Young Globular Clusters Research Articles

A Direct Distance to the Large Magellanic Cloud Cepheid HV 12198 from the Infrared Surface Brightness Technique.

We report on the first application of the infrared surface brightness technique on a Cepheid in the Large Magellanic Cloud, the variable HV 12198 in the young globular cluster NGC 1866. From this one star, we determine a distance modulus of 18.42+/-0.10 (random and systematic uncertainty) to the cluster. When the results on further member Cepheids in NGC 1866 become available, we expect to derive the distance to the LMC with a +/-3%-4% accuracy, including systematic errors, from this technique.

The Radio Supernebula in NGC 5253.

We present 1.3 cm and 2 cm subarcsecond resolution VLA images of the dwarf galaxy NGC 5253. Within the central starburst, we detect high-brightness [Tb&parl0;2 cm&parr0; approximately 100-12,000 K] radio continuum sources. These appear to be very dense, "compact" H ii regions. The dominant radio source is a nebula approximately 1-2 pc in size, requiring several thousand O stars within the volume to maintain its ionization. This nebula has no obvious optical counterpart. The number of ionizing photons we find for this cluster is nearly 2 orders of magnitude larger than indicated by Halpha fluxes, and the deduced stellar content accounts for a significant fraction of the total infrared luminosity of the galaxy. This cluster is a strong candidate for a globular cluster in the process of formation, perhaps the youngest globular cluster known.

Young Globular Clusters and Dwarf Spheroidals

Most of the globular clusters in the main body of the Galactic halo were formed almost simultaneously. However, globular cluster formation in dwarf spheroidal galaxies appears to have extended over a significant fraction of a Hubble time. This suggests that the factors which suppressed late-time formation of globulars in the main body of the Galactic halo were not operative in dwarf spheroidal galaxies. Possibly the presence of significant numbers of ``young'' globulars at R_{GC} > 15 kpc can be accounted for by the assumption that many of these objects were formed in Sagittarius-like (but not Fornax-like) dwarf spheroidal galaxies, that were subsequently destroyed by Galactic tidal forces. It would be of interest to search for low-luminosity remnants of parental dwarf spheroidals around the ``young'' globulars Eridanus, Palomar 1, 3, 14, and Terzan 7. Furthermore multi-color photometry could be used to search for the remnants of the super-associations, within which outer halo globular clusters originally formed. Such envelopes are expected to have been tidally stripped from globulars in the inner halo.

The Formation and Evolution of Candidate Young Globular Clusters in NGC 3256

We present images of the recent galaxy merger NGC 3256 obtained with the Wide Field Planetary Camera 2 of the Hubble Space Telescope in B and I filters. We show that there is a large population of more than 1000 compact, bright, blue objects in this galaxy within the 7 kpc × 7 kpc region studied. These objects have sizes, colors, and luminosities like those expected for young Galactic globular clusters, with ages ranging from a few to several hundred megayears. On this basis, we identify at least some fraction of the compact, bright, blue objects in NGC 3256 as young globular clusters. The young cluster system makes up a significant fraction of the total luminosity of the galaxy within the region studied—15%–20% in B and half that in I, indicating a high efficiency of cluster formation on a galaxy-wide scale. In order to determine the properties of this young cluster system, the selection effects in size, color, and luminosity are carefully modeled. We find that the intrinsic color distribution is broad and there is no significant trend of color with magnitude. The combination of the broad range of observed colors and the lack of a trend of redder colors at fainter magnitudes cannot be fitted solely by a broad age distribution and/or differential reddening, although the latter is clearly present. The observations can be accounted for by either the preferential depletion/destruction of lower mass clusters as they age or a very young age (≲20 Myr) for the cluster population, comparable to or less than the dynamical time of the region in which the clusters are observed. We also find that the luminosity function of the young cluster system can be roughly fitted by a power law with an exponent of -1.8, with tentative evidence that it flattens at faint magnitudes. The clusters are compact in size, with typical estimated half-light radii of 5–10 pc, but there is no obvious cutoff for larger radii and only a shallow trend of size with luminosity. We discuss the implications of these results for models of the formation and dynamical evolution of globular clusters, as well as for interpretation of the properties of older globular cluster systems.

Deep [ITAL]Hubble Space Telescope[/ITAL] Observations of Blue Star Clusters in NGC 3597

We have analyzed HST/WFPC2 images of NGC 3597 and find ~700 compact objects surrounding the galaxy with an average (B-R)_0~0.6. We propose that the majority of these objects are young globular clusters. They have a spread in colors that is consistent with that expected for a population of young clusters with a common age and spread induced by photometric errors and reddening within NGC 3597. If these objects were similar to the Galactic globular cluster system seen at a younger age, we would predict a turnover in the luminosity function at B~23. However, we find that the luminosity function for the clusters is increasing to the limits of our photometry (B~27).

The Very Young Starburst Merger System NGC 1741

We use Hubble Space Telescope (HST) Faint Object Camera (FOC) ultraviolet (UV) and WFPC2 optical images in conjunction with UV spectroscopic observations taken with the Goddard High Resolution Spectrograph to examine the star formation history and properties of the interacting galaxy system NGC 1741 in the Hickson Compact Group 31. The high spatial resolution afforded by HST has allowed us to identify a large number of starburst knots, or super?star clusters (SSCs), in the starburst regions of this system. Photometry of these SSCs in the UV and optical bands indicates that most of these objects have ages of a few Myr, with a few up to ?100 Myr, and masses between 104 and 106 M?. The estimated age is confirmed by a spectral synthesis analysis of one knot for which we have obtained a UV spectrum. The V-band luminosity function of the SSCs is well represented by a power law (L) ~ L-? with an index of -1.85, with no evidence of a turnover brighter than the completeness limit. These properties are in good agreement with those found for SSCs in other starburst galaxies. Our results support the suggestion that some of these SSCs may be extremely young globular clusters formed in a relatively recent starburst episode that has been triggered by a merger event.

Stellar Populations in the Magellanic Clouds

There seems to be no strong evidence that the young globular clusters in the MC have metallicities differing significantly from the metallicities of MC field stars of the same age. The old globular clusters in the LMC are of the same age as, or slightly younger than, those in the outer halo of our Galaxy. It is suggested that the increase in the SFR in the LMC ∼ 4 Gyr ago was related to the collapse of the system to a plane. Evidence for a spread in metallicities amongst young stars in either Cloud remains tentative. There is no strong evidence for bursts of star formation being triggered by LMC-SMC-Galaxy interactions but the possibility is raised that the SFR in the SMC has been strongly affected by this interaction.

NLTE Abundances from Magellanic Cloud B Stars

Nine MC young globular cluster and field stars are analysed using fully line-blanketed LTE ATLAS 9 atmospheres and the DETAIL/SURFACE non-LTE line formation code. CNO mixing seems to occur much earlier than predicted by standard theory.

Ages and Metallicities of Young Globular Clusters in the Merger Remnant NGC 7252

Ultraviolet-to-visual spectra of eight young star clusters in the merger remnant and protoelliptical galaxy NGC 7252, obtained with the 4 m Blanco Telescope on Cerro Tololo, are presented. These clusters lie at projected distances of 3–15 kpc from the center and move with a velocity dispersion of 140 ± 35 km s-1 in the line of sight. Seven of the clusters show strong Balmer absorption lines in their spectra [EW(Hβ) = 6–13 A], while the eighth lies in a giant H II region and shows no detectable absorption features. Based on comparisons with model cluster spectra computed by Bruzual & Charlot and Bressan, Chiosi, & Tantalo, six of the absorption-line clusters have ages in the narrow range of 400–600 Myr, indicating that they formed early in the recent merger. These clusters, and probably also the seventh absorption-line cluster, are globular clusters, as judged by their small effective radii and ages corresponding to ~102 core crossing times. The one emission-line object is 10 Myr old and may be a nascent globular cluster or an OB association. The mean metallicities measured for three clusters are solar to within about ±0.15 dex, suggesting that the merger of two likely Sc galaxies in NGC 7252 formed a globular cluster system with a bimodal metallicity distribution. Since NGC 7252 itself shows the characteristics of a 0.5–1 Gyr old protoelliptical galaxy, its second-generation solar-metallicity globular clusters provide direct evidence that giant elliptical galaxies with bimodal globular cluster systems can form through major mergers of gas-rich disk galaxies. A puzzling property of the observed young globular clusters are the high masses of (1–35)M(ω Cen) implied by their luminosities and ages (for an assumed Salpeter IMF). A spectrum of a candidate superluminous globular cluster in the elliptical galaxy NGC 1700, obtained with the Hiltner Telescope at MDM Observatory, shows this object to be a foreground star.

Deep Optical Imaging of the Bright Seyfert Galaxy NGC 5548: A Long, Very Low Surface Brightness Tail

Deep Hubble Space Telescope Wide Field Camera (WFC-1) imaging of a region 37 east of the V = 13 mag Seyfert galaxy NGC 5548 in the F555W and F785LP bands, and deep ground-based V and I imaging, show a new extended, blue, low surface brightness structure, apparently a remnant of a tidal interaction associated with NGC 5548. If this straight, low surface brightness (V ~ 27–28 mag arcsec-2) tidal tail is associated with NGC 5548, it extends at least 80 kpc from it and has an absolute magnitude of MV ≈ -16.4 (H0 = 65 km s-1 Mpc-1). Previous imaging surveys of Seyfert galaxies would have missed such low surface brightness tails. Morphologically similar to some brighter tails seen in other interacting systems and in simulations of merging galaxies, this faint tail is a useful diagnostic of an earlier interaction and of the progenitors' halo-to-disk mass ratios. Luminous ripples and a brighter tail wrapped around the galaxy are seen in the inner 1–10 kpc region. Unresolved blue objects in the long tail have the colors and absolute magnitudes of young globular clusters. Taken together, the two tails and other features suggest that two spirals merged less than ≈1 Gyr ago. The inner luminosity profile of NGC 5548 is a good fit to a de Vaucouleurs profile with reff = 5.8 kpc. Recent simulations of merged galaxies with high-mass halos fail to form lasting tidal tails, suggesting that the NGC 5548 progenitor spirals had modest halo masses.

The Low-Mass Stellar Luminosity Function of the 30 Dor Starburst Cluster

AbstractDiffraction limited near-infrared H-band (1.6 μm) NICMOS HST images are scheduled to be obtained in mid-October 1997 of the young cluster NGC 2070 (age 3.5 Myr) in the 30 Dor giant HII region in the LMC. The aim is to search for the low-mass (M < 2 Mʘ) low-luminosity, red pre-Main Sequence stellar population and to establish the H-band infrared luminosity function. With the NICMOS we can now determine whether the IMF in this prototypical extragalactic starburst cluster is deficient in subsolar low-mass stars or not. The best ground-based data can sample only M > 2 Mʘ. In principle, NICMOS in the H-band (F160W) is sensitive enough to reach a magnitude of ~ 23.5 in a relatively short integration time, which indeed corresponds to the fantastic possibility to detect young stellar objects with masses near the hydrogen burning limit (M=0.1 Mʘ) according to pre-Main Sequence evolutionary models. Even if we could reach only H = 22.5 (i.e. M=0.4 Mʘ), our observations will still go a long way in directly answering, by star counts, whether the IMF in starburst galaxies is low-mass deficient or not, with all the corresponding far-reaching implications.The observations would also tell us whether the 30 Dor cluster can be regarded as a prototype young globular cluster. This possibility would be ruled out, if we found NGC 2070 to be low-mass deficient, because old globular clusters do have a rich population of low-mass stars.

Residual gas expulsion from young globular clusters

The results of N-body simulations of the expulsion of residual gas from young globular clusters are presented. Globular clusters with a variety of initial masses, Galactocentric radii, concentration and initial mass function slope with star formation efficiencies (SFEs) below 50% were investigated. The residual gas in the clusters was simply treated as an external potential acting upon the star particles which was reduced in a number of ways to simulate the loss of the gas. The states of the clusters after ~50 Myr was compared to the results of Chernoff & Shapiro (1987) in order to estimate if the clusters would be able to survive for a Hubble time or would be disrupted. The rapid expulsion of large amounts of a cluster's initial mass is found to considerably effect the structures of clusters, but they may be able to survive with SFEs far lower than 50%. It is suggested that the Galactic globular cluster population could have formed from a proto-cluster populations with SFEs ~40% and central densities approximately the same as those found in giant molecular clouds.

Reddenings, Metallicities, and Possible Abundance Anomalies in Young Globular Clusters

Reddenings, Metallicities, and Possible Abundance Anomalies in Young Globular Clusters

The initial conditions of young globular clusters in the Large Magellanic Cloud

N-body simulations are used to model the early evolution of globular clusters. These simulations include residual gas which was not turned into stars which is expelled from the globular cluster by the actions of massive stars. The results of these simulations are compared to observations of 8 LMC globular clusters less than 100 Myr old. These observations are used to constrain the initial conditions that may have produced these clusters. It is found that the entire variety of young LMC globular clusters may be explained in a model where they form from a fairly uniform population of roughly spherical, relaxed proto-cluster clouds very similar to Giant Molecular Clouds in the Galaxy, with star formation efficiencies between 25% and 60%. This paper has been accepted for publication in MNRAS.

Hubble Space Telescope Observations of Candidate Young Globular Clusters and Stellar Associations in the Recent Merger Remnant NGC 3921

Hubble Space Telescope Observations of Candidate Young Globular Clusters and Stellar Associations in the Recent Merger Remnant NGC 3921

Dynamical Evidence for a Massive, Young Globular Cluster in NGC 1569

Recent high-resolution observations with the Hubble Space Telescope (HST) reveal that star of extraordinary luminosity and compactness are commonly found in a variety of starburst systems. There has been much speculation that these represent present-day analogs of young globular clusters. Using the HIRES echelle spectrograph on the Keck 10 m telescope, we obtained high-dispersion optical spectra of one of the super star clusters (cluster A) in the nearby dwarf galaxy NGC 1569. The size of the cluster is known from published HST images. The line-of-sight velocity dispersion (σ* = 15.7 ± 1.5 km s-1) has been measured from a cross-correlation analysis of its integrated spectrum at visual wavelengths. If the cluster is gravitationally bound and the velocities are isotropic, application of the virial theorem implies that the cluster has a total stellar mass of (3.3 ± 0.5) × 105 M☉. This object's mass, mass density, and probable mass-to-light ratio after aging 10-15 Gyr are fully consistent with the typical values of Galactic globular clusters. Our result strongly suggests that at least some of the luminous, compact, young star being discovered with HST will indeed evolve into normal globular of the type seen in the Milky Way.

The Discovery of Young, Luminous, Compact Stellar Clusters in the Starburst Galaxy NGC 253

We report the discovery of four young, luminous, compact stellar clusters in the central region of the starburst spiral galaxy NGC 253. One cluster has an intrinsic M_v of about -15 or brighter, an inferred mass in excess of 1.5X10^6 M_☉, assuming a Salpeter IMF from 0.08M_☉ to l00M_☉, and is resolved with a half-light radius of about 2.5 parsec if it has a radial profile similar to a Galactic globular cluster. The others have intrinsic M_v of about -11 or brighter, inferred masses in excess of 5 X 10^4 M_☉, and are similarly compact. These clusters are excellent candidate young globular clusters. The brighter cluster is similar in luminosity, inferred mass, and compactness to the super star clusters found in other sites of ongoing or recent intense star formation. The location of these clusters in the midst of a starburst region suggests that intense star formation may be a key ingredient in their creation. There are hints that these candidate young globular clusters may be forming with a high specific frequency.

Hubble Space Telescope Planetary Camera Images of NGC 1316 (Fornax A)

We present HST Planetary Camera V and I~band images of the central region of the peculiar giant elliptical galaxy NGC 1316. The inner profile is well fit by a nonisothermal core model with a core radius of 0.41" +/- 0.02" (34 pc). At an assumed distance of 16.9 Mpc, the deprojected luminosity density reaches $\sim 2.0 \times 10^3 L_{\sun}$ pc$^{-3}$. Outside the inner two or three arcseconds, a constant mass-to-light ratio of $\sim 2.2 \pm 0.2$ is found to fit the observed line width measurements. The line width measurements of the center indicate the existence of either a central dark object of mass $2 \times 10^9 M_{\sun}$, an increase in the stellar mass-to-light ratio by at least a factor of two for the inner few arcseconds, or perhaps increasing radial orbit anisotropy towards the center. The mass-to-light ratio run in the center of NGC 1316 resembles that of many other giant ellipticals, some of which are known from other evidence to harbor central massive dark objects (MDO's). We also examine twenty globular clusters associated with NGC 1316 and report their brightnesses, colors, and limits on tidal radii. The brightest cluster has a luminosity of $9.9 \times 10^6 L_{\sun}$ ($M_V = -12.7$), and the faintest detectable cluster has a luminosity of $2.4 \times 10^5 L_{\sun}$ ($M_V = -8.6$). The globular clusters are just barely resolved, but their core radii are too small to be measured. The tidal radii in this region appear to be $\le$ 35 pc. Although this galaxy seems to have undergone a substantial merger in the recent past, young globular clusters are not detected.

Colliding and Merging Galaxies. III. The Dynamically Young Merger Remnant NGC 3921

view Abstract Citations (103) References (126) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Colliding and Merging Galaxies. III. The Dynamically Young Merger Remnant NGC 3921 Schweizer, Francois Abstract This paper presents imaging, photometric, and spectroscopic observations of NGC 3921 = Mrk 430 gathered over many years with five optical telescopes. This luminous galaxy (M_V_= -22.8 for H_0_ = 50) at cz_hel_=5926 +/- 15 km s^-1^ features a single nucleus, a main body with complex fine structure (ripples, loops, fan-shaped protrusions), and a pair of ~100 kpc long, crossed tidal tails indicative of two former disk galaxies of near-equal mass. These galaxies have essentially merged. The main body of the remnant shows a typical post-starburst spectrum dominated in the blue by A 3-5 V stars. The inferred burst age is 0.5-1 Gyr and the burst strength ~10% (by mass). Surrounding the nucleus is extremely centrally concentrated ionized gas that can be traced out to ~12" (7 kpc), emits ~> 1.5 x 10^41^ ergs s^-1^ in Hα, and shows signs of both rotational and chaotic motions. The bright semistellar nucleus appears strikingly off-centered relative to the main body, which itself features "sloshing" isophotes. That is, the centers of successive isophotes shift position by ~>2 kpc, causing the nucleus to appear eccentric by up to 23% relative to a nearly half-light isophote. The luminous matter has clearly not yet equilibrated, and this merger remnant is dynamically young. Nevertheless, the mean light distribution of the main body is already well described by an r^1/4^ law. This distribution plus the luminosity, UBV colors, color gradients, velocity dispersion, spectroscopic line strengths, and fine-structure index all agree with the notion that NGC 3921, which is a member of a small, tight group of four galaxies, is a 0.7+/-0.3 Gyr old protoelliptical (reckoned since close passage that started the merger). Both it and its kin NGC 7252 are nearby analogs of distant galaxies with "E+A"-type spectra in Butcher-Oemler clusters. A search for star clusters and associations in NGC 3921 reveals 19 candidate OB associations, but only five candidate young globular clusters with M_V_ = -12 to -14. Thus, NGC 3921 appears to have distinctly fewer and certainly less luminous young globular clusters than NGC 7252. This less extreme population of young globulars may reflect a paucity of gas in one of the two merging component disks of this suspected S0-Sc or Sa-Sc merger (Hibbard & van Gorkom, AJ, in press). Such gas paucity may explain the weaker starburst and may have supplied fewer giant molecular clouds for globular cluster formation. Hence, the Hubble types and gas contents of component galaxies appear to play an important role in determining the cluster populations in merger remnants. Publication: The Astronomical Journal Pub Date: January 1996 DOI: 10.1086/117765 Bibcode: 1996AJ....111..109S Keywords: GALAXIES: INTERACTIONS; GALAXIES: INDIVIDUAL: NGC 3921 full text sources ADS | data products SIMBAD (48) NED (11) Related Materials (2) Part 1: 1982ApJ...252..455S Part 2: 1983AJ.....88..909S

The Young Globular Clusters of the Milky Way and the Local Group Galaxies: Playing with Great Circles

view Abstract Citations (92) References (51) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Young Globular Clusters of the Milky Way and the Local Group Galaxies: Playing with Great Circles Fusi Pecci, F. ; Bellazzini, M. ; Cacciari, C. ; Ferraro, F. R. Abstract The small group of galactic globular clusters (GGC) (Pal 12, Terzan 7, Ruprecht 106, Arp 2) recently discovered to be significantly younger (by ~3-4 Gyr) than the average cluster population of the Galaxy are shown to lie near planes passing in the vicinity of some satellite galaxies of the Milky Way and through the galactic center itself. Assuming that these configurations represent a fossil record of interactions between the Galaxy and its companions from which these clusters originated, we identified, along one of them, another candidate "young" GGC, i.e., IC 4499, whose color-magnitude diagram is presented. Various hypotheses on the possible origin of "young" GGC are also briefly discussed within a framework where the location on preferential planes may be seen as a general characteristic for the Local Group members. Publication: The Astronomical Journal Pub Date: October 1995 DOI: 10.1086/117639 arXiv: arXiv:astro-ph/9507065 Bibcode: 1995AJ....110.1664F Keywords: LOCAL GROUP; GLOBULAR CLUSTERS: GENERAL; GALAXIES: INTERACTIONS; Astrophysics E-Print: 15 pages+1 table, plain TEX, 6 figures available via ftp://boas3.bo.astro.it or http://www.bo.astro.it/ or upon request to appear in The Astronomical Journal, October issue full text sources arXiv | ADS | data products SIMBAD (69) NED (22)