Comparison of AGN and Nuclear Starburst Activity in Seyfert 1 and 2 Galaxies over a Wide Luminosity Range Based on Near-infrared 2-4 micrometer Spectroscopy

We present near-infrared K- (1.9-2.5 micrometer) and L- (2.8-4.2 micrometer) band spectroscopy of 22 Seyfert nuclei. We use two methods to investigate the presence of nuclear starbursts: (1) the Polycyclic Aromatic Hydrocarbon (PAH) emission feature at lambda_rest = 3.3 micrometer in the rest frame of L-band spectrum (a starburst indicator) and (2) the CO absorption feature at lambda_rest = 2.3-2.4 micrometer in the rest frame of the K-band spectrum, originating in the CO molecule. We clearly detected the 3.3 micrometer PAH emission features in five objects and the CO absorption features in 17 objects. Seyfert 2 galaxies tend to show bluer K-L colors compared with Seyfert 1 galaxies. We interpret the discrepancy as resulting from relative strength of stellar emission because AGN emission is affected by dust extinction. The 3.3 micrometer PAH emission luminosity (L_3.3PAH) distributions for the Seyfert 1s and Seyfert 2s are very similar when normalized to the AGN power. Star-formation rates estimated from L_3.3PAH could be large enough to inflate the dusty torus by supernova explosion. We find that L_3.3PAH positively correlates with N-band luminosity with small aperture over a wide AGN luminosity range, and is independent of physical area we probed. The results suggest that nuclear region has a concentration of star formation and the star formation would control AGN activity.Comment: 21 pages, 9 figures, 3 tables, accepted for publication in PAS

Comparison of AGN and Nuclear Starburst Activity in Seyfert 1 and 2 Galaxies over a Wide Luminosity Range Based on Near-infrared 2-4 micrometer Spectroscopy

We present near-infrared K- (1.9-2.5 micrometer) and L- (2.8-4.2 micrometer) band spectroscopy of 22 Seyfert nuclei. We use two methods to investigate the presence of nuclear starbursts: (1) the Polycyclic Aromatic Hydrocarbon (PAH) emission feature at lambda_rest = 3.3 micrometer in the rest frame of L-band spectrum (a starburst indicator) and (2) the CO absorption feature at lambda_rest = 2.3-2.4 micrometer in the rest frame of the K-band spectrum, originating in the CO molecule. We clearly detected the 3.3 micrometer PAH emission features in five objects and the CO absorption features in 17 objects. Seyfert 2 galaxies tend to show bluer K-L colors compared with Seyfert 1 galaxies. We interpret the discrepancy as resulting from relative strength of stellar emission because AGN emission is affected by dust extinction. The 3.3 micrometer PAH emission luminosity (L_3.3PAH) distributions for the Seyfert 1s and Seyfert 2s are very similar when normalized to the AGN power. Star-formation rates estimated from L_3.3PAH could be large enough to inflate the dusty torus by supernova explosion. We find that L_3.3PAH positively correlates with N-band luminosity with small aperture over a wide AGN luminosity range, and is independent of physical area we probed. The results suggest that nuclear region has a concentration of star formation and the star formation would control AGN activity.Comment: 21 pages, 9 figures, 3 tables, accepted for publication in PAS

Infrared 3-4 Micron Spectroscopy of Nearby PG QSOs and AGN-Nuclear Starburst Connections in High-luminosity AGN Populations

We present the results of infrared L-band (3-4 micron) slit spectroscopy of 30 PG QSOs at z < 0.17, the representative sample of local high-luminosity, optically selected AGNs. The 3.3 micron polycyclic aromatic hydrocarbon (PAH) emission feature is used to probe nuclear (< a few kpc) starburst activity and to investigate the connections between AGNs and nuclear starbursts in PG QSOs. The 3.3 micron PAH emission is detected in the individual spectra of 5/30 of the observed PG QSOs. We construct a composite spectrum of PAH-undetected PG QSOs and discern the presence of the 3.3 micron PAH emission therein. We estimate the nuclear-starburst and AGN luminosities from the observed 3.3 micron PAH emission and 3.35 micron continuum luminosities, respectively, and find that the nuclear-starburst-to-AGN luminosity ratios in PG QSOs are similar to those of previously studied AGN populations with lower luminosities, suggesting that AGN-nuclear starburst connections are valid over the wide luminosity range of AGNs in the local universe. The observed nuclear-starburst-to-AGN luminosity ratios in PG QSOs with available supermassive black hole masses are comparable to a theoretical prediction based on the assumption that the growth of a supermassive black hole is controlled by starburst-induced turbulence.Comment: 10 pages, 5 figures, accepted for publication in PASJ (No. 63, 2011 March, Subaru special issue

Predicting dust extinction properties of star-forming galaxies from H-alpha/UV ratio

Using star-forming galaxies sample in the nearby Universe (0.02<z<0.10) selected from the SDSS (DR7) and GALEX all-sky survey (GR5), we present a new empirical calibration for predicting dust extinction of galaxies from H-alpha-to-FUV flux ratio. We find that the H-alpha dust extinction (A(Ha)) derived with H-alpha/H-beta ratio (Balmer decrement) increases with increasing H-alpha/UV ratio as expected, but there remains a considerable scatter around the relation, which is largely dependent on stellar mass and/or H-alpha equivalent width (EW(Ha)). At fixed H-alpha/UV ratio, galaxies with higher stellar mass (or galaxies with lower EW(Ha)) tend to be more highly obscured by dust. We quantify this trend and establish an empirical calibration for predicting A(Ha) with a combination of H-alpha/UV ratio, stellar mass and EW(Ha), with which we can successfully reduce the systematic uncertainties accompanying the simple H-alpha/UV approach by ~15-30%. The new recipes proposed in this study will provide a convenient tool for predicting dust extinction level of galaxies particularly when Balmer decrement is not available. By comparing A(Ha) (derived with Balmer decrement) and A(UV) (derived with IR/UV luminosity ratio) for a subsample of galaxies for which AKARI FIR photometry is available, we demonstrate that more massive galaxies tend to have higher extra extinction towards the nebular regions compared to the stellar continuum light. Considering recent studies reporting smaller extra extinction towards nebular regions for high-redshift galaxies, we argue that the dust geometry within high-redshift galaxies resemble more like low-mass galaxies in the nearby Universe.Comment: 14 pages, 14 figures, Accepted for publication in MNRA