The Astrophysical Journal, 617:214–231, 2004 December 10 # 2004. The American Astronomical Society. All rights reserved. Printed in U.S.A. COMPARISON OF NUCLEAR STARBURST LUMINOSITIES BETWEEN SEYFERT 1 AND 2 GALAXIES BASED ON NEAR-INFRARED SPECTROSCOPY Masatoshi Imanishi1, 2 and Keiichi Wada1 National Astronomical Observatory, 2-21-1, Osawa, Mitaka, Tokyo 181-8588, Japan; [email protected], [email protected] Received 2004 June 2; accepted 2004 Auggust 20 ABSTRACT We report on infrared K- (2–2.5 m) and L-band (2.8–4.1 m) slit spectroscopy of 23 Seyfert 1 galaxies in the CfA and 12 m samples. A polycyclic aromatic hydrocarbon (PAH) emission feature at 3.3 mintheL band is primarily used to investigate nuclear star-forming activity in these galaxies. The 3.3 m PAH emission is detected in 10 sources (=43%), demonstrating that detection of nuclear star formation in a significant fraction of Seyfert 1 galaxies is now feasible. For the PAH-detected nuclei, the surface brightness values of the PAH emission are as high as those of typical starbursts, suggesting that the PAH emission probes the putative nuclear starbursts in the dusty tori around the central active galactic nuclei (AGNs). The magnitudes of the nuclear starbursts are quan- titatively estimated from the observed 3.3 m PAH emission luminosities. The estimated starburst luminosities relative to some indicators of AGN powers in these Seyfert 1 galaxies are compared with 32 Seyfert 2 galaxies in the same samples that we have previously observed. We find that there is no significant difference in nuclear starburst to AGN luminosity ratios of Seyfert 1 and 2 galaxies and that nuclear starburst luminosity positively correlates with AGN power in both types. Our results favor a slightly modified AGN unification model, which predicts that nuclear starbursts occurring in the dusty tori of Seyfert galaxies are physically connected to the central AGNs, rather than the classical unification paradigm, in which the dusty tori simply hide the central AGNs of Seyfert 2 galaxies and reprocess AGN radiation as infrared dust emission in Seyfert galaxies. No significant differences in nuclear star formation properties are recognizable between Seyfert 1 galaxies in the CfA and 12 m samples. Subject headinggs: galaxies: active — galaxies: nuclei — galaxies: Seyfert — infrared: galaxies 1. INTRODUCTION extended (tens of parsecs) dusty torus can produce an inflated, turbulent torus around a central supermassive black hole. They Seyfert galaxies constitute a population that shows bright also suggested the possibility that the nuclear starbursts in the optical nuclei (Seyfert 1943) and is the most numerous class of torus may be more powerful in Seyfert 2 than in Seyfert 1 active galactic nuclei (AGNs) in the local universe. It is gener- galaxies, because in a more inflated torus with stronger nuclear ally believed that they contain a mass-accreting supermassive black hole as a central engine, which is surrounded by toroi- starbursts, the chance that the central engine is obscured and that, therefore, the galaxy would be classified as a Seyfert 2, dally distributed dust and molecular gas (the so-called dusty would be higher. From a study of the equivalent width of the torus). There are two types of Seyfert galaxies: type 1 (which show broad optical emission lines) and type 2 (which do not). iron K emission at 6.4 keV in X-rays, Levenson et al. (2002) made a similar suggestion. To obtain a deeper understanding of The different emission properties are explained by the AGN AGNs, it is important to observationally constrain the putative unification paradigm, in which the central engine of a Seyfert 2 nuclear starbursts in the dusty tori of Seyfert galaxies and to test galaxy is intrinsically the same as that in a Seyfert 1 galaxy but whether this slightly modified AGN model (with nuclear star- is obscured from our line of sight by a dusty torus (Antonucci bursts in the dusty tori) is more representative of actual AGNs 1993). In this classical AGN unification paradigm, intrinsic than the classical model. torus properties are assumed to be the same for the two types Some observations and simple theoretical considerations of Seyfert galaxies, and the primary role of the dusty tori is of gravitational instability suggest that nuclear starbursts in twofold: they simply (1) hide the central AGNs in Seyfert 2 Seyfert galaxies are likely to occur at the outer part of the dusty galaxies and (2) absorb AGN radiation and reradiate as infrared dust emission in both Seyfert 1 and 2 galaxies (Pier & Krolik torus (Heckman et al. 1997; Imanishi 2003). Attempts have been made to detect nuclear starbursts in the tori of Seyfert galaxies 1992). by several groups in the UV–optical (Gonzalez Delgado et al. Since a dusty torus is rich in molecular gas, nuclear starbursts k < : may occur there, and energy feedback from the nuclear star- 2001) and near-infrared at 2 5 m (Oliva et al. 1999). De- tection has been successful only in type 2 sources, because bursts could work to inflate the torus (Fabian et al. 1998). Using unattenuated AGN emission from Seyfert 1 galaxies weakens high-resolution three-dimensional hydrodynamic simulations, the signatures of the nuclear starbursts in the observed spectra Wada & Norman (2002) showed that the nuclear starbursts in an and makes their detection difficult; however, it is also possible that Seyfert 1 galaxies have weaker nuclear starbursts than do 1 Visiting Astronomer at the Infrared Telescope Facility, which is operated Seyfert 2 galaxies (Wada & Norman 2002). Until very recently, by the University of Hawaii under cooperative agreement NCC 5-538 with the very little about nuclear starbursts in Seyfert 1 galaxies has been National Aeronautics and Space Administration, Office of Space Science, Planetary Astronomy Program. constrained observationally. 2 Based in part on data collected at Subaru Telescope, which is operated by To study nuclear starbursts in Seyfert galaxies, infrared the National Astronomical Observatory of Japan. L-band (2.8–4.1 m) spectroscopy can be a very powerful tool, 214 NUCLEAR STARBURSTS IN SEYFERT GALAXIES 215 TABLE 1 Detailed Information on the Observed Seyfert 1 Galaxies f12 f25 f60 f100 log LIR(logLIR=L ) fN fN (1B5) log f8:4 GHz Object Redshift (Jy) (Jy) (Jy) (Jy) (ergs sÀ1) (Jy) (Jy) (mJy beamÀ1) Remarks (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) CfA Sample Mrk 335 ................................ 0.025 0.30 0.38 0.34 <0.57 44.2 (10.6) 0.21 (i) 0.15 1.89 CfA, 12 m NGC 863 (Mrk 590) ............ 0.027 0.19 0.22 0.49 1.46 44.2 (10.6) 0.15 (ii) ... 3.44 CfA NGC 3786 (Mrk 744) .......... 0.009 0.10a 0.39a 1.20a 3.00a 43.4 (9.8) 0.059 (ii) ... 0.86 CfA NGC 4235............................. 0.008 <0.13 <0.16 0.32 0.65 42.5–43.0 (8.9–9.4) 0.034 (ii) ... 5.22 CfA NGC 4253 (Mrk 766)b......... 0.013 0.39 1.30 4.03 4.66 44.2 (10.6) 0.24 (ii) 0.25 4.77 CfA, 12 m NGC 5548............................. 0.017 0.40 0.77 1.07 1.61 44.2 (10.6) 0.20 (iii) 0.29 2.07 CfA, 12 m Mrk 817 ................................ 0.031 0.34 1.18 2.12 2.27 44.8 (11.2) ... 0.23 3.45 CfA, 12 m NGC 5940............................. 0.034 <0.17 0.11 0.74 1.75 44.3–44.4 (10.7–10.8) 0.026 (ii) ... ... CfA 2237+07 (UGC 1238) .......... 0.025 0.20 0.34 0.83 <2.49 44.2–44.3 (10.6–10.7) 0.063 (ii) ... 1.73 CfA NGC 7469............................. 0.016 1.35 5.79 25.87 34.90 45.1 (11.5) 0.60 (i) 0.31 13.63 CfA, 12 m Mrk 530 (NGC 7603) .......... 0.029 0.16 <0.25 0.85 2.04 44.3–44.4 (10.7–10.8) 0.077 (i) 0.10 2.78 CfA, 12 m 12 m Sample NGC 931 (Mrk 1040) .......... 0.016 0.61 1.32 2.56 4.55 44.4 (10.8) 0.33 (ii) 0.21 ... 12 m F03450+0055 ........................ 0.031 0.28 0.51 0.47 <3.24 44.4–44.6 (10.8–11.0) 0.17 (ii) 0.10 4.2 12 m 3C 120 .................................. 0.033 0.29 0.64 1.28 2.79 44.7 (11.1) 0.22 (i) 0.11 ... 12 m Mrk 618 ................................ 0.035 0.34 0.79 2.71 4.24 44.9 (11.3) 0.27 (ii) <0.06 1.7 12 m MCG À5-13-17 .................... 0.013 0.22 0.57 1.40 1.99 43.8 (10.2) ... 0.14 1.4 12 m Mrk 79 .................................. 0.022 0.31 0.76 1.50 2.36 44.4 (10.8) 0.26 (ii) 0.24 0.8 12 m NGC 2639............................. 0.011 0.16 0.21 1.99 7.06 43.8 (10.2) 0.008 (ii) <0.14 90.6 12 m Mrk 704 ................................ 0.030 0.35 0.53 0.36 <0.77 44.4–44.5 (10.8–10.9) 0.27 (ii) 0.30 0.7 12 m NGC 2992............................. 0.008 0.59 1.37 6.87 14.4 44.0 (10.4) 0.20 (iv) 0.16 3.9 12 m Mrk 1239b............................. 0.019 0.65 1.14 1.34 <2.42 44.4 (10.8) 0.60 (ii) 0.29 6.8 12 m UGC 7064............................