Engineering topics

Lee, Jong Hwan

Publications and source records attributed to Lee, Jong Hwan.

4 records

DOE OSTI2015

OPTICAL SPECTROSCOPY OF SUPERNOVA REMNANTS IN M81 AND M82

We present spectroscopy of 28 supernova remnant (SNR) candidates as well as one H ii region in M81 and two SNR candidates in M82. Twenty-six of the M81 candidates turn out to be genuine SNRs, and two in M82 may be shocked condensations in the galactic outflow or SNRs. The distribution of [N ii]/Hα ratios of M81 SNRs is bimodal. M81 SNRs are divided into two groups in the spectral line ratio diagrams: an [O iii]-strong group and an [O iii]-weak group. The latter are larger and may have faster shock velocities. [N ii]/Hα ratios of the SNRs show a strong correlation with [S ii]/Hα ratios. They show a clear radial gradient in their [N ii]/Hα and [S ii]/Hα ratios: dLog ([N ii]/Hα)/dLog R = −0.018 ± 0.008 dex kpc{sup −1} and dLog ([S ii]/Hα)/dLog R = −0.016 ± 0.008 dex kpc{sup −1}, where R is the deprojected galactocentric distance. We estimate the nitrogen and oxygen abundances of the SNRs from comparison with shock-ionization models. We obtain a value for the nitrogen radial gradient of dLog(N/H)/dLog R= −0.023 ± 0.009 dex kpc{sup −1}, and find little evidence for an oxygen gradient. This nitrogen abundance shows a gradient that is a few times flatter than those of the planetary nebulae and H ii regions. We find that five SNRs are matched with X-ray sources. Their X-ray hardness colors are consistent with thermal SNRs.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2014

Properties of optically selected supernova remnant candidates in M33

Narrowband images covering strong emission lines are efficient for surveying supernova remnants (SNRs) in nearby galaxies. Using the narrowband images provided by the Local Group Galaxy Survey, we searched for SNRs in M33. Culling the objects with enhanced [S II]/Hα and round morphology in the continuum-subtracted Hα and [S II] images, we produced a list of 199 sources. Among them, 79 are previously unknown. Their progenitor and morphology types were classified. A majority of the sample (170 objects) are likely remnants of core-collapse supernovae (SNe), and 29 are remnants of Type Ia SNe. The cumulative size distribution of these objects is found to be similar to that of the M31 remnants derived in a similar way. We obtain a power-law slope, α = 2.38 ± 0.05. Thus, a majority of the sources are considered to be in the Sedov-Taylor phase, consistent with previous findings. The histogram of the emission-line ratio ([S II]/Hα) of the remnants has two concentrations at [S II]/Hα ∼ 0.55 and ∼0.8, as in M31. Interestingly, L {sub X} (and L {sub 20cm}) of the compact center-bright objects are correlated with their optical luminosity. The remnants with X-ray emission have brighter optical surface brightnesses and smaller diameters than those without X-ray emission.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2014

A new optical survey of supernova remnant candidates in M31

We present a survey of optically emitting supernova remnants (SNRs) in M31 based on Hα and [S II] images in the Local Group Survey. Using these images, we select objects that have [S II]:Hα > 0.4 and circular shapes. We identify 156 SNR candidates, of which 76 are newly found objects. We classify these SNR candidates according to two criteria: the SNR progenitor type (Type Ia and core-collapse (CC) SNRs) and the morphological type. Type Ia and CC SNR candidates make up 23% and 77%, respectively, of the total sample. Most of the CC SNR candidates are concentrated in the spiral arms, while the Type Ia SNR candidates are rather distributed over the entire galaxy, including the inner region. The CC SNR candidates are brighter in Hα and [S II] than the Type Ia SNR candidates. We derive a cumulative size distribution of the SNR candidates, finding that the distribution of the candidates with 17 <D < 50 pc is fitted well by a power law with the power-law index α = 2.53 ± 0.04. This indicates that most of the SNR candidates identified in this study appear to be in the Sedov-Taylor phase. The [S II]:Hα distribution of the SNR candidates is bimodal, with peaks at [S II]:Hα ∼ 0.4 and ∼0.9. The properties of these SNR candidates vary little with the galactocentric distance. The Hα and [S II] surface brightnesses show a good correlation with the X-ray luminosity of the SNR candidates that are center-bright.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2012

DO BARS TRIGGER ACTIVITY IN GALACTIC NUCLEI?

We investigate the connection between the presence of bars and active galactic nucleus (AGN) activity, using a volume-limited sample of {approx}9000 late-type galaxies with axis ratio b/a > 0.6 and M{sub r} < -19.5 + 5 log h at low redshift (0.02 {<=} z {approx}< 0.055), selected from Sloan Digital Sky Survey Data Release 7. We find that the bar fraction in AGN-host galaxies (42.6%) is {approx}2.5 times higher than in non-AGN galaxies (15.6%), and that the AGN fraction is a factor of two higher in strong-barred galaxies (34.5%) than in non-barred galaxies (15.0%). However, these trends are simply caused by the fact that AGN-host galaxies are on average more massive and redder than non-AGN galaxies because the fraction of strong-barred galaxies (f{sub SB}) increases with u - r color and stellar velocity dispersion. When u - r color and velocity dispersion (or stellar mass) are fixed, both the excess of f{sub SB} in AGN-host galaxies and the enhanced AGN fraction in strong-barred galaxies disappears. Among AGN-host galaxies we find no strong difference of the Eddington ratio distributions between barred and non-barred systems. These results indicate that AGN activity is not dominated by the presence of bars, and that AGN power is not enhanced by bars. In conclusion, we do not find clear evidence that bars trigger AGN activity.

79 ASTRONOMY AND ASTROPHYSICS↗