Publications of the Astronomical Society of the Pacific

Armus, L.; Mazzarella, J. M.; Evans, A. S.; Surace, J. A.; Sanders, D. B.; Iwasawa, K.; Frayer, D. T.; Howell, J. H.; Chan, B.; Petric, A.; Vavilkin, T.; Kim, D. C.; Haan, S.; Inami, H.; Murphy, E. J.; Appleton, P. N.; Barnes, J. E.; Bothun, G.; Bridge, C. R.; Charmandaris, V.; Jensen, J. B.; Kewley, L. J.; Lord, S.; Madore, B. F.; Marshall, J. A.; Melbourne, J. E.; Rich, J.; Satyapal, S.; Schulz, B.; Spoon, H. W. W.; Sturm, E.; U, V.; Veilleux, S.; Xu, K.

doi: 10.1086/600092pmid: N/A

The Great Observatories All-Sky LIRG Survey (GOALS20) combines data from NASA’s Spitzer Space Telescope,Chandra X-Ray Observatory,Hubble Space Telescope (HST), and Galaxy Evolution Explorer (GALEX) observatories, together with ground-based data, into a comprehensive imaging and spectroscopic survey of over 200 low-redshift (z < 0.088), Luminous Infrared Galaxies (LIRGs). The LIRGs are a complete subset of the IRAS Revised Bright Galaxy Sample (RBGS), which comprises 629 extragalactic objects with 60 μm flux densities above 5.24 Jy, and Galactic latitudes above five degrees. The LIRGs targeted in GOALS span the full range of nuclear spectral types defined via traditional optical line-ratio diagrams (type-1 and type-2 AGN, LINERs, and starbursts) as well as interaction stages (major mergers, minor mergers, and isolated galaxies). They provide an unbiased picture of the processes responsible for enhanced infrared emission in galaxies in the local Universe. As an example of the analytic power of the multiwavelength GOALS data set, we present Spitzer, Chandra, HST, and GALEX images and spectra for the interacting system VV 340 (IRAS F14547 + 2449). The Spitzer MIPS imaging data indicates that between 80–95% of the total far-infrared emission (or about 5 × 1011 L⊙) originates in VV 340 north. While the Spitzer IRAC colors of VV 340 north and south are consistent with star-forming galaxies, both the Spitzer IRS and Chandra ACIS data indicate the presence of an AGN in VV 340 north. The observed line fluxes, without correction for extinction, imply that the AGN accounts for less than 10%–20% of the observed infrared emission. The X-ray data are consistent with a heavily absorbed (NH≥1024 cm-2) AGN. The GALEX far and near-UV fluxes imply a extremely large infrared “excess” (IRX) for the system (FIR/Ffuv ∼ 81) which is well above the correlation seen in starburst galaxies. Most of this excess is driven by VV 340 N, which has an IR excess of nearly 400. The VV 340 system seems to be comprised of two very different galaxies: an infrared luminous edge-on galaxy (VV 340 north) that dominates the long-wavelength emission from the system, which hosts a buried AGN; and a face-on starburst (VV 340 south) that dominates the short-wavelength emission.

Martell, Sarah L.; Smith, Graeme H.

doi: 10.1086/599979pmid: N/A

We present a comparison of CN band strength variations in the high-metallicity globular clusters NGC 6356 and NGC 6528 with those measured in the old open clusters NGC 188, NCG 2158 and NGC 7789. Star-to-star abundance variations, of which CN differences are a readily observable sign, are commonplace in moderate-metallicity halo globular clusters but are unseen in the field or in open clusters. We find that the open clusters have narrow, unimodal distributions of CN band strength, as expected from the literature, while the globular clusters have broad, bimodal distributions of CN band strength, similar to moderate-metallicity halo globular clusters. This result has interesting implications for the various mechanisms proposed to explain the origin of globular cluster abundance inhomogeneities, and suggests that the local environment at the epoch of cluster formation plays a vital role in regulating intracluster enrichment processes.

Fatuzzo, Marco; Melia, Fulvio

doi: 10.1086/603529pmid: N/A

Molecular clouds at the Galactic center (GC) have environments considerably different from their disk counterparts. The GC may therefore provide important clues about how the environment affects star formation. Interestingly, while the inner 50 pc of our Galaxy include a remarkable population of high-mass stars, the initial mass function (IMF) appears to be consistent with a Salpeter slope down to ∼1 M⊙. We show here that the loss of turbulent pressure due to ambipolar diffusion and the damping of Alfvén and fast MHD waves can lead to the formation of dense condensations exceeding their Jeans limit. The fragmentation and subsequent collapse of these condensations is similar to the diffusion-driven protostellar collapse mechanism expected to occur within nearby “regular” molecular clouds. As such, a Salpeter IMF at the GC is not surprising, though the short dynamical timescales associated with the GC molecular clouds may help explain the lower star formation efficiency observed from this region.

Mauerhan, Jon C.; Van Dyk, Schuyler D.; Morris, Pat W.

doi: 10.1086/603544pmid: N/A

We report new results from our effort to identify obscured Wolf-Rayet stars in the Galaxy. Candidates were selected by their near-infrared (Two Micron All Sky Survey; 2MASS) and mid-infrared (Spitzer/GLIMPSE) color excesses, which are consistent with free-free emission from ionized stellar winds and thermal excess from hot dust. We have confirmed 12 new Wolf-Rayet stars in the Galactic disk, including nine of the nitrogen subtype (WN), and three of the carbon subtype (WC); this raises the total number of Wolf-Rayet stars discovered with our approach to 27. We classify one of the new stars as a possible dust-producing WC9d + OBI colliding-wind binary, as evidenced by an infrared excess resembling that of known WC9d stars, the detection of OBI features superimposed on the WC9 spectrum, and hard X-ray emission detected by XMM-Newton. A WC8 star in our sample appears to be a member of the stellar cluster Danks 1, in contrast to the rest of the confirmed Wolf-Rayet stars that generally do not appear to reside within dense stellar clusters. Either the majority of the stars are runaways from clusters, or they formed in relative isolation. We briefly discuss prospects for the expansion and improvement of the search for Wolf-Rayet stars throughout the Milky Way Galaxy.

Welsh, Barry Y.; Wheatley, Jonathan; Lallement, Rosine

doi: 10.1086/599846pmid: N/A

We present high resolution (R = 60000) measurements of the Na I D1 & D2 (5890 Å) and Ca II K (3933 Å) interstellar absorption line profiles recorded toward several post-AGB stars located within the M13 and M15 globular clusters, supplemented with a lower resolution spectrum of the Ca II K-line observed in absorption toward an Ofpe/WN9 star in the central region of the M33 galaxy. The normalized interstellar absorption profiles have been fit with cloud component velocities, doppler widths, and column densities in order to investigate the kinematics and physical conditions of the neutral and partially ionized gas observed along each sight line. Our Ca II observations toward M13 have revealed three absorption components that can be identified with galactic Intermediate Velocity Clouds (IVCs) spanning the -50 > Vlsr > -80 km s-1 range. The Na I/Ca II ratio for these IVCs is < 0.2, which characterizes the gas as being warm (T ∼ 103 K) and partially ionized. Similar observations toward two stars within M15 have revealed absorption due to a galactic IVC at Vlsr ∼ +65 km s-1. This IVC requires at least three different cloud components to fit the observed Na I and Ca II profiles, which is consistent with the theoretical scenario in which possible cloud-to-cloud collisions could result in triggering the star formation process within such complexes. Ca II K-line observations of a sight line toward the center of the M33 galaxy have revealed at least six gas cloud components. A cloud at Vlsr ∼ -130 km s-1 is either an IVC associated with the M33 galaxy occurring at +45 km s-1 with respect to the M33 local standard of rest, or it is a newly discovered HVC associated with our own Galaxy. In addition, three clouds have been discovered in the -170 > Vlsr > -205 km s-1 range. Two of these clouds are identified with the disk gas of M33, whereas the cloud at -202 km s-1 could be IVC gas in the surrounding halo of M33.

Peek, Kathryn M. G.; Johnson, John Asher; Fischer, Debra A.; Marcy, Geoffrey W.; Henry, Gregory W.; Howard, Andrew W.; Wright, Jason T.; Lowe, Thomas B.; Reffert, Sabine; Schwab, Christian; Williams, Peter K. G.; Isaacson, Howard; Giguere, Matthew J.

Srivastava, Mudit K.; Prabhudesai, Swapnil M.; Tandon, Shyam N.

doi: 10.1086/603543pmid: N/A

Ultra Violet Imaging Telescope (UVIT) is one of the five payloads aboard the Indian Space Research Organization (ISRO)’s ASTROSAT space mission, with broad science objectives extending from individual hot stars and star-forming regions to active galactic nuclei. The imaging performance of UVIT would depend on several factors in addition to the optics: e.g., resolution of the detectors, satellite drift and jitter, image frame acquisition rate, sky background, source intensity, etc. The use of intensified CMOS-imager–based photon counting detectors in UVIT put their own complexity over reconstruction of the images. All these factors could lead to several systematic effects in the reconstructed images. A study has been done through numerical simulations with artificial point sources and archival image of a galaxy from the GALEX data archive, to explore the effects of the above-mentioned parameters on the reconstructed images. In particular, the issues of angular resolution, photometric accuracy, and photometric nonlinearity associated with the intensified CMOS-imager–based photon counting detectors have been investigated. The photon events in image frames are detected by three different centroid algorithms with some energy thresholds. Our results show that in the presence of bright sources, reconstructed images from UVIT would suffer from photometric distortion in a complex way, and the presence of overlapping photon events could lead to complex patterns near the bright sources. Further, the angular resolution, photometric accuracy, and distortion would depend on the values of various thresholds chosen to detect photon events.

Blair, William P.; Oliveira, Cristina; LaMassa, Stephanie; Gutman, Serah; Danforth, Charles W.; Fullerton, Alex W.; Sankrit, Ravi; Gruendl, Robert

doi: 10.1086/600387pmid: N/A

We present an atlas of stellar sight line data from the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite for 287 stars in the Magellanic Clouds, obtained from eight years of satellite operations. The intent of our project is to make this rich ensemble data set accessible to a broad community of researchers in a standardized format that will enable easy identification of subsets of these data that are appropriate for pursuing specific science programs. We present the data in a standardized manner, showing key interstellar lines on a velocity scale, optical (MCELS) and infrared (Spitzer) sight line context images, and overview plots of the spectral region containing the O VI λλ1032, 1038 doublet, and the entire 905–1187 Å spectral range observed with FUSE. Objects with multiple observations have had their data summed to directly provide the highest signal-to-noise ratio available. These data are accessible online as a High Level Science Product through the Multimission Archive at Space Telescope (MAST). In this article, we describe the data sets and processing, the atlas materials, and the MAST interface in detail, and also provide examples of how to use these materials.

Dolphin, Andrew E.

doi: 10.1086/600028pmid: N/A

Charge transfer loss on the Wide Field Planetary Camera 2 (WFPC2) onboard the Hubble Space Telescope is a primary source of uncertainty in stellar photometry obtained with this camera. This effect, discovered shortly after the camera was installed, has grown over time and can dim stars by several tenths of a magnitude (or even more, in particularly bad cases). The impact of CTE loss on WFPC2 stellar photometry was characterized by several studies between 1998 and 2000, but has received diminished attention since ACS became HST’s primary imager. After the failure of ACS in January 2007, WFPC2 once again became the primary imaging instrument on board HST, restoring the importance of ensuring accurate CTE corrections. This paper reexamines the CTE loss of WFPC2, with three significant changes over previous studies. First, the present study considers calibration data obtained through 2007, thus increasing the confidence in the reliability of the CTE corrections when applied to recent observations. Second, the change in CTE loss during readout is accounted for analytically. Finally, a reanalysis of the CTE dependencies on counts, background, and observation date was made. The resulting correction is significantly more accurate than that provided in the WFPC2 Instrument Handbook (Dolphin 2002 and updates through 2004), resulting in photometry that can be enhanced by over 5% in certain circumstances.

Travouillon, T.; Jolissaint, L.; Ashley, M. C. B.; Lawrence, J. S.; Storey, J. W. V.

doi: 10.1086/600077pmid: N/A

The unique atmospheric conditions present at sites such as Dome C on the Antarctic plateau are very favorable for high spatial resolution astronomy. At Dome C, the majority of the optical turbulence is confined to a 30 to 40 m thick stable boundary layer that results from the strong temperature inversion created by the heat exchange between the air and the ice-covered ground. To fully realize the potential of the exceptionally calm free atmosphere, this boundary layer must be overcome. In this article we compare the performance of two methods proposed to beat the boundary layer: mounting a telescope on a tower that physically puts it above the turbulent layer, and installing a telescope at ground level with a ground-layer adaptive optics system. A case is also made to combine these two methods to further improve the image quality.