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David Axon

Publications and source records attributed to David Axon.

17 recordsLinked to original sources
arXiv2011

An H-alpha nuclear spiral structure in the E0 active galaxy Arp102B

We report the discovery of a two-armed mini-spiral structure within the inner kiloparsec of the E0 LINER/Seyfert 1 galaxy Arp102B. The arms are observed in H-alpha emission and located East and West of the nucleus, extending up to about 1 kpc from it. We use narrow-band imaging from the Hubble Space Telescope Advanced Camera for Surveys, in combination with archival VLA radio images at 3.6 and 6 cm to investigate the origin of the nuclear spiral. From the H-alpha luminosity of the spiral, we obtain an ionized gas mass of the order of one million solar masses. One possibility is that the nuclear spiral represents a gas inflow triggered by a recent accretion event which has replenished the accretion disk, giving rise to the double-peaked emission-line profiles characteristic of Arp102B. However, the radio images show a one-sided curved jet which correlates with the eastern spiral arm observed in the H-alpha image. A published milliarcsecond radio image also shows one-sided structure at position angle about 40 degrees, approximately aligned with the inner part of the eastern spiral arm. The absence of a radio counter-part to the western spiral arm is tentatively interpreted as indicating that the jet is relativistic, with an estimated speed of 0.45c. Estimates of the jet kinetic energy and the ionizing luminosity of the active nucleus indicate that both are capable of ionizing the gas along the spiral arms. We conclude that, although the gas in the nuclear region may have originated in an accretion event, the mini-spiral is most likely the result of a jet-cloud interaction rather than an inflowing stream.

astro-ph.CO↗
arXiv2010

Hubble Space Telescope Near Infrared Snapshot Survey of 3CR radio source counterparts III: Radio galaxies and quasars in context

We compare the near-infrared (NIR) H band photometric and morphological properties of low-redshift (z<0.3) 3CR radio galaxies with samples of BL Lac object and quasar host galaxies, merger remnants, quiescent elliptical galaxies, and brightest cluster galaxies drawn from the literature. In general the 3CR host galaxies are consistent with luminous (~L*) elliptical galaxies. The vast majority of FR II's (~80%) occupy the most massive ellipticals and form a homogeneous population that is comparable to the population of radio-loud quasar (RLQ) host galaxies in the literature. However, a significant minority (~20%) of the 3CR FR II's appears under-luminous with respect to quasar host galaxies. All FR II objects in this faint tail are either unusually red, or appear to be the brightest objects within a group. We discuss the apparent differences between the radio galaxy and RLQ host galaxy populations. RLQs appear to require >1E11 M_sun host galaxies (and ~1E9 M_sun black holes), whereas radio galaxies and RQQs can exist in galaxies down to 3E10 M_sun. This may be due to biases in the measured quasar host galaxy luminosities or populations studied, or due to a genuine difference in host galaxy. If due to a genuine difference, it would support the idea that radio and optical active galactic nucleii are two separate populations with a significant overlap.

astro-ph.CO↗
arXiv2008

HST NIR Snapshot Survey of 3CR Radio Source Counterparts II: An Atlas and Inventory of the Host Galaxies, Mergers and Companions

We present the second part of an H-band (1.6 microns) atlas of z<0.3 3CR radio galaxies, using the Hubble Space Telescope Near Infrared Camera and Multi-Object Spectrometer (HST NICMOS2). We present new imaging for 21 recently acquired sources, and host galaxy modeling for the full sample of 101 (including 11 archival) -- an 87% completion rate. Two different modeling techniques are applied, following those adopted by the galaxy morphology and the quasar host galaxy communities. Results are compared, and found to be in excellent agreement, although the former breaks down in the case of strongly nucleated sources. Companion sources are tabulated, and the presence of mergers, tidal features, dust disks and jets are catalogued. The tables form a catalogue for those interested in the structural and morphological dust-free host galaxy properties of the 3CR sample, and for comparison with morphological studies of quiescent galaxies and quasar host galaxies. Host galaxy masses are estimated, and found to typically lie at around 2*10^11 solar masses. In general, the population is found to be consistent with the local population of quiescent elliptical galaxies, but with a longer tail to low Sersic index, mainly consisting of low-redshift (z<0.1) and low-radio-power (FR I) sources. A few unusually disky FR II host galaxies are picked out for further discussion. Nearby external sources are identified in the majority of our images, many of which we argue are likely to be companion galaxies or merger remnants. The reduced NICMOS data are now publicly available from our website (http://archive.stsci.edu/prepds/3cr/)

astro-ph↗
arXiv2008

The effect of radiation pressure on virial black hole mass estimates and the case of Narrow Line Seyfert 1 galaxies

We consider the effect of radiation pressure from ionizing photons on black hole (BH) mass estimates based on the application of the virial theorem to broad emission lines in AGN spectra. BH masses based only on the virial product V^2R and neglecting the effect of radiation pressure can be severely underestimated especially in objects close to the Eddington limit. We provide an empirical calibration of the correction for radiation pressure and we show that it is consistent with a simple physical model in which BLR clouds are optically thick to ionizing radiation and have average column densities of NH~10^23 cm^-2. This value is remarkably similar to what is required in standard BLR photoionization models to explain observed spectra. With the inclusion of radiation pressure the discrepancy between virial BH masses based on single epoch spectra and on reverberation mapping data drops from 0.4 to 0.2 dex rms. The use of single epoch observations as surrogates of reverberation mapping campaigns can thus provide more accurate BH masses than previously thought. Finally, we show that Narrow Line Seyfert 1 (NLS1) galaxies have apparently low BH masses because they are radiating close to their Eddington limit. After the radiation pressure correction, NLS1 galaxies have BH masses similar to other broad line AGNs and follow the same MBH-sigma/L relations as other active and normal galaxies. Radiation forces arising from ionizing photon momentum deposition constitute an important physical effect which must be taken into account when computing virial BH masses.

astro-ph↗
arXiv2006

Hubble Space Telescope Near-Infrared Snapshot Survey of 3CR radio source counterparts at low redshift

We present newly acquired images of the near-infrared counterpart of 3CR radio sources. All the sources were selected to have a redshift of less than 0.3 to allow us to obtain the highest spatial resolution. The observations were carried out as a snapshot program using the Near-Infrared Camera and Multiobject Spectrograph (NICMOS) on-board the Hubble Space Telescope (HST). In this paper we describe 69 radio galaxies observed for the first time with NICMOS during HST cycle 13. All the objects presented here are elliptical galaxies. However, each of them has unique characteristics such as close companions, dust lanes, unresolved nuclei, arc-like features, globular clusters and jets clearly visible from the images or with basic galaxy subtraction.

astro-ph↗
arXiv2006

An optical-IR jet in 3C133

We report the discovery of a new optical-IR synchrotron jet in the radio galaxy 3C133 from our HST/NICMOS snapshot survey. The jet and eastern hotspot are well resolved, and visible at both optical and IR wavelengths. The IR jet follows the morphology of the inner part of the radio jet, with three distinct knots identified with features in the radio. The radio-IR SED's of the knots are examined, along with those of two more distant hotspots at the eastern extreme of the radio feature. The detected emission appears to be synchrotron, with peaks in the NIR for all except one case, which exhibits a power-law spectrum throughout.

astro-ph↗
arXiv2006

The Warped Nuclear Disk of Radio Galaxy 3C 449

Among radio galaxies containing nuclear dust disks, the bipolar jet axis is generally observed to be perpendicular to the disk major axis. The FR I radio source 3C 449 is an outlier to this statistical majority, as it possesses a nearly parallel jet/disk orientation on the sky. We examine the 600 pc dusty disk in this galaxy with images from the Hubble Space Telescope. We find that a colormap of the disk exhibits a twist in its isocolor contours (isochromes). We model the colormap by integrating galactic starlight through an absorptive disk, and find that the anomalous twist in the isochromes can be reproduced in the model with a vertically thin, warped disk. The model predicts that the disk is nearly perpendicular to the jet axis within 100 pc of the nucleus. We discuss physical mechanisms capable of causing such a warp. We show that precessional models or a torque on the disk arising from a possible binary black hole in the AGN causes precession on a timescale that is too long to account for the predicted disk morphology. However, we estimate that the pressure in the X-ray emitting interstellar medium is large enough to perturb the disk, and argue that jet-driven anisotropy in the excited ISM may be the cause of the warp. In this way, the warped disk in 3C 449 may be a new manifestation of feedback from an active galactic nucleus.

astro-ph↗
arXiv2005

The nature of the HE0450-2958 System

Magain et al. (2005) argued that the host galaxy of the quasar in HE0450-2958 is substantially under-luminous given the likely mass of its nuclear black hole. Using kinematical information from the spectra of the quasar and the companion galaxy, an ultra-luminous infrared galaxy, we test the hypothesis that the black hole powering the quasar was ejected from the companion galaxy during a merger. We find that the ejection model can be securely ruled out, since the kick velocity required to remove the black hole from the galaxy is greater than about 500 km/s, inconsistent with the presence of narrow emission line gas at the same redshift as the quasar nucleus. We also show that the quasar in HE0450-2958 has the spectral characteristics of a narrow-line Seyfert 1 galaxy and calculate a mass for its black hole that is roughly an order of magnitude smaller than estimated by Magain et al. The predicted luminosity of the host galaxy is then consistent with the upper limits inferred by those authors.

astro-ph↗
arXiv2004

The supermassive black hole in the Seyfert 2 galaxy NGC 5252

We present results from HST/STIS long-slit spectroscopy of the gas motions in the nuclear region of the Seyfert 2 galaxy NGC 5252. The observed velocity field is consistent with gas in regular rotation with superposed localized patches of disturbed gas. The dynamics of the circumnuclear gas can be accurately reproduced by adding to the stellar mass component a compact dark mass of MBH = 0.95 (-0.45;+1.45) 10E9 M(sun), very likely a supermassive black hole. Contrarily to results obtained in similar studies rotational broadening is sufficient to reproduce also the behaviour of line widths. The MBH estimated for NGC 5252 is in good agreement with the correlation between MBH and bulge mass. The comparison with the MBH vs sigma relationship is less stringent (mostly due to the relatively large error in sigma); NGC 5252 is located above the best fit line by between 0.3 and 1.2 dex, i.e. 1 - 4 times the dispersion of the correlation. Both the galaxy's and MBH of NGC 5252 are substantially larger than those usually estimated for Seyfert galaxies but, on the other hand, they are typical of radio-quiet quasars. Combining the determined MBH with the hard X-ray luminosity, we estimate that NGC 5252 is emitting at a fraction ~ 0.005 of L(Edd). In this sense, this active nucleus appears to be a quasar relic, now probably accreting at a low rate, rather than a low black hole mass counterpart of a QSO.

astro-ph↗
arXiv2003

Hubble Space Telescope Observations of NGC 6240: a Case Study of an Ultra-Luminous Infrared Galaxy with Obscured Activity

We present results from an HST study of the morphology and kinematics of NGC 6240. This merging galaxy with a double nucleus is one of the nearest and best-studied ultraluminous infrared galaxies. HST resolves both nuclei into seperate components. The distance between the northern and southern optical/near-infrared components is greater than that observed in radio and X-ray studies, arguing that even in K-band we may not be seeing all the way through the dust to the true nuclei. The ionized gas does not display rotation around either of the nuclei, or equilibrium motion in general. There is a strong velocity gradient between the nuclei, similar to what is seen in CO data. There is no such gradient in our stellar kinematics. The velocity dispersion of the gas is larger than expected for a cold disk. We also map and model the emission-line velocity field at an off-nuclear position where a steep velocity gradient was previously detected in ground-based data. Overall, the data indicate that line-of-sight projection effects, dust absorption, non-equilibrium merger dynamics, and the possible influence of a wind may be playing an important role in the observed kinematics. Chandra observations of hard X-rays have shown that both of the nuclei contain an Active Galactic Nucleus (AGN). The HST data show no clear sign of the two AGNs: neither continuum nor narrow-band imaging shows evidence for unresolved components in the nuclei, and there are no increased emission line widths or rapid rotation near the nuclei. This underscores the importance of X-ray data for identifying AGNs in highly dust-enshrouded environments.

astro-ph↗
arXiv2000

Peering through the dust: Evidence for a supermassive Black Hole at the nucleus of Centaurus A from VLT IR spectroscopy

We used the near infrared spectrometer ISAAC at the ESO 'Very Large Telescope' to map the velocity field of Centaurus A (NGC 5128) at several position angles and locations in the central 20" of the galaxy. The high spatial resolution (~0.5") velocity fields from both ionized and molecular gas (PaBeta, [FeII], BrGamma, and H2) are not compromised by either excitation effects or obscuration. We identify three distinct kinematical systems: (i) a rotating 'nuclear disk' of ionized gas, confined to the inner 2", the counterpart of the PaAlpha feature previously revealed by HST/NICMOS imaging; (ii) a ring-like system with a ~6" inner radius detected only in H2, likely the counterpart of the 100pc-scale structure detected in CO by other authors; (iii) a normal extended component of gas rotating in the galactic potential. The nuclear disk is in keplerian rotation around a central mass concentration, dark (M/L>20 Msun/LsunK) and point-like at the spatial resolution of the data (R<0.25" ~4pc). We interpret this mass concentration as a supermassive black hole. Its dynamical mass based on the line velocities and disk inclination (i>15deg) is M=2(+3.0;-1.4) 10^8 Msun. The ring-like system is probably characterized by non-circular motions; a 'figure-of-8' pattern observed in the H2 position-velocity diagram might provide kinematical evidence for the presence of a nuclear bar.

astro-ph↗
arXiv1999

Unveiling the Active Nucleus of Centaurus A

We report new HST WFPC2 and NICMOS observations of the center of the nearest radio galaxy Centaurus A (NGC 5128) and discuss their implications for our understanding of the active nucleus and jet. We detect the active nucleus in the near-IR (K and H) and, for the first time, in the optical (I and V), deriving the spectral energy distribution of the nucleus from the radio to X-rays. The optical and part of the near-IR emission can be explained by the extrapolation of the X-ray power law reddened by A_V~14mag, a value consistent with other independent estimates. The 20pc-scale nuclear disk discovered by Schreier et al. (1998) is detected in the [FeII] 1.64mic line and presents a morphology similar to that observed in Pa alpha with a [FeII]/Pa alpha ratio typical of low ionization Seyfert galaxies and LINERs. NICMOS 3 Pa alpha observations in a 50"x50" circumnuclear region suggest enhanced star formation (~0.3Msun/yr) at the edges of the putative bar seen with ISO, perhaps due to shocks driven into the gas. The light profile, reconstructed from V, H and K observations, shows that Centaurus A has a core profile with a resolved break at ~4" and suggests a black--hole mass of ~10^9 Msun. A linear blue structure aligned with the radio/X-ray jet may indicate a channel of relatively low reddening in which dust has been swept away by the jet.

astro-ph↗
arXiv1998

Evidence for a 20pc disk at the nucleus of Centaurus A

We report HST NICMOS observations of the central region of NGC 5128 at 2.2mic and in Paschen Alpha (PA). The continuum images show extended emission typical of an elliptical galaxy and a strong unresolved central source we identify as the nucleus of the galaxy. Its position is consistent with ground-based IR and radio data, and with the peak of reddening found with WF/PC-1. In PA, we detect a prominent elongated structure, centered on the nucleus, extended by ~2" at a position angle of 33deg, and with a major to minor axis ratio of ~2. We interpret this as an inclined, ~40 parsec diameter, thin nuclear disk of ionized gas rather than a jet-gas cloud interaction. We do see several weaker PA features, some of which may be circumnuclear gas clouds shocked by the X-ray/radio jet. The disk is one of the smallest ever observed at the nucleus of an AGN. It is not perpendicular to the jet, but consistent with being oriented along the major axis of the bulge. If it represents the warped outer portion of an accretion disk around a black hole, we conclude that even on the scale of a few parsecs, the disk is dominated by the galaxy gravitational potential and not directly related to the symmetry axis of the AGN.

astro-ph↗