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Register a new userNuclear Radio Continuum Emission of Low-Luminosity Active Galactic Nucleus NGC 4258
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The nearby low-luminosity active galactic nucleus (LLAGN) NGC 4258 has a weak radio continuum emission at the galactic center. Quasi-simultaneous multi-frequency observations using the Very Large Array (VLA) from 5 GHz (6 cm) to 22 GHz (1.3 cm) showed inverted spectra in all epochs, which were intra-month variable, as well as complicated spectral features that cannot be represented by a simple power law, indicating multiple blobs in nuclear jets. Using the Nobeyama Millimeter Array (NMA), we discovered a large amplitude variable emission at 100 GHz (3 mm), which had higher flux densities at most epochs than those of the VLA observations. A James Clerk Maxwell Telescope (JCMT) observation at 347 GHz (850 micron) served an upper limit of dust contamination. The inverted radio spectrum of the nucleus NGC 4258 is suggestive of an analogy to our Galactic center Sgr A*, but with three orders of magnitude higher radio luminosity. In addition to the LLAGN M 81, we discuss the nucleus of NGC 4258 as another up-scaled version of Sgr A*.
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The nearby low-luminosity active galactic nucleus (LLAGN) NGC 4258 has a weak radio continuum component at the galactic center. We investigate its radio spectral properties on the basis of our new observations using the Nobeyama Millimeter Array at 100 GHz and archival data from the Very Large Array (VLA) at 1.7-43 GHz and the James Clerk Maxwell telescope at 347 GHz. The NGC 4258 nuclear component exhibits (1) an intra-month variable and complicated spectral feature at 5-22 GHz and (2) a slightly inverted spectrum at 5-100 GHz (a spectral index of ~0.3) in time-averaged flux densities, which are also apparent in the closest LLAGN M81. These similarities between NGC 4258 and M81 in radio spectral natures in addition to previously known core shift in their AU-scale jet structures produce evidence that the same mechanism drives their nuclei. We interpret the observed spectral property as the superposition of emission spectra originating at different locations with frequency-dependent opacity along the nuclear jet. Quantitative differences between NGC 4258 and M81 in terms of jet/counter jet ratio, radio loudness, and degree of core shift can be consistently understood by fairly relativistic speeds (bulk Lorentz factors of >~ 3) of jet and their quite different inclinations. The picture established from the two closest LLAGNs is useful for understanding the physical origin of unresolved and flat/inverted spectrum radio cores that are prevalently found in LLAGNs, including Sgr A*, with starved supermassive black holes in the present-day universe.
In 2006 June, the obscured low luminosity active galactic nucleus in the nearby Seyfert 1.9 galaxy NGC 4258 was observed with Suzaku for ~ 100 ks. Utilizing the XIS and the HXD, the nucleus emission was detected over 2 to 40 keV range, with an unabsorbed 2--10 keV luminosity of 8 x 10 40 erg / s, and varied by a factor of ~ 2 during the observation. Its 2--40 keV spectrum is reproduced by a single power law with photon index of ~ 2.0, absorbed by an equivalent hydrogen column of ~ 1.0 x 10 23 cm2. The spectrum within 4 of the nucleus required also a softer thin-thermal emission, as well as an intermediate hardness component attributable to integrated point sources. A weak neutral Fe-Kalpha florescence line was detected at an equivalent width of ~ 40 eV. The cold reflection component was not required by the data, with the reflector solid angle Omega seen from the nucleus constrained as Omega / 2 pi < 0.3 assuming a general case of 60 deg inclination. The results suggest that the cold reflecting material around the nucleus is localized along our line of sight, rather than forming a thick torus.
We present high-resolution mid-infrared (MIR) imaging, nuclear spectral energy distributions (SEDs) and archival Spitzer spectra for 22 low-luminosity active galactic nuclei (LLAGN; Lbol lesssim 10^42 erg/sec). Infrared (IR) observations may advance our understanding of the accretion flows in LLAGN, the fate of the obscuring torus at low accretion rates, and, perhaps, the star formation histories of these objects. However, while comprehensively studied in higher-luminosity Seyferts and quasars, the nuclear IR properties of LLAGN have not yet been well-determined. We separate the present LLAGN sample into three categories depending on their Eddington ratio and radio emission, finding different IR characteristics for each class. (I) At the low-luminosity, low-Eddington ratio (log Lbol/LEdd < -4.6) end of the sample, we identify host-dominated galaxies with strong polycyclic aromatic hydrocarbon bands that may indicate active (circum-)nuclear star formation. (II) Some very radio-loud objects are also present at these low Eddington ratios. The IR emission in these nuclei is dominated by synchrotron radiation, and some are likely to be unobscured type 2 AGN that genuinely lack a broad line region. (III) At higher Eddington ratios, strong, compact nuclear sources are visible in the MIR images. The nuclear SEDs of these galaxies are diverse; some resemble typical Seyfert nuclei, while others lack a well-defined MIR dust bump. Strong silicate emission is present in many of these objects. We speculate that this, together with high ratios of silicate strength to hydrogen column density, could suggest optically thin dust and low dust-to-gas ratios, in accordance with model predictions that LLAGN do not host a Seyfert-like obscuring torus.
Low-luminosity Active Galactic Nuclei, i.e. L_bol/L_edd ~ 10^-6 - 10^-3, constitute the bulk population of Active Galactic Nuclei (AGNs). Powerful jets, common in these objects, are a crucial source of feedback energy driving mass outflows into the host galaxy and the intergalactic medium. This paper reports the first direct measurement of powerful mass outflows traced by the forbidden high ionization gas in the low luminosity AGN NGC1386 at scales of a few parsecs from the central engine. The high angular resolution of the data allows us to directly measure the location, morphology and kinematic of the outflow. This the form of two symmetrical expanding hot gas shells moving in opposite directions along the line of sight. The co-spatiality of the gas shells with radio emission seen at the same parsec scales and with X-rays indicates that this is a shock-driven outflow induced by an incipient core-jet. With a minimum number of assumptions, we derive a mass outflow rate of 11 solar masses/yr, comparable to those of powerful AGN. The result has strong implications in the global accounting of feedback mass and energy driven by a low-luminosity AGN into the medium and the corresponding galaxy evolution.
Observations made during the last ten years with the Chandra X-ray Observatory have shed much light on the cooling gas in the centers of clusters of galaxies and the role of active galactic nucleus (AGN) heating. Cooling of the hot intracluster medium in cluster centers can feed the supermassive black holes found in the nuclei of the dominant cluster galaxies leading to AGN outbursts which can reheat the gas, suppressing cooling and large amounts of star formation. AGN heating can come in the form of shocks, buoyantly rising bubbles that have been inflated by radio lobes, and the dissipation of sound waves.
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