Low-luminosity Globular Cluster Research Articles

RR Lyrae Stars Belonging to the Candidate Globular Cluster Patchick 99

Patchick 99 is a candidate globular cluster located in the direction of the Galactic bulge, with a proper motion almost identical to the field and extreme field star contamination. A recent analysis suggests it is a low-luminosity globular cluster with a population of RR Lyrae stars. We present new spectra of stars in and around Patchick 99, targeting specifically the three RR Lyrae stars associated with the cluster as well as the other RR Lyrae stars in the field. A sample of 53 giant stars selected from proper motions and a position on the color–magnitude diagram are also observed. The three RR Lyrae stars associated with the cluster have similar radial velocities and distances, and two of the targeted giants also have radial velocities in this velocity regime and [Fe/H] metallicities that are slightly more metal-poor than the field. Therefore, if Patchick 99 is a bona fide globular cluster, it would have a radial velocity of −92 ± 10 km s−1, a distance of 6.7 ± 0.4 kpc (as determined from the RR Lyrae stars), and an orbit that confines it to the inner bulge.

A new low-luminosity globular cluster discovered in the Milky Way with the VVVX survey

Context. The VISTA Variables in the Via Láctea Extended Survey (VVVX) allows us to probe previously unexplored regions of the inner Milky Way (MW), especially those that are affected by stellar crowding and strong extinction. Aims. Our long-term goal is to identify new star clusters and investigate them to reveal their true nature. In particular, we are looking for new candidate globular clusters (GCs) located in the Galactic bulge and disk, with the aim of completing the census of the MW GC system. Methods. We searched and characterised new GCs, using a combination of the near-infrared (IR) VVVX survey and Two Micron All Sky survey (2MASS) datasets, and the optical Gaia Early Data Release 3 (EDR3) photometry and its precise proper motions (PMs). Results. We report the discovery of a new Galactic GC, named Garro 02, situated at RA = 18:05:51.1, Dec = −17:42:02 and l = 12.°042, b = +1.°656. Performing a PM-decontamination procedure, we built a final catalogue with all cluster members, on which we performed a photometric analysis. We calculated a reddening of E(J − Ks) = 1.07 ± 0.06 mag and extinction of AKs = 0.79 ± 0.04 mag in the near-IR; while E(BP − RP) = 2.40 ± 0.01 mag and AG = 4.80 ± 0.02 mag in optical passbands. Its heliocentric distance is D = 5.6 ± 0.8 kpc, which places Garro 02 at a Galactocentric distance of RG = 2.9 kpc and Z = 0.006 kpc above the Galactic plane. We also estimated the metallicity and age by comparison with known GCs and by fitting PARSEC isochrones, finding [Fe/H] = −1.30 ± 0.2 dex and age = 12 ± 2 Gyr. We derived the mean cluster PM of (µα*,µδ) = (−6.07 ± 0.62, −6.15 ± 0.75) mas yr−1. We calculated the cluster luminosity in the near-IR of MKs = −7.52 ± 1.23 mag, which is equivalent to MV = −5.44 mag. The core and tidal radii from the radial density profile are rc = 1.25 ± 0.27 arcmin (2.07 pc) and rt = 7.13 ± 3.83 arcmin (11.82 pc), respectively. Conclusions. We confirm Garro 02 as a new genuine Galactic GC, located in the MW bulge. It is a low-luminosity, metal-poor, and old GC, and it is a lucky survivor of the strong dynamical processes that occurred during the MW’s entire life.

Hidden in the haystack: low-luminosity globular clusters towards the Milky Way bulge

ABSTRACT Recent wide-area surveys have enabled us to study the Milky Way with unprecedented detail. Its inner regions, hidden behind dust and gas, have been partially unveiled with the arrival of near-infrared (IR) photometric and spectroscopic data sets. Among recent discoveries, there is a population of low-mass globular clusters, known to be missing, especially towards the Galactic bulge. In this work, five new low-luminosity globular clusters located towards the bulge area are presented. They were discovered by searching for groups in the multidimensional space of coordinates, colours, and proper motions from the Gaia EDR3 catalogue and later confirmed with deeper VVV survey near-IR photometry. The clusters show well-defined red giant branches and, in some cases, horizontal branches with their members forming a dynamically coherent structure in proper motion space. Four of them were confirmed by spectroscopic follow-up with the MUSE instrument on the ESO VLT. Photometric parameters were derived, and when available, metallicities, radial velocities, and orbits were determined. The new clusters Gran 1 and 5 are bulge globular clusters, while Gran 2, 3 and 4 present halo-like properties. Preliminary orbits indicate that Gran 1 might be related to the Main Progenitor, or the so-called ‘low-energy’ group, while Gran 2, 3 and 5 appears to follow the Gaia-Enceladus/Sausage structure. This study demonstrates that the Gaia proper motions, combined with the spectroscopic follow-up and colour–magnitude diagrams, are required to confirm the nature of cluster candidates towards the inner Galaxy. High stellar crowding and differential extinction may hide other low-luminosity clusters.

Eight more low luminosity globular clusters in the Sagittarius dwarf galaxy

Context. The Sagittarius (Sgr) dwarf galaxy is merging with the Milky Way, and the study of its globular clusters (GCs) is important to understand the history and outcome of this ongoing process. Aims. Our main goal is to characterize the GC system of the Sgr dwarf galaxy. This task is hampered by high foreground stellar contamination, mostly from the Galactic bulge. Methods. We performed a GC search specifically tailored to find new GC members within the main body of this dwarf galaxy using the combined data of the VISTA Variables in the Via Lactea Extended Survey near-infrared survey and the Gaia Early Data Release 3 optical database. Results. We applied proper motion cuts to discard foreground bulge and disk stars, and we found a number of GC candidates in the main body of the Sgr dwarf galaxy. We selected the best GCs as those objects that have significant overdensities above the stellar background of the Sgr galaxy and that possess color-magnitude diagrams with well-defined red giant branches consistent with the distance and reddening of this galaxy. Conclusions. We discover eight new GC members of the Sgr galaxy, which adds up to 29 total GCs known in this dwarf galaxy. This total number of GCs shows that the Sgr dwarf galaxy hosts a rather rich GC system. Most of the new GCs appear to be predominantly metal-rich and have low luminosity. In addition, we identify ten other GC candidates that are more uncertain and need more data for proper confirmation.

Confirmation and physical characterization of the new bulge globular cluster Patchick 99 from the VVV and Gaia surveys

Context. Globular clusters (GCs) are recognised as important tools for understanding the formation and evolution of the Milky Way (MW) because they are the oldest objects in our Galaxy. Unfortunately, the known sample in our MW is still incomplete, especially towards the innermost regions, because of the high differential reddening, extinction, and stellar crowding. Therefore, the discovery of new GC candidates and the confirmation of their true nature are crucial for the census of the MW GC system. Aims. Our main goal is to confirm the physical nature of two GC candidates: Patchick 99 and TBJ 3. Both are located towards the Galactic bulge. We use public data in the near-infrared(IR) passband from the VISTA Variables in the Via Láctea Survey (VVV), the VVV eXtended Survey, and the Two Micron All Sky Survey along with deep optical data from the Gaia Mission DR 2 in order to estimate their main astrophysical parameters, such as reddening and extinction, distance, total luminosity, mean cluster proper motions, size, metallicity, and age. Methods. We investigated both candidates at different wavelengths, allowing us to discard TBJ3 as a possible GC. We use near-IR (Ks vs. (J − Ks)) and optical (G vs. (BP − RP)) colour–magnitude diagrams (CMDs) in order to analyse Patchick 99. First, we decontaminated CMDs, following a statistical procedure, as well as selecting only stars which have similar proper motions (PMs) and are situated within 3′ of the centre. Mean PMs were measured from Gaia DR 2 data. Reddening and extinction were derived by adopting optical and near-IR reddening maps, and were used to estimate the distance modulus and the heliocentric distance. Metallicity and age were evaluated by fitting theoretical stellar isochrones. Results. Reddening and extinction values for Patchick 99 are E(J − Ks) = (0.12 ± 0.02) mag and AKs = (0.09 ± 0.01) mag from the VVV data, whereas we calculate E(BP − RP) = (0.21 ± 0.03) mag and AG = (0.68 ± 0.08) mag from Gaia DR 2 data. We use those values and the magnitude of the RC to estimate the distance, finding good agreement between the near-IR and optical measurements. In fact, we obtain (m − M)0 = (14.02 ± 0.01) mag, equivalent to a distance D = (6.4 ± 0.2) kpc in near-IR and (m − M)0 = (14.23 ± 0.1) mag and so D = (7.0 ± 0.2) kpc in optical. In addition, we derive the metallicity and age for Patchick 99 using our distance and extinction values and fitting PARSEC isochrones. We find [Fe/H]=(−0.2 ± 0.2) dex and t = (10 ± 2) Gyr. The mean PMs for Patchick 99 are μα = ( − 2.98 ± 1.74) mas yr−1 and μδ = ( − 5.49 ± 2.02) mas yr−1 using the Gaia DR 2 data. These are consistent with the bulge kinematics. We also calculate the total luminosity of our cluster and confirm that it is a low-luminosity GC, with MKs = ( − 7.0 ± 0.6) mag. The radius estimation is performed building the radial density profile and we find its angular radius rP99 ∼ 10′. We also recognise seven RR Lyrae star members within 8.2 arcmin from the Patchick 99 centre, but only three of them have PMs matching the mean GC PM, confirming the distance found by other methods. Conclusions. We find that TBJ 3 shows mid-IR emissions that are not present in GCs. We therefore discard TBJ 3 as a GC candidate and focus our work on Patchick 99. We conclude that Patchick 99 is an old metal-rich GC situated in the Galactic bulge. TBJ 3 is a background galaxy.

3FGL DEMOGRAPHICS OUTSIDE THE GALACTIC PLANE USING SUPERVISED MACHINE LEARNING: PULSAR AND DARK MATTER SUBHALO INTERPRETATIONS

ABSTRACT Nearly one-third of the sources listed in the Third Fermi Large Area Telescope (LAT) catalog (3FGL) remain unassociated. It is possible that predicted and even unanticipated gamma-ray source classes are present in these data waiting to be discovered. Taking advantage of the excellent spectral capabilities achieved by the Fermi LAT, we use machine-learning classifiers (Random Forest and XGBoost) to pinpoint potentially novel source classes in the unassociated 3FGL sample outside the Galactic plane. Here we report a total of 34 high-confidence Galactic candidates at . The currently favored standard astrophysical interpretations for these objects are pulsars or low-luminosity globular clusters hosting millisecond pulsars (MSPs). Yet these objects could also be interpreted as dark matter annihilation taking place in ultra-faint dwarf galaxies or dark matter subhalos. Unfortunately, Fermi LAT spectra are not sufficient to break degeneracies between the different scenarios. Careful visual inspection of archival optical images reveals no obvious evidence for low-luminosity globular clusters or ultra-faint dwarf galaxies inside the 95% error ellipses. If these are pulsars, this would bring the total number of MSPs at to 106, down to an energy flux ≈4.0 × 10−12 erg cm−2 s−1 between 100 MeV and 100 GeV. We find this number to be in excellent agreement with predictions from a new population synthesis of MSPs that predicts 100–126 high-latitude 3FGL MSPs depending on the choice of high-energy emission model. If, however, these are dark matter substructures, we can place upper limits on the number of Galactic subhalos surviving today and on dark matter annihilation cross sections. These limits are beginning to approach the canonical thermal relic cross section for dark matter particle masses below ∼100 GeV in the bottom quark ( ) annihilation channel.

THE TWO-DIMENSIONAL SPATIAL DISTRIBUTIONS OF THE GLOBULAR CLUSTERS AND LOW-MASS X-RAY BINARIES OF NGC 4649

We report significant anisotropies in the projected two-dimensional (2D) spatial distributions of Globular Clusters (GCs) of the giant Virgo elliptical galaxy NGC4649 (M60). Similar features are found in the 2D distribution of low-mass X-ray binaries (LMXBs), both associated with GCs and in the stellar field. Deviations from azimuthal symmetry suggest an arc-like excess of GCs extending north at 4-15 kpc galactocentric radii in the eastern side of major axis of NGC4649. This feature is more prominent for red GCs, but still persists in the 2D distribution of blue GCs. High and low luminosity GCs also show some segregation along this arc, with high-luminosity GCs preferentially located in the southern end and low-luminosity GCs in the northern section of the arc. GC-LMXBs follow the anisotropy of red-GCs, where most of them reside; however, a significant overdensity of (high-luminosity) field LMXBs is present to the south of the GC arc. These results suggest that NGC4649 has experienced mergers and/or multiple accretions of less massive satellite galaxies during its evolution, of which the GCs in the arc may be the fossil remnant. We speculate that the observed anisotropy in the field LMXB spatial distribution indicates that these X-ray binaries may be the remnants of a star formation event connected with the merger, or maybe be ejected from the parent red GCs, if the bulk motion of these clusters is significantly affected by dynamical friction. We also detect a luminosity enhancement in the X-ray source population of the companion spiral galaxy NGC4647. We suggest that these may be younger high mass X-ray binaries formed as a result of the tidal interaction of this galaxy with NGC4649.

Segue 3: the youngest globular cluster in the outer halo★

Deep Galileo (Telescopio Nazionale Galileo) B, V and I images of Segue 3, reaching V ∼ 25, reveal that it is the youngest globular cluster known so far in the Galaxy. A young age of 3.2 Gyr is found, differently from a previous estimate of 12 Gyr. It also appears to be moderately metal rich with [Fe/H] ∼ −0.8, rather than [Fe/H] ∼ −1.7, as previously suggested by Fadely et al. A main difference in the age derivation relative to Fadely et al. comes from the consideration of subgiant branch stars in the isochrone fitting. A deduced distance of d⊙ = 29.1 kpc is compatible with the outer halo location of other low luminosity globular clusters.

The Discovery of Two Extremely Low Luminosity Milky Way Globular Clusters

We report the discovery of two extremely low luminosity globular clusters in the Milky Way Halo. These objects were detected in the Sloan Digital Sky Survey Data Release 5 and confirmed with deeper imaging at the Calar Alto Observatory. The clusters, Koposov 1 and Koposov 2, are located at $\sim 40-50$ kpc and appear to have old stellar populations and luminosities of only $M_V \sim -1$ mag. Their observed sizes of $\sim 3$ pc are well within the expected tidal limit of $\sim$10 pc at that distance. Together with Palomar 1, AM 4 and Whiting 1, these new clusters are the lowest luminosity globulars orbiting the Milky Way, with Koposov 2 the most extreme. Koposov 1 appears to lie close to distant branch of the Sagittarius stream. The half-mass relaxation times of Koposov 1 and 2 are only $\sim 70$ and $\sim 55$ Myr respectively (2 orders of magnitude shorter than the age of the stellar populations), so it would seem that they have undergone drastic mass segregation. Since they do not appear to be very concentrated, their evaporation timescales may be as low as $\sim 0.1 t_{\rm Hubble}$. These discoveries show that the structural parameter space of globular clusters in the Milky Way halo is not yet fully explored. They also add, through their short remaining survival times, significant direct evidence for a once much larger population of globular clusters.

ChandraObservation of the Globular Cluster NGC 6440 and the Nature of Cluster X‐Ray Luminosity Functions

As part of our campaign to determine the nature of the various source populations of the low-luminosity globular cluster X-ray sources, we have obtained a Chandra X-Ray Observatory ACIS-S3 image of the globular cluster NGC 6440. We detect 24 sources to a limiting luminosity of ~2 × 1031 ergs s-1 (0.5-2.5 keV) inside the cluster's half-mass radius, all of which lie within ~2 core radii of the cluster center. We also find excess emission in and around the core that could be due to unresolved point sources. Based on X-ray luminosities and colors, we conclude that there are 4-5 likely quiescent low-mass X-ray binaries and that most of the other sources are cataclysmic variables. We compare these results to Chandra results from other globular clusters and find the X-ray luminosity functions differ among the clusters.

Palomar 13: An Unusual Stellar System in the Galactic Halo

We report the first results of a program to study the internal kinematics of globular clusters in the outer halo of the Milky Way. Using the Keck telescope and High Resolution Echelle Spectrometer, we have measured precise radial velocities for 30 candidate red giants in the direction of Palomar 13, an object traditionally cataloged as a compact, low-luminosity globular cluster. We have combined these radial velocities with published proper motion membership probabilities and new CCD photometry from the Keck and Canada-France-Hawaii telescopes to isolate a sample of 21 probable members. We find a systemic velocity of vrs = 24.1 ? 0.5 km s-1 and a projected, intrinsic velocity dispersion of ?p = 2.2 ? 0.4 km s-1. Although modest, this dispersion is nevertheless several times larger than that expected for a globular cluster of this luminosity and central concentration. Taken at face value, it implies a mass-to-light ratio of V = 40 based on the best-fit King-Michie model. The surface density profile of Palomar 13 also appears unusual compared to most Galactic globular clusters; depending upon the details of background subtraction and model-fitting, Palomar 13 either contains a substantial population of extratidal stars, or is considerably more spatially extended than previously suspected. The full surface density profile is equally well fitted by a King-Michie model having a high concentration and large tidal radius, or by a Navarro-Frenk-White model. We examine?and tentatively reject?a number of possible origins for the observed characteristics of Palomar 13 (e.g., velocity jitter among the red giant branch stars, spectroscopic binary stars, nonstandard mass functions, modified Newtonian dynamics) and conclude that the two leading explanations are either catastrophic heating during a recent perigalacticon passage or the presence of a dark matter halo. The available evidence therefore suggests that Palomar 13 is either a globular cluster that is now in the process of dissolving into the Galactic halo or a faint, dark matter-dominated stellar system.

ROSATHRI X‐Ray Observations of the Open Globular Cluster NGC 288

A ROSAT HRI X-ray image was obtained of the open globular cluster NGC 288, which is located near the South Galactic Pole. This is the first deep X-ray image of this system. We detect a low-luminosity globular cluster X-ray source (LLGCX) RX J005245.0-263449 with an X-ray luminosity of (5.5 ± 1.4) × 1032 ergs s-1 (0.1-2.0 keV), which is located very close to the cluster center. There is evidence for X-ray variability on a timescale of 1 day. The presence of this LLGCX in such an open globular cluster suggests that dense stellar systems with high interaction rates are not needed to form LLGCXs. We also searched for diffuse X-ray emission from NGC 288. Upper limits on the X-ray luminosities are L < 9.5 × 1032 ergs s-1 (0.52-2.02 keV) and L < 9.3 × 1032 ergs s-1 (0.11-0.41 keV). These imply upper limits to the diffuse X-ray-to-optical light ratios in NGC 288, which are lower than the values observed for X-ray faint early-type galaxies. This indicates that the soft X-ray emission in these galaxies is due either to a component that is not present in globular clusters (e.g., interstellar gas or a stellar component that is not found in low-metallicity Population II systems) or to a relatively small number of bright low-mass X-ray binaries.

C J0907-372 (PYXIS): A NEW DISTANT GALACTIC GLOBULAR CLUSTER

ABSTRACT The prime focus camera of the Anglo-Australian Telescope has been used to image the object listed by Weinberger (PASP, 107, 58 [1995]) as a possible distant star cluster or dwarf spheroidal galaxy. The CCD data reveal this object to be a distant globular cluster of low central concentration. In particular, the color-magnitude diagram for the cluster core shows a sparse giant branch, a prominent red horizontal branch at R=18.75 and a main sequence turnoff near the limit of the data at R=22.0. A population of possible blue stragglers is also evident. Based on comparisons with the giant branches of standard globular clusters, the reddening to this cluster, designated C J0907-372 (Pyxis), is estimated to be 0.25 ≤ E(B-V) ≤ 0.40. The cluster is then approximately 35 kpc from the Sun and 37 kpc from the galactic center. With a core radius of ~14 pc, an estimated absolute magnitude MV=-5.7 and a dominant red horizontal branch, C J0907-372 (Pyxis) has very similar properties to the other low luminosity globular clusters of the outer galactic halo.

Predictions of a population of cataclysmic variables in globular clusters

We have studied the number of cataclysmic variables (CVs) that should be active in globular clusters during the present epoch as a result of binary formation via two-body tidal capture. We predict the orbital period and luminosity distributions of CVs in globular clusters. The results arebased on Monte Carlo simulations combined with evolution calculations appropriate to each system formed during the lifetime of two specific globular clusters, omega Cen and 47 Tuc. From our study of these two clusters, which represent the range of core densities and states of mass segregation that are likely to be interesting, we extrapolate our results to the Galactic globlular cluster system. Although there is at present little direct observational evidence of CVs in globular clusters, we find that there should be a large number of active systems. We predict that there should be more than approximately 100 CVs in both 47 Tuc and omega Cen and several thousand in the Galactic globular cluster system. These numbers are based on two-body processes alone and represent a lower bound on the number of systems that may have been formed as a result of stellar interaction within globular clusters. The relation between these calculations and the paucity of optically detected CVs in globular clusters is discussed. Should future observations fail to find convincing evidence of a substantial population of cluster CVs, then the two-body tidal capture scenario is likely to be seriously constrained. Of the CVs we espect in 47 Tuc and omega Cen, approximately 45 and 20, respectively, should have accretion luminosities above 10(exp 33) ergs/s. If one utilizes a relation for converting accretion luminosity to hard X-ray luminosity that is based on observations of Galactic plane CVs, even these sources will not exhibit X-ray luminosities above 10(exp 33) ergs/s. While we cannot account directly for the most luminous subset of the low-luminosity globular cluster X-ray sources without assuming an evolutionary pattern that is different from that of the majority of CVs in the disk, we are able to account for all of the observed lower luminosity subset of these sources, many of which have been recently discovered through ROSAT observations. In order for our predicted integrated cluster X-ray luminosities to be consistent with observational upper limits, the relation between accretion and X-ray luminosities should be something like that inferred from the Galactic plane population of CVs. Our calculations predict a large number of systems with L(sub acc) is less than 10(exp 32) ergs/s. Although our calculations imply that globular clusters should have an enhancement of CVs relative to the number thought to be present in the Galactic disk, this enhancement is at most roughly an order of magnitude, not comparable to the factor of approximately 100 for low-mass X-ray binaries (LMXBs).

The nature of the low-luminosity globular cluster X-ray sources

view Abstract Citations (65) References (50) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The nature of the low-luminosity globular cluster X-ray sources. Hertz, P. ; Wood, K. S. Abstract An X-ray survey of 134 galactic globular clusters was performed with the Large-Area Sky Survey Experiment aboard HEAO 1. No new globular cluster X-ray sources were discovered, confirming the gap in the luminosity function discussed, obtaining upper limits on X-ray emission from 67 globular clusters. Statistical analyses reveal that the presence of both high-luminosity and low-luminosity globular cluster X-ray sources is correlated with the time scale for binary formation via the tidal capture mechanism within globular clusters. Among other predictions, a turnover in the luminosity function below 10 to the 32nd erg/s is expected. Finally, the implications of the model presented on the evolution of globular cluster X-ray sources and on low-mass X-ray binaries and cataclysmic variables in the galactic plane. Publication: The Astrophysical Journal Pub Date: March 1985 DOI: 10.1086/162971 Bibcode: 1985ApJ...290..171H Keywords: Astronomical Spectroscopy; Binary Stars; Galactic Clusters; Globular Clusters; Stellar Luminosity; X Ray Sources; Cataclysmic Variables; Emission Spectra; Heao 1; Heao 2; Sky Surveys (Astronomy); Spaceborne Astronomy; Stellar Mass Accretion; White Dwarf Stars; Astrophysics full text sources ADS | data products SIMBAD (140)