Structural Parameters of Seven SMC Intermediate-Age and Old Star Clusters

We present structural parameters for the seven intermediate-age and old star clusters NGC121, Lindsay 1, Kron 3, NGC339, NGC416, Lindsay 38, and NGC419 in the Small Magellanic Cloud. We fit King profiles and Elson, Fall, and Freeman profiles to both surface-brightness and star count data taken with the Advanced Camera for Surveys aboard the Hubble Space Telescope. Clusters older than 1 Gyr show a spread in cluster core radii that increases with age, while the youngest clusters have relatively compact cores. No evidence for post core collapse clusters was found. We find no correlation between core radius and distance from the SMC center, although consistent with other studies of dwarf galaxies, some relatively old and massive clusters have low densities. The oldest SMC star cluster, the only globular NGC121, is the most elliptical object of the studied clusters. No correlation is seen between ellipticity and distance from the SMC center. The structures of these massive intermediate-age (1-8 Gyr) SMC star clusters thus appear to primarily result from internal evolutionary processes.Comment: 16 pages, 13 figure

A spectroscopy-based Age-Metallicity Relation of the SMC

The Small Magellanic Cloud (SMC) is the only dwarf galaxy in the Local Group that is known to have formed and preserved populous star clusters continuously over the past 12 Gyr. Due to its proximity (≈ 60 kpc), stars can be resolved well below the oldest main sequence turnoff points. This facilitates accurate age and metallicity determinations without suffering from the age-metallicity degeneracy. Therefore, the SMC star clusters provide a unique closely spaced set of single-age, single-metallicity tracers to derive a well-sampled age-metallicity relation required for the understanding of the star formation history of this satellite galaxy. Up to date spectroscopically based metallicity estimates exist only for the small number of 7 clusters (Da Costa & Hatzidimitriou 1998). Our project now more than doubles the available data set by the observation of 10 additional cluster

Present-day Mass Function of Six Small Magellanic Cloud Intermediate-age and Old Star Clusters

We determined the present-day mass functions (PDMFs) of the five intermediate-age star clusters Lindsay 1, Kron 3, NGC339, NGC416, and Lindsay 38 and the old star cluster NGC121 in the Small Magellanic Cloud (SMC) based on observations with the Hubble Space Telescope Advanced Camera for Surveys. The global PDMFs are well matched by Salpeter-like power laws from their main-sequence turnoffs to 0.6 M with a power-law exponent α ranging from 1.51 0.11 (Lindsay 1) to 2.29 0.15 (NGC339). We derive total stellar masses of 105 M , except for Lindsay 38, whose mass is of the order of 104 M. Differences between the PDMFs most likely reflect the varying stages of dynamical evolution of the clusters. These SMC clusters do not follow the α versus concentration parameter c correlation as found for Galactic globular clusters of similar mass. This might be an age effect or due to their location in a galaxy where bulge and disk crossings do not play a role. No correlation is found between α and the cluster core and tidal radii (rc and rt , respectively), the half-light radii rh , age, central surface brightness, metallicity, and galactocentric radius rgc. All six clusters mass-segregated to different degrees. The two clusters Lindsay 1 and Kron 3 barely show signs for mass segregation, but have low-mass star deficient global PDMFs and might be the remnants of star clusters whose outer parts were stripped. A trend exists between the degree of mass segregation and the ratio age/relaxation time tr, h, which indicates the stage of dynamical evolution for a cluster. Our data thus suggest that the SMC clusters in the present sample had a range of initial densities and presumably different amounts of mass loss that led to different rates of dynamical evolution. The clusters' positions in the rh, m/rt versus r0/rh, m plane imply that all of the clusters are tidally filled. Our SMC clusters with projected distances larger than 3kpc from the SMC center should have Jacobi radii significantly larger than their observed King tidal radii. The clusters also have higher mean densities than the estimated central density of the SMC. It is possible that these clusters formed in a denser overall environment of the younger SMC, or that the cluster structures were unusually strongly influenced by encounters with giant molecular clouds

Age Determination of Six Intermediate-age SMC Star Clusters with HST/ACS

We present a photometric analysis of the star clusters Lindsay 1, Kron 3, NGC339, NGC416, Lindsay 38, and NGC419 in the Small Magellanic Cloud (SMC), observed with the Hubble Space Telescope Advanced Camera for Surveys (ACS) in the F555W and F814W filters. Our color magnitude diagrams (CMDs) extend ~3.5 mag deeper than the main-sequence turnoff points, deeper than any previous data. Cluster ages were derived using three different isochrone models: Padova, Teramo, and Dartmouth, which are all available in the ACS photometric system. Fitting observed ridgelines for each cluster, we provide a homogeneous and unique set of low-metallicity, single-age fiducial isochrones. The cluster CMDs are best approximated by the Dartmouth isochrones for all clusters, except for NGC419 where the Padova isochrones provided the best fit. The CMD of NGC419 shows several main-sequence turn-offs, which belong to the cluster and to the SMC field. We thus derive an age range of 1.2-1.6 Gyr for NGC419. Interestingly, our intermediate-age star clusters have a metallicity spread of ~0.6 dex, which demonstrates that the SMC does not have a smooth, monotonic age-metallicity relation. We find an indication for centrally concentrated blue straggler star candidates in NGC416, while for the other clusters these are not present. Using the red clump magnitudes, we find that the closest cluster, NGC419 (~50kpc), and the farthest cluster, Lindsay 38 (~67kpc), have a relative distance of ~17kpc, which confirms the large depth of the SMC.Comment: 25 pages, 45 Figure

Virgo cluster early-type dwarf galaxies with the Sloan Digital Sky Survey. III. Subpopulations: distributions, shapes, origins

From a quantitative analysis of 413 Virgo cluster early-type dwarf galaxies (dEs) with Sloan Digital Sky Survey imaging data, we find that the dE class can be divided into multiple subpopulations that differ significantly in their morphology and clustering properties. Three dE subclasses are shaped like thick disks and show no central clustering: (1) dEs with disk features like spiral arms or bars, (2) dEs with central star formation, and (3) ordinary, bright dEs that have no or only a weak nucleus. These populations probably formed from infalling progenitor galaxies. In contrast, ordinary nucleated dEs follow the picture of classical dwarf elliptical galaxies in that they are spheroidal objects and are centrally clustered like E and S0 galaxies, indicating that they have resided in the cluster since a long time, or were formed along with it. These results define a morphology-density relation within the dE class. We find that the difference in the clustering properties of nucleated dEs and dEs with no or only a weak nucleus is not caused by selection biases, as opposed to previously reported suggestions. The correlation between surface brightness and observed axial ratio favors oblate shapes for all subclasses, but our derivation of intrinsic axial ratios indicates the presence of at least some triaxiality. We discuss possible interrelations and formation mechanisms (ram-pressure stripping, tidally induced star formation, harassment) of these dE subpopulations.Comment: 12 pages + 9 figures. Accepted for publication in Ap

Virgo cluster early-type dwarf galaxies with the Sloan Digital Sky Survey. II. Early-type dwarfs with central star formation

Despite the common picture of an early-type dwarf (dE) as a quiescent galaxy with no star formation and little gas, we identify 23 dEs that have blue central colors caused by recent or ongoing star formation in our sample of 476 Virgo cluster dEs. In addition, 14 objects that were mostly classified as (candidate) BCDs have similar properties. Among the certain cluster members, the dEs with blue centers reach a fraction of more than 15% of the dE population at brighter (B<=16) magnitudes. A spectral analysis of the centers of 16 galaxies reveals in all cases an underlying old population that dominates the mass, with M(old)>=90% for all but one object. Therefore the majority of these galaxies will appear like ordinary dEs within ~one Gigayear or less after the last episode of star formation. Their overall gas content is less than that of dwarf irregular galaxies, but higher than that of ordinary dEs. Their flattening distribution suggests the shape of a thick disk, similar to what has been found for dEs with disk features in Paper I of this series. Their projected spatial distribution shows no central clustering, and their distribution with projected local density follows that of irregular galaxies, indicative of an unrelaxed population. This is corroborated by their velocity distribution, which displays two side peaks characteristic of recent infall. We discuss possible formation mechanisms (ram-pressure stripping, tidally induced star formation, harassment) that might be able to explain both the disk shape and the central star formation of the dEs with blue centers.Comment: 16 pages + 15 figures. Accepted for publication in AJ. We recommend downloading the full resolution version from http://www.virgo-cluster.com/lisker2006b.ps.g

Stellar populations of Virgo cluster early-type dwarf galaxies with and without discs: a dichotomy in age?

[Abridged] Using VLT/FORS2 spectroscopy, we have studied the properties of the central stellar populations of a sample of 38 nucleated early-type dwarf (dE) galaxies in the Virgo Cluster. We find that these galaxies do not exhibit the same average stellar population characteristics for different morphological subclasses. The nucleated galaxies without discs are older and more metal poor than the dEs with discs . The alpha-element abundance ratio appears consistent with the solar value for both morphological types. Besides a well-defined relation of metallicity and luminosity, we also find a clear anti-correlation between age and luminosity. More specifically, there appears to be a bimodality: brighter galaxies, including the discy ones, exhibit significantly younger ages than fainter dEs. Therefore, it appears less likely that fainter and brighter dEs have experienced the same evolutionary history, as the well-established trend of decreasing average stellar age when going from the most luminous ellipticals towards low-luminosity Es and bright dEs is broken here. The older and more metal-poor dEs could have had an early termination of star formation activity, possibly being "primordial" galaxies in the sense that they have formed along with the protocluster or experienced very early infall. By contrast, the younger and relatively metal-rich brighter dEs, most of which have discs, might have undergone structural transformation of infalling disc galaxies.Comment: Accepted by MNRAS; 22 pages, 20 figure

Dissecting the Shared Genetic Architecture of Suicide Attempt, Psychiatric Disorders, and Known Risk Factors

Background Suicide is a leading cause of death worldwide, and nonfatal suicide attempts, which occur far more frequently, are a major source of disability and social and economic burden. Both have substantial genetic etiology, which is partially shared and partially distinct from that of related psychiatric disorders. Methods We conducted a genome-wide association study (GWAS) of 29,782 suicide attempt (SA) cases and 519,961 controls in the International Suicide Genetics Consortium (ISGC). The GWAS of SA was conditioned on psychiatric disorders using GWAS summary statistics via multitrait-based conditional and joint analysis, to remove genetic effects on SA mediated by psychiatric disorders. We investigated the shared and divergent genetic architectures of SA, psychiatric disorders, and other known risk factors. Results Two loci reached genome-wide significance for SA: the major histocompatibility complex and an intergenic locus on chromosome 7, the latter of which remained associated with SA after conditioning on psychiatric disorders and replicated in an independent cohort from the Million Veteran Program. This locus has been implicated in risk-taking behavior, smoking, and insomnia. SA showed strong genetic correlation with psychiatric disorders, particularly major depression, and also with smoking, pain, risk-taking behavior, sleep disturbances, lower educational attainment, reproductive traits, lower socioeconomic status, and poorer general health. After conditioning on psychiatric disorders, the genetic correlations between SA and psychiatric disorders decreased, whereas those with nonpsychiatric traits remained largely unchanged. Conclusions Our results identify a risk locus that contributes more strongly to SA than other phenotypes and suggest a shared underlying biology between SA and known risk factors that is not mediated by psychiatric disorders.Peer reviewe

Star clusters as age tracers of the age-metallicity relation of the small magellanic cloud

In my Thesis, I determined ages of Small Magellanic Cloud (SMC) star clusters that have formed during the galaxys entire lifetime. The youngest cluster ages (�10 Myr<age<1 Gyr) were derived using ground-based photometric data. For the six intermediate-age clusters Lindsay 1, Kron 3, NGC339, NGC416, Lindsay 38, and NGC419 and the only old globular cluster (GC), NGC121, observations obtained with the Hubble Space Telescope exists. This work was part of a ground-based and spacebased program to uncover the age-metallicity evolution of the SMC. In the first three parts of my Thesis, I presented accurate ages, distance estimates, and structural parameters for the seven intermediateage and old SMC star clusters. The cluster ages were determined fitting di�erent isochrone models to the observed color-magnitude diagrams (CMDs). The CMDs reach at least 3 mag below the mainsequence turno�-points, which makes it the deepest available photometry for these clusters obtained so far. Only for a few SMC clusters ages had been determined in previous studies using spacebased data. The ground-based spectroscopy was obtained with the Very Large Telescope (VLT). My photometric results are combined with these spectroscopic metallicity determinations to obtain a well-sampled age-metallicity relation. I measured structural parameters of these seven star clusters and extended the sample of known SMC clusters having accurate age measurements and structural parameters enormously. The SMC hosts a large number of intermediate-age and young star clusters, but only one ‘old’ GC, NGC121, for which I determined an age of �10.5 Gyr. Consequently, NGC121 is 2–3 Gyr younger than the oldest GCs in the Large Magellanic Cloud (LMC) and the Milky Way (MW). For comparison, the GC system of the MW exhibits a range of ages between �10.5 and 14 Gyr, similar to the LMC, with the oldest populations belonging to the most ancient surviving stellar systems. NGC121 is similar in age to the youngest GC in the Fornax dSph and to several of the young Galactic halo clusters. On the other hand, NGC121 is not as young as some of the Sgr dwarf galaxy’s GCs or the youngest Galactic GCs. With the SMC having no ‘truly’ old star cluster, it appears that the SMC has a delayed cluster formation compared to its companions. The isochrone that fitted best the CMD of NGC121 was �-enhanced. NGC121 is the only known �-enhanced star cluster in the SMC, a property that it shares with many of the old outer Galactic halo globulars and which indicates an early rapid chemical enrichment. In a subsequent project additional SMC clusters will be analyzed for possible �-abundances and elements produced through r-processes (rapid neutron capture). These �-elements and r-process elements are synthesized in quickly evolving high-mass stars. The outcoming results can be compared to chemical evolution models, which calculate chemical abundances in detail (e.g., Pagel & Tautvaiˇsien˙e 1998). With these models the chemical evolution of the SMC can be further constrained and analyzed. It is also intriguing that NGC121 is not as metal-poor as the oldest LMC andMWglobulars. The SMC must have experienced substantial enrichment prior to the formation of NGC121. In the LMC, two main epochs of the formation of compact populous clusters are observed. In the first epoch, GCs with ages and metallicities similar to the oldest MW GCs are found, but also very old GCs having a metallicity similar to NGC121, indicating very early chemical enrichment also in this galaxy. In the second epoch, numerous stellar populations with ages less than 3-4 Gyr developed. The two epochs are separated by an ‘age gap’ of �4-9 Gyr in which no star cluster has formed. Also theMWcontains old GCs that have similarly high metallicities as the younger NGC121. Evidently, the conditions for and the e�ciency of star formation varied in these three galaxies at early epochs. The fact that this cluster is younger than the Galactic mean, relatively metal-rich, and enhanced in �-elements has interesting implications for the early development of the SMC. The SMC is the only dwarf galaxy in the Local Group in which populous star clusters formed and survived for most of its lifetime. The intermediate-age clusters in the SMC appear to be capable of surviving a Hubble time, due to their high mass and the structure of the SMC (no bulge or disk to be passed). After the formation of NGC121, there is a gap of �3 Gyr and thus likely in cluster formation activity. The second oldest star cluster is Lindsay 1 for which I determined an age of �7.5 Gyr. Since then compact populous star clusters formed fairly continuously until the present day in the SMC - a contrast to both the LMC and the MW. For the youngest cluster in my sample, NGC419, me and Elena Sabbi found indication for a multiple stellar population. A more detailed analysis is in progress and the study will be published by Elena Sabbi. Only a few multiple stellar populations are known in theMW, the LMC, and the SMC, but their number is increasing also due to the improved instruments. Combining the newly derived ages with age and metallicity estimates adopted from di�erent sources in the literature, it is possible to present a well-sampled age-metallicity relation (AMR) for the SMC, which is fully based on space-based age determinations and spectroscopic metallicity measurements. The SMC has experienced an early enrichment as can be seen in the relatively metal-rich oldest SMC star cluster NGC121. The most striking feature in the AMR is the wide metallicity spread for clusters with ages around 6 Gyr indicating that the SMC was not very well mixed in the past. The mean metallicity, however, remains relatively constant for about 4 Gyr, but rises for star clusters that have formed within the past 2–3 Gyr to a present day metallicity of [Fe/H]�-0.70. From the apparent magnitudes of the cluster’s red clumps, I provided an estimate of direct distances for the clusters. Together with cluster distances from the literature that were obtained using the same approach and that are based on space-based observations, I confirmed the large depth extent of the SMC along the line-of-sight. The three oldest clusters (age>7 Gyr) are located in the north-western part of the SMC. NGC361 is a candidate for having an age older than 8 Gyr, but the age determination found in the literature is associated with large uncertainties and new space-based photometry of this cluster is needed. The youngest clusters (age<1 Gyr) lie near the SMC main body in active star forming regions. The number of intermediate-age and old SMC clusters having accurate structural parameters and reliable ages was extended enormously in this study. The galactic environment causes external perturbations such as tidal shocking that occurs as star clusters cross the disk or pass near the bulge. These processes tend to decrease the cluster mass and therefore change its structural parameters. I confirmed previous findings (Mackey & Gilmore 2003a,b) that some of the older objects in LMC and SMC have experienced a significant change in core radius, while for other old objects the core radii apparently have almost remained unchanged. The core radii of SMC clusters show a trend of older clusters having a larger spread in core radii than the younger population. Even though I extended the sample with structural parameters from the literature, the sample is highly incomplete, because only for a few intermediate-age clusters both reliable ages and corresponding profiles are available. The analysis of structural parameters of additional SMC clusters is necessary. Clusters in the LMC have experienced a similar evolution, even though the two galaxies show strong di�erences in various other aspects. The two confirmed Sagittarius clusters as well as the five Fornax clusters show the same spread in core radii. The oldest clusters in the MW, however, modified their original structure during their lifetime and have developed small cores. The largest di�erence between GCs in the MW, the LMC, and the SMC is that the MW GCs clusters are subject to much larger tidal e�ects. The biggest dynamical influence on most MW globular halo clusters is the tidal shocking that occurs when they cross the disk of the MW. Tidal shocking is likely much less e�ective in the LMC and probably even less so in the SMC. Therefore, the main reason for the smaller core radii of MW globulars is the di�erent morphology of the three galaxies. The di�erent morphologies might also be the reason for the di�erent flattening distributions of star clusters in the SMC, the LMC, and theMW. SMC clusters are more flattened than clusters in theMW and even more flattened than those in the LMC. I found that only NGC121 and Lindsay 38 exhibit a significant flattening. Galactic GCs modify their original structure and become more spherical with increasing age, while LMC and SMC clusters maintain their original shape. This might be explained with the di�erent dynamical influence and therefore the varying strength of the tidal field of the parent galaxy. The tidal fields of the LMC and SMC might not be strong enough to modify the shape of their clusters significantly. No relation between cluster age, distance from the SMC center, and ellipticity was found, but this point needs further analysis because only for a few SMC star clusters reliable ages, ellipticities and distances are available. Finally, I provided today’s largest catalog of young SMC star clusters containing ages and luminosities. The catalog covers an age range between 10 Myr and 1 Gyr. Star clusters are claimed to be produced through strong shock compressions induced by the collision of their host galaxies which causes enhanced star formation during close encounters. The most recent model calculations (e.g., Bekki & Chiba 2005, Kallivayalil et al. 2006a,b) showed that the SMC, the LMC, and the MW have only interacted long enough to produce the Magellanic Stream. The models predict the last close encounter between LMC and SMC around 200 Myr ago due to which enhanced cluster formation can be expected. The cluster age distribution combining my results with the cluster ages provided by Chiosi et al. (2006) shows indeed evidence for episodic star formation. The second of two peaks in the age distribution coincides with the model predicted closest approach of the LMC. The origin of the first peak about 6.5 Myr ago might have been triggered by internal mechanisms. Looking at their spatial distribution, the young clusters are assembled in the two large star forming HI super-shells and in the inter-shell region. Their formation might have been triggered by the expansion of the shells through gas compression. I found no indication of cluster dissolution. As mentioned above, SMC clusters evolve di�erently from MW clusters. Due to the di�erent morphologies of the parent galaxies, the tidal field of the SMC has no big influence on its star clusters. It is most likely that SMC clusters decrease their mass through stellar evolution with time until the clusters finally dissolve

Meta-analysis of COMT val158met in panic disorder: Ethnic heterogeneity and gender specificity

There is strong evidence for a genetic contribution to the pathogenesis of panic disorder, with the functional catechol-O-methyltransferase (COMT) val158met polymorphism having been suggested as a potential susceptibility factor. In the present study, a meta-analysis of six available case-control studies (557 patients with panic disorder and 763 healthy controls in total) on the role of the COMT val158met polymorphism in panic disorder was conducted in an attempt to reconcile previous conflicting results and to facilitate evaluation of the role of COMT gene variation in panic disorder. Overall, no significant association, but strong between-study heterogeneity, was discerned. Analysis of studies pooled by ancestry yielded a significant association of the COMT 158val allele with panic disorder in Caucasian samples and, conversely, a trend towards association of the COMT 158met allele with the disorder in Asian samples. Interestingly, stratification for gender as well as ethnicity revealed that association of the 158val allele in Caucasians and, reciprocally, the 158met allele in Asian samples was restricted to females. The present meta-analysis provides tentative support for the COMT val158met polymorphism as a possible risk factor for panic disorder, with differential effects in Caucasian and Asian populations, and suggests a female-specific effect. However, given the relatively small number of case-control studies presently available, several more association studies, preferably including a larger number of family-based studies, are warranted for conclusive evaluation of the COMT val158met polymorphism as a vulnerability factor in panic disorder