C.-H. R. Chen

University of Maryland, College Park, Maryland, United States

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Publications (25)50.9 Total impact

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    ABSTRACT: \abstract{ High-excitation compact \hii\ regions of the Magellanic Clouds highlight the most recent massive star formation events in these galaxies. Detailed study of these regions and their environments using high-spatial resolution observations is necessary to better understand massive star formation, which is still an unsolved problem. } { We present a detailed study of the compact \hii\ region N26 in the the Small Magellanic Cloud (SMC) using high- and medium-resolution observations at several wavelengths. We also study a much larger ($\sim$ 50 x 76 pc) region of active star formation enclosing N26 and four other \hii\ regions. This work aims to investigate the morphology, environment, and stellar content of N26, as well as to identify and characterize Young Stellar Object (YSO) candidates. } % methods heading (mandatory) { This study is based on high spatial resolution imaging ($\sim$ 0\asec.1-0\asec.3) in \textit{JHKs} and $L'$ bands, using the VLT equipped with the NAOS adaptive optics system. The field was also imaged at medium-spatial resolution, using the ESO 2.2m telescope in optical wavelengths. We also used the \textit{JHKs} archival data from the IRSF survey (Kato et al. 2007) and the \textit{Spitzer} Space Telescope SAGE-SMC survey (3.6, 4.5, 5.8, 8.0, 24, 70, and 160 $\mu$m bands; Gordon et al. 2011). We derive nebular parameters from spectra, and using color-magnitude and color-color diagrams we identify stellar sources that show significant near-IR excess emission, in order to identify the best YSO candidates. } % results heading (mandatory) { Our high-resolution \textit{JHKs} data of N26, which is a compact high-excitation \hii\ region ($\sim$\,4\asec\ in diameter) reveal a bright, red component C between the two bright optical components A and B (Testor 2001). A and C are complex, formed by two bright stars separated by 0\asec.4 for A and by at least three stars for C. Centered on N26A, a small cluster of 4$\arcsec$ diameter formed by faint hardly resolved stars is detected. From spectroscopy A is found to be the main ionization source of N26 and coincides perfectly with the radio source B0046-7333. A new faint and compact \hii\ region with a very faint \oiii\ $\lambda$5007 emission is discovered. In the mid-IR, our field resembles a shell formed of filaments and dust clumps, coinciding with the molecular cloud SMCB2. Among the extended \hii\ regions, N22, located in the center of the bubble, is the most excited and seems to have created a cavity in SMCB2. Based on the multi-wavelength photometry we identified several YSO candidates with bright 5.8 $\mu$m emission, faint optical counterparts, and very red MIR colors [5.8]-[8.0]\,$\geq$ 1.2.
    Astronomy and Astrophysics 01/2014; A&A(564):A31. · 5.08 Impact Factor
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    ABSTRACT: The Spitzer Space Telescope Legacy Program SAGE-SMC allows global studies of resolved stellar populations in the SMC in a different environment than our Galaxy. Using the SAGE-SMC IRAC (3.6-8.0 μm) and MIPS (24 and 70 μm) catalogs and images combined with near-infrared (JHK s ) and optical (UBVI) data, we identified a population of ~1000 intermediate- to high-mass young stellar objects (YSOs) in the SMC (three times more than previously known). Our method of identifying YSO candidates builds on the method developed for the Large Magellanic Cloud by Whitney et al. with improvements based on what we learned from our subsequent studies and techniques described in the literature. We perform (1) color-magnitude cuts based on five color-magnitude diagrams (CMDs), (2) visual inspection of multi-wavelength images, and (3) spectral energy distribution (SED) fitting with YSO models. For each YSO candidate, we use its photometry to calculate a measure of our confidence that the source is not a non-YSO contaminant, but rather a true YSO, based on the source's location in the color-magnitude space with respect to non-YSOs. We use this CMD score and the SED fitting results to define two classes of sources: high-reliability YSO candidates and possible YSO candidates. We found that, due to polycyclic aromatic hydrocarbon emission, about half of our sources have [3.6]-[4.5] and [4.5]-[5.8] colors not predicted by previous YSO models. The YSO candidates are spatially correlated with gas tracers.
    The Astrophysical Journal 11/2013; 778(1):15-. · 6.73 Impact Factor
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    ABSTRACT: We present an overview of the HERschel Inventory of The Agents of Galaxy Evolution (HERITAGE) in the Magellanic Clouds project, which is a Herschel Space Observatory open time key program. We mapped the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) at 100, 160, 250, 350, and 500 μm with the Spectral and Photometric Imaging Receiver (SPIRE) and Photodetector Array Camera and Spectrometer (PACS) instruments on board Herschel using the SPIRE/PACS parallel mode. The overriding science goal of HERITAGE is to study the life cycle of matter as traced by dust in the LMC and SMC. The far-infrared and submillimeter emission is an effective tracer of the interstellar medium (ISM) dust, the most deeply embedded young stellar objects (YSOs), and the dust ejected by the most massive stars. We describe in detail the data processing, particularly for the PACS data, which required some custom steps because of the large angular extent of a single observational unit and overall the large amount of data to be processed as an ensemble. We report total global fluxes for the LMC and SMC and demonstrate their agreement with measurements by prior missions. The HERITAGE maps of the LMC and SMC are dominated by the ISM dust emission and bear most resemblance to the tracers of ISM gas rather than the stellar content of the galaxies. We describe the point source extraction processing and the criteria used to establish a catalog for each waveband for the HERITAGE program. The 250 μm band is the most sensitive and the source catalogs for this band have ~25,000 objects for the LMC and ~5500 objects for the SMC. These data enable studies of ISM dust properties, submillimeter excess dust emission, dust-to-gas ratio, Class 0 YSO candidates, dusty massive evolved stars, supernova remnants (including SN1987A), H II regions, and dust evolution in the LMC and SMC. All images and catalogs are delivered to the Herschel Science Center as part of the community support aspects of the project. These HERITAGE images and catalogs provide an excellent basis for future research and follow up with other facilities.
    The Astronomical Journal 08/2013; 146(3):62. · 4.97 Impact Factor
  • C.-H. R. Chen, M. Messineo, K. Menten, F. Wyrowski
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    ABSTRACT: The relationship between star formation rate (SFR) and the interstellar gas surface density (Σgas) is fundamental to galaxy evolution. To achieve physical understanding of this relationship, it is necessary to study the detailed star formation properties of giant molecular clouds (GMCs). Here we report preliminary results of the case study of W31 and W43, two obscured starburst regions in the Milky Way. We have used IR spectroscopic observations to determine the type and nature of luminous sources in these two regions. Progressions of massive star formation are seen in both W31 and W43. We will extend this work to more starbursts associated with GMCs and aim at assessing the SFR-Σgas relation in the Galaxy.
    01/2013;
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    ABSTRACT: The relationship between star formation rate (SFR) and the gas surface density (Σgas) is one of the most critical links between star formation and galaxy evolution. The observed SFR- Σgas relation, the “Schmidt-Kennicutt (S-K) law”, is tight when properties are averaged over kpc, but breaks down at the scale of giant molecular clouds (GMCs). To understand the physics governing the variations at GMC scales and the tight correlation at kpc scales, spatially and temporally resolved data covering a wide range of linear scale are needed. We have used the Spitzer surveys of the Large Magellanic Cloud and Magellanic Bridge to identify massive young stellar objects (YSOs), estimate “instantaneous” SFRs, and compare them to the S-K relation. These instantaneous SFRs are further compared to that estimated from integrated Hα and 24 μm luminosities to examine how SFRs vary on 10 Myr timescales. We have also used SINFONI near-IR integral field spectra of two Galactic mini-starbursts W31 and W43 to determine their underlying massive stellar content, estimate the SFRs, and compare to the S-K relation. To investigate evironmental effects on star formation, we have used complete YSO samples in the LMC and the Bridge to estimate global star formation efficiencies (SFE) in these two systems.
    Proceedings of the International Astronomical Union 08/2012; 8(S292).
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    ABSTRACT: Supernova remnant (SNR) candidates in the giant spiral galaxy M101 have been previously identified from ground-based H-alpha and [SII] images. We have used archival Hubble Space Telescope (HST) H-alpha and broad-band images as well as stellar photometry of 55 SNR candidates to examine their physical structure, interstellar environment, and underlying stellar population. We have also obtained high-dispersion echelle spectra to search for shocked high-velocity gas in 18 SNR candidates, and identified X-ray counterparts to SNR candidates using data from archival observations made by the Chandra X-ray Observatory. Twenty-one of these 55 SNR candidates studied have X-ray counterparts, although one of them is a known ultra-luminous X-ray source. The multi-wavelength information has been used to assess the nature of each SNR candidate. We find that within this limited sample, ~16% are likely remnants of Type Ia SNe and ~45% are remnants of core-collapse SNe. In addition, about ~36% are large candidates which we suggest are either superbubbles or OB/HII complexes. Existing radio observations are not sensitive enough to detect the non-thermal emission from these SNR candidates. Several radio sources are coincident with X-ray sources, but they are associated with either giant HII regions in M101 or background galaxies. The archival HST H-alpha images do not cover the entire galaxy and thus prevents a complete study of M101. Furthermore, the lack of HST [SII] images precludes searches for small SNR candidates which could not be identified by ground-based observations. Such high-resolution images are needed in order to obtain a complete census of SNRs in M101 for a comprehensive investigation of the distribution, population, and rates of SNe in this galaxy.
    The Astronomical Journal 02/2012; 143(4). · 4.97 Impact Factor
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    ABSTRACT: We present the classification of 197 point sources observed with the Infrared Spectrograph in the SAGE-Spec Legacy programme on the Spitzer Space Telescope. We introduce a decision-tree method of object classification based on infrared spectral features, continuum and spectral energy distribution shape, bolometric luminosity, cluster membership and variability information, which is used to classify the SAGE-Spec sample of point sources. The decision tree has a broad application to mid-infrared spectroscopic surveys, where supporting photometry and variability information are available. We use these classifications to make deductions about the stellar populations of the Large Magellanic Cloud and the success of photometric classification methods. We find 90 asymptotic giant branch (AGB) stars, 29 young stellar objects, 23 post-AGB objects, 19 red supergiants, eight stellar photospheres, seven background galaxies, seven planetary nebulae, two HII regions and 12 other objects, seven of which remain unclassified. (1 data file).
    VizieR Online Data Catalog. 09/2011; 741:11597.
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    ABSTRACT: We present the classification of 197 point sources observed with the Infrared Spectrograph in the SAGE-Spec Legacy programme on the Spitzer Space Telescope. We introduce a decision-tree method of object classification based on infrared spectral features, continuum and spectral energy distribution shape, bolometric luminosity, cluster membership and variability information, which is used to classify the SAGE-Spec sample of point sources. The decision tree has a broad application to mid-infrared spectroscopic surveys, where supporting photometry and variability information are available. We use these classifications to make deductions about the stellar populations of the Large Magellanic Cloud and the success of photometric classification methods. We find 90 asymptotic giant branch (AGB) stars, 29 young stellar objects, 23 post-AGB objects, 19 red supergiants, eight stellar photospheres, seven background galaxies, seven planetary nebulae, two H ii regions and 12 other objects, seven of which remain unclassified.
    Monthly Notices of the Royal Astronomical Society 02/2011; 411(3):1597 - 1627. · 5.52 Impact Factor
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    ABSTRACT: The Spitzer SAGE-Spec program (PID: 40159) consists of 224.6hr of spectroscopic observations of targets in the LMC. The targets included point sources and extended regions, both of which were observed using the IRS low-resolution and MIPS SED modes. Observations were done in the IRS staring mode for 196 point sources, and 48 point sources were observed in MIPS SED mode. In addition, 10 extended regions were mapped in both the MIPS SED and IRS observing modes. (4 data files).
    VizieR Online Data Catalog. 11/2010; 612:20683.
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    ABSTRACT: The HERschel Inventory of The Agents of Galaxy Evolution (HERITAGE) of the Magellanic Clouds will use dust emission to investigate the life cycle of matter in both the Large and Small Magellanic Clouds (LMC and SMC). Using the Herschel Space Observatory's PACS and SPIRE photometry cameras, we imaged a 2x8 square degree strip through the LMC, at a position angle of ~22.5 degrees as part of the science demonstration phase of the Herschel mission. We present the data in all 5 Herschel bands: PACS 100 and 160 {\mu}m and SPIRE 250, 350 and 500 {\mu}m. We present two dust models that both adequately fit the spectral energy distribution for the entire strip and both reveal that the SPIRE 500 {\mu}m emission is in excess of the models by 6 to 17%. The SPIRE emission follows the distribution of the dust mass, which is derived from the model. The PAH-to-dust mass (f_PAH) image of the strip reveals a possible enhancement in the LMC bar in agreement with previous work. We compare the gas mass distribution derived from the HI 21 cm and CO J=1-0 line emission maps to the dust mass map from the models and derive gas-to-dust mass ratios (GDRs). The dust model, which uses the standard graphite and silicate optical properties for Galactic dust, has a very low GDR = 65(+15,-18) making it an unrealistic dust model for the LMC. Our second dust model, which uses amorphous carbon instead of graphite, has a flatter emissivity index in the submillimeter and results in a GDR = 287(+25,-42) that is more consistent with a GDR inferred from extinction. Comment: To be published in Astronomy & Astrophysics, Herschel First Results Issue
    06/2010;
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    ABSTRACT: We study the structure of the medium surrounding sites of high-mass star formation to determine the interrelation between the HII regions and the environment from which they were formed. The density distribution of the surroundings is key in determining how the radiation of the newly formed stars interacts with the surrounds in a way that allows it to be used as a star formation tracer. We present new Herschel/SPIRE 250, 350 and 500 mum data of LHA 120-N44 and LHA 120-N63 in the LMC. We construct average spectral energy distributions (SEDs) for annuli centered on the IR bright part of the star formation sites. The annuli cover ~10-~100 pc. We use a phenomenological dust model to fit these SEDs to derive the dust column densities, characterise the incident radiation field and the abundance of polycyclic aromatic hydrocarbon molecules. We see a factor 5 decrease in the radiation field energy density as a function of radial distance around N63. N44 does not show a systematic trend. We construct a simple geometrical model to derive the 3-D density profile of the surroundings of these two regions. Herschel/SPIRE data have proven very efficient in deriving the dust mass distribution. We find that the radiation field in the two sources behaves very differently. N63 is more or less spherically symmetric and the average radiation field drops with distance. N44 shows no systematic decrease of the radiation intensity which is probably due to the inhomogeneity of the surrounding molecular material and to the complex distribution of several star forming clusters in the region. Comment: Accepted for publication in A&A letters (Herschel special issue)
    Astronomy and Astrophysics 05/2010; · 5.08 Impact Factor
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    ABSTRACT: The SAGE-Spec Spitzer Legacy program is a spectroscopic follow-up to the SAGE-LMC photometric survey of the Large Magellanic Cloud carried out with the Spitzer Space Telescope. We present an overview of SAGE-Spec and some of its first results. The SAGE-Spec program aims to study the life cycle of gas and dust in the Large Magellanic Cloud, and to provide information essential to the classification of the point sources observed in the earlier SAGE-LMC photometric survey. We acquired 224.6 hours of observations using the InfraRed Spectrograph and the SED mode of the Multiband Imaging Photometer for Spitzer. The SAGE-Spec data, along with archival Spitzer spectroscopy of objects in the Large Magellanic Cloud, are reduced and delivered to the community. We discuss the observing strategy, the specific data reduction pipelines applied and the dissemination of data products to the scientific community. Initial science results include the first detection of an extragalactic "21 um" feature towards an evolved star and elucidation of the nature of disks around RV Tauri stars in the Large Magellanic Cloud. Towards some young stars, ice features are observed in absorption. We also serendipitously observed a background quasar, at a redshift of z~0.14, which appears to be host-less. Comment: 33 pages, 12 figures, 8 tables; accepted for publication by PASP
    04/2010;
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    ABSTRACT: This table contains the names, coordinates, 2MASS (JHKs), Spitzer (IRAC 3.6, 4.5, 5.8, 8.0um; MIPS 24, 70um), and Herschel (PACS 100, 160; SPIRE 250, 350, 500um) fluxes for 4 embedded YSOs analyzed in the paper. Source fluxes were determined by aperture photometry on 2MASS, Spitzer, and Herschel images using apertures scaled to the instrumental resolution. Details on aperture sizes and aperture corrections can be found in Sect. 2 of the paper. The last column of the table gives alternative names of the sources from literature. (1 data file).
    VizieR Online Data Catalog. 04/2010; 351:89073.
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    ABSTRACT: We demonstrate the unique capabilities of Herschel to study very young luminous extragalactic young stellar objects (YSOs) by analyzing a central strip of the Large Magellanic Cloud obtained through the HERITAGE science demonstration program. We combine PACS 100 and 160, and SPIRE 250, 350, and 500 mum photometry with 2MASS (1.25-2.17 mum) and Spitzer IRAC and MIPS (3.6-70 mum) to construct complete spectral energy distributions (SEDs) of compact sources. From these, we identify 207 candidate embedded YSOs in the observed region, ~40% never-before identified. We discuss their position in far-infrared color-magnitude space, comparing with previously studied, spectroscopically confirmed YSOs and maser emission. All have red colors indicating massive cool envelopes and great youth. We analyze four example YSOs, determining their physical properties by fitting their SEDs with radiative transfer models. Fitting full SEDs including the Herschel data requires us to increase the size and mass of envelopes included in the models. This implies higher accretion rates (≳10-4 M_&sun;yr-1), in agreement with previous outflow studies of high-mass protostars. Our results show that Herschel provides reliable longwave SEDs of large samples of high-mass YSOs; discovers the youngest YSOs whose SEDs peak in Herschel bands; and constrains the physical properties and evolutionary stages of YSOs more precisely than was previously possible. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.Table 1 and Figs. 3-5 are only available in electronic form at http://www.aanda.org
    Astronomy and Astrophysics 01/2010; · 5.08 Impact Factor
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    ABSTRACT: We present spectroscopic observations of a sample of 15 embedded young stellar objects (YSOs) in the Large Magellanic Cloud (LMC). These observations were obtained with the Spitzer Infrared Spectrograph (IRS) as part of the SAGE-Spec Legacy program. We analyze the two prominent ice bands in the IRS spectral range: the bending mode of CO2 ice at 15.2 μm and the ice band between 5 and 7 μm that includes contributions from the bending mode of water ice at 6 μm among other ice species. The 5-7 μm band is difficult to identify in our LMC sample due to the conspicuous presence of polycyclic aromatic hydrocarbon emission superimposed onto the ice spectra. We identify water ice in the spectra of two sources; the spectrum of one of those sources also exhibits the 6.8 μm ice feature attributed in the literature to ammonium and methanol. We model the CO2 band in detail, using the combination of laboratory ice profiles available in the literature. We find that a significant fraction (50%) of CO2 ice is locked in a water-rich component, consistent with what is observed for Galactic sources. The majority of the sources in the LMC also require a pure-CO2 contribution to the ice profile, evidence of thermal processing. There is a suggestion that CO2 production might be enhanced in the LMC, but the size of the available sample precludes firmer conclusions. We place our results in the context of the star formation environment in the LMC.
    The Astrophysical Journal 12/2009; 707(2):1269. · 6.73 Impact Factor
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    ABSTRACT: We have used Spitzer IRAC and MIPS observations of N44 to identify young stellar objects (YSOs). Sixty YSO candidates with masses 4M are identified. We have compared the distribution of YSOs with those of the ionized gas, molecular clouds, and HI gas to study the properties of star formation.
    Proceedings of the International Astronomical Union 07/2006; 2:401 - 401.
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    ABSTRACT: We have analyzed Chandra data for supernova remnants (SNRs) within the N19 H II complex in the Small Magellanic Cloud, supplemented by optical emission-line images and echelle spectroscopy. Our observations include the known SNR MCRX J0047.2-7308 (SNR B0045-73.4), confirm the SNR candidate MCRX J0046.6-7308 (SNR B0045-7325), and strengthen the case for a large, extended SNR identified as candidate MCRX J0047.5-7308. We find that the abundances inferred from spectral model fits to the X-ray emission from these SNRs and SNR candidates are consistent in each case with remnants of massive-star progenitors. We observe a possible point source within J0047.2-7308, embedded in a region of hard emission, suggestive of a possible embedded PWN. As these three remnants, all with massive-star origins, appear to be spatially located within the N19 H II complex, we infer that a small OB association may be the source of these phenomena. To expand on this assessment, we estimate the number of massive stars within N19 and project their energy input to the region. We find that a stellar-wind created superbubble is unlikely to have formed, but that the further expansion of the SNRs in the region may lead to superbubble formation on a timescale of < 0.3 Myr from the first supernova. RMW gratefully acknowledges support from SAO grant G03-4094A, and NASA grant NNG05GC97G issued through the LTSA Program.
    American Astronomical Society Meeting; 12/2005
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    ABSTRACT: It is often hypothesized that star formation can be triggered or propagates from nearby sites that have recently formed stars. The superbubble LHalpha 120-N 51D (N51D) in the Large Magellanic Cloud contains two OB associations, LH 51 and LH 54 and appears to be a good site to search for evidence of triggered/propagating star formation. A previous study by Oey and Smedley (1998) find ages for LH 51 and LH 54 of 1-2 Myr and ˜3 Myr, respectively. They also determine that the shell kinematics are consistent with the origin of this superbubble being the combined stellar winds of the massive stars in LH 54 and speculate that this may have triggered the formation of LH 51. We have examined the on-going star formation in N51D using combined observations from the Spitzer Space Telescope, the Magellanic Clouds Emission Line Survey, the Magellanic Clouds Photometric Survey and 2MASS. We find sites of on-going star formation both along the periphery of the the expanding superbubble shell and projected within the superbubble interior.
    American Astronomical Society Meeting; 12/2005
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    ABSTRACT: Using Spitzer Space Telescope and Hubble Space Telescope observations of the superbubble N51D, we have identified three young stellar objects (YSOs) in dust globules, and made the first detection of a Herbig-Haro object outside the Galaxy. The spectral energy distributions of these YSOs suggest young massive stars with disk, envelope, and outflow cavities. The interstellar conditions are used to assess whether the star formation was spontaneous or induced by external pressure. Comment: 12 pages, 4 figures, accepted for ApJL, December 1, 2005 High-resolution figures can be requested via e-mail to chu@astro.uiuc.edu. Replacement corrects author name
    The Astrophysical Journal 11/2005; · 6.73 Impact Factor
  • C.-H. R. Chen, S. Wang, M. A. Guerrero, Y.-H. Chu
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    ABSTRACT: Chandra, with its unprecedented resolution and sensitivity, is able to resolve and study point sources and diffuse emission in galaxies as distant as tens of Mpc. To date, Chandra has observed ˜100 galaxies within 30 Mpc with ACIS-S or ACIS-I for exposure times greater than 20 ks. This Chandra Archive of Nearby Galaxies provides an invaluable resource to study the X-ray properties of galaxies of different types. We are using this archive to produce an X-ray atlas and determine the relative contribution of nuclear source, X-ray binaries, and hot gas to the total X-ray emission for a wide range of galaxies. For the late-type normal galaxies, we further analyze the diffuse X-ray emission to determine the distribution and physical conditions of the hottest component of the interstellar medium, and use the underlying interstellar structures and stellar content derived from optical wavelengths to assess the origin and evolution of the hot interstellar medium. This poster presents a progress report of our study of X-ray properties of ˜60 nearby galaxies without nuclear activity.
    05/2003;

Publication Stats

57 Citations
50.90 Total Impact Points

Institutions

  • 2013
    • University of Maryland, College Park
      • Department of Astronomy
      Maryland, United States
  • 2009–2011
    • University of Virginia
      • Department of Astronomy
      Charlottesville, VA, United States
  • 2005
    • University of Illinois, Urbana-Champaign
      • Department of Astronomy
      Urbana, Illinois, United States