Faint Star Clusters Research Articles

Magellan/M2FS and MMT/Hectochelle Spectroscopy of Dwarf Galaxies and Faint Star Clusters within the Galactic Halo* * This paper presents data gathered with the Magellan Telescopes at Las Campanas Observatory, Chile, and the MMT Observatory, a joint facility of the Smithsonian Institution, and the University of Arizona.

We present spectroscopic data for 16,369 stellar targets within and/or toward 38 dwarf spheroidal galaxies and faint star clusters within the Milky Way halo environment. All spectra come from observations with the multiobject, fiber-fed echelle spectrographs M2FS at the Magellan/Clay telescope or Hectochelle at the MMT, reaching a typical limiting magnitude G ≲ 21. Data products include processed spectra from all observations and catalogs listing estimates—derived from template model fitting—of line-of-sight velocity (median uncertainty 1.4 km s−1) effective temperature (255 K), (base-10 logarithm of) surface gravity (0.59 dex in cgs units), [Fe/H] (0.4 dex) and [Mg/Fe] (0.27 dex) abundance ratios. The sample contains multiepoch measurements for 3720 sources, with up to 15 epochs per source, enabling studies of intrinsic spectroscopic variability. The sample contains 6087 likely red giant stars (based on surface gravity), and 4492 likely members (based on line-of-sight velocity and Gaia-measured proper motion) of the target systems. The number of member stars per individual target system ranges from a few, for the faintest systems, to ∼850 for the most luminous. For most systems, our new samples extend over wider fields than have previously been observed; of the likely members in our samples, 820 lie beyond 2 times the projected half-light radius of their host system, and 42 lie beyond 5 R half.

First evidence of a stripped star cluster from the Small Magellanic Cloud

ABSTRACT We present results on the recently discovered stellar system YMCA-1, for which physical nature and belonging to any of the Magellanic System galaxies have been irresolutely analysed. We used SMASH and Gaia Early Data Release 3 data sets to conclude that we are dealing with a small star cluster. Its reddening-free, field star decontaminated colour–magnitude diagram was explored in order to obtain the cluster parameters. We found that YMCA-1 is a small (435 ${\rm \, M_\odot }$), moderately old (age = 9.6 Gyr), moderately metal-poor ([Fe/H] = −1.16 dex) star cluster, located at a nearly Small Magellanic Cloud (SMC) distance (60.9 kpc) from the Sun, at ∼17.1 kpc to the east from the Large Magellanic Cloud (LMC) centre. The derived cluster brightness and size would seem to suggest some resemblance to the recently discovered faint star clusters in the Milky Way (MW) outer halo, although it does not match their age–metallicity relationship, nor those of MW globular clusters formed in situ or ex situ, nor that of LMC clusters either, but is in agreement with that of SMC old star clusters. We performed numerical Monte Carlo simulations integrating its orbital motion backwards in the MW–LMC–SMC system with radially extended dark matter haloes that experience dynamical friction, and by exploring different radial velocity (RV) regimes for YMCA-1. For RVs ≳ 300 km s−1, the cluster remains bound to the LMC during the last 500 Myr. The detailed tracked kinematic of YMCA-1 suggests that it could have been stripped by the LMC from the SMC during any of the close interactions between both galaxies, a scenario previously predicted by numerical simulations.

Deep SOAR follow-up photometry of two Milky Way outer-halo companions discovered with Dark Energy Survey

We report the discovery of a new star cluster, DES 3, in the constellation of Indus, and deeper observations of the previously identified satellite DES J0222.7$-$5217 (Eridanus III). DES 3 was detected as a stellar overdensity in first-year Dark Energy Survey data, and confirmed with deeper photometry from the 4.1 metre Southern Astrophysical Research (SOAR) telescope. The new system was detected with a relatively high significance and appears in the DES images as a compact concentration of faint blue point sources. We determine that DES 3 is located at a heliocentric distance of $\sim 76\,\mathrm{kpc}$ and it is dominated by an old ($\simeq 9.8\,\mathrm{Gyr}$) and metal-poor ($\mathrm{[Fe/H]}\simeq -1.88$) population. While the age and metallicity values of DES 3 are similar to globular clusters, its half-light radius ($r_\mathrm{h}\sim 6.5\,\mathrm{pc}$) and luminosity ($M_V \sim -1.9$) are more indicative of faint star clusters. Based on the apparent angular size, DES 3, with a value of $r_\mathrm{h}\sim 0.\!^{\prime}3$, is among the smallest faint star clusters known to date. Furthermore, using deeper imaging of DES J0222.7$-$5217 taken with the SOAR telescope, we update structural parameters and perform the first isochrone modeling. Our analysis yields the first age ($\simeq 12.6\,\mathrm{Gyr}$) and metallicity ($\mathrm{[Fe/H]}\simeq -2.01$) estimates for this object. The half-light radius ($r_\mathrm{h}\sim 10.5\,\mathrm{pc}$) and luminosity ($M_V\sim -2.7$) of DES J0222.7$-$5217 suggest that it is likely a faint star cluster. The discovery of DES 3 indicates that the census of stellar systems in the Milky Way is still far from complete, and demonstrates the power of modern wide-field imaging surveys to improve our knowledge of the Galaxy's satellite population.

KIM 3: AN ULTRA-FAINT STAR CLUSTER IN THE CONSTELLATION OF CENTAURUS

ABSTRACT We report the discovery of an ultra-faint star cluster in the constellation of Centaurus. This new stellar system, Kim 3, features a half-light radius of r h = 2.29 − 0.52 + 1.28 ?> pc and a total luminosity of M V = +0.7 ± 0.3. Approximately 26 stars are identified as candidate member stars down to four magnitudes below the main-sequence turn-off, which makes Kim 3 the least luminous star cluster known to date. The compact physical size and extreme low luminosity place it close to faint star clusters in the size–luminosity plane. The stellar population of Kim 3 appears to be relatively young ( 9.5 − 1.7 + 3.0 ?> Gyr) and metal-poor ([Fe/H] = − 1.6 − 0.30 + 0.45 ?> ) at a heliocentric distance of 15.14 − 0.28 + 1.00 ?> kpc. The cluster lacks a well-defined center, and a small but prominent group of stars consistent with the Kim 3 isochrone is present approximately 9.7 pc in projection south of the cluster center. Both are signs of the cluster being in the final stage of tidal disruption.

A NEW MILKY WAY HALO STAR CLUSTER IN THE SOUTHERN GALACTIC SKY

We report on the discovery of a new Milky Way companion stellar system located at (RA, Dec) = (22h10m43.15s, +14:56:58.8). The discovery was made using the eighth data release of SDSS after applying an automated method to search for overdensities in the Baryon Oscillation Spectroscopic Survey footprint. Follow-up observations were performed using CFHT-MegaCam, which reveal that this system is comprised of an old stellar population, located at a distance of 31.9+1.0-1.6 kpc, with a half-light radius of r_h = 7.24+1.94-1.29 pc and a concentration parameter of c = 1.55. A systematic isochrone fit to its color-magnitude diagram resulted in log(age) = 10.07+0.05-0.03 and [Fe/H] = -1.58+0.08-0.13 . These quantities are typical of globular clusters in the MW halo. The newly found object is of low stellar mass, whose observed excess relative to the background is caused by 96 +/- 3 stars. The direct integration of its background decontaminated luminosity function leads to an absolute magnitude of MV = -1.21 +/- 0.66. The resulting surface brightness is uV = 25.9 mag/arcsec2 . Its position in the M_V vs. r_h diagram lies close to AM4 and Koposov 1, which are identified as star clusters. The object is most likely a very faint star cluster - one of the faintest and lowest mass systems yet identified.

The Stellar Content of Nearby Star‐forming Galaxies. III. Unravelling the Nature of the Diffuse Ultraviolet Light

We investigate the nature of the diffuse intracluster ultraviolet light seen in 12 local starburst galaxies, using long-slit ultraviolet spectroscopy obtained with the Space Telescope Imaging Spectrograph (STIS) aboard the Hubble Space Telescope (HST). We take this faint intracluster light to be the field in each galaxy and compare its spectroscopic signature with Starburst99 evolutionary synthesis models and with neighboring star clusters. Our main result is that the diffuse ultraviolet light in 11 of the 12 starbursts lacks the strong O star wind features that are clearly visible in spectra of luminous clusters in the same galaxies. The difference in stellar features dominating cluster and field spectra indicates that the field light comes primarily from a different stellar population and not from scattering of UV photons originating in the massive clusters. A cut along the spatial direction of the UV spectra establishes that the field light is not smooth but rather shows numerous bumps and wiggles. Roughly 30%-60% of these faint peaks seen in field regions of the closest (<4 Mpc) starbursts appear to be resolved, suggesting a contribution from superpositions of stars and/or faint star clusters. Complementary WFPC2 UVI imaging for the three nearest target galaxies, NGC 4214, NGC 4449, and NGC 5253, is used to obtain a broader picture and establish that all three galaxies have a dispersed population of unresolved, luminous blue sources. Because the field spectra are dominated by B stars, we suggest that the individual sources observed in the WFPC2 images are individual B stars (rather than O stars) or small star clusters. We consider several scenarios to understand the lack of observed massive stars in the field and their implications for the origin of the field stellar population. If the field stellar populations formed in situ, the field must have either an IMF that is steeper than Salpeter (α ~ -3.0 to -3.5) or a Salpeter slope with an upper mass cutoff of 30-50 M☉. If star formation occurs primarily in star clusters, the field could be composed of older, faded clusters and/or a population that is coeval with the luminous clusters but lower in mass. We use these benchmark populations to place constraints on the field stellar population origin. Although the field probably includes stars of different ages, the UV light is dominated by the youngest stellar populations in the field. If the field is composed of older, dissolving clusters, we estimate that star clusters (regardless of mass) need to dissolve on timescales 7-10 Myr to create the field. If the field is composed of young clusters that fall below the detection limit of individual sources in our spectroscopy, they would have to be several hundred solar masses or less, in order to be deficient in O stars, despite their extreme youth.

AM-3: An Intermediate-Age Star Cluster in the Extreme Outskirts of the SMC

The faint star cluster AM-3 is one of three such objects discovered by Madore &amp; Arp (1979). AM-1 has since been shown to be one of the most distant of the Galaxy's halo globular clusters, while AM-2 is now recognized as a distant, reddened Galactic open cluster. AM-3, however, has been largely ignored despite the fact that it lies only ~4.5 degrees from the center of the SMC making association with the SMC a possibility. This possibility was strengthened by the results of Irwin (1990) who showed that AM-3 lies (in projection) just within the outermost density contours of the SMC field population.

E2 - an intermediate-age LMC cluster

view Abstract Citations (6) References (35) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS E2: an intermediate-age LMC cluster. Schommer, Robert A. ; Olszewski, Edward W. ; Aaronson, Marc Abstract The authors have measured a color-magnitude diagram for the faint star cluster E2, located near the tip of the Magellanic Stream. The main-sequence turnoff, a few giants, and a giant clump can be discerned. Comparisons with VandenBerg models show E2 to be a 1.5 Gyr old cluster with abundance Z ≡ 0.01 at the distance of the LMC, and thus similar to the more populous intermediate-age LMC clusters. The existence of cluster formation at a large distance from the LMC center (E2 is at almost 8° radius) is discussed in the context of the star-formation history, structure, and kinematics of the outer "halo" of the Large Magellanic Cloud. Publication: The Astronomical Journal Pub Date: December 1986 DOI: 10.1086/114265 Bibcode: 1986AJ.....92.1334S Keywords: Color-Magnitude Diagram; Magellanic Clouds; Star Clusters; Star Formation; Stellar Spectrophotometry; Data Reduction; Giant Stars; Main Sequence Stars; Stellar Models; Astronomy; C-M Diagrams:Star Clusters; Magellanic Clouds:Star Clusters; Star Clusters:Ages; Star Clusters:C-M Diagrams; Star Clusters:Magellanic Clouds full text sources ADS | data products SIMBAD (7) NED (1)

A 4-m census of faint star clusters in the Large Magellanic Cloud

view Abstract Citations (16) References (14) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A 4-m census of faint star clusters in the Large Magellanic Cloud. Hodge, P. W. Abstract A survey of five centers of the Large Magellanic Cloud (LMC) with the CTIO 4-m telescope has tripled the number of known clusters as compared with catalogued clusters (Hodge and Sexton, 1966), the total population of detectable clusters being now equal to 4800. With allowance for clusters undetectable because of absorption and severe crowding in certain areas of the LMC the previous prediction of 6500 clusters remains a very reasonable value. The average size of the new clusters has been found to be pc, the smallest being equal to 1.5 pc, whereas the limiting magnitude of star brightness of each center averaged 22.5. The size distribution of the open clusters in the LMC and our Galaxy has been compared and the differences in distribution were interpreted as due to the weak gravitation tidal force of the LMC. Publication: The Astronomical Journal Pub Date: April 1980 DOI: 10.1086/112691 Bibcode: 1980AJ.....85..423H Keywords: Magellanic Clouds; Star Clusters; Star Distribution; Astronomical Catalogs; Astronomical Photography; Milky Way Galaxy; Open Clusters; Stellar Magnitude; Astronomy full text sources ADS | data products SIMBAD (1) NED (1)

Three new faint star clusters

view Abstract Citations (31) References (12) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Three new faint star clusters. Madore, B. F. ; Arp, H. C. Abstract Descriptions and photographs are presented for three faint star clusters. The first is extremely distant and may constitute the first identification of a truly intergalactic globular cluster. The second, a low-galactic-latitude object, may also be a globular cluster or a low-mass dwarf member galaxy of the Local Group. Finally, the third object is a far outlying member of the Small Magellanic Cloud, probably an open cluster. Publication: The Astrophysical Journal Pub Date: January 1979 DOI: 10.1086/182876 Bibcode: 1979ApJ...227L.103M Keywords: Globular Clusters; Star Clusters; Astronomical Photography; Intergalactic Media; Magellanic Clouds; Open Clusters; Astronomy; Magellanic Clouds:Open Clusters full text sources ADS | data products SIMBAD (3) NED (3)

NOTES ON FAINT STAR CLUSTERS

Faint star clusters, mostly discovered on Palomar Sky Survey plates, have been reported by several writers.1·2»3 RRLyrae variables have been found in four,4·5»6·12 namely, Abell 3, 5, 12, and 13, and two long-period variables (P > 130a) have been found in Abell 4.4>6 The variable-star content, high galactic latitudes, and (for Abell 3 and 4) the color-magnitude diagrams4 suggest that many of these objects are globular clusters. A study of them might reveal possible differences between them and the globular clusters nearer the galactic center. These notes give preliminary results of a variable-star search in some of these clusters, mainly with the Lick 120-inch reflector. Cluster profiles derived from star counts are also given.