Circinus Galaxy Research Articles

Iron and nickel line properties in the X-ray-reflecting region of the Circinus galaxy

We discuss the iron and nickel properties in the nuclear X-ray-reflecting region of the Circinus galaxy, studied with XMM-Newton. The main results are as follows. (i) From the depth of the Fe Ka edge, a value of A F e = 1.7 in number with respect to the cosmic value (as for Anders & Grevesse) is measured, if the (not directly visible) illuminating spectrum is assumed to be that measured by BeppoSAX. If the slope of the primary power law is left free to vary, a steeper spectrum and a lower iron abundance (about 1.2) are found. (ii) From the Ni to Fe Kα line flux ratio, a nickel-to-iron abundance ratio of 0.055-0.075 is found. (iii) The Fe Kβ/Kα flux ratio is slightly lower than expected, possibly due to a mild ionization of iron (which, however, cannot be much more ionized than x). (iv) The presence of the Fe Ka Compton shoulder, already discovered by Chandra, is confirmed, its relative flux implying Compton-thick matter. This further supports the identification of the reflecting region with the absorber.

The Discovery of H 2 O Maser Emission in Seven Active Galactic Nuclei and at High Velocities in the Circinus Galaxy

We report the discovery of H2O maser emission at 1.35 cm wavelength in seven active galactic nuclei (at distances of up to 80 Mpc) during a survey conducted at the 70 m diameter antenna of the NASA Deep Space Network near Canberra, Australia. The detection rate was ~4%. Two of the maser sources are particularly interesting because they display satellite high-velocity emission lines, which are a signature of emission from the accretion disks of supermassive black holes when seen edge-on. Three of the masers are coincident, to within uncertainties of 02, with continuum emission sources that we observed at about λ = 1.3 cm. We also report the discovery of new spectral features in the Circinus galaxy H2O maser that broaden the known velocity range of emission therein by a factor of ~1.7. If the new spectral features originate in the Circinus accretion disk, then molecular material must survive at radii ~3 times smaller than had been believed previously (~0.03 pc or ~2 × 105 Schwarzschild radii).

Flux and spectral variations in the Circinus Galaxy

We report on a dramatic flux (~ increase in the LECS and MECS band) and spectral variation between two BeppoSAX observations of the Circinus Galaxy performed almost three years apart. Through the analysis of all Chandra observations available in the archive, including a new DDT observation on May 2001, we show that a high flux state of an extremely variable Ultra Luminous X-ray source (CG X-1: Bauer et al. [CITE]), which is within the adopted BeppoSAX source extraction region of , is the most likely explanation for most of the observed variation. However, the presence of a high flux 6.7 keV line and the spectral variation of the PDS in the new BeppoSAX data could be partly due to intrinsic variation of the nucleus. Comparing the longest Chandra observation and the BeppoSAX one, we find that the long-term flux variability of CG X-1 is not accompanied by a significant spectral variability. We also re-analysed the Chandra HEG nuclear spectra and report on the presence of a Compton shoulder with a flux of about the line core, in agreement with theoretical expectations for Compton-thick matter.

Infrared spectroscopy around 4 μm of Seyfert 2 galaxies: Obscured broad line regions and coronal lines

The state of the matter that is obscuring a small circumnuclear region in active galactic nuclei can be probed by ob- servations of its broad emission lines. Infrared lines are particularly useful since they penetrate significant columns of obscuring matter, the properties of which can be constrained by comparing infrared and X-ray obscuration. We report on new 4m spec- troscopy with ISAAC at the ESO VLT of a sample of 12 Seyfert 2 galaxies, probing for broad components to the Brackett 4.05m hydrogen recombination line. Broad components are observed in 3 to 4 objects. All objects with a broad component exhibit relatively low X-ray obscuring columns, and our results are consistent with a Galactic ratio of 4m obscuration to the BLR and X-ray column. In combination with observations of a non-Galactic ratio of visual obscuration of BLRs and X-ray obscuring column in Seyferts, and interpreted in a unified AGN scheme, this result can be reconciled with two interpretations. Either the properties of dust near the AGN are modified towards larger grains, for example through coagulation, in a way that significantly flattens the optical/IR extinction curve, or the ratio of dust obscuration to X-ray column varies for dierent viewing angles with respect to the axis of symmetry of the putative torus. Our spectra also provide a survey of emission in the (Si IX) 3.94m coronal line, finding variation by an order of magnitude in its ratio to Br. The first extragalactic detection of the (Ca VII) 4.09m and (Ca V) 4.16m coronal lines is reported in the spectrum of the Circinus galaxy.

Variability of Water Megamasers in AGN

AbstractPowerful water masers have been detected in 23 active galactic nuclei (AGN) to date. Most of the sources that have been monitored show evidence for variability, which may be either intrinsic (response of the masers to a time-varying pump or background source) or extrinsic (the effects of scintillation in the interstellar medium of the Galaxy). I briefly review the examples in which these mechanisms may be operating (interstellar scintillation in the Circinus galaxy, a time-varying background source in Mrk 348, and a time-varying AGN luminosity in NGC 1068).

N‐Band Imaging of Seyfert Nuclei and the Mid‐Infrared–X‐Ray Correlation

We present new mid-infrared (N-band) images of a sample of eight nearby Seyfert galaxies. In all of our targets, we detect a central unresolved source, which in some cases has been identified for the first time. In particular, we have detected the mid-infrared emission from the active nucleus of NGC 4945, which previously remained undetected at any wavelength but hard X-rays. We also detect circumnuclear extended emission in the Circinus galaxy along its major axis and find marginal evidence for extended circumnuclear emission in NGC 3281. The high spatial resolution (17) of our data allows us to separate the flux of the nuclear point sources from the extended circumnuclear starburst (if present). We complement our sample with literature data for a number of nonactive starburst galaxies and relate the nuclear N-band flux to published hard (2-10 keV) X-ray fluxes. We find tight and well-separated correlations between the nuclear N-band flux and X-ray flux for both Seyfert and starburst nuclei that span over 3 orders of magnitude in luminosity. We demonstrate that these correlations can be used as a powerful classification tool for galactic nuclei. For example, we find strong evidence against NGC 1808 currently harboring an active Seyfert nucleus based on its position in the mid-infrared-X-ray diagram. On the other hand, we confirm that NGC 4945 is in fact a Seyfert 2 galaxy.

AChandraObservation of the Circinus Galaxy

We report on a recent Chandra ACIS-S observation of the Circinus galaxy. These observations confirm that the nuclear spectrum results from reflection of a hard X-ray continuum by "neutral" matter. The nuclear X-ray emission is extended by ~60 pc in the general direction of the optical "ionization cone." An image in the Fe Kα line has been made and shows that this emission extends up to 200 pc from the nucleus. There is also large-scale X-ray emission both along and perpendicular to the galaxy disk. Thermal plasma models for this extended gas indicate temperatures kT ~ 0.6 keV, though cooler photoionized gas is also possible. The X-ray emission from gas in the disk is probably associated with the starburst ring of radius 150-250 pc. The gas extending ~600 pc perpendicular to the disk is closely correlated with the high-excitation optical line emission. In addition to its soft X-ray emission, we tentatively detect a hard component from the gas above the plane; this hard emission may represent nuclear X-rays scattered into our line of sight by electrons in the outflowing wind. A total of 10 compact sources are found in the central kiloparsec of the galaxy. The most luminous has an X-ray luminosity of ≃1040 ergs s-1 and seems to be an X-ray binary in the Circinus galaxy with a black hole mass exceeding 80 M☉.

A [ITAL]CHANDRA[/ITAL][ITAL]Chandra[/ITAL] Study of the Circinus Galaxy Point-Source Population

We have used the Chandra X-Ray Observatory to resolve spatially and spectrally the X-ray emission from the Circinus galaxy. We report here on the nature of the X-ray emission from the off-nuclear point sources associated with the disk of Circinus, which make up ≈34% of the total 0.5–10 keV emission. We find that many of the serendipitous X-ray sources are concentrated along the optical disk of the galaxy, although few have optical counterparts within 1'' of their X-ray positions down to limiting magnitudes of mV = 23–25. At the distance of Circinus (≈3.8 Mpc), their intrinsic 0.5–10 keV luminosities range from ≈2 × 1037 ergs s-1 to ≈4 × 1039 ergs s-1. One-fourth of the sources are variable over the duration of the 67 ks observation, and spectral fitting of these off-nuclear sources shows a diverse range of spectral properties. The overall characteristics of the point sources suggest that most are X-ray binaries and/or ultraluminous supernova remnants within Circinus. We are able to analyze the two strongest off-nuclear sources in greater detail and find both to have remarkable properties. The average X-ray luminosities of the two sources are 3.7 × 1039 ergs s-1 and 3.4 × 1039 ergs s-1. The former displays large and periodic flux variations every 7.5 hr and is well fitted by a multicolor blackbody accretion disk model with Tin = 1.35 keV, properties consistent with an eclipsing ≳50 M⊙ black hole binary. The latter appears to be a young supernova remnant, as it coincides with a nonthermal radio counterpart and an Hα-detected H II region. This source exhibits both long-term (≈4 yr) X-ray variability and a 6.67–6.97 keV iron emission-line blend with a 1.6 keV equivalent width. These two objects further support the notion that super-Eddington X-ray sources in nearby galaxies can be explained by a mixture of intermediate-mass black holes in X-ray binaries and young supernova remnants.

Circumnuclear obscuration of the Circinus galaxy by a starburst-radiation supported wall

We consider the radiation-hydrodynamical formation of a dusty wall in the circumnuclear regions of the Circinus Seyfert 2 galaxy. We focus on the radiative flux force due to the circumnuclear starburst, the nuclear starburst and the active galactic nucleus (AGN), and analyze the equilibrium configuration and stability of the dusty gas in the circumnuclear regions. The mass distributions used to model the gravitational fields are determined by the observed rotation velocities of the stars and of the gas. Moreover, by using the simple stellar evolution in the circumnuclear starburst, the bolometric luminosity of the starburst is estimated as a function of time. As a result, it is shown that the radiation force by the circumnuclear starburst does play an important role to build up a radiatively-supported dusty wall of $\la$ 100 pc, which obscures the circumnuclear regions with $A_V$ of a few magnitudes. It is also found that the age of a circumnuclear starburst when the circumnuclear regions are enshrouded by the dusty wall is constrained to be $\la 10^8$ yr, which is consistent with the age observationally inferred for the starburst, $4 10^7$ yr $-1.5 10^8$ yr.

Constraints on the nuclear emission of the Circinus galaxy: the torus

In the context of the unified model of Seyfert galaxies, we use observations from the literature and a radiative transfer model to investigate the near–IR to mm emission produced by the presumed torus in the Circinus galaxy, from 2µm to 1.3mm. From the infrared SED modelling, we find that the total luminosity (LIR) in this wavelength range, consists of similar contributions from the torus and starburst with a ratio of nuclear luminosity to starburst luminosity (LNUC/LSB) � 0.8. By using a similar torus model to that of NGC1068, but without the conical dust, we find an upper limit to the outer torus radius of �12pc with a best fit of �2pc. The upper limit torus size estimated from the radiative transfer modelling is consistent with the 16pc torus radius estimated from near–IR imaging polarimetry of Circinus.

Molecular gas conditions in NGC 4945 and the Circinus galaxy

We present results of a multi-transition study of the dense molecular gas in the central part of the hybrid star-burst/Seyfert galaxies NGC 4945 and the Circinus galaxy. From the results of radiative transfer calculations, we estimate in NGC 4945 $n_{{\rm H}_2}=3 10^3{-}10^4$ cm -3 and $T_{\rm kin}\approx100$ K and in Circinus $n_{{\rm H}_2}=2 10^3{-}10^5$ cm -3 and $T_{\rm kin}\approx50{-}80$ K for the molecular hydrogen density and kinetic temperature, respectively. As well as density/temperature tracing molecules, we have observed C 17 O and C 18 O in each galaxy and the value of C 18 O/C$^{17}{\rm O}\approx6$ for the isotopic column density ratio suggests that both have relatively high populations of massive stars. Finally, although star formation is present, the radiative transfer results combined with the high HCN/CO and (possibly) HCN/FIR, radio/FIR ratios may suggest that, in comparison with Circinus, a higher proportion of the dense gas emission in NGC 4945 may be located in the hypothesised central nuclear disk as opposed to dense star forming cloud cores. Contrary to the literature, which assumes that all of the far-infrared emission arises from star formation, our results suggest that in NGC 4945 some of this emission could arise from an additional source, and so we believe that a revision of the star formation rate estimates may be required for these two galaxies.

Accretion and Outflow Traced by H2O Masers in the Circinus AGN

The first VLBI images of H2O maser emission in the Circinus Galaxy AGN show both a warped, edge-on accretion disk and an outflow 0.1 to 1 pc from the central engine. The inferred central mass is 1.3 × 106 M⊙, while the disk mass may be on the order of 105 M⊙, based on a nearly Keplerian rotation curve. The bipolar, wide-angle outflow appears to contain “bullets” ejected from within < 0.1 pc of the central mass. The positions of filaments and bullets observed in the AGN ionization cone on kpc-scales suggest that the disk channels the flow to a radius of ∼ 0.4 pc, at which the flow appears to disrupt the disk.

The circumnuclear X-ray reflectors in NGC 1068 and the Circinus galaxy

The ASCA and BeppoSAX spectra of the Circinus Galaxy and NGC 1068 are analysed and compared with photoionization models based on cloudy. In the case of Circinus, a single, mildly ionized reflector can account for the line spectrum, while in NGC 1068 at least three different reflectors (with different ionization states) are needed. We suggest that the reflector in Circinus, and the low ionized one in NGC1068 are the inner and visible part of the material responsible for the X-ray absorption. With this assumption, we estimate for the inner radius of the absorber a value of 0.2 pc for Circinus and of a few parsecs for NGC 1068.

The Physical Conditions of the X‐Ray Emission Line Regions in the Circinus Galaxy

We present a detailed X-ray spectral analysis of the Circinus Galaxy using archival data obtained with the ASCA satellite. The spectrum shows numerous emission lines in the soft X-ray band from highly ionized ions, as well as Compton reflection and fluorescent lines from neutral or near-neutral matter. We analyze the spectrum in the context of a self-consistent recombination cascade model and find that a nearly flat differential emission measure (DEM) distribution in ionization parameter fits the data. For a fixed solid angle distribution of matter surrounding a point source, this corresponds to a run of electron density of the form, n(r) ~ r^(-3/2), which is suggestive of Bondi accretion onto a central compact mass. Using this density profile and comparing the resulting emission spectra with the ASCA data, the size of the X-ray emission line region is estimated to be < 1 kpc. We also show that the derived density as a function of radius is compatible with the X-ray recombination line emission region being the confining medium of the coronal line regions.

The X-ray spectra of Compton-thick Seyfert 2 galaxies as seen by BeppoSAX

Results from BeppoSAX observations of Compton--thick Seyfert 2 galaxies are summarized and reviewed, and their general properties derived and discussed. In five out of the seven observed sources, the nucleus is directly visible at high X-ray energies, where the photons penetrate absorbers with column densities in the range 1.1-4.3x10^{24} cm-2 (in the other two sources, NGC 1068 and NGC 7674, the nucleus is instead totally obscured at all energies, implying even larger column densities). In most sources there is unambiguous evidence of a reflection component from optically thick, cold matter, while in two (or maybe four) cases there is also evidence of reflection from ionized matter. For the sources with a measured X-ray luminosity, a comparison with the infrared luminosity is made; while in two cases (the Circinus galaxy and NGC 4945) the IR emission appears to be dominated by starburst activity, in the other three sources (NGC 6240, Mrk 3 and TOL 0109-383) it is likely to be dominated by reprocessing of the UV and X-ray photons emitted by an AGN.

The X-ray spectra of Compton-thick Seyfert 2 galaxies as seen by BeppoSAX

Results from BeppoSAX observations of Compton-thick Seyfert 2 galaxies are summarized and reviewed, and their general properties derived and discussed. In five out of the seven observed sources, the nucleus is directly visible at high X-ray energies, where the photons penetrate absorbers with column densities in the range 1.1–4.3×1024 cm−2 (in the other two sources, NGC 1068 and NGC 7674, the nucleus is instead totally obscured at all energies, implying even larger column densities). In most sources there is unambiguous evidence of a reflection component from optically thick, cold matter, while in two (or maybe four) cases there is also evidence of reflection from ionized matter. For the sources with a measured X-ray luminosity, a comparison with the infrared luminosity is made; while in two cases (the Circinus galaxy and NGC 4945) the IR emission appears to be dominated by starburst activity, in the other three sources (NGC 6240, Mrk 3 and TOL 0109-383) it is likely to be dominated by reprocessing of the UV and X-ray photons emitted by an active galactic nucleus.

ITAL]HUBBLE SPACE TELESCOPE[/ITAL][ITAL]Hubble Space Telescope[/ITAL] Imaging of the Circinus Galaxy

We present a Hubble Space Telescope imaging study of the nearby (4 Mpc distant) Circinus galaxy, which contains the nearest type 2 Seyfert nucleus and prominent circumnuclear star formation. Images have been obtained in the [O III] ?5007, H?, and H2 v = 1?0 S(1) emission lines, and in the green (5470 ?), red (8140 ?), and near-infrared (2.04 and 2.15 ?m) continua. An image in the [Fe II] ?1.644 ?m line has been taken with a ground-based telescope. The [O III] and H? images reveal the detailed structure of the complex of streamers and knots of high-excitation gas, which extends out of the galaxy disk. The morphology some 250 pc from the nucleus strongly suggests that the high-excitation gas is concentrated on the surface of a hollow cone with apex close to the nucleus. Such a structure may result through entrainment of dense gas from a circumnuclear torus in the galaxy disk by a low-density, outflowing wind or jet. Within 40 pc of the nucleus, the high-excitation gas takes the form of a striking, filled V-shaped structure extending in the same direction as the larger scale high-excitation emission. This V can be described as an ionization cone, though a matter-bounded structure is also possible. The implied collimation of the ionizing photons or gaseous outflow must occur within 2 pc of the apex of the cone, presumed to be the location of the nucleus. The H? image shows a complex structure of H II regions, including the well-known starburst ring of radius 150?270 pc. In addition, there is a more compact (40 pc radius), elliptical ring of H II regions around the ionization cone. We argue that this latter ring, which we call the nuclear ring, is intrinsically circular and located in the plane of the galaxy disk. Much of the [Fe II] emission is associated with this nuclear, star-forming ring and is presumably powered by supernova remnants. Hot molecular hydrogen extends to within 10 pc of the nucleus, and possibly closer. The intrinsic infrared?optical continuum colors in the inner regions of the Circinus galaxy are, in many locations, bluer than is typical of bulges, indicating a relatively young stellar population is present. We confirm the presence of a compact (<2 pc), very red nuclear source in the K band. Its properties are consistent with a type 1 Seyfert nucleus viewed through an obscuration of AV = 28 ? 7 mag.

H [CSC]i[/CSC]–bright Galaxies in the Southern Zone of Avoidance

A blind survey for H I–bright galaxies in the southern zone of avoidance (212° ≤ l ≤ 36°, |b| ≤ 5°) has been made with the 21 cm multibeam receiver on the Parkes 64 m radio telescope. The survey, sensitive to normal spiral galaxies to a distance of ∼40 Mpc and more nearby dwarfs, detected 110 galaxies. Of these, 67 have no counterparts cataloged in the NASA/IPAC Extragalactic Database. In general, the uncataloged galaxies lie behind thicker obscuration than do the cataloged objects. All the newly discovered galaxies have H I flux integrals that are more than an order of magnitude lower than those of the Circinus galaxy. The survey recovers the Puppis Cluster and foreground group, and the Local Void remains empty. The H I mass function derived for the sample is satisfactorily fitted by a Schechter function with parameters α = 1.51 ± 0.12, Φ* = 0.006 ± 0.003, and log M* = 9.7 ± 0.10.

Constraints on the nuclear emission of the Circinus galaxy: optical and near-IR spectropolarimetry★

‘The definitive version is available at www.blackwell-synergy.com.’ Copyright Blackwell Publishing / Royal Astronomical Society. DOI: 10.1046/j.1365-8711.2000.03285.x [Full text of this article is not available in the UHRA]

A near-infrared polarized bipolar cone in the Circinus galaxy

We present near-infrared broad-band polarization images of the nuclear regions of the Circinus galaxy in the J, H and K bands. For the first time the south-eastern reflection cone is detected in polarized light, which is obscured at optical wavelengths behind the galactic disk. This biconical structure is clearly observed in J and H band polarized flux whilst in the K band a more compact structure is detected. Total flux J-K and H-K colour maps reveal a complex colour gradient toward the south-east direction (where the Circinus galactic disk is nearer to us). We find enhanced extinction in an arc shaped structure, at about 200pc from the nucleus, probably part of the star-formation ring. We model the polarized flux images with the scattering and torus model of Young etal, with the same basic input parameters as used by Alexander etal in the spectropolarimetry modelling of Circinus. The best fit to the polarized flux is achieved with a torus radius of ~16pc, and a visual extinction Av, through the torus, to the near-infrared emission regions of >66 mags.