Lutherville, MD, United States
Lutherville, MD, United States

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Ryan Jr. R.E.,University of California at Davis | McCarthy P.J.,Observatories of the Carnegie Institute of Washington | Cohen S.H.,Arizona State University | Yan H.,Ohio State University | And 28 more authors.
Astrophysical Journal | Year: 2012

We present the size evolution of passively evolving galaxies at z 2 identified in Wide-Field Camera 3 imaging from the Early Release Science program. Our sample was constructed using an analog to the passive BzK galaxy selection criterion, which isolates galaxies with little or no ongoing star formation at z ≳ 1.5. We identify 30 galaxies in 40arcmin2 to H < 25mag. By fitting the 10-band Hubble Space Telescope photometry from 0.22 μm ≲ λobs ≲ 1.6 μm with stellar population synthesis models, we simultaneously determine photometric redshift, stellar mass, and a bevy of other population parameters. Based on the six galaxies with published spectroscopic redshifts, we estimate a typical redshift uncertainty of 0.033(1 + z). We determine effective radii from Sérsic profile fits to the H-band image using an empirical point-spread function. By supplementing our data with published samples, we propose a mass-dependent size evolution model for passively evolving galaxies, where the most massive galaxies (M * 1011 M) undergo the strongest evolution from z 2 to the present. Parameterizing the size evolution as (1 + z)-α, we find a tentative scaling of α (- 0.6 0.7) + (0.9 0.4)log (M */109 M), where the relatively large uncertainties reflect the poor sampling in stellar mass due to the low numbers of high-redshift systems. We discuss the implications of this result for the redshift evolution of the M *-Re relation for red galaxies. © 2012. The American Astronomical Society All rights reserved.

Liu G.,University of Massachusetts Amherst | Liu G.,Johns Hopkins University | Calzetti D.,University of Massachusetts Amherst | Hong S.,University of Massachusetts Amherst | And 9 more authors.
Astrophysical Journal Letters | Year: 2013

We present Hubble Space Telescope/WFC3 narrow-band imaging of the starburst galaxy M83 targeting the hydrogen recombination lines (Hβ, Hα, and Paβ), which we use to investigate the dust extinction in the H II regions. We derive extinction maps with 6 pc spatial resolution from two combinations of hydrogen lines (Hα/Hβ and Hα/Paβ), and show that the longer wavelengths probe larger optical depths, with AV values larger by ≳1 mag than those derived from the shorter wavelengths. This difference leads to a factor ≳2 discrepancy in the extinction-corrected Hα luminosity, a significant effect when studying extragalactic H II regions. By comparing these observations to a series of simple models, we conclude that a large diversity of absorber/emitter geometric configurations can account for the data, implying a more complex physical structure than the classical foreground "dust screen" assumption. However, most data points are bracketed by the foreground screen and a model where dust and emitters are uniformly mixed. When averaged over large (≳100-200 pc) scales, the extinction becomes consistent with a "dust screen," suggesting that other geometries tend to be restricted to more local scales. Moreover, the extinction in any region can be described by a combination of the foreground screen and the uniform mixture model with weights of 1/3 and 2/3 in the center (≲2 kpc), respectively, and 2/3 and 1/3 for the rest of the disk. This simple prescription significantly improves the accuracy of the dust extinction corrections and can be especially useful for pixel-based analyses of galaxies similar to M83. © 2013. The American Astronomical Society. All rights reserved.

Kim H.,Arizona State University | Whitmore B.C.,US Space Telescope Science Institute | Chandar R.,University of Toledo | Saha A.,U.S. National Optical Astronomy Observatories | And 23 more authors.
Astrophysical Journal | Year: 2012

We present a multi-wavelength photometric study of 15,000 resolved stars in the nearby spiral galaxy M83 (NGC5236, D = 4.61Mpc) based on Hubble Space Telescope Wide Field Camera 3 observations using four filters:F336W, F438W, F555W, and F814W. We select 50 regions (an average size of 260pc by 280pc) in the spiral arm and inter-arm areas of M83 and determine the age distribution of the luminous stellar populations in each region. This is accomplished by correcting for extinction toward each individual star by comparing its colors with predictions from stellar isochrones. We compare the resulting luminosity-weighted mean ages of the luminous stars in the 50 regions with those determined from several independent methods, including the number ratio of red-to-blue supergiants, morphological appearance of the regions, surface brightness fluctuations, and the ages of clusters in the regions. We find reasonably good agreement between these methods. We also find that young stars are much more likely to be found in concentrated aggregates along spiral arms, while older stars are more dispersed. These results are consistent with the scenario that star formation is associated with the spiral arms, and stars form primarily in star clusters and then disperse on short timescales to form the field population. The locations of Wolf-Rayet stars are found to correlate with the positions of many of the youngest regions, providing additional support for our ability to accurately estimate ages. We address the effects of spatial resolution on the measured colors, magnitudes, and age estimates. While individual stars can occasionally show measurable differences in the colors and magnitudes, the age estimates for entire regions are only slightly affected. © 2012. The American Astronomical Society. All rights reserved.

Kaviraj S.,Imperial College London | Kaviraj S.,University of Oxford | Mark Crockett R.,University of Oxford | Whitmore B.C.,US Space Telescope Science Institute | And 9 more authors.
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2012

We combine near-ultraviolet (NUV; 2250 Å) and optical (U, B, V, I) imaging from the Wide Field Camera 3 (WFC3), on-board the Hubble Space Telescope (HST), to study the globular cluster (GC) population in NGC 4150, a sub-L* (M B ~-18.48 mag) early-type minor-merger remnant in the Coma I cloud. We use broad-band NUV-optical photometry from the WFC3 to estimate individual ages, metallicities, masses and line-of-sight extinctions [E(B - V)] for 63 bright (M V < -5 mag) GCs in this galaxy. In addition to a small GC population with ages greater than 10 Gyr, we find a dominant population of clusters with ages centred around 6 Gyr, consistent with the expected peak of stellar mass assembly in faint early types residing in lowdensity environments. The old and intermediate-age GCs in NGC 4150 are metal poor, with metallicities less than 0.1 Z, and reside in regions of low extinction [E(B - V) < 0.05 mag]. We also find a population of young, metal-rich (Z > 0.3 Z) clusters that have formed within the last Gyr and reside in relatively dusty [E(B - V) > 0.3 mag] regions that are coincident with the part of the galaxy core that hosts significant recent star formation. Cluster disruption models (in which ~80-90 per cent of objects younger than a few ×10 8 yr dissolve every dex in time) suggest that the bulk of these young clusters are a transient population. © 2012 The Authors. Monthly Notices of the Royal Astronomical Society © 2012 RAS.

Andrews J.E.,University of Massachusetts Amherst | Calzetti D.,University of Massachusetts Amherst | Chandar R.,University of Toledo | Lee J.C.,US Space Telescope Science Institute | And 12 more authors.
Astrophysical Journal | Year: 2013

The production rate of ionizing photons in young (≤8 Myr), unresolved stellar clusters in the nearby irregular galaxy NGC 4214 is probed using multi-wavelength Hubble Space Telescope WFC3 data. We normalize the ionizing photon rate by the cluster mass to investigate the upper end of the stellar initial mass function (IMF). We have found that within the uncertainties the upper end of the stellar IMF appears to be universal in this galaxy, and that deviations from a universal IMF can be attributed to stochastic sampling of stars in clusters with masses 103MO. Furthermore, we have found that there does not seem to be a dependence of the maximum stellar mass on the cluster mass. We have also found that for massive clusters, feedback may cause an underrepresentation in Hα luminosities, which needs to be taken into account when conducting this type of analysis. © 2013. The American Astronomical Society. All rights reserved.

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