An ASCA Observation of M51 (NGC 5194): Iron K Emission from an Obscured Active Galactic Nucleus
TL;DR: In this paper, the authors present an ASCA observation of the nearby spiral galaxy M51 (NGC 5194) and detect hard X-ray emission with a photon index of 1.4 and a luminosity of LX ~ 1.1 × 1040 ergs s-1 in the 2-10 keV band (assuming a distance of 9.6 Mpc).
Abstract: We present an ASCA observation of the nearby spiral galaxy M51 (NGC 5194). We detected hard X-ray emission with a photon index of ~1.4 and a luminosity of LX ~ 1.1 × 1040 ergs s-1 in the 2-10 keV band (assuming a distance of 9.6 Mpc). A strong fluorescent iron K line (equivalent width ~900 eV) was detected at 6.4 keV in the X-ray spectra. Such an intense iron line is characteristic of Seyfert 2 galaxies and strongly suggests the presence of a heavily obscured active galactic nucleus (AGN). However, the X-ray image is extended even at energies above 2 keV. From the strong iron line and the extended hard X-ray image we speculate that the AGN is obscured by matter with a hydrogen column density more than several times 1023 cm-2 and that the observed 2-10 keV X-ray flux is not dominated by emission from the AGN but rather by other components, such as low-mass X-ray binaries, which typically dominate the X-ray emission of normal spiral galaxies. Emission lines from O K, Ne K, Fe L, Mg K and Si K were detected in the soft energy spectra, which indicate the presence of hot gas. The soft component is well represented by a Raymond-Smith thermal plasma model (with kT ~ 0.4 keV), which suggests a lower iron abundance (<0.1 solar) than other elements (~0.1-0.4 solar), or by a two temperature (kT ~ 0.3 keV and kT ~ 0.8 keV) model with ~0.1 solar abundance, which is reminiscent of the X-ray-emitting gas in starburst galaxies.
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TL;DR: A significant fraction of nearby galaxies show evidence of weak nuclear activity unrelated to normal stellar processes as discussed by the authors, which supports the notion that most, perhaps all, bulges host a central supermassive black hole, although the existence of active nuclei in at least some late type galaxies suggests that a classical bulge is not a prerequisite to seed a nuclear black hole.
Abstract: A significant fraction of nearby galaxies show evidence of weak nuclear activity unrelated to normal stellar processes. Recent high-resolution, multiwavelength observations indicate that the bulk of this activity derives from black hole accretion with a wide range of accretion rates. The low accretion rates that typify most low-luminosity active galactic nuclei induce significant modifications to their central engine. The broad-line region and obscuring torus disappear in some of the faintest sources, and the optically thick accretion disk transforms into a three-component structure consisting of an inner radiatively inefficient accretion flow, a truncated outer thin disk, and a jet or outflow. The local census of nuclear activity supports the notion that most, perhaps all, bulges host a central supermassive black hole, although the existence of active nuclei in at least some late-type galaxies suggests that a classical bulge is not a prerequisite to seed a nuclear black hole.
1,145 citations
Cites background or methods from "An ASCA Observation of M51 (NGC 519..."
...…thoroughly been investigated using ASCA (Yaqoob et al. 1995; Ishisaki et al. 1996; Iyomoto et al. 1996, 1997, 1998a, 1998b; Ptak et al. 1996, 1999; Terashima et al. 1998a, 1998b, 2000a, 2000b, 2002; Ho et al. 1999a; Terashima, Ho & Ptak 2000; Roberts, Schurch & Warwick 2001), with important…...
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...The marginally broad iron lines discovered with ASCA in M81 (Ishisaki et al. 1996) and NGC 4579 (Terashima et al. 1998a) has now been resolved into multiple components (Dewangan et al. 2004; Page et al. 2004; Young et al. 2007), none of which can be associated with a canonical disk....
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TL;DR: In this article, a three-dimensional diagnostic diagram for Seyfert 2 galaxies obtained using X-ray and [O III] data on a large sample of objects was presented.
Abstract: We present and discuss a three-dimensional diagnostic diagram for Seyfert 2 galaxies obtained using X-ray and [O III] data on a large sample of objects (reported in the Appendix). The diagram shows the Kα iron line equivalent width as a function of both the column density derived from the photoelectric cutoff and the 2-10 keV flux normalized to the [O III] optical-line flux (the latter corrected for extinction and assumed to be a true indicator of the source intrinsic luminosity). We find that the hard X-ray properties of type 2 objects depend on a single parameter, the absorbing column density along the line of sight, in accordance with the unified model. The diagram can be used to identify Compton-thick sources and to isolate and study peculiar objects. From this analysis we have obtained a column density distribution of Seyfert 2 galaxies that is thought to be a good approximation of the real distribution. A large population of heavily absorbed objects is discovered, including many Compton-thick candidates. Our results indicate that the mean log NH cm-2 in type 2 Seyfert galaxies is 23.5 and that as many as 23%-30% of sources have NH ≥ 1024 cm-2.
495 citations
Additional excerpts
...…1996; ( 21 )Ptak et al., 1996; ( 22 )this work; ( 23 )Makishima et al., 1994; ( 24 )Iwasawa et al., 1997a; ( 25 )Comastri et al., 1998; ( 26 )Terashima et al., 1998a; ( 27 ) Done, Madejski, & Smith, 1996; ( 28 )Terashima et al., 1998b; ( 29 )Akiyama et al., 1998; ( 30 )Turner et al., 1997;…...
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...…et al., 1994; ( 24 )Iwasawa et al., 1997a; ( 25 )Comastri et al., 1998; ( 26 )Terashima et al., 1998a; ( 27 ) Done, Madejski, & Smith, 1996; ( 28 )Terashima et al., 1998b; ( 29 )Akiyama et al., 1998; ( 30 )Turner et al., 1997; ( 31 )Terashima et al., 1998c; ( 32 )Cappi et al., 1996; ( 33 )Ueno…...
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...…et al., 1998a; ( 27 ) Done, Madejski, & Smith, 1996; ( 28 )Terashima et al., 1998b; ( 29 )Akiyama et al., 1998; ( 30 )Turner et al., 1997; ( 31 )Terashima et al., 1998c; ( 32 )Cappi et al., 1996; ( 33 )Ueno et al., 1996; ( 34 )Matt et al., 1996b; ( 35 )Iwasawa & Comastri, 1998; ( 36 )Iwasawa,…...
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TL;DR: In this paper, a comprehensive study of the X-ray properties of low-ionization nuclear emission-line regions (LINERs) and low-luminosity Seyfert galaxies based on observations obtained with the ASCA satellite is presented.
Abstract: This paper presents a comprehensive study of the X-ray properties of low-ionization nuclear emission-line regions (LINERs) and low-luminosity Seyfert galaxies based on observations obtained with the ASCA satellite. We analyzed data of 53 observations of 21 LINERs and 17 low-luminosity Seyferts. X-ray emission has been detected in all but one object. The X-ray luminosities in the 2-10 keV band range from 4 × 1039 to 5 × 1041 ergs s-1, which are 1-3 orders of magnitude smaller than in classical Seyfert galaxies. The X-ray spectra of most objects are well described by a canonical model which consists of (1) a soft component from a thermal plasma with kT < 1 keV and (2) a hard component represented by a power law with a photon index of Γ ≈ 1.8 or thermal bremsstrahlung emission with kT ≈ 10 keV. Several objects do not require the soft thermal component, and their continua are well fitted by a single power-law model. Some objects show heavy absorption with column densities in excess of 1023 cm-2. We detect in several objects Fe K line emission with equivalent widths ranging from 50 eV to 2 keV. Variability on timescales less than a day is uncommon in our sample. By comparing multiple observations made with ASCA or with published 2-10 keV observations from other satellites, we show that at least eight objects are variable on timescales of a week to several years. We find that the morphologies of many objects, both in the soft and hard bands, are consistent with being pointlike relative to the telescope point-spread function; a few are clearly extended in either or both energy bands. The second paper of this series will discuss the physical interpretation of the X-ray emission and its implications for low-luminosity active galactic nuclei.
218 citations
Cites background or methods from "An ASCA Observation of M51 (NGC 519..."
...— Detailed analyses of threeASCA observations are presented in Terashima et al. (1998a, 2000c)....
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...In the SIS observations, in which both the faint and bright modes were used, the two data sets were combined after the faint-mode data were convertedto the format of the bright-mode data; the only exception was the 1995 observation of NGC 4579 (see Terashima et al. 1998a)....
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...— Results of the firstASCA observation is presented in Terashima et al. (1998b)....
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...Alternatively, it is possible that the hard X-ray emission comes mainly from X-ray binaries, whatever contribution from an obscured AGN being strongly diluted, as in the case of NGC 5194 (Terashima et al. 1998b; Terashima & Wilson 2001; Fukazawa et al. 2001)....
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TL;DR: In this paper, the results of the analysis of 0.4-10.0 keV ASCA spectral analysis of a sample of low-luminosity active galactic nuclei (AGNs) (LLAGNs; M51, NGC 3147, and NGC 4258), low-ionization nuclear emission-line regions (LINERs; NGC 3079, NGN 3310, NGA 3998, NCA 4579, and nGC 4594), and starburst galaxies (M82, NCL 253, N GC 3628,
Abstract: The results of the analysis of 0.4-10.0 keV ASCA spectral analysis of a sample of low-luminosity active galactic nuclei (AGNs) (LLAGNs; M51, NGC 3147, and NGC 4258), low-ionization nuclear emission-line regions (LINERs; NGC 3079, NGC 3310, NGC 3998, NGC 4579, and NGC 4594), and starburst galaxies (M82, NGC 253, NGC 3628, and NGC 6946) are presented. In spite of the heterogeneous optical classifications of these galaxies, the X-ray spectra are fitted well by a canonical model consisting of an optically thin Raymond-Smith plasma soft component with T ~ 7 ? 106 K and a hard component that can be modeled by either a power law with a photon index ? ~ 1.7 or a thermal bremsstrahlung with T ~ 6 ? 107 K. The soft-component absorption is typically less than 1021 cm-2, while the hard component is typically absorbed by an additional column on the order of 1022 cm-2. The soft-component 0.4-10 keV intrinsic luminosities tend to be on the order 1039?40 ergs s-1, while the hard-component luminosities tend to be on the order of 1040?41 ergs s-1. The abundances inferred from the fits to the soft component are significantly subsolar. The Fe abundance can be measured independently of the other elemental abundances (dominated by ?-process elements) in M51, M82, NGC 253, and NGC 4258. In these galaxies the Fe abundance relative to ?-process elements is also (statistically) significantly subsolar. There is some indication (at a low statistical significance) that the abundance properties of starburst emission from starburst galaxies differs from the starburst emission from low-luminosity AGNs. However, these results on abundances are model dependent. Significant Fe K line emission is observed in M51, M82, NGC 3147, NGC 4258, and NGC 4579. An analysis of the short-term variability properties was given in Ptak et al. and detailed interpretation of these results will be given in Paper II in this series.
138 citations
Cites background from "An ASCA Observation of M51 (NGC 519..."
...A detailed analysis of the PV-phase ASCA is given in Terashima et al. (1998b) (note that this paper also includes the later AO-1 observation of M51)....
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...Significant Fe-K emission was detected in five galaxies: M51 (see also Terashima et al. 1998a), M82 (see also Ptak et al. 1997), NGC 3147 (see also Ptak et al. 1996), NGC 4579 (see also Terashima et al. 1998b), and NGC 4258 (see also Makishima et al. 1994)....
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...A detailed analysis of the ASCA data presented in this paper is given in Terashima et al. (1998b)....
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TL;DR: In this article, Chandra archive data was used to detect the presence of an active galactic nucleus in NGC-2273, UGC-5101, and NGC 3393 with isotropic luminosities of 7, 1500, and 400.
Abstract: Having conducted a search for the λ ~ 1.3 cm (22 GHz) water vapor line towards galaxies with nuclear activity, large nuclear column densities or high infrared luminosities, we present H2 O spectra for NGC 2273, UGC 5101, and NGC 3393 with isotropic luminosities of 7, 1500, and 400 . The H2 O maser in UGC 5101 is by far the most luminous yet found in an ultraluminous infrared galaxy. NGC 3393 reveals the classic spectrum of a “disk maser”, represented by three distinct groups of Doppler components. As in all other known cases except NGC 4258, the rotation velocity of the putative masing disk is well below 1000 km s-1 . Based on the literature and archive data, X-ray absorbing column densities are compiled for the 64 galaxies with reported maser sources beyond the Magellanic Clouds. For NGC 2782 and NGC 5728, we present Chandra archive data that indicate the presence of an active galactic nucleus in both galaxies. Modeling the hard nuclear X-ray emission, NGC 2782 is best fit by a high energy reflection spectrum with N H 1024 cm-2 . For NGC 5728, partial absorption with a power law spectrum indicates N H ~ 8 1023 cm-2 . The correlation between absorbing column and H2 O emission is analyzed. There is a striking difference between kilo- and megamasers with megamasers being associated with higher column densities. All kilomasers ( cm-2 . Among the H2 O megamasers, 50% arise from Compton-thick and 85% from heavily obscured (>1023 cm-2 ) active galactic nuclei. These values are not larger but consistent with those from samples of Seyfert 2 galaxies not selected on the basis of maser emission. The similarity in column densities can be explained by small deviations in position between maser spots and nuclear X-ray source and a high degree of clumpiness in the circumnuclear interstellar medium.
111 citations
Cites background from "An ASCA Observation of M51 (NGC 519..."
...NGC 5194 appears to be variable from marginally Compton-thick in 1993 (Terashima et al. 1998; Bassani et al. 1999) to Compton-thick in 2000 (Fukazawa et al. 2001) and NGC 2273, the newly detected maser source, is a Compton-thick source detected by BeppoSAX (Maiolino et al. 1998) and XMM-Newton…...
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TL;DR: In this paper, an optical spectroscopic survey of the nuclear regions (r < 200 pc) of a large sample of nearby galaxies is presented, along with a compilation of the global properties of all 486 galaxies in the survey.
Abstract: We have completed an optical spectroscopic survey of the nuclear regions (r < 200 pc) of a large sample of nearby galaxies. Although the main objectives of the survey are to search for low-luminosity active galactic nuclei and to quantify their luminosity function, the database can be used for a variety of other purposes. This paper presents measurements of the spectroscopic parameters for the 418 emission-line nuclei, along with a compilation of the global properties of all 486 galaxies in the survey. Stellar absorption generally poses a serious obstacle to obtaining accurate measurement of emission lines in nearby galactic nuclei. We describe a procedure for removing the starlight from the observed spectra in an efficient and objective manner. The main parameters of the emission lines (intensity ratios, fluxes, profile widths, equivalent widths) are measured and tabulated, as are several stellar absorption-line and continuum indices useful for studying the stellar population. Using standard nebular diagnostics, we determine the probable ionization mechanisms of the emission-line objects. The resulting spectral classifications provide extensive information on the demographics of emission-line nuclei in the local universe. This new catalog contains over 200 objects showing spectroscopic evidence for recent star formation and an equally large number of active galactic nuclei, including 46 which show broad H-alpha emission. These samples will serve as the basis of future studies of nuclear activity in nearby galaxies.
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Abstract: The spectrum of radiation emitted by a hot optically thin plasma with abundances and equilibrium ionization balance appropriate to interstellar conditions has been calculated. The results at wavelengths shorter than 200 A are discussed for material in the electron-temperature range from 160,000 to 100 million K.
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Abstract: Optical spectroscopic data are presented on the ionized nebulae associated with 14 galaxies that are strong far-IR emitters. It is found that the data provide both qualitative and quantitative support for the 'superwind' model in which the kinetic energy provided by SNe and winds from massive stars in a central starburst drives a large-scale outflow that can shock heat and accelerate ambient interstellar and circumgalactic gas. Clear kinematic signatures of an outflow along the galaxy's minor axis are found for the three nearest far-IR galaxies (FIRGs). The FIRG nebulae are highly overpressured relative to the Galactic ISM, with the pressure dropping systematically with distance from the nucleus. Superwinds are energetically adequate to power both the observed optical and X-ray nebulae, and the relative emission-line intensities and their radial variations are consistent with ionization by wind-driven shocks, but not with photoionization by normal O stars or an AGN. The possible astrophysical implications of superwinds are discussed.
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TL;DR: In this article, the observed spectra consist of both direct radiation from the primary X-ray source and reflection from the surrounding accretion flow, and features imprinted in it by photoabsorption, iron fluorescence and Compton scattering.
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927 citations