Clusters Of Remnants Research Articles

Dynamical Simulations of the First Globular Clusters

Abstract A Milky Way–like halo is simulated with tidally limited star clusters inserted in the dark matter halos present at high redshift. An n-body code augmented with velocity relaxation in the star clusters evolves the system. The stripped stars and remnant clusters that began in the lower mass subhalos have a distribution somewhat more extended than the dark matter halo, with a mean galactic radius of about 60 kpc inside 150 kpc, whereas the distribution of all stripped stars and clusters is more centrally concentrated than the dark matter. The clusters from low mass subhalos, those with a peak circular velocity of 12–18 , also produce most of the population’s thin stellar streams. Together these suggest a search strategy for extremely metal-poor cluster stars and their remnants. The dependence of the stellar population distribution on subhalo mass is not seen in simulations that start clusters at lower redshift. The half mass radii of the clusters are set by the tidal fields of the initial subhalo, “dwarf galaxy” location, which causes the average half mass–radius to decrease with increasing redshift of formation. Starting clusters at greater than redshift 4 leads to cluster half mass radii approximately as seen in the Milky Way.

Ultraviolet Imaging Telescope View of Dwarf Irregular Galaxy IC 2574: Is the Star Formation Triggered Due to Expanding H i Shells?

Abstract Star-forming dwarf galaxies in the local volume are diverse and ideal test beds to understand the details of star formation in a variety of environments. Here we present a deep far-UV (FUV) imaging study of the nearby dwarf irregular galaxy IC 2574 using the Ultraviolet Imaging Telescope. We identified 419 FUV-bright regions with radii between 15 and 285 pc in the galaxy and found that 28.6% of them are located in H i shells, 12.6% inside holes, and 60.1% away from the holes. The H i column density is found to be more than 1021 cm−2 for 82.3% of the identified regions. Thirty of the 48 H i holes show triggered star formation in their shells, while 16 holes do not show any related FUV emission. Cross-matching with Hα emission, we found that 23 holes have both FUV and Hα emission in their shells, signifying a very recent trigger. Therefore, star formation in the galaxy has been partly triggered due to the expanding H i holes, whereas in the majority of the sites, it is driven by other mechanisms. Irrespective of the location, larger star-forming complexes were found to have multiple substructures. We report two resolved components for the remnant cluster of the supergiant shell and estimate their masses. The star formation rate of IC 2574 is found to be 0.57 M ⊙ yr−1, which is slightly higher compared to the average value of other nearby dwarf irregular galaxies.

Histology of the Ovary of the Laying Hen (Gallus domesticus).

The laying hen (Gallus domesticus) is a robust animal model for epithelial ovarian cancer. The use of animal models is critical in identifying early disease markers and developing and testing chemotherapies. We describe the microscopic characteristics of the normally functioning laying hen ovary and proximal oviduct to establish baselines from which lesions associated with ovarian cancer can be more readily identified. Ovaries and oviducts were collected from 18-month-old laying hens (n = 18) and fixed in 10% neutral buffered formalin. Hematoxylin- and eosin-stained sections were examined by light microscopy. Both post-ovulatory follicular regression and atresia of small follicles produce remnant clusters of vacuolated cells with no histological evidence that scar tissue persists. Infiltrates of heterophils are associated with atresia of small follicles, a relationship not previously documented in laying hen ovaries. Because these tissues can be mistaken for cancerous lesions, we present a detailed histological description of remnant Wolffian tissues in the laying hen ovary. Immunohistochemical staining for pancytokeratin produced a positive response in ovarian surface epithelium and staining for vimentin produced a positive response in granulosa cells of follicles. Epithelial cells lining glands of the remnant epoöphoron had a positive response to both pancytokeratin and vimentin, a result also observed in women.

The growth of massive stars via stellar collisions in ensemble star clusters

Recent simulations and observations suggest that star clusters form via the assembling of smaller sub-clusters. Because of their short relaxation time, sub-clusters experience core collapse much earlier than virialized solo-clusters, which have similar properties of the merger remnant of the assembling clusters. As a consequence it seems that the assembling clusters result in efficient multiple collisions of stars in the cluster core. We performed a series of $N$-body simulations of ensemble and solitary clusters including stellar collisions and found that the efficiency of multiple collisions between stars are suppressed if sub-clusters assemble after they experience core collapse individually. In this case, sub-clusters form their own multiple collision stars which experienced a few collisions, but they fail to collide with each other after their host sub-clusters assemble. The multiple collision stars scatter each other and escape, and furthermore the central density of the remnant clusters had already been depleted for the stars to experience more collisions. On the other hand, if sub-clusters assemble before they experience core collapse, the multiple collisions of stars proceed efficiently in the remnant cluster, and the collision products are more massive than virialized solo-clusters and comparable in mass to cold solo-clusters.

THE CHEMICAL SIGNATURE OF A RELIC STAR CLUSTER IN THE SEXTANS DWARF SPHEROIDAL GALAXY—IMPLICATIONS FOR NEAR-FIELD COSMOLOGY

We present tentative evidence for the existence of a dissolved star cluster at [Fe/H]=-2.7 in the Sextans dwarf spheroidal galaxy. We use the technique of chemical tagging to identify stars that are highly clustered in a multi-dimensional chemical abundance space (C-space). In a sample of six stars, three, possibly four stars are identified as potential cluster stars. The initial stellar mass of the parent cluster is estimated from two independent observations to M*,init=1.9^{+1.5}_{-0.9} (1.6^{+1.2}_{-0.8}) x 10^5 M_sol, assuming a Salpeter (Kroupa) initial mass function (IMF). If corroborated by follow-up spectroscopy, this star cluster is the most metal-poor system identified to date. Chemical signatures of remnant clusters in dwarf galaxies like Sextans provide us with a very powerful probe to the high-redshift Universe. From available observational data, we argue that the average star cluster mass in the majority of the newly discovered ultra-faint dwarf galaxies was notably lower than it is in the Galaxy today and possibly lower than in the more luminous, classical dwarf spheroidal galaxies. Furthermore, the mean cumulative metallicity function of the dwarf spheroidals falls below that of the ultra-faints, which increases with increasing metallicity as predicted from our stochastic chemical evolution model. These two findings, together with a possible difference in the <[Mg/Fe]> ratio suggest that the ultra-faint dwarf galaxy population, or a significant fraction thereof, and the dwarf spheroidal population, were formed in different environments and would thus be distinct in origin.

Star formation in the Galactic bulge: the role of collisions

AbstractPrompted by the possibility that we have observed star formation triggered by globular cluster and dwarf spheroidal transits through galactic disks, we have examined kinematic evidence as to whether the superstar clusters in the Galactic bulge could have been formed from such transits. From their trajectores, we cannot exclude such a possibility. We note also that the high frequency of these transits may have generated rapid star formation at early times, and that remnant cluster cores may themselves nucleate further star formation.

Removal of chylomicron remnants in transgenic mice overexpressing normal and membrane-anchored hepatic lipase

The LDL receptor and the LDL receptor-related protein (LRP) mediate the removal of chylomicron remnants. The LRP pathway involves sequestration of particles in the space of Disse. It has been proposed that either alone or in combination with other factors, such as apolipoprotein E and proteoglycans, hepatic lipase (HL) may contribute to the sequestration of chylomicron remnants. To test this hypothesis, we generated two lines of transgenic mice producing rat HL as a native or as a membrane-anchored form. These animals express HL at levels similar to normal rat. Chylomicron remnants were perfused in a single nonrecirculating pass into the livers of the rat HL transgenic, HL-deficient, and wild-type (WT) mice for 20 min, and the rate of chylomicron remnant removal was measured. Chylomicron remnants were removed at a rate of approximately 50% per pass in WT mice. It was slightly increased in both transgenic mice and reduced in HL-deficient mice compared with the WT mice. Confocal microscopy of liver sections showed that a modest amount of HL colocalized with chylomicron remnant clusters in the transgenic mice, suggesting that HL is a component of the LRP-proteoglycan clusters. These data suggest that HL helps to direct cholesterol to the tissues in which it is localized by a nonenzymatic mechanism.

Population consequences of mutual attraction between settling and adult barnacles

Summary Spatial patterns of recruitment were compared between populations of the rocky shore barnacles (Crustacea: Cirripedia) Semibalanus balanoides, an obligate cross‐fertilizer, and Chthamalus montagui, which can self‐fertilize. We tested the hypothesis that recruitment depends on a behaviourally mediated interaction at settlement between the effects of adult background density and adult spacing, which limit free space for settlement and mating opportunities, respectively. Recruitment to patches of cleared rock (10‐ or 30‐cm diameter) was compared between replicate shores with background densities of adult barnacles classed as low (≤20 per 25 cm2) and high (&gt;4× low). Replicate patches were cleared of all barnacles surrounding a remnant cluster, comprising 0, 1, 2, 4, 8, 16 or 32 adults. For S. balanoides, settlement and subsequent recruitment over 5 months varied in direct proportion to remnant cluster size, except on the shore with the highest background density where recruitment was inversely proportional to cluster size. We interpret this inversion to indicate attraction to potential mates at low adult densities (positive density dependence, an Allee effect) switching to attraction to free space at high density (negative density dependence). The strengths and slopes of the regressions increased from shores with the lowest to the highest overall recruitment of barnacles, but retained significance over a five‐fold range in recruitment. Positive effects of cluster size on recruitment were consistent between consecutive years, despite considerable variation in recruitment densities. In contrast, recruitment of C. montagui was generally more weakly proportional to cluster size, except for a strong positive correlation at the shore with the highest recruitment. Dispersion of recruits within treatment patches was accurately modelled by a computer simulation that allowed each barnacle to settle at random between fixed minimum and maximum distances from the nearest other settled barnacle. The model estimated threshold distances by maximum likelihood fit to observed recruitment into concentric annuli around the central adult cluster. Upper thresholds of separation corresponded to penis length for 65% of S. balanoides and 42% of C. montagui patches. Lower thresholds were ∼2× cyprid length for 75% of S. balanoides patches, but were larger than this for C. montagui patches.

LDL receptor-related protein mediates cell-surface clustering and hepatic sequestration of chylomicron remnants in LDLR-deficient mice.

It has been proposed that in the liver, chylomicron remnants (lipoproteins carrying dietary lipid) may be sequestered before being internalized by hepatocytes. To study this, chylomicron remnants labeled with a fluorescent dye were perfused into isolated livers of LDL receptor-deficient (LDLR-deficient) mice (Ldlr(-/-)) and examined by confocal microscopy. In contrast to livers from normal mice, there was clustering of the chylomicron remnants on the cell surface in the space of DISSE: These remnant clusters colocalized with clusters of LDLR-related protein (LRP) and could be eliminated by low concentrations of receptor-associated protein, an inhibitor of LRP. When competed with ligands of heparan sulfate proteoglycans (HSPGs), the remnant clusters still appeared but were fewer in number, although syndecans (membrane HSPGs) colocalized with the remnant clusters. This suggests that the clustering of remnants is not dependent on syndecans but that the syndecans may modify the binding of remnants. These results establish that sequestration is a novel process, the clustering of remnants in the space of DISSE: The clustering involves remnants binding to the LRP, and this may be stabilized by binding with syndecans, eventually followed by endocytosis.

Apoptosis of postovulatory cumulus granulosa cells of the rat.

The process of apoptosis in the postovulatory cumulus granulosa cells was investigated in pregnant rats. Mature female Wistar rats, exhibiting a regular 4-day oestrous cycle, were placed with males on the day of pro-oestrus. The following day, on which spermatozoa were found in vaginal smears, was designated day 1 of pregnancy. The animals were killed just before ovulation (24.00 hours), on days 1 (5.00, 11.00, and 18.00 hours), and 2 ( 11.00 hours) of pregnancy. Excised ovaries and oviducts were submitted to a routine histological procedure and paraplast sections were subjected to detection of apoptotic cells using the TUNEL method. The cumulus granulosa cells of preovulatory follicles (24.00 hours) were negative for apoptotic staining. However, 5 h later a positive staining was observed in the oviduct ampulla and included the cumulus granulosa cells lying in the peripheral parts of postovulatory cumuli oophori, and the oviductal epithelial cells of this region. On the evening of day 1 almost all cumulus granulosa cells showed strong immunostaining while on day 2 at 11.00 hours only immunonegative clusters of remnants of cumulus granulosa cells were present in the distended ampulla region, while naked, two or more cell embryos were present in the further parts of oviduct. These results indicate that in the rat apoptosis of cumulus granulosa cells starts shortly after ovulation in the peripheral region. Epithelial ampullary cells surrounding ovulated cumuli show a massive apoptosis.

A Test of the Standard Hypothesis for the Origin of the H [CSC]i[/CSC] Holes in Holmberg II

The nearby irregular galaxy Holmberg II (Ho II, DDO 50) has been extensively mapped in H I using the Very Large Array, revealing intricate structure in its interstellar gas component, as reported by Puche et al. in 1992. An analysis of these structures shows the neutral gas to contain a number of expanding H I holes. The formation of the H I holes has been attributed to multiple supernova (SN) events occurring within wind-blown shells around young, massive star clusters, with as many as 10–200 SNe required to produce many of the holes. From the sizes and expansion velocities of the holes, Puche et al. assigned ages of ~107 to 108 years. If the SN scenario for the formation of the H I holes is correct, it implies the existence of star clusters with a substantial population of late B, A, and F main-sequence stars at the centers of the holes. Many of these clusters should be detectable in deep ground-based CCD images of the galaxy. To test the SN hypothesis for the formation of the H I holes, we have obtained and analyzed deep broadband BVR and narrowband Hα images of Ho II. We compare the optical and H I data and search for evidence of the expected star clusters in and around the H I holes. We also use the H I data to constrain models of the expected remnant stellar population. Assuming that the H I holes are created by multiple SNe, that the number of SNe required can be determined from the observed energetics of the holes, and that the SNe represent the high-mass population of a cluster with a normal initial mass function, we show that in several of the holes the observed upper limits for the remnant cluster brightness are strongly inconsistent with the SN hypothesis described by Puche et al. Moreover, many of the H I holes are located in regions of very low optical surface brightness that show no indication of recent star formation. Here we present our findings, discuss their implications, and explore possible alternative explanations for the existence of the H I holes in Ho II, including the recent suggestion that some of the holes could be produced by gamma-ray burst events.

Microscopic imaging of recorded patterns subjected to an external dc erasing magnetic field

Magnetic structures in thin-film recording media were imaged with sub-micron resolution while undergoing de erasure. Distinct changes in the patterns occurred at applied fields as low as 150 Oe below the media coercivity, and characteristic spatial variations were observed with applied fields approaching and extending beyond the coercivity. The results show that the onset of erasure is initiated by the rotation of magnetic moments at the track edges. As the field is increased, the regions magnetized parallel to the applied field expand in the direction of the field while those oriented antiparallel to it contact. Above the coercivity the unfavorably magnetized areas break up into remnant clusters whose sizes decrease as the entire region coalesces into a single magnetization structure. This paper correlates these microscopic results with macroscopic measurements of the medium and suggests possible mechanisms that lead to data erasure through media saturation.