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Register a new userPrevalence of Tidal Interactions among Local Seyfert Galaxies
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No mechanisms have hitherto been conclusively demonstrated to be responsible for initiating optically-luminous nuclear (Seyfert) activity in local disk galaxies. Only a small minority of such galaxies are visibly disturbed in optical starlight, with the observed disturbances being at best marginally stronger than those found in matched samples of inactive galaxies. Here, we report the first systematic study of an optically-selected sample of twenty-three active galaxies in atomic hydrogen (HI) gas, which is the most sensitive and enduring tracer known of tidal interactions. Eighteen of these galaxies are (generally) classified as Seyferts, with over half (and perhaps all) having [OIII] luminosities within two orders of magnitude of Quasi-Stellar Objects. Only ~28% of these Seyfert galaxies are visibly disturbed in optical DSS2 images. By contrast, ~94% of the same galaxies are disturbed in HI, in nearly all cases not just spatially but also kinematically on galactic (>~20 kpc) scales. In at least ~67% and perhaps up to ~94% of cases, the observed HI disturbances can be traced to tidal interactions with neighboring galaxies detected also in HI. The majority of these neighboring galaxies have projected separations of <~ 100 kpc and differ in radial velocities by <~100 km/s from their respective Seyfert galaxies, and many have optical luminosities ranging from the Small to Large Magellanic Clouds. In a companion paper, we show that only ~15% of a matched control sample of inactive galaxies display comparable HI disturbances. Our results suggest that: i) most Seyfert galaxies (with high nuclear luminosities) have experienced tidal interactions in the recent past; ii) in most cases, these tidal interactions are responsible for initiating events that lead to their nuclear activity.
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We test whether there is a relation between the observed tidal interactions and Seyfert activity by imaging in HI twenty inactive galaxies at the same spatial resolution and detection threshold as the Seyfert sample. This control sample of inactive galaxies were closely matched in Hubble type, range in size and inclination, and have roughly comparable galaxy optical luminosity to the Seyfert galaxies. We find that only ~15% of the galaxies in our control sample are disturbed in HI, whereas the remaining ~85% show no disturbances whatsoever in HI. Even at a spatial resolution of ~10 kpc, none of the latter galaxies show appreciable HI disturbances reminiscent of tidal features. In a companion paper (Kuo et al. 2008), we report results from the first systematic imaging survey of Seyfert galaxies in atomic hydrogen (HI) gas. We find that only ~28% of the eighteen Seyfert galaxies in that sample are visibly disturbed in optical starlight. By contrast, ~94% of the same Seyfert galaxies are disturbed spatially and usually also kinematically in HI gas on galactic scales of >~20 kpc. In at least ~67% and up to perhaps ~94% of cases, the observed disturbances can be traced to tidal interactions with neighboring galaxies detected also in HI. The dramatic contrast between the observed prevalence of HI disturbances in the Seyfert and control samples implicates tidal interactions in initiating events that lead to luminous Seyfert activity in a large fraction of local disk galaxies.
We present an imaging survey of the CO(1--0), HCN(1--0), and HCO$^+$(1--0) lines in the nearby Seyfert galaxies using the Nobeyama Millimeter Array and RAINBOW Interferometer. Some of the observed Seyfert galaxies including NGC 1068, NGC 1097, NGC 5033, and NGC 5194 exhibit strong HCN(1--0) emission on a few 100 pc scales. The observed HCN(1--0)/CO(1--0) and HCN(1--0)/HCO$^+$(1--0) line ratios in the Seyfert nuclei ($>$0.2 and $>$1.8, respectively) have never been observed in the central regions of nuclear starburst galaxies. On the other hand, the molecular line ratios in the nuclei of NGC 3079, NGC 3227, NGC 4051, NGC 6764, NGC 7479, and NGC 7469 are comparable with those in the nuclear starburst galaxies. We propose that the elevated HCN emission originates from the X-ray irradiated dense molecular tori or XDRs close to the active nuclei. Our HCN/CO and HCN/HCO$^+$ diagrams will provide a new powerful diagnostic of the nuclear power source in active galaxies. Based on our diagnostic, we observe 3 of 5 type-1 Seyferts (6 of 10 in total) host compact nuclear starbursts. Our results are also supported by observations at other wavelengths such as those by L-band PAH spectroscopy. The proposed method will be crucial for investigating extremely dusty nuclei, such as ULIRGs and high-z submm galaxies, because these molecular lines are devoid of dust extinction. As an example, we present the HCN and HCO$^+$ observations of the LIRG NGC 4418, which suggests the presence of a buried active nucleus.
We present a three-dimensional study of the local (<100 h^-1} kpc) and the large scale (<1 h^{-1} Mpc) environment of the two main types of Seyfert AGN galaxies. For this purpose we use 48 Sy1 galaxies (with redshifts in the range 0.007<z<0.036) and 56 Sy2 galaxies (with 0.004<z<0.020), located at high galactic latitudes, as well as two control samples of non-active galaxies having the same morphological, redshift, and diameter size distributions as the corresponding Seyfert samples. Using the Center for Astrophysics (CfA2) and Southern Sky Redshift Survey (SSRS) galaxy catalogues (m_B~15.5) and our own spectroscopic observations (m_B~18.5), we find that within a projected distance of 100 h^-1 kpc and a radial velocity separation of dv<600 km/sec around each of our AGNs, the fraction of Seyfert 2 galaxies with a close neighbor is significantly higher than that of their control (especially within 75 h^{-1} kpc) and Seyfert 1 galaxy samples, confirming a previous two-dimensional analysis of Dultzin-Hacyan et al. We also find that the large-scale environment around the two types of Seyfert galaxies does not vary with respect to their control sample galaxies. However, in the Seyfert 2 and control galaxy samples do differ significantly when compared to the corresponding Seyfert 1 samples. Since the main difference between these samples is their morphological type distribution, we argue that the large-scale environmental difference cannot be attributed to differences in nuclear activity but rather to their different type of host galaxies.
We study the properties of infrared-selected QSOs (IR QSOs), optically-selected QSOs (PG QSOs) and Narrow Line Seyfert 1 galaxies (NLS1s). We compare their properties from the infrared to the optical and examine various correlations among the black hole mass, accretion rate, star formation rate and optical and infrared luminosities. We find that the infrared excess in IR QSOs is mostly in the far infrared, and their infrared spectral indices suggest that the excess emission is from low temperature dust heated by starbursts rather than AGNs. The infrared excess is therefore a useful criterion to separate the relative contributions of starbursts and AGNs. We further find a tight correlation between the star formation rate and the accretion rate of central AGNs for IR QSOs. The ratio of the star formation rate and the accretion rate is about several hundred for IR QSOs, but decreases with the central black hole mass. This shows that the tight correlation between the stellar mass and the central black hole mass is preserved in massive starbursts during violent mergers. We suggest that the higher Eddington ratios of NLS1s and IR QSOs imply that they are in the early stage of evolution toward classical Seyfert 1s and QSOs, respectively.
We present a detailed comparative systematic study using a sample of 221 Narrow-line Seyfert 1 (NLSy1) galaxies in comparison to a redshift matched sample of 154 Broad-line Seyfert 1 (BLSy1) galaxies based on their observations using ROSAT and/or XMM-Newton telescopes in soft X-ray band (0.1-2.0 keV). A homogeneous analysis is carried out to estimate their soft X-ray photon indices ($Gamma^{s}_{X}$) and its correlations with other parameters of nuclear activities such as Eddington ratios (R$_mathrm{Edd}$), bolometric luminosities (L$_mathrm{bol}$), black hole masses (M$_mathrm{BH}$) and the widths of the broad component of H$beta$ lines (FWHM(H$beta$)). In our analysis, we found clear evidence of the difference in the $Gamma^{s}_{X}$ and R$_mathrm{Edd}$ distributions among NLSy1 and BLSy1 galaxies, with steeper $Gamma^{s}_{X}$ and higher R$_mathrm{Edd}$ for the former. Such a difference also exists in the spectral indices distribution in hard X-ray ($Gamma^{h}_{X}$), based on the analysis of 53 NLSy1 and 46 BLSy1 galaxies in the 2-10 keV energy band. The difference in R$_mathrm{Edd}$ distributions does exist even after applying the average correction for the difference in the inclination angle of NLSy1 and BLSy1 galaxies. We also estimated R$_mathrm{Edd}$, based on SED fitting of 34 NLSy1 and 30 BLSy1 galaxies over the 0.3-10 keV energy band and found that results are still consistent with R$_mathrm{Edd}$ estimates based on the optical bolometric luminosity. Our analysis suggests that the higher R$_mathrm{Edd}$ in NLSy1 is responsible for its steeper X-ray spectral slope compared to the BLSy1, consistent with the disc-corona model as proposed for the luminous AGNs.
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