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Register a new userNew infrared star clusters in the Northern and Equatorial Milky Way with 2MASS
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Added by
Carlos Maximiliano Dutra
Publication date
2003
fields
Physics
and research's language is
English
Authors
E. Bica
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We carried out a survey of infrared star clusters and stellar groups on the 2MASS J, H and K_s all-sky release Atlas in the Northern and Equatorial Milky Way (350 < l < 360, 0 < l < 230). The search in this zone complements that in the Southern Milky Way (Dutra et al. 2003a). The method concentrates efforts on the directions of known optical and radio nebulae. The present study provides 167 new infrared clusters, stellar groups and candidates. Combining the two studies for the whole Milky Way, 346 infrared clusters, stellar groups and candidates were discovered, whereas 315 objects were previously known. They constitute an important new sample for future detailed studies.
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We carried out a 2MASS J, H and K_s survey of infrared star clusters in the Milky Way sector 230$^{circ}$ $< ell <$ 350$^{circ}$. This zone was the least studied in the literature, previously including only 12 infrared clusters or stellar groups with $|b|<$ 10$^{circ}$, according to the recent catalogue by Bica et al. (2003). We concentrated efforts on embedded clusters, which are those expected in the areas of known radio and optical nebulae. The present study provides 179 new infrared clusters and stellar groups, which are interesting targets for detailed future infrared studies. The sample of catalogued infrared clusters and stellar groups in the Galaxy is now increased by 63%.
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 $10^6$ $M_{odot}$.
AIMS: In their 1st extension to the Milky Way Star Clusters (MWSC) survey, Schmeja et al. applied photometric filters to the 2MASS to find new cluster candidates that were subsequently confirmed or rejected by the MWSC pipeline. To further extend the MWSC census, we aimed at discovering new clusters by conducting an almost global search in proper motion catalogues as a starting point. METHODS: We first selected high-quality samples from the PPMXL and UCAC4 for comparison and verification of the proper motions. For 441 circular proper motion bins (radius 15 mas/yr) within $pm$50 mas/yr, the sky outside a thin Galactic plane zone ($|b|$$<$5$^{circ}$) was binned in small areas (sky pixels) of 0.25$times$0.25 deg$^2$. Sky pixels with enhanced numbers of stars with a certain common proper motion in both catalogues were considered as cluster candidates. After visual inspection of the sky images, we built an automated procedure that combined these representations of the sky for neighbouring proper motion subsamples after a background correction. RESULTS: About half of our 692 candidates overlapped with known clusters (46 globular and 68 open clusters in the Galaxy, about 150 known clusters of galaxies) or the Magellanic Clouds. About 10% of our candidates turned out to be 63 new open clusters confirmed by the MWSC pipeline. They occupy predominantly the two inner Galactic quadrants and have apparent sizes and numbers of high-probable members slightly larger than those of the typically small MWSC clusters, whereas their other parameters (ages, distances, tidal radii) fall in the typical ranges. As our search aimed at finding compact clusters, we did not find new very nearby (extended) clusters. (abridged)
An earlier analysis of the Milky Way Star Cluster (MWSC) catalogue revealed an apparent lack of old (> 1 Gyr) open clusters in the solar neighbourhood (< 1 kpc). To fill this gap we undertook a search for hitherto unknown star clusters, assuming that the missing old clusters reside at high Galactic latitudes |b|> 20{deg}. We were looking for stellar density enhancements using a star count algorithm on the 2MASS point source catalogue. To increase the contrast between potential clusters and the field, we applied filters in colour-magnitude space according to typical colour-magnitude diagrams of nearby old open clusters. The subsequent comparison with lists of known objects allowed us to select thus far unknown cluster candidates. For verification they were processed with the standard pipeline used within the MWSC survey for computing cluster membership probabilities and for determining structural, kinematic, and astrophysical parameters. In total we discovered 782 density enhancements, 522 of which were classified as real objects. Among them 139 are new open clusters with ages 8.3 < log (t [yr]) < 9.7, distances d < 3 kpc, and distances from the Galactic plane 0.3 < Z < 1 kpc. This new sample has increased the total number of known high latitude open clusters by about 150%. Nevertheless, we still observe a lack of older nearby clusters up to 1 kpc from the Sun. This volume is expected to still contain about 60 unknown clusters that probably escaped our detection algorithm, which fails to detect sparse overdensities with large angular size.
Gravitational wave emission by coalescing black holes (BHs) kicks the remnant BH with a typical velocity of hundreds of km/s. This velocity is sufficiently large to remove the remnant BH from a low-mass galaxy but is below the escape velocity from the Milky Way (MW) galaxy. If central BHs were common in the galactic building blocks that merged to make the MW, then numerous BHs that were kicked out of low-mass galaxies should be freely floating in the MW halo today. We use a large statistical sample of possible merger tree histories for the MW to estimate the expected number of recoiled BH remnants present in the MW halo today. We find that hundreds of BHs should remain bound to the MW halo after leaving their parent low-mass galaxies. Each BH carries a compact cluster of old stars that populated the core of its original host galaxy. Using the time-dependent Fokker-Planck equation, we find that present-day clusters are ~< 1 pc in size, and their central bright regions should be unresolved in most existing sky surveys. These compact systems are distinguishable from globular clusters by their internal (Keplerian) velocity dispersion greater than one hundred km/s and their high mass-to-light ratio owing to the central BH. An observational discovery of this relic population of star clusters in the MW halo, would constrain the formation history of the MW and the dynamics of BH mergers in the early Universe. A similar population should exist around other galaxies, and may potentially be detectable in M31 and M33.
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