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Lee, J C

Publications and source records attributed to Lee, J C.

6 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

SN 2008jb: A 'LOST' CORE-COLLAPSE SUPERNOVA IN A STAR-FORMING DWARF GALAXY AT {approx}10 Mpc

We present the discovery and follow-up observations of SN 2008jb, a core-collapse supernova in the southern dwarf irregular galaxy ESO 302-14 (M{sub B} = -15.3 mag) at 9.6 Mpc. This nearby transient was missed by galaxy-targeted surveys and was only found in archival optical images obtained by the Catalina Real-time Transient Survey and the All-Sky Automated Survey. The well-sampled archival photometry shows that SN 2008jb was detected shortly after explosion and reached a bright optical maximum, V{sub max} {approx_equal} 13.6 mag (M{sub V,max} {approx_equal} -16.5). The shape of the light curve shows a plateau of {approx}100 days, followed by a drop of {approx}1.4 mag in the V band to a slow decline with an approximate {sup 56}Co decay slope. The late-time light curve is consistent with 0.04 {+-} 0.01 M{sub Sun} of {sup 56}Ni synthesized in the explosion. A spectrum of the supernova obtained two years after explosion shows a broad, boxy H{alpha} emission line, which is unusual for normal Type II-Plateau supernovae at late times. We detect the supernova in archival Spitzer and WISE images obtained 8-14 months after explosion, which show clear signs of warm (600-700 K) dust emission. The dwarf irregular host galaxy, ESO 302-14, has a low gas-phase oxygen abundance, 12 + log(O/H) = 8.2 ({approx}1/5 Z{sub Sun }), similar to those of the Small Magellanic Cloud and the hosts of long gamma-ray bursts and luminous core-collapse supernovae. This metallicity is one of the lowest among local ({approx}< 10 Mpc) supernova hosts. We study the host environment using GALEX far-UV, R-band, and H{alpha} images and find that the supernova occurred in a large star formation complex. The morphology of the H{alpha} emission appears as a large shell (R {approx_equal} 350 pc) surrounding the FUV and optical emission. Using the H{alpha}-to-FUV ratio and FUV and R-band luminosities, we estimate an age of {approx}9 Myr and a total mass of {approx}2 Multiplication-Sign 10{sup 5} M{sub Sun} for the star formation complex, assuming a single-age starburst. These properties are consistent with the expanding H{alpha} supershells observed in many well-studied nearby dwarf galaxies, which are tell-tale signs of feedback from the cumulative effect of massive star winds and supernovae. The age estimated for the star-forming region where SN 2008jb exploded suggests a relatively high-mass progenitor star with an initial mass M {approx} 20 M{sub Sun} and warrants further study. We discuss the implications of these findings in the study of core-collapse supernova progenitors.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

THE SUZAKU VIEW OF THE DISK-JET CONNECTION IN THE LOW-EXCITATION RADIO GALAXY NGC 6251

We present results from an 87 ks Suzaku observation of the canonical low-excitation radio galaxy (LERG) NGC 6251. We have previously suggested that LERGs violate conventional active galactic nucleus unification schemes: they may lack an obscuring torus and are likely to accrete in a radiatively inefficient manner, with almost all of the energy released by the accretion process being channeled into powerful jets. We model the 0.5-20 keV Suzaku spectrum with a single power law of photon index {Gamma} = 1.82{sup +0.04} {sub -0.05}, together with two collisionally ionized plasma models whose parameters are consistent with the known galaxy- and group-scale thermal emission. Our observations confirm that there are no signatures of obscured, accretion-related X-ray emission in NGC 6251, and we show that the luminosity of any such component must be substantially sub-Eddington in nature.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

THE SPIN OF THE SUPERMASSIVE BLACK HOLE IN NGC 3783

The Suzaku AGN Spin Survey is designed to determine the supermassive black hole spin in six nearby active galactic nuclei (AGNs) via deep Suzaku stares, thereby giving us our first glimpse of the local black hole spin distribution. Here, we present an analysis of the first target to be studied under the auspices of this Key Project, the Seyfert galaxy NGC 3783. Despite complexity in the spectrum arising from a multi-component warm absorber, we detect and study relativistic reflection from the inner accretion disk. Assuming that the X-ray reflection is from the surface of a flat disk around a Kerr black hole, and that no X-ray reflection occurs within the general relativistic radius of marginal stability, we determine a lower limit on the black hole spin of a {>=} 0.88 (99% confidence). We examine the robustness of this result to the assumption of the analysis and present a brief discussion of spin-related selection biases that might affect flux-limited samples of AGNs.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2010

A MULTIWAVELENGTH STUDY ON THE FATE OF IONIZING RADIATION IN LOCAL STARBURSTS

The fate of ionizing radiation is vital for understanding cosmic ionization, energy budgets in the interstellar and intergalactic medium, and star formation rate indicators. The low observed escape fractions of ionizing radiation have not been adequately explained, and there is evidence that some starbursts have high escape fractions. We examine the spectral energy distributions (SEDs) of a sample of local star-forming galaxies, containing 13 local starburst galaxies and 10 of their ordinary star-forming counterparts, to determine if there exist significant differences in the fate of ionizing radiation in these galaxies. We find that the galaxy-to-galaxy variations in the SEDs are much larger than any systematic differences between starbursts and non-starbursts. For example, we find no significant differences in the total absorption of ionizing radiation by dust, traced by the 24 {mu}m, 70 {mu}m, and 160 {mu}m MIPS bands of the Spitzer Space Telescope, although the dust in starburst galaxies appears to be hotter than that of non-starburst galaxies. We also observe no excess ultraviolet flux in the Galaxy Evolution Explorer bands that could indicate a high escape fraction of ionizing photons in starburst galaxies. The small H{alpha} fractions of the diffuse, warm ionized medium (WIM) in starburst galaxies are apparently due to temporarily boosted H{alpha} luminosity within the star-forming regions themselves, with an independent, constant WIM luminosity. This independence of the WIM and starburst luminosities contrasts with WIM behavior in non-starburst galaxies and underscores our poor understanding of radiation transfer in both ordinary and starburst galaxies.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2010

A CHANDRA OBSERVATION OF 3C 288-REHEATING THE COOL CORE OF A 3 keV CLUSTER FROM A NUCLEAR OUTBURST at z = 0.246

We present results from a 42 ks Chandra/ACIS-S observation of the transitional FR I/FR II radio galaxy 3C 288 at z = 0.246. We detect {approx}3 keV gas extending to a radius of {approx}0.5 Mpc with a 0.5-2.0 keV luminosity of 6.6 x 10{sup 43} erg s{sup -1}, implying that 3C 288 lies at the center of a poor cluster. We find multiple surface brightness discontinuities in the gas indicative of either a shock driven by the inflation of the radio lobes or a recent merger event. The temperature across the discontinuities is roughly constant with no signature of a cool core, thus disfavoring either the merger cold front or sloshing scenarios. We argue therefore that the discontinuities are shocks due to the supersonic inflation of the radio lobes. If they are shocks, the energy of the outburst is {approx}10{sup 60} erg, or roughly 30% of the thermal energy of the gas within the radius of the shock, assuming that the shocks are part of a front produced by a single outburst. The cooling time of the gas is {approx}10{sup 8} yr, so that the energy deposited by the nuclear outburst could have reheated and efficiently disrupted a cool core.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2010

A METHOD FOR MEASURING VARIATIONS IN THE STELLAR INITIAL MASS FUNCTION

We present a method for investigating variations in the upper end of the stellar initial mass function (IMF) by probing the production rate of ionizing photons in unresolved, compact star clusters with ages {<=}10{sup 7} yr and with different masses. We test this method by performing a pilot study on the young cluster population in the nearby galaxy NGC5194 (M51a), for which multi-wavelength observations from the Hubble Space Telescope are available. Our results indicate that the proposed method can probe the upper end of the IMF in galaxies located out to at least {approx}10 Mpc, i.e., a factor {approx}200 further away than what is possible by counting individual stars in young compact clusters. Our results for NGC5194 show no obvious dependence of the upper mass end of the IMF on the mass of the star cluster down to {approx}10{sup 3} M{sub sun}, although more extensive analyses involving lower mass clusters and other galaxies are needed to confirm this conclusion.

79 ASTRONOMY AND ASTROPHYSICS↗