Joseph Silk

Abstract:

Bayesian model averaging is a procedure to obtain parameter constraints that account for the uncertainty about the correct cosmological model. We use recent cosmological observations and Bayesian model averaging to derive tight limits on the curvature parameter, as well as robust lower bounds on the curvature radius of the Universe and its minimum size, while allowing for the possibility of an evolving dark energy component. Because flat models are favoured by Bayesian model selection, we find that model-averaged constraints on the curvature and size of the Universe can be considerably stronger than non-model-averaged ones. For the most conservative prior choice (based on inflationary considerations), our procedure improves on non-model-averaged constraints on the curvature by a factor of ~2. The curvature scale of the Universe is conservatively constrained to be Rc > 42 Gpc (99 per cent), corresponding to a lower limit to the number of Hubble spheres in the Universe NU > 251 (99 per cent). © 2011 The Authors. Monthly Notices of the Royal Astronomical Society © 2011 RAS.

Abstract:

We used near-infrared data obtained with the Wide Field Camera 3 on the Hubble Space Telescope to identify objects having the colors of brown dwarfs (BDs) in the field of the massive galactic cluster NGC3603. These are identified through a combination of narrow- and medium-band filters which span the J and H bands and are particularly sensitive to the presence of the 1.3-1.5 μm H2O molecular band unique to BDs. We provide a calibration of the relationship between effective temperature and color for both field stars and BDs. This photometric method provides effective temperatures for BDs to an accuracy of 350 ± K relative to spectroscopic techniques. This accuracy is shown to be not significantly affected by either stellar surface gravity or uncertainties in the interstellar extinction. We identify nine objects having effective temperatures between 1700 and 2200 K, typical of BDs, observed J-band magnitudes in the range 19.5-21.5, and that are strongly clustered toward the luminous core of NGC3603. However, if these are located at the distance of the cluster, they are far too luminous to be normal BDs. We argue that it is unlikely that these objects are either artifacts of our data set, normal field BDs/M-type giants, or extragalactic contaminants and, therefore, might represent a new class of stars having the effective temperatures of BDs but with luminosities of more massive stars. We explore the interesting scenario in which these objects would be normal stars that have recently tidally ingested a hot Jupiter, the remnants of which are providing a short-lived extended photosphere to the central star. In this case, we would expect them to show the signature of fast rotation. © 2011 The American Astronomical Society. All rights reserved.

Abstract:

We investigate the ionization structure of the nebular gas in M83 using the line diagnostic diagram, [O III](5007)/Hβ versus [S II](6716+6731)/ Hα, with the newly available narrowband images from the Wide Field Camera 3 (WFC3) of the Hubble Space Telescope (HST). We produce the diagnostic diagram on a pixel-by-pixel (02 × 02) basis and compare it with several photo- and shock-ionization models. We select four regions from the center to the outer spiral arm and compare them in the diagnostic diagram. For the photoionized gas, we observe a gradual increase of the log ([O III]/Hβ) ratios from the center to the spiral arm, consistent with the metallicity gradient, as the H II regions go from super-solar abundance to roughly solar abundance from the center out. Using the diagnostic diagram, we separate the photoionized from the shock-ionized component of the gas. We find that the shock-ionized Hα emission ranges from ∼2% to about 15%-33% of the total, depending on the separation criteria used. An interesting feature in the diagnostic diagram is a horizontal distribution around log ([O III]/Hβ) 0. This feature is well fit by a shock-ionization model with 2.0 Z metallicity and shock velocities in the range of 250-350 km s-1. A low-velocity shock component, <200 km s-1, is also detected and is spatially located at the boundary between the outer ring and the spiral arm. The low-velocity shock component can be due to (1) supernova remnants located nearby, (2) dynamical interaction between the outer ring and the spiral arm, and (3) abnormal line ratios from extreme local dust extinction. The current data do not enable us to distinguish among those three possible interpretations. Our main conclusion is that, even at the HST resolution, the shocked gas represents a small fraction of the total ionized gas emission at less than 33% of the total. However, it accounts for virtually all of the mechanical energy produced by the central starburst in M83. © 2011. The American Astronomical Society. All rights reserved.

Abstract:

We describe the Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) Early Release Science (ERS) observations in the Great Observatories Origins Deep Survey (GOODS) South field. The new WFC3 ERS data provide calibrated, drizzled mosaics in the UV filters F225W, F275W, and F336W, as well as in the near-IR filters F098M (Ys ), F125W (J), and F160W (H) with 1-2 HST orbits per filter. Together with the existing HST Advanced Camera for Surveys (ACS) GOODS-South mosaics in the BViz filters, these panchromatic 10-band ERS data cover 40-50arcmin2 at 0.2-1.7 μm in wavelength at 007-015 FWHM resolution and 0090 Multidrizzled pixels to depths of AB≃ 26.0-27.0mag (5σ) for point sources, and AB≃ 25.5-26.5mag for compact galaxies. In this paper, we describe (1) the scientific rationale, and the data taking plus reduction procedures of the panchromatic 10-band ERS mosaics, (2) the procedure of generating object catalogs across the 10 different ERS filters, and the specific star-galaxy separation techniques used, and (3) the reliability and completeness of the object catalogs from the WFC3 ERS mosaics. The excellent 007-015 FWHM resolution of HST/WFC3 and ACS makes star-galaxy separation straightforward over a factor of 10 in wavelength to AB≃ 25-26mag from the UV to the near-IR, respectively. Our main results are: (1) proper motion of faint ERS stars is detected over 6 years at 3.06 ± 0.66masyear -1 (4.6σ), consistent with Galactic structure models; (2) both the Galactic star counts and the galaxy counts show mild but significant trends of decreasing count slopes from the mid-UV to the near-IR over a factor of 10 in wavelength; (3) combining the 10-band ERS counts with the panchromatic Galaxy and Mass Assembly survey counts at the bright end (10mag ≲ AB≲ 20mag) and the Hubble Ultra Deep Field counts in the BVizYsJH filters at the faint end (24mag ≲ AB≲ 30mag) yields galaxy counts that are well measured over the entire flux range 10mag ≲ AB≲ 30mag for 0.2-2 μm in wavelength; (4) simple luminosity+density evolution models can fit the galaxy counts over this entire flux range. However, no single model can explain the counts over this entire flux range in all 10 filters simultaneously. More sophisticated models of galaxy assembly are needed to reproduce the overall constraints provided by the current panchromatic galaxy counts for 10mag ≲ AB≲ 30mag over a factor of 10 in wavelength. © 2011. The American Astronomical Society. All rights reserved.

Abstract:

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 Hα emission to estimate the ages of clusters younger than τ ≈ 10 Myr. It is based on the simple premise that the gas in very young (τ< a few Myr) clusters is largely coincident with the cluster stars, is in a small, ring-like structure surrounding the stars in slightly older clusters since massive star winds and supernovae have had time to push out the natal gas (e.g., τ 5 Myr), and is in a larger ring-like bubble for still older clusters (i.e., 5-10 Myr). If no Hα is associated with a cluster it is generally older than 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. Older clusters therefore have a smoother appearance and smaller pixel-to-pixel variations. The youngest clusters also have lower flux variations, hence the relationship is double valued. This degeneracy in age can be broken using other age indicators such as Hα morphology. 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 UBVIHα 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 τ 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" H II regions in M83, with central stars having ages ≈ 4 Myr. © 2011. The American Astronomical Society. All rights reserved..