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Seyfert Galaxies with Circumnuclear/Nuclear Starbursts

In this paper, we present our preliminary results on Seyfert galaxies with circumnuclear/nuclear starburst(SB) activity. We have searched the recent available literature and found 76 active galaxies with clear evidence of nuclear SB activity, among which 16 are Seyfert 1s, 51 Seyfert 2s, and 9 LINERs. After studying the 51 Seyfert 2s, we find that those Seyfert 2s with hidden Seyfert 1 nuclei, have similar Infrared-Radio properties as Seyfert 1 galaxies, and are different from"real"Seyfert 2s without a hidden Seyfert 1 nucleus. The later are similar to starburst galaxies.


INTRODUCTION
The connection between starburst(SB) activity and AGNs is one of the most important and hotly debated issues in the study of active galaxies. On the theoretical side, Norman & Scoville (1988) have suggested that the circumnuclear star-forming activity could drive gas into the innermost nuclear region to feed the central black hole. However, Terlevich (1992) and Terlevich et al. (1992) have proposed that Seyfert 2s and LINERs could be well reproduced by violent star formation in a metal-rich environment, such as in the nuclei of earlytype galaxies, which has been well known as the Starburst model for AGN (see e.g. the recent review by Cid Fernandes 1997 and references therein). In a series of papers (Dultzin-Hacyan 1995, Dultzin-Hacyan & Ruano 1996, and Dultzin-Hacyan et al. 1999, it has been shown that there are several lines of evidence against the simplest formulation of a "Unified Scheme" for Seyfert galaxies, according to which, all Seyfert 2s have a hidden Sy 1 nucleus. In this paper, we add new evidence to the existence of two kinds of Sy 2s: The hidden Sy 1, and the "real" or "pure" Sy 2. According to these authors the difference between Sy 1 and "real" Sy 2 is relative decrease in the accretion power in the first versus a relative increase of nuclear/circumnuclear SB activity in the second. On the other hand, more and more high-quality observations from HST and large telescopes on ground, support the presence of active star formation activity around the nuclei in dozens of Seyfert galaxies. For example, in NGC 1068, a prototype of Seyfert 2 galaxy, the circumnuclear starburst accounts for at least 50% of the whole bolometric luminosity, 81% at far UV(∼ 1500Å), and 83 % at near UV (∼ 2500Å) bands(Gonzalez .  and Gonzalez Delgado et al. (1998) have presented high resolution UV images of 4 Seyfert 2 galaxies, and found compact nuclear starburst in all of them. In Seyfert 2 galaxies with nuclear starburst, the observed UV fluxes, as suggested by , are dominated not by AGN, but by the young massive stars in the nuclear region.
There are several ways to detect star-forming activities in Seyfert galaxies (Wilson 1987). Narrow-band Hα imaging , high-resolution radio continuum mapping(Forbes & Norris 1998), high S/N spectroscopy on central region and using stellar population synthesis technique , Boisson et al. 2000, and Jimenez-Benito et al. 2000, the shape of the IRAS spectra, and infrared(IR) spectroscopics searching for absorption lines from red supergiants (Terlevich et al. 1990, Oliva et al. 1995.
In order to study the role of starburst in Seyfert galaxies and the connection between starburst and AGNs, we collect all active galaxies with circumnuclear and/or nuclear starburst as completely as possible from the recent literature, which are listed in Table 1.
This paper is organized as follows. In Section 2, we present the basic information of our sample of 76 active galaxies with circumnuclear/nuclear starburst, and the IR and radio properties of 51 Seyfert 2 galaxies are presented and discussed in Section 3. Finally, Section 4 summarizes our main results.

THE SAMPLE OF ACTIVE GALAXIES
From the recent literature, we collect the active galaxies with the clear evidence of nuclear and/or circumnuclear starburst activities, such as nuclear star-forming rings, spectroscopic signature (high order Balmer absorption lines, Wolf-Rayet features) of young massive stars, etc, and find 76 such active galaxies, which are presented in Table 1. Table 1 contains the list of all 76 active galaxies with increasing right ascension, which includes: galaxy name (column 1); 2000 right ascension and declination (columns 2 and 3); redshift (4) and activity type (5) from NASA/IPAC Extragalactic Database(NED); distance (6), for nearby galaxies (z < 0.01), it is taken from the database of the CfA redshift survey, and for those with z > 0.01, it is calculated by V GSR /h 0 , where h 0 is equal to 75 km/s/Mpc; B magnitude (7) and the recessional velocities corrected to the Galactic Center (V GSR )(8) from RC3 (de Vaucouleurs et al. 1991), and finally references to evidence of starburst in column 9.
Among these 76 active galaxies, the activity types of NGC 2681 and NGC 6574 in NED are given to be "Sy", so we assign NGC 2681 to be LINER as in Veron & Veron (2000), and NGC 6574, "Sy2", as in Kotilainen et al. (2000). Following Maiolino & Rieke (1995), we take Seyfert 1 + 1.2 + 1.5 as Seyfert 1 galaxies, and Seyfert 1.8 + 1.9 + 2.0 as Seyfert 2s, so we get 16 Seyfert 1s, 51 Seyfert 2s, and 9 LINERs. Though our sample cannot be considered complete in any way, and there exists an observational bias (it is difficult to detect the nuclear starburst in Seyfert 1s), our results confirm that nuclear starburst activities are more common in Seyfert 2s than in Seyfert 1s , Maiolino et al. 1997, and Dultzin-Hacyan et al. 1999.
In Table 2, we summarize the IRAS fluxes and 1.49 GHz radio emission from the NRAO/VLA Sky Survey (NVSS) (Condon et al. 1998), and T, the numerical index of Hubble type, incl, the inclination angle, and W20, 21-cm HI line width at 20 percent of the peak, are taken from the Lyon-Meudon Extragalactic Database (LEDA).
In the following text, we define "SS" for Seyfert 2 galaxies with Starbursting activity and "PA" for Seyfert 2s with Pure AGN activity in order to avoid confusion.

Two types of Seyfert 2s ?
The discovery of broad polarized Balmer lines in some Seyfert 2 galaxies (Antonucci & Miller 1985, Miller & Goodrich 1990, and Tran 1995a, leads to the now well-known standard unified model for Seyfert galaxies, where Seyfert 1 and 2 galaxies are intrinsically the same, the observed differences are just the result of varying orientation relative to the line of sight, and the broad-line region in Seyfert 2s is hidden by an obscuring dusty molecular torus whose size is around a few pc. If it were the case, there would be no statistical difference between their host galaxies. However, recent studies have indicated that the host galaxies of Seyfert 1 are different from those of Seyfert 2s, including (1) the Hubble type, Seyfert 1's is earlier than Seyfert 2's ( Malkan et al. 1998, Hunt & Malkan 1999); (2) the environment, the relative densities of Seyfert 2s are 1.6 to 2.7 times higher than those of Seyfert 1s (Laurikainen & Salo 1995, Dultzin-Hacyan et al. 1999; (3) the circumnuclear Fig. 1. Distribution of Seyfert 2 galaxies with nuclear starburst in the IR diagram. Open circle represents Seyfert 2 galaxy without hidden Seyfert 1 nucleus, and filled circle for Seyfert 2 with hidden Seyfert 1 nucleus.
star-forming activity, there is more starburst activity in Seyfert 2s than in Seyfert 1s (Maiolino et al. 1997, Ohsuga & Umemura 1999; and (4) the bar percentage in Seyfert 2s is also higher than that in Seyfert 1s (Pogge 1989, Maiolino et al. 1997). All these evidence can't be explained by this simplified unified model.
On the other hand, Hutchings & Neff (1991) and Neff & Hutchings(1992) even earlier have suggested that there are two distinct types of Seyfert 2s, those with hidden Seyfert 1 nuclei and those without. During studying the origin of the ultraviolet(UV)continuum in Seyfert 2 galaxies, Heckman et al. (1995) have also suggested the possibility of two entirely different kinds of type 2 Seyfert galaxies(see also Antonucci 1993).
Among these 51 Sy 2s, there are 9 PA galaxies as indicated in Veron & Veron (2000) and 42 SS galaxies, where 8 sources with upper limits of 25 µm fluxes, so we need to use the survival analysis methods (ASURV Rev 1.2, Isobe, Feigelson & Nelson 1986) for the following statistical study. We find that the probability for these two samples (PA and SS) to be extracted from the same parent population is less than 0.0001 %, and the mean values of s 25µm /s 60µm are 0.484±0.082 and 0.170±0.018 for PA and SS, respectively.
Following Hutchings & Neff (1991), we plot these 51 Seyfert 2s in Fig 1, where we show the distribution of 60 µm luminosity vs. the flux ratio of 25 µm to 60 µm. As predicted, most of them are located in the SS region. We find that there are 12 sources located in the PA region, eight of them (NGC 262, NGC 1068, NGC 1275, NGC 4507, NGC 5506, IC 3639, Mark 477 and Mark 1210) have been detected polarized broad emission lines, except IC 4995, Mark 607, NGC 5347 and NGC 7672. It is very interesting to notice that the 60 µm luminosities of these four sources are all less than 6.3 ×10 9 L ⊙ , and also less than that of those with detectable hidden BLRs, which indicate that there might also exist hidden BLRs, but too faint to be detected (David Alexander, 2000, private communication). It is consistent for NGC 7672 that according to Miller & Goodrich (1990), NGC 7672 might have hidden BLRs, but at a flux level below the detection threshold. For NGC 5347, Gonzalez Delgado & Perez (1996) have observed very-high-excitation lines, such as [Ne V], He II, [Fe VII] and [Fe X], which are photoionized by a hard AGN-like continuum. For the other two sources, IC 4995 and Mark 607, there are no spectropolarimetric observations yet. Thus, our results confirm that there might exist two distinct Seyfert 2s: one is the "pure" Seyfert 2 and the other is the hidden Seyfert 1.

IR and Radio properties
In order to find out the differences between PA and SS galaxies, we study their radio and IR properties. It is well known that there exists a tight relation between radio and far infrared (FIR) emission for normal, starburst and Seyfert galaxies (the later with more scatter). We define q to be the ratio of FIR to 1.49 GHz emission as in Helou et al. (1985), which is : Where FIR = 1.26 × 10 −14 (2.58 × s 60µm + s 100µm ), s 60µm and s 100µm are IRAS fluxes at 60 µm and 100 µm, respectively. Since IRAS was taken with a fixed aperture (at 60 µm, its resolution is 1.5' × 4.75 ', Neugebauer et al. 1984), in general, for the most nearby galaxies, the circumnuclear starburst rings could lie outside the aperture. However, introspection of our sample shows that for the three closest galaxies in our sample: Circinus (4.2 Mpc), NGC 4945 (3.7 Mpc), and NGC 5128 (4.9 Mpc), their star-forming rings are located at about 10, 5.6 and 50 arcseconds, respectively (Marconi et al. 1994, Marconi et al. 2000a, which are far smaller than the IRAS aperture. And for the rest of galaxies, the distance is more than 10 Mpc, and 1.5' corresponds to larger than 4.36 Kpc. So the IRAS aperture is not a problem for our sample of galaxies. For (radio-loud) AGN, q is less than 2.0 (Sopp & Alexander 1991), while in starburst it is around 2.21 Fig. 2. Distribution of Seyfert 2 galaxies with nuclear starburst in the diagram of ratio of FIR to radio (q) vs. the IR spectral index. The symbols have the same meaning as in Fig. 1. (Condon et al. 1991, Forbes & Norris 1998 3 . On the other hand, according to de Grijp et al. (1985), the spectral index between 25 µm and 60 µm, α(25,60) defined by f ν ∝ ν α , is also a powerful parameter for discriminating AGNs from starburst, with AGN-like colour, -1.5 < α(25,60) < 0.0. Thus, we plot 51 Seyfert 2s in the diagram q vs. α(25,60) (see Figure 2). In Fig. 2, we define two regions: the SB region (α(25,60) < -1.5 and 2.0 < q < 2.8 ) and the AGNs region ( α(25,60) > -1.5 and q < 2.0 ). It is consistent with Fig. 1 in that most of SS galaxies are located in the SB region, since in these galaxies, both the FIR and radio emissions are dominated by star-forming activities (Sopp & Alexander 1991, Roy et al. 1998).
There are eight galaxies in the upper-left AGNs region, seven of them are Seyfert 2s with hidden Seyfert 1 nuclei (NGC 262, NGC 1068, NGC 1275, NGC 4507, NGC 5506, Mark 477 and Mark 1210. The only one without detection of hidden BLRs is NGC 3393, which might be due to the huge amount of obscuring matter in the line of sight (N H > 10 25 cm −2 , Bassani et al. 1999), and recently, Cooke et al. (1997) report that they have detected the broad emission lines, so it might contain a Seyfert 1 nucleus in the center. There is also one exception with the obscured Seyfert 1 nucleus but lying outside the AGNs region, which is IC 3639. But it is located very nearby. For four galaxies with possible weak hidden BLRs as indicated in Fig. 1, there is no 1.49GHz observation data for IC 4995, NGC 7672 is much closer to the AGNs region, and the other two sources (NGC 5347 and Mark 607), due to the weakness of the hidden Seyfert 1 nuclei and the presence of nuclear starburst, they locate neither in the AGNs region nor the starburst region. There is one galaxy (NGC 4639) to the right side of SB region, which has the largest FIR excess, and needs further consideration. It is shown that these galaxies have smaller q, which means that they have a radio excess, which might be related to the central AGN (Roy et al. 1998).
It indicates again that SS galaxies have similar properties as starburst galaxies, while PA galaxies are similar to Seyfert 1 galaxies.

CONCLUSIONS
We have collected 76 active galaxies with circumnuclear or nuclear star-forming activities, among which, 16 are Seyfert 1s, 51 Seyfert 2s and 9 LINERs. After studying their IR and radio properties of 51 Seyfert 2s, we confirm that there are two distinct types of Seyfert 2 galaxies, those with hidden Seyfert 1 nuclei share similar properties with Seyfert 1s, and those without, are similar to Starburst galaxies.