Ultraviolet‐Optical Observations of the Seyfert 2 Galaxies NGC 7130, NGC 5135, and IC 3639: Implications for the Starburst–Active Galactic Nucleus Connection (original) (raw)
Related papers
1998
The data provide direct evidence of the existence of a central nuclear starburst that dominates the UV light, and that seem to be responsible for the origin of the so called featureless continuum. These starbursts are dusty and compact. They have sizes (from less than 100 pc to a few hundred pc) much smaller and closer to the nucleus than that seen in the prototype Seyfert 2 galaxy NGC 1068. The bolometric luminosity of these starbursts is similar to the estimated bolometric luminosities of their obscured Seyfert 1 nuclei, and thus they contribute in the same amount to the overall energetics of these galaxies.
The nuclear starburst activity in the Seyfert 2 galaxy NGC 7679
Astronomy and Astrophysics, 2001
We present our recent spectrophotometric results of the infrared luminous Seyfert 2 galaxy NGC 7679. We find compelling evidence of the presence of a recent nuclear starburst, revealed by the observations of a) the spectral features of high order Balmer absorption lines, b) the weak equivalent widths for CaIIK λ3933, CN λ4200, G-band λ4300 and MgIb λ5173, and c) the suggested UV stellar wind resonance lines (N V λ1240, Si IV λ1400 and C IV λ1550) in the IUE spectrum. Using the simple stellar population synthesis model, we find that in the nuclear 2" × 2" region, the contributions from the old, intermediate and young components are 21.7%, 42.9% and 35.4%, respectively. These nuclear starburst activities might be triggered by the close encounter with NGC 7682, as suggested by recent numerical simulations.
Compact Nuclear Starbursts in Seyfert 2 Galaxies from the CfA and 12 Micron Samples
The Astrophysical Journal, 2003
We present infrared 2.8-4.1 µm slit spectra of 32 Seyfert 2 galaxies in the CfA and 12 µm samples. The 3.3 µm polycyclic aromatic hydrocarbon (PAH) emission feature was used to estimate the absolute magnitude of a compact nuclear starburst (less than a few hundred parsecs in size) that is presumed to have occurred in the outer region of an obscuring dusty molecular torus around a central supermassive black hole. We detected 3.3 µm PAH emission in 11 of the 32 Seyfert 2 nuclei in our sample, providing evidence for the presence of compact nuclear starbursts in a significant fraction of Seyfert 2 nuclei. However, the rest-frame equivalent widths of the 3.3 µm PAH emission, and the 3.3 µm PAH-to-infrared luminosity ratios measured in this study suggest that compact nuclear starbursts generally do not contribute significantly to the observed 3-4 µm nuclear fluxes or to the infrared luminosities of Seyfert 2 galaxies. Absorption features at 3.4 µm from bare dust were clearly detected in only two of the nuclei, and features at 3.1 µm from ice-covered dust were detected in only one nucleus. If the dust properties in the direction of these Seyfert 2 nuclei do not differ significantly from the Galactic interstellar medium, then these small absorption optical -2depths suggest that dust extinction toward the 3-4 µm continuum-emitting region in the innermost part of the obscuring dusty torus is modest: A V < 50-60 mag. Finally, the 3.3 µm PAH emission luminosities measured in this study were found to be significantly correlated with IRAS 12-and 25-µm, and nuclear Nband (10.6 µm) luminosities. If these three luminosities trace the power of the active galactic nucleus (AGN), then the luminosities of compact nuclear starbursts and AGNs are correlated. This correlation is in agreement with theories predicting that the presence of a compact nuclear starburst in the torus leads to an enhancement of the mass accretion rate onto the central supermassive black hole.
The Astrophysical Journal, 2004
We report on infrared K-(2-2.5 µm) and L-band (2.8-4.1 µm) slit spectroscopy of 23 Seyfert 1 galaxies in the CfA and 12 µm samples. A polycyclic aromatic hydrocarbon (PAH) emission feature at 3.3 µm in the L band is primarily used to investigate nuclear star-forming activity in these galaxies. The 3.3 µm 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 quantitatively estimated from the observed 3.3 µm PAH emission luminosities. The estimated starburst luminosities relative to some indicators of AGN powers in these Seyfert 1s are compared with 32 Seyfert 2s in the same samples that we have previously observed. We find that there is no significant difference in nuclear starburst to AGN luminosity ratios between Seyfert 1 and 2 galaxies, and that nuclear starburst luminosity positively correlates with AGN power in both types of Seyferts. Our results favor a slightly-modified AGN unification model, which predicts that nuclear starbursts occurring in the dusty tori of Seyferts are physically connected to the central AGNs, rather than the classical unification
On the Origin of the Ultraviolet Continuum in Seyfert 2 Galaxies
Astrophysical Journal - ASTROPHYS J, 1997
High-resolution Hubble Space Telescope ultraviolet images of the Seyfert 2 galaxies NGC 1667, NGC 3982, NGC 5427, and NGC 5953, previously known to have circumnuclear star-forming rings, are presented. The UV images of all four Seyfert 2 galaxies show the presence of several compact UV-bright knots at distances of ~0.7-2 kpc from the nucleus and associated to the circumnuclear star-forming ring. The optically bright Seyfert 2 nucleus is barely detected in the UV. The UV images provide the first direct empirical evidence that the UV flux emitted by galaxies having a Seyfert 2-type nucleus surrounded by a circumnuclear star-forming ring is dominated by radiation coming from clusters of young hot stars distributed along the star-forming ring. The contribution of the Seyfert 2 nucleus amounts to ~1%-10% of the observed UV flux in these four galaxies. If circumnuclear star-forming rings of 2-3 kpc in diameter, or smaller, are a common feature of Seyfert 2 galaxies, we conclude that (a) the observed UV flux in Seyfert 2 galaxies is not dominated by the radiation from the nuclear ionizing source and (b) the observed UV flux is dominated by emission from clusters of young hot stars located in circumnuclear star-forming regions. Whatever the nature of the nuclear ionizing source in these Seyfert 2 galaxies, this must be obscured. The UV and Hα luminosities associated to the Seyfert 2 nucleus in these galaxies are consistent with the scenario of the nuclear ionizing source being an obscured power-law ionizing source or a young (2-4 Myr) obscured high-metallicity (>=Zsolar) nuclear starburst.
Publications of the Astronomical Society of Japan, 2010
We present near-infrared K-(1.9-2.5µm) and L-(2.8-4.2µm) 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 λ rest = 3.3 µm in the rest frame of L-band spectrum (a starburst indicator) and (2) the CO absorption feature at λ rest = 2.3-2.4 µm in the rest frame of the K-band spectrum, originating in the CO molecule. We clearly detected the 3.3 µm 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 µm PAH emission luminosity (L 3.3P AH ) distributions for the Seyfert 1s and Seyfert 2s are very similar when normalized to the AGN power. Star-formation rates estimated from L 3.3P AH could be large enough to inflate the dusty torus by supernova explosion. We find that L 3.3P AH 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.
Seyfert 2 galaxies in the GeV band: jets and starburst
2010
Context. The Fermi/LAT collaboration recently reported the detection of starburt galaxies in the high energy γ-ray domain, as well as radio-loud narrow-line Seyfert 1 objects. Aims. Motivated by the presence of sources close to the location of composite starburst/Seyfert 2 galaxies in the first year Fermi/LAT catalogue, we aim at studying high energy γ-ray emission from such objects, and at disentangling the emission of starburst and Seyfert activity. Methods. We analysed 1.6 years of Fermi/LAT data from NGC 1068 and NGC 4945, which count among the brightest Seyfert 2 galaxies. We search for potential variability of the high energy signal, and derive a spectrum of these sources. We also analyse public INTEGRAL IBIS/ISGRI data over the last seven years to derive their hard X-ray spectrum. Results. We find an excess of high energy γ-rays of 8.3 σ and 9.2 σ for 1FGL J0242.7+0007 and 1FGL J1305.4−4928, which are found to be consistent with the position of the Seyfert 2 galaxies NGC 1068 and NGC 4945, respectively. The energy spectrum of the sources can be described by a power law with a photon index of Γ = 2.31 ± 0.13 and a flux of F 100 MeV−100 GeV =(8.60 ± 2.27) × 10 −12 erg cm −2 s −1 for NGC 1068, while for NGC 4945, we obtain a photon index of Γ = 2.31 ± 0.10 and a flux of F 100 MeV−100 GeV =(1.58 ± 0.32) × 10 −11 erg cm −2 s −1 . For both sources, we detect no significant variability nor any indication of a curvature of the spectrum. While the high energy emission of NGC 4945 is consistent with starburst activity, that of NGC 1068 is an order of magnitude above expectations, suggesting dominant emission from the active nucleus. We show that a leptonic scenario can account for the multi-wavelength spectral energy distribution of NGC 1068. Conclusions. High energy γ-ray emission is revealed for the first time in a Seyfert 2 galaxy. If this result is confirmed in other objects, new perspectives would be opened up into the GeV band, with the discovery of a new class of high energy γ-ray emitters.
Publ Astron Soc Jpn, 2010
We present near-infrared K- (1.9-2.5μm) and L- (2.8-4.2μm) band spectroscopy of 22 Seyfert nuclei. We used two methods to investigate the presence of nuclear starbursts: (1) the Polycyclic Aromatic Hydrocarbon (PAH) emission feature at λrest = 3.3μm in the rest frame of the K-band spectrum (a starburst indicator) and (2) the CO absorption feature at λrest = 2.3-2.4μm in the rest frame of the K-band spectrum, originating in the CO molecule. We clearly detected the 3.3μm 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 the relative strength of stellar emission because AGN emission is affected by dust extinction. The 3.3μm PAH emission luminosity (L3.3PAH) distributions for the Seyfert 1s and Seyfert 2s are very similar when normalized to the AGN power. The star-formation rates estimated from L3.3PAH could be large enough to inflate the dusty torus by a supernova explosion. We find that L3.3PAH positively correlates with the N-band luminosity with small aperture over a wide AGN luminosity range, and is independent of the physical area we probed. The results suggest that the nuclear region has a concentration of star formation and it would control the AGN activity.
The Astrophysical Journal Supplement Series, 2009
We have analyzed FUSE (905-1187Å) spectra of a sample of 16 local starburst galaxies. These galaxies cover almost three orders of magnitude in starformation rates and over two orders of magnitude in stellar mass. Absorption features from the stars and interstellar medium are observed in all the spectra. The strongest interstellar absorption features are generally blue-shifted by ∼ 50 to 300 km s −1 , implying the almost ubiquitous presence of starburst-driven galactic winds in this sample. The outflow velocites increase with both the star formation rate and the star formation rate per unit stellar mass, consistent with a galactic wind driven by the population of massive stars. We find outflowing coronal-phase gas (T ∼ 10 5.5 K) detected via the O VI absorption-line in nearly every galaxy. The O VI absorption-line profile is optically-thin, is generally weak near the galaxy systemic velocity, and has a higher mean outflow velocity than seen in the lower ionization lines. The relationship between the line width and column density for the O VI absorbing gas is in good agreement with expectations for radiatively cooling and outflowing gas. Such gas will be created in the interaction of the hot out-rushing wind seen in X-ray emission and cool dense ambient material. O VI emission is not generally detected in our sample, suggesting that radiative cooling by the coronal gas is not dynamically significant in draining energy from galactic winds. We find that the measured outflow velocities in the HI and HII phases of the interstellar gas in a given galaxy increase with the strength (equivalent width) of the absorption feature and not with the a FUV luminosity derived from FUSE LiF 2A continuum flux at 1150Å. Values are corrected for MW extinction using the method of and the E(B-V) values given in this table.
Probing nuclear starburst activity in a sample of nearby spiral galaxies
Astronomy and Astrophysics
As part of a systematic study of the UV properties of galaxies in the IUE library, we present in this paper an analysis of nuclear stellar populations in spiral galaxies with radial velocity ≤ 5 000 km s −1 . In this sample of 60 galaxies the IUE aperture probes a mean 1.0 kpc × 2.1 kpc area. Prior to any comparison of galaxy spectra in the range covered by IUE (1200-3200Å), we have formed subsets according to the absolute magnitude and morphological type of the studied galaxies. In a second step, and within each subset, we have co-added the spectra, and hence the objects into groups of similar spectral properties in the UV, also taking into account their spectral properties in the visible/nearinfrared ranges. As a result, high signal-to-noise ratio templates have been obtained, and information on spectral features can now be extracted and interpreted. We distinguish 4 groups for Sa, 8 for Sb, and 4 for Sc galaxies. We have carried out population syntheses using as base elements: H ii regions, integrated star clusters, and far-UV weak elliptical galaxies as representative of bulge stellar population. The variety of UV spectral types found in the central regions of spiral galaxies can be readily explained by different mixtures of bulge, circumnuclear burst and disc populations. Across different morphological types, similar templates can also be found. This is due to compensation effects of bulge contribution with the disc and circumnuclear burst ones. Flux fractions derived from the population synthesis have been converted into mass contributions and inferences have been made on the star-formation histories. In the central kpc of the galaxies with strong UV flux, we find that the mass stored in the young components (t < 500 Myr) is typically ≈ 10 7 M . We confirm that such star-formation enhancements occur preferentially in barred spirals. Internal reddening in the templates has been studied and inferences have been made on the corresponding reddening laws. We find cases where an SMC-like law applies and others where a faint λ2200Å absorption feature occurs resembling the reddening law of the LMC. The interest of the IUE data set resides in its rather large entrance aperture which samples a large portion of nearby galaxies, and is therefore quite suitable for the interpretation of large redshift galaxies.