VLBA Imaging of NGC 4261: Symmetric Parsec-scale Jets and the Inner Accretion Region

We observed the nuclear region of NGC 4261 (3C270) with the VLBA at two frequencies (1.6 and 8.4 GHz) simultaneously. We find that the position angle of the parsec-scale radio axis agrees, within the errors, with the position angle of the VLA-scale jet. Our observations also reveal basically symmetric radio structures at both 1.6 and 8.4 GHz. Analysis of these images shows that most of the central 10 pc of this source is not significantly affected by free-free absorption, even though HST images show that the nucleus contains a nearly edge-on disk of gas and dust on larger scales. Our highest angular resolution image shows a very narrow gap in emission, which we interpret as an absorption feature, just east of the radio core. This suggests that there may be a small, dense inner accretion disk whose width is less than 0.1 pc. If the inclination of this inner disk is close to that of the larger-scale HST disk it becomes optically thin to 8.4 GHz radiation at a deprojected radius of about 0.8 pc. The brightness of the pc-scale jets falls off very rapidly on both sides of the core, suggesting that the jets are rapidly expanding during the the first several pc of their travel. It appears that there is a small dense inner disk centered on the radio core (the base of the jets; less than 1 pc), a low density bubble filling most of the the inner several pc of the nucleus (within which the radio jets expand rapidly; ~10 pc), and a surrounding cool, higher density region (of which the HST absorption disk is part; tens to hundreds of pc) within which the transverse expansion of the radio jets, as implied by the rate of decrease in jet brightness, is nearly halted.Comment: Accepted by the Astrophysical Journa

Two tails in NGC 3656, and the major merger origin of shell and minor axis dust lane ellipticals

I report on the discovery of two faint (~ 26.8 Rmag/arcsec^2) tidal tails around the shell elliptical NGC 3656 (Arp 155). This galaxy had previously been interpreted as a case of accretion, or minor merger. The two tidal tails are inconsistent with a minor merger, and point instead to a disk-disk major merger origin. NGC 3656 extends Toomre's merger sequence toward normal elliptical galaxies, and hints at a major merger origin for shells and minor-axis dust lanes. A dwarf galaxy lies at the tip of one of the tidal tails. A prominent shell, which shows sharp azymuthal color discontinuities, belongs to a rotating dynamical component of young stars which includes the inner dust lane.Comment: 9 pages, 2 plates, 1 figure, uses aaspp.sty, accepted for publication in The Astrophysical Journal Letters. Files also available by anonymous ftp at ftp.iac.es, directory ./pub/balcell

A Search for Kinematic Evidence of Radial Gas Flows in Spiral Galaxies

CO and HI velocity fields of seven nearby spiral galaxies, derived from radio-interferometric observations, are decomposed into Fourier components whose radial variation is used to search for evidence of radial gas flows. Additional information provided by optical or near-infrared isophotes is also considered, including the relationship between the morphological and kinematic position angles. To assist in interpreting the data, we present detailed modeling that demonstrates the effects of bar streaming, inflow, and a warp on the observed Fourier components. We find in all of the galaxies evidence for either elliptical streaming or a warped disk over some range in radius, with deviations from pure circular rotation at the level of ~20-60 km/s. Evidence for kinematic warps is observed in several cases well inside R_{25}. No unambiguous evidence for radial inflows is seen in any of the seven galaxies, and we are able to place an upper limit of ~5-10 km/s (3-5% of the circular speed) on the magnitude of any radial inflow in the inner regions of NGC 4414, 5033 and 5055. We conclude that the inherent non-axisymmetry of spiral galaxies is the greatest limitation to the direct detection of radial inflows.Comment: 22 emulateapj pages with bitmapped colour figures, to appear in ApJ (April 2004). For full resolution figures go to http://www.atnf.csiro.au/people/twong/preprints

HI in the shell elliptical NGC 3656

VLA neutral hydrogen observations of the shell elliptical NGC 3656 reveal an edge-on, warped minor axis gaseous disk (M_HI ~ 2.10^9 Msun) extending 7 kpc. HI is also found outside the optical image, on two complexes to the North-East and North-West that seem to trace an outer broken HI disk or ring, or possibly one or two tidal tails. Integral-field optical fiber spectroscopy at the region of the bright southern shell of NGC 3656 has provided a determination of the stellar velocities of the shell. The shell has traces of HI with velocities bracketing the stellar velocities, providing evidence for a dynamical association of HI and stars at the shell. Within the errors the stars have systemic velocity, suggesting a possible phase wrapping origin for the shell. We detect five dwarf galaxies with HI masses ranging from 2.10^8 Msun to 2.10^9 Msun all within 180 kpc from NGC 3656 and all within the velocity range (450 \kms) of the HI of NGC 3656. For the NGC 3656 group to be bound requires a total mass of 3-7.4x10^{12} Msun, yielding a mass to light ratio from 125 to 300. The overall HI picture presented by NGC 3656 supports the hypothesis of a disk-disk merger origin, or possibly an ongoing process of multiple merger with nearby dwarfs.Comment: 24 pages, 10 figures, 3 in color, to appear in the Astronomical Journal, high resolution version available at http://www.iac.es/galeria/balcells/publ_mbc.htm

Ellipticals with Kinematically-Distinct Cores: (V-I) Color Images with WFPC2

We have analysed HST/WFPC2 F555W and F814W images for fifteen elliptical galaxies with kinematically-distinct cores. For each of them we have derived surface brightness and isophotal parameter profiles in the two bands, color maps, and radial profiles in (V-I). We have detected photometric evidence for faint stellar disks, on scales of a few tens to a few arcseconds, in seven galaxies, namely NGC 1427, 1439, 1700, 4365, 4406, 4494 and 5322. In NGC 1700, the isophotes are slightly boxy at the scale of the counter-rotating component, and disky at larger radii. We find no difference in (V-I) color greater than 0.02 mag between these disks and the surrounding galactic regions. Hence the stellar populations in the kinematically distinct cores are not strongly deviant from the population of the main body. For one galaxy, NGC 4365, the innermost region is bluer than the surrounding regions. This area extends to about 15pc, and contains a luminosity of 2.5x10^6 L$_\odot$. If interpreted as a stellar population effect, an age difference of $\sim$ 3-4 Gyrs, or an $[Fe/H]$ variation of about 0.2 dex, is derived. The nuclear intensity profiles show a large variety: some galaxies have steep cusp profiles, others have shallow cusps and a ``break radius''. The nuclear cusps of galaxies with kinematically-distinct cores follow the same trends as the nuclei of normal galaxies. We have not been able to identify a unique, qualifying feature in the WFPC2 images which distinguish the galaxies with kinematically distinct cores from the kinematically normal cores. [shortened]Comment: 56 pages, latex, 17 figures; figure 1 available upon request; ApJ, 481 in pres

Bipolar-Hyper-Shell Galactic Center Statrburst Model: Further Evidence from ROSAT Data and New Radio and X-ray Simulations

Using the all-sky ROSAT soft X-ray and 408-MHz radio continuum data, we show that the North Polar Spur and its western and southern counter-spurs draw a giant dumbbell-shape necked at the galactic plane. We interpret these features as due to a shock front originating from a starburst 15 million years ago with a total energy of the order of $\sim 10^{56}$ ergs or $10^5$ type II supernovae. We simulate all-sky distributions of radio continuum and soft X-ray intensities based on the bipolar-hyper-shell galactic center starburst model. The simulations can well reproduce the radio NPS and related spurs, as well as radio spurs in the tangential directions of spiral arms. Simulated X-ray maps in 0.25, 0.75 and 1.5 keV bands reproduce the ROSAT X-ray NPS, its western and southern counter-spurs, and the absorption layer along the galactic plane. We propose to use the ROSAT all-sky maps to probe the physics of gas in the halo-intergalactic interface, and to directly date and measure the energy of a recent Galactic Center starburst.Comment: To appear in ApJ, Latex MS in ApJ macro, 8 figures in jpg (original quality ps figs available on request

A Radio Study of the Seyfert Galaxy IC 5063: Evidence for Fast Gas Outflow

New radio continuum (8 GHz and 1.4 GHz) and HI 21 cm line observations of the Seyfert 2 galaxy IC 5063 (PKS 2048-572) were obtained with the Australia Telescope Compact Array (ATCA). The 8 GHz image reveals a linear triple structure (~4'', 1.5 kpc) oriented perpendicular to the optical polarization position angle. It is aligned with the inner dust lane and shows strong morphological association with the narrow emission line region (NLR). At 21 cm, very broad (~700 km/s) HI absorption is observed against the strong continuum source. This absorption is almost entirely blueshifted, indicating a fast net outflow, but a faint and narrow redshifted component is also present. In IC 5063 we see clear evidence for strong shocks resulting from the radio plasma-ISM interaction in the central few kpc. However, the energy flux in the radio plasma is an order of magnitude smaller than the energy emitted in emission lines. Thus, shocks are unlikely to account solely for the global ionization of the emission line region, particularly at large distances. The HI emission outlines a warped disk associated with the system of dust lanes some ~2' (~38 kpc) in radius. The lack of kinematically disturbed gas outside the central few kpc, coupled with the disk warp and close morphological connection of the inner dust lanes and the large-scale ionized gas, support the idea that the gas at large radii is photoionized by the central region, while shadowing effects are important in defining its X-shaped morphology. The kinematics of the ionized and of the neutral gas suggests the existence of a dark halo.Comment: 18 pages, 8 Postscript figures, 3 jpeg figures, Postscript preprint is available from http://jhufos.pha.jhu.edu/~zlatan/papers.htm

Central Structural Parameters of Early-Type Galaxies as Viewed with HST/NICMOS

We present surface photometry for the central regions of a sample of 33 early-type (E, S0, and S0/a) galaxies observed at 1.6 microns (H band) using the Hubble Space Telescope (HST). We employ a new technique of two-dimensional fitting to extract quantitative parameters for the bulge light distribution and nuclear point sources, taking into consideration the effects of the point-spread function. Parameterizing the bulge profile with a ``Nuker'' law, we confirm that the central surface-brightness distributions largely fall into two categories, each of which correlates with the global properties of the galaxies. ``Core'' galaxies tend to be luminous ellipticals with boxy or pure elliptical isophotes, whereas ``power-law'' galaxies are preferentially lower luminosity systems with disky isophotes. Unlike most previous studies, however, we do not find a clear gap in the distribution of inner cusp slopes; several objects have inner cusp slopes (0.3 < gamma < 0.5) which straddle the regimes conventionally defined for core and power-law type galaxies. The nature of these intermediate objects is unclear. We draw attention to two objects in the sample which appear to be promising cases of galaxies with isothermal cores that are not the brightest members of a cluster. Unresolved nuclear point sources are found in about 50% of the sample galaxies, roughly independent of profile type, with magnitudes in the range m^{nuc}_H = 12.8 to 17.4 mag, which correspond to M_H^{nuc} = -12.8 to -18.4 mag. (Abridged)Comment: To appear in The Astronomical Journal. Latex, 24 pages and 17 JPEG image

Ultraviolet Signposts of Resonant Dynamics in the Starburst-Ringed Sab Galaxy, M94 (NGC 4736)

M94 (NGC 4736) is investigated using images from the Ultraviolet Imaging Telescope (FUV-band), Hubble Space Telescope (NUV-band), Kitt Peak 0.9-m telescope (H-alpha, R, and I bands), and Palomar 5-m telescope (B-band), along with spectra from the International Ultraviolet Explorer and Lick 1-m telescopes. The wide-field UIT image shows FUV emission from (a) an elongated nucleus, (b) a diffuse inner disk, where H-alpha is observed in absorption, (c) a bright inner ring of H II regions at the perimeter of the inner disk (R = 48 arcsec. = 1.1 kpc), and (d) two 500-pc size knots of hot stars exterior to the ring on diametrically opposite sides of the nucleus (R= 130 arcsec. = 2.9 kpc). The HST/FOC image resolves the NUV emission from the nuclear region into a bright core and a faint 20 arcsec. long ``mini-bar'' at a position angle of 30 deg. Optical and IUE spectroscopy of the nucleus and diffuse inner disk indicates an approximately 10^7 or 10^8 yr-old stellar population from low-level starbirth activity blended with some LINER activity. Analysis of the H-alpha, FUV, NUV, B, R, and I-band emission along with other observed tracers of stars and gas in M94 indicates that most of the star formation is being orchestrated via ring-bar dynamics involving the nuclear mini-bar, inner ring, oval disk, and outer ring. The inner starburst ring and bi-symmetric knots at intermediate radius, in particular, argue for bar-mediated resonances as the primary drivers of evolution in M94 at the present epoch. Similar processes may be governing the evolution of the ``core-dominated'' galaxies that have been observed at high redshift. The gravitationally-lensed ``Pretzel Galaxy'' (0024+1654) at a redshift of approximately 1.5 provides an important precedent in this regard.Comment: revised figure 1 (corrected coordinate labels on declination axis); 19 pages of text + 19 figures (jpg files); accepted for publication in A

A Counter-rotating Bulge in the Sb Galaxy NGC 7331

We have found that the bulge of the large, nearby Sb galaxy NGC 7331 rotates retrograde to its disk. Analysis of spectra in the region of the near-IR Ca II triplet along the major axis shows that, in the radial range between 5'' and ~20'', the line of sight velocity distribution of the absorption lines has two distinct peaks, and can be decomposed into a fast-rotating component with v/sigma > 3, and a slower rotating, retrograde component with v/sigma between 1 -- 1.5. The radial surface brightness profile of the counter-rotating component follows that of the bulge, obtained from a 2-dimensional bulge-disk decomposition of a near-infrared K-band image, while the fast rotating component follows the disk. At the radius where the disk starts to dominate the isophotes change from being considerably boxy to very disky. Although a number of spiral galaxies have been found that contain cold, couter-rotating disks, this is the first galaxy known to have a boxy, probably triaxial, fairly warm, counter-rotating component, which is dominating in the central regions. If it is a bar seen end-on, this bar has to be thicker than the disk. We find that NGC 7331, even though it is a fairly early-type spiral, does not have a conventional, co-rotating bulge. The fact that the inner component is retrograde makes us believe that it was formed from infalling material, in either stellar or gaseous form (e.g. Balcells & Quinn 1990). Another possibility however is that the structure has been there since the formation of the galaxy. In this case it will be a challenge to explain the large change in orientation of the angular momentum when going outward radially.Comment: 13 pages latex, including 4 figures and 1 B/W plate. Accepted for Astrophysical Journal Letters. Revised version incorporating some small last-minute change