NIR as well as ancillary information such as reddening, metallicity, and radial velocities. In particular, this is the most extensive and updated catalog of UV integrated photometry for M31 star clusters. Ages and masses of star clusters are derived by fitting the multi-band photometry with model spectral energy distribution (SED); UV photometry enables more accurate age estimation of young clusters. Our catalog includes 182 young clusters with ages less than 1 Gyr. Our estimated ages and masses of young clusters are in good agreement with previously determined values in the literature. The mean age and mass of young clusters are about 300 Myr and 10^4 M_sun, respectively. We found that the compiled [Fe/H] values of young clusters included in our catalog are systematically lower (by more than 1 dex) than those from recent high-quality spectroscopic data and our SED fitting result. We confirm that most of the young clusters kinematics show systematic rotation around the minor axis and association with the thin disk of M31. The young clusters distribution exhibits a distinct peak in the M31 disk around 10 - 12 kpc from the center and follow a spatial distributions similar to other tracers of disk structure such as OB stars, UV star-forming regions, and dust. Some young clusters also show concentration around the ring splitting regions found in the southern part of the M31 disk and most of them have systematically younger (< 100 Myr) ages.
A new population of extended, luminous globular clusters has recently been discovered in the outskirts of M31. These objects have luminosities typical of classical globular clusters, but much larger half-light radii. We report the first results from deep ACS imaging of four such clusters, one of which is a newly-discovered example lying at a projected distance of ~60 kpc from M31. Our F606W, F814W colour-magnitude diagrams extend ~3 magnitudes below the horizontal branch level, and clearly demonstrate, for the first time, that all four clusters are composed of >10 Gyr old, metal-poor stellar populations. No evidence for multiple populations is observed. From a comparison with Galactic globular cluster fiducials we estimate metallicities in the range -2.2 < [Fe/H] < -1.8. The observed horizontal branch morphologies show a clear second parameter effect between the clusters. Preliminary radial luminosity profiles suggest integrated magnitudes in the range -6.6 < M_V < -7.7, near the median value of the globular cluster luminosity function. Our results confirm that these four objects are bona fide old, metal-poor globular clusters, albeit with combined structures and luminosities unlike those observed for any other globular clusters in the Local Group or beyond.
The large majority of extragalactic star cluster studies performed to date essentially use multi-colour photometry, combined with theoretical stellar synthesis models, to derive ages, masses, extinction estimates, and metallicities. M31 offers a unique laboratory for studies of globular cluster (GC) systems. In this paper, we obtain new age estimates for 91 M31 globular clusters, based on improved photometric data, updated theoretical stellar synthesis models and sophisticated new fitting methods. In particular, we used photometric measurements from the Two Micron All Sky Survey (2MASS), which, in combination with optical photometry, can partially break the well-known age-metallicity degeneracy operating at ages in excess of a few Gyr. We show robustly that previous age determinations based on photometric data were affected significantly by this age-metallicity degeneracy. Except for one cluster, the ages of our other sample GCs are all older than 1 Gyr. Their age distribution shows populations of young and intermediate-age GCs, peaking at $sim3$ and 8 Gyr respectively, as well as the usual complement of well-known old GCs, i.e., GCs of similar age as the majority of the Galactic GCs. Our results also show that although there is significant scatter in metallicity at any age, there is a noticeable lack of young metal-poor and old metal-rich GCs, which might be indicative of an underlying age-metallicity relationship among the M31 GC population.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
