We present the highest spatial resolution (~0.5) CO (3-2) observations to date of the overlap region in the merging Antennae galaxies (NGC 4038/39), taken with the ALMA. We report on the discovery of a long (3 kpc), thin (aspect ratio 30/1), filament
of CO gas which breaks up into roughly ten individual knots. Each individual knot has a low internal velocity dispersion (~10 km/s), and the dispersion of the ensemble of knots in the filament is also low (~10 km/s). At the other extreme, we find that the individual clouds in the Super Giant Molecular Cloud 2 region discussed by Wilson and collaborators have a large range of internal velocity dispersions (10 to 80 km/s), and a large dispersion amongst the ensemble (~80 km/s). We use a combination of optical and near-IR data from HST, radio continuum observations taken with the VLA, and CO data from ALMA to develop an evolutionary classification system which provides a framework for studying the sequence of star cluster formation and evolution, from diffuse SGMCs, to proto, embedded, emerging, young, and intermediate/old clusters. The relative timescales have been assessed by determining the fractional population of sources at each evolutionary stage. Using the evolutionary framework, we estimate the maximum age range of clusters in a single SGMC is ~10 Myr, which suggests that the molecular gas is removed over this timescale resulting in the cessation of star formation and the destruction of the GMC within a radius of about 200 pc. (abridged)
We study star clusters in two fields in the nearby spiral galaxy M83 using broad and narrow band optical imaging taken with the WFC3 on-board HST. We present results based on several different catalogs of star clusters in an inner and outer field, an
d conclude that different methods of selection do not strongly impact the results, particularly for clusters older than $approx$10 Myr. The age distributions can be described by a power law, $dN/dtau proptotau^{gamma}$, with $gammaapprox -$0.84$pm$0.12 in the inner field, and $gammaapprox -$0.48$pm$0.12 in the outer field for $taugtrsim$10 Myr. We bracket the difference, $Delta gamma$, between the two fields to be in the range 0.18$-$0.36, based on estimates of the relative star formation histories. The mass functions can also be described by a power law, $dN/dMpropto M^{beta}$, with $betaapprox -$1.98$pm$0.14 and $betaapprox $2.34$pm$0.26 in the inner and outer fields, respectively. We conclude that the shapes of the mass and age distributions of the clusters in the two fields are similar, as predicted by the quasi-universal model. Any differences between the two fields are at the $approx$2$-$3$sigma (approx$1$-$2$sigma)$ level for the age (mass) distributions. Therefore any dependence of these distributions on the local environment is probably weak. We compare the shapes of the distributions with those predicted by two popular cluster disruption models, and find that both show evidence that the clusters are disrupted at a rate that is approximately independent of their mass, but that the observational results do not support the earlier disruption of lower mass clusters relative to their higher mass counterparts.
Luminosity functions have been determined for star cluster populations in 20 nearby (4-30 Mpc), star-forming galaxies based on ACS source lists generated by the Hubble Legacy Archive. Comparisons are made with other recently generated cluster catalog
s demonstrating that the HLA-generated catalogs are of similar quality, but in general do not go as deep. A typical cluster luminosity function can be approximated by a power-law, $dN/dLpropto L^{alpha}$, with an average value for $alpha$ of $-2.37$ and RMS scatter = 0.18 when using the F814W ($I$) band. We find that galaxies with high rates of star formation (or equivalently, with the brightest or largest numbers of clusters) have a slight tendency to have shallower values of $alpha$. In particular, the Antennae galaxy (NGC4038/39), a merging system with a relatively high star formation rate, has the second flattest luminosity function in the sample. A tentative correlation may also be present between Hubble Type and values of $alpha$, in the sense that later type galaxies (i.e., Sd and Sm) appear to have flatter luminosity functions. Hence, while there do appear to be some weak correlations, the relative similarity in the values of $alpha$ for a large number of star-forming galaxies suggests that, to first order, the LFs are fairly universal. We examine the bright end of the luminosity functions and find evidence for a downturn, although it only pertains to about 1% of the clusters. Our uniform database results in a small scatter ($approx$0.4 to 0.5 mag) in the correlation between the magnitude of the brightest cluster ($M_mathrm{brightest}$) and log of the number of clusters brighter than $M_{I}=-9$ (log N). We also examine the magnitude of the brightest cluster vs. log SFR for a sample including both dwarfs galaxies and ULIRGS.
Detailed spectral classifications are presented for 352 O-B0 stars in the VLT-FLAMES Tarantula Survey, of which 213 O-type are of sufficient quality for further morphological analysis. Among them, six subcategories of special interest are distinguish
ed. (1) Several new examples of the earliest spectral types O2-O3 have been found. (2) A group of extremely rapidly rotating main-sequence objects has been isolated, including the largest $vsin i$ values known, the spatial and radial-velocity distributions of which suggest ejection from the two principal ionizing clusters. (3) Several new examples of the evolved, rapidly rotating Onfp class show similar evidence. (4) No fewer than 48 members of the Vz category, hypothesized to be on or near the ZAMS, are found in this sample; in contrast to the rapid rotators, they are strongly concentrated to the ionizing clusters, supporting their interpretation as very young objects, as do their relatively faint absolute magnitudes. (5) A surprisingly large fraction of the main-sequence spectra belong to the recently recognized V((fc)) class, with C III emission lines of similar strength to the usual N III in V((f)) spectra; there are also six objects with very high-quality data but no trace of either mission feature, presenting new challenges to physical interpretations. (6) Five spectra with morphologically enhanced nitrogen lines have been detected. Absolute visual magnitudes have been derived for each star with individual extinction laws, and composite HRDs provide evidence of the multiple generations present in this field. Associations with X-ray sources are noted. Further analyses of this unique dataset underway will provide new insights into the evolution of massive stars and starburst clusters.
The most luminous Spitzer point sources in the 30 Doradus triggered second generation are investigated coherently in the 3-8 micron region. Remarkable diversity and complexity in their natures are revealed. Some are also among the brightest JHK sourc
es, while others are not. Several of them are multiple when examined at higher angular resolutions with HST NICMOS and WFPC2/WFC3 as available, or with VISTA/VMC otherwise. One is a dusty compact H II region near the far northwestern edge of the complex, containing a half dozen bright I-band sources. Three others appear closely associated with luminous WN stars and causal connections are suggested. Some are in the heads of dust pillars oriented toward R136, as previously discussed from the NICMOS data. One resides in a compact cluster of much fainter sources, while another appears monolithic at the highest resolutions. Surprisingly, one is the brighter of the two extended mystery spots associated with Knot 2 of Walborn et al. Masses are derived from YSO models for unresolved sources and lie in the 10-30 M_{sun} range. Further analysis of the IR sources in this unique region will advance understanding of triggered massive star formation, perhaps in some unexpected and unprecedented ways.
We describe circular polarization as a remote sensing diagnostic of chiral signatures which may be applied to Mars. The remarkable phenomenon of homochirality provides a unique biosignature which can be amenable to remote sensing through circular pol
arization spectroscopy. The natural tendency of microbes to congregate in close knit communities would be beneficial for such a survey. Observations of selected areas of the Mars surface could reveal chiral signatures and hence explore the possibility of extant or preserved biological material. We describe a new instrumental technique that may enable observations of this form.
We present an approach to spectropolarimetry which requires neither moving parts nor time dependent modulation, and which offers the prospect of achieving high sensitivity. The technique applies equally well, in principle, in the optical, UV or IR. T
he concept, which is one of those generically known as channeled polarimetry, is to encode the polarization information at each wavelength along the spatial dimension of a 2D data array using static, robust optical components. A single two-dimensional data frame contains the full polarization information and can be configured to measure either two or all of the Stokes polarization parameters. By acquiring full polarimetric information in a single observation, we simplify polarimetry of transient sources and in situations where the instrument and target are in relative motion. The robustness and simplicity of the approach, coupled to its potential for high sensitivity, and applicability over a wide wavelength range, is likely to prove useful for applications in challenging environments such as space.
The physical relationship between low-excitation gas filaments at ~10^4 K, seen in optical line emission, and diffuse X-ray emitting coronal gas at ~10^7 K in the centers of many galaxy clusters is not understood. It is unclear whether the ~10^4 K fi
laments have cooled and condensed from the ambient hot (~10^7 K) medium or have some other origin such as the infall of cold gas in a merger, or the disturbance of an internal cool reservoir of gas by nuclear activity. Observations of gas at intermediate temperatures (~10^5-10^6 K) can potentially reveal whether the central massive galaxies are gaining cool gas through condensation or losing it through conductive evaporation and hence identify plausible scenarios for transport processes in galaxy cluster gas. Here we present spectroscopic detection of ~10^5 K gas spatially associated with the H-alpha filaments in a central cluster galaxy, M87 in the Virgo Cluster. The measured emission-line fluxes from triply ionized carbon (CIV 1549 A) and singly ionized helium (HeII 1640 A) are consistent with a model in which thermal conduction determines the interaction between hot and cold phases.
With new data from the Galactic O-Star Spectroscopic Survey, we confirm and expand the ONn category of late-O, nitrogen-enriched (N), rapidly rotating (n) giants. In particular, we have discovered two clones (HD 102415 and HD 117490) of one of the mo
st rapidly rotating O stars previously known (HD 191423, Howarths Star). We compare the locations of these objects in the theoretical HR Diagram to those of slowly rotating ON dwarfs and supergiants. All ON giants known to date are rapid rotators, whereas no ON dwarf or supergiant is; but all ON stars are small fractions of their respective spectral-type/luminosity-class/rotational subcategories. The ONn giants, displaying both substantial processed material and high rotation at an intermediate evolutionary stage, may provide significant information about the development of those properties. They may have preserved high initial rotational velocities or been spun up by TAMS core contraction; but alternatively and perhaps more likely, they may be products of binary mass transfer. At least some of them are also runaway stars.
We use new WFC3 observations of the nearby grand design spiral galaxy M83 to develop two independent methods for estimating the ages of young star clusters. The first method uses the physical extent and morphology of Halpha emission to estimate the a
ges of clusters younger than tau ~10 Myr. It is based on the simple premise that the gas in very young (tau < few Myr) clusters is largely coincident with the cluster stars, is in a small, ring-like structure surrounding the stars in slightly older clusters (e.g., tau ~5 Myr), and is in a larger ring-like bubble for still older clusters (i.e., ~5-10 Myr). The second method is based on an observed relation between pixel-to-pixel flux variations within clusters and their ages. This method relies on the fact that the brightest individual stars in a cluster are most prominent at ages around 10 Myr, and fall below the detection limit (i.e., M_V < -3.5) for ages older than about 100 Myr. These two methods are the basis for a new morphological classification system which can be used to estimate the ages of star clusters based on their appearance. We compare previous age estimates of clusters in M83 determined from fitting UBVI Halpha measurements using predictions from stellar evolutionary models with our new morphological categories and find good agreement at the ~95% level. The scatter within categories is ~0.1 dex in log tau for young clusters (<10 Myr) and ~0.5 dex for older (>10 Myr) clusters. A by-product of this study is the identification of 22 single-star HII regions in M83, with central stars having ages ~4 Myr.
