J. M. Wrobel

The Expanded Very Large Array: A New Telescope for New Science

Since its commissioning in 1980, the Very Large Array (VLA) has consistently demonstrated its scientific productivity. However, its fundamental capabilities have changed little since 1980, particularly in the key areas of sensitivity, frequency coverage, and velocity resolution. These limitations have been addressed by a major upgrade of the array, which began in 2001 and will be completed at the end of 2012. When completed, the Expanded VLA?the EVLA?will provide complete frequency coverage from 1 to 50?GHz, a continuum sensitivity of typically 1 ?Jy beam?1 (in 9 hr with full bandwidth), and a modern correlator with vastly greater capabilities and flexibility than the VLAs. In this Letter, we describe the goals of the EVLA project, its current status, and the anticipated expansion of capabilities over the next few years. User access to the array through the Open Shared Risk Observing and Resident Shared Risk Observing programs is described. The following papers in this special issue, derived from observations in its early science period, demonstrate the astonishing breadth of this most flexible and powerful general-purpose telescope.

A SUBKILOPARSEC DISK IN MARKARIAN 231

We present imaging with the Very Long Baseline Array of the neutral hydrogen 21 cm absorption-line system seen toward the nuclear regions of Mrk 231 at z☉ = 0.04217, and imaging of the radio continuum emission at 1.4 GHz on scales ranging from a few parsecs to a few hundred parsecs. These data indicate the existence of a subkiloparsec gas disk in Mrk 231, as seen in H I 21 cm absorption and in radio continuum emission. The radio continuum morphology is consistent with a disk of maximum radius of 440 mas (260 h-1 pc), at an inclination angle of 45°, with a major axis oriented east-west. The H I 21 cm absorption shows an east-west gradient in position and velocity of about ±110 km s-1 out to radii of 100 mas (60 h-1 pc). We identify this H I and radio continuum disk as the inner part of the molecular disk seen on a factor of 3 larger scale. The physical conditions for the thermal and nonthermal gas in the subkiloparsec disk of Mrk 231 are similar to those proposed for compact nuclear starburst galaxies and, in particular, to the conditions proposed for the subkiloparsec gas disk in Arp 220. From the neutral hydrogen velocity field we derive a gravitational mass enclosed within a 50 h-1 pc radius of 3 × 108 h-1 M☉, and from the radio continuum emission we derive a massive star formation rate in the disk of 60 M☉ yr-1. We also present a search for H I 21 cm absorption associated with the optical broad absorption line (BAL) systems toward Mrk 231. We do not detect H I 21 cm absorption associated with any of the optical BAL systems. These negative results require that the neutral atomic gas in the BAL clouds be fairly warm (spin temperature Ts > 50 K), unless the Na I abundance is higher than solar, or the dust-to-gas ratio is higher than Galactic, or the observed extinction toward the nucleus of Mrk 231 is not due to the BAL gas.

THE NUCLEAR REGIONS OF THE SEYFERT GALAXY NGC 4151: PARSEC-SCALE H i ABSORPTION AND A REMARKABLE RADIO JET

Sensitive high angular and linear resolution radio images of the 240 pc radio jet in NGC 4151, imaged at linear resolutions of 0.3-2.6 pc using the VLBA and phased VLA at ?21 cm, are presented and reveal for the first time a faint, highly collimated jet (diameter 1.4 pc) underlying discrete components, seen in lower resolution MERLIN and VLA images, that appear to be shocklike features associated with changes in direction as the jet interacts with small gas clouds within the central ~100 pc of the galaxy. In addition, ?21 cm spectral line imaging of the neutral hydrogen in the nuclear region reveals the spatial location, distribution, and kinematics of the neutral gas detected previously in a lower resolution MERLIN study. Neutral hydrogen absorption is detected against component C4W (E+F) as predicted by Mundell et al, but the absorption, extending over 3 pc, is spatially and kinematically complex on subparsec scales, suggesting the presence of small, dense gas clouds with a wide range of velocities and column densities. The main absorption component matches that detected in the MERLIN study, close to the systemic velocity (998 km s-1) of the galaxy, and is consistent with absorption through a clumpy neutral gas layer in the putative obscuring torus, with higher velocity blue- and redshifted systems with narrow line widths also detected across E+F. In this region, average column densities are high, lying in the range 2.7 ? 1019TS < NH < 1.7 ? 1020TS cm-2 K-1 (TS is the spin temperature), with average radial velocities in the range 920 < Vr < 1050 km s-1. The spatial location and distribution of the absorbing gas across component E+F rules out component E as the location of the active galactic nucleus (AGN) (as suggested by Ulvestad et al.) and, in combination with the well-collimated continuum structures seen in component D, suggests that component D (possibly subcomponent D3) is the most likely location for the AGN. We suggest that components C and E are shocks produced in the jet as the plasma encounters, and is deviated by, dense clouds with diameters smaller than ~1.4 pc. Comparison of the radio jet structure and the distribution and kinematics of ionized gas in the narrow-line region (NLR) suggests that shock excitation by passage of the radio jet is not the dominant excitation mechanism for the NLR. We therefore favor nuclear photoionization to explain the structure of the NLR, although it is interesting to note that a small number of clouds with low-velocity and high-velocity dispersion are seen to bound the jet, particularly at positions of jet direction changes, suggesting that some NLR clouds are responsible for bending the jet. Alternatively, compression by a cocoon around the radio jet due to pressure stratification in the jet bow shock could explain the bright, compressed optical line-emitting clouds surrounding the cloud-free channel of the radio jet, as modeled by Steffen et al.

Radio Continuum Evidence for Outflow and Absorption in the Seyfert 1 Galaxy Markarian 231

The Very Long Baseline Array (VLBA) and the Very Large Array (VLA) have been used to image the continuum radio emission from Mrk 231, a Seyfert 1 galaxy and the brightest infrared galaxy in the local universe. The smallest VLBA scales reveal a double, or possibly triple, source less than 2 pc in extent. The components of this central source have minimum brightness temperatures of 109-1010 K and spectral turnovers between 2 and 10 GHz and appear to define the galaxy nucleus plus the inner regions of a jet. The strongest component is probably synchrotron self-absorbed, while the weaker component to the northeast may be either free-free absorbed or synchrotron self-absorbed. On larger VLBA scales, the images confirm a previously known north-south triple source extending 40 pc and elongated perpendicular to a 350 pc starburst disk traced by H I and CO. Both lobes of the triple show evidence for free-free absorption near 2 GHz, probably due to ionized gas with a density of (1-2) × 103 cm-3 in the innermost regions of the starburst disk. This free-free absorption resembles that toward the counterjet of 3C 84 but requires ionized gas at lower density located considerably farther from the central source. The absorbing gas may be ionized by the active nucleus or by local regions of enhanced star formation, possibly in the inner part of the starburst disk. The elongation position angle of the 40 pc triple differs by 65° from that of the 2 pc source. Unless the radio source is seen nearly end-on, the different symmetry axes on different scales in Mrk 231 imply a dramatic curvature in the inner part of the Mrk 231 radio jet. A comparison of VLBA and VLA flux densities indicates that the radio continuum from the 350 pc disk has a spectral index near -0.4 at frequencies above 1.4 GHz and is plausibly energized by a massive burst of star formation, with the overall spectrum flattened somewhat by a contribution from free-free absorption. On VLA scales, asymmetric and diffuse emission extends for more than 25 kpc. This emission has a steep spectrum, exhibits linear polarization exceeding 50% at some locations, and shares the symmetry axis of the 40 pc triple, but on a scale larger by 3 orders of magnitude. The large-scale radio emission extends beyond the bulk of the optical galaxy but has an initial axis similar to a series of optical star-forming knots several kiloparsecs from the nucleus. This diffuse radio source is probably generated by energy deposition from a slow-moving nuclear jet, which conceivably could help energize the off-nuclear starburst as well.

VLBA Observations of a Sample of Nearby FR I Radio Galaxies

We observed 17 nearby low-luminosity FR I radio galaxies using the NRAO Very Long Baseline Array (VLBA) at 1.67 GHz, as part of a multiwavelength study of a complete sample of 21 sources selected by radio flux density from the Uppsala General Catalogue of Galaxies. We detected radio emission from all 17 galaxies. At a FWHM resolution of ≈10 × 4 mas, five galaxies show only an unresolved radio core, 10 galaxies show core-jet structures, and two galaxies show twin-jet structures. Comparing these VLBA images with images previously obtained with the NRAO VLA, we find that all detected VLBA jets are well aligned on parsec scales with the VLA jets on kiloparsec scales and that the jet-to-counterjet surface brightness ratios, or the sidedness, decreases systematically with increasing distance along the jet. We attribute the sidedness to the Doppler boosting effect and its decline to the deceleration of the jets. We show that a distribution of Lorentz factor centered near Γ = 5 can reproduce our VLBA detection statistics for core, core-jet, and twin-jet sources. We also note that the luminosity per unit length, Lj, of the VLBA jets drops quickly with distance, r, along the jet, approximately as Lj ∝ r-2.0. We discuss three different mechanisms to explain this jet fading: (1) the decrease of Doppler boosting due to jet deceleration, (2) synchrotron losses, and (3) expansion losses in constant velocity but adiabatically spreading jets. Mechanisms (1) and (2) are inconsistent with the observations, while mechanism (3) is consistent with the observations provided the magnetic field lines in the jets are aligned perpendicular to the jet axis. This implies that the deceleration of the jets required by the unified scheme does not occur on the tens of parsec scales but must occur on larger scales.

Detection of a High Brightness Temperature Radio Core in the Active-galactic-nucleus-driven Molecular Outflow Candidate NGC 1266

We present new high spatial resolution Karl G. Jansky Very Large Array (VLA) H I absorption and Very Long Baseline Array (VLBA) continuum observations of the active-galactic-nucleus-(AGN-)driven molecular outflow candidate NGC 1266. Although other well-known systems with molecular outflows may be driven by star formation (SF) in a central molecular disk, the molecular mass outflow rate of 13 M_☉ yr^(–1) in NGC 1266 reported by Alatalo et al. exceeds SF rate estimates from a variety of tracers. This suggests that an additional energy source, such as an AGN, may play a significant role in powering the outflow. Our high spatial resolution H I absorption data reveal compact absorption against the radio continuum core co-located with the putative AGN, and the presence of a blueshifted spectral component re-affirms that gas is indeed flowing out of the system. Our VLBA observations at 1.65 GHz reveal one continuum source within the densest portion of the molecular gas, with a diameter d 1.5 × 10^7 K that is most consistent with an AGN origin. The radio continuum energetics implied by the compact VLBA source, as well as archival VLA continuum observations at lower spatial resolution, further support the possibility that the AGN in NGC 1266 could be driving the molecular outflow. These findings suggest that even low-level AGNs may be able to launch massive outflows in their host galaxies.

An Attempt to Probe the Radio Jet Collimation Regions in NGC 4278, NGC 4374 (M84), and NGC 6166

NRAO Very Long Baseline Array (VLBA) observations of NGC 4278, NGC 4374 (M84), NGC 6166, and M87 (NGC 4486) have been made at 43 GHz in an effort to image the jet collimation region. This is the first attempt to image the first three sources at 43 GHz using very long baseline interferometry (VLBI) techniques. These three sources were chosen because their estimated black hole mass and distance implied a Schwarzschild radius with large angular size, giving hope that the jet collimation regions could be studied. Phase referencing was utilized for the three sources because of their expected low flux densities. M87 was chosen as the calibrator for NGC 4374 because it satisfied the phase-referencing requirements: near the source and sufficiently strong. Having observed M87 for a long integration time, we have detected its subparsec jet, allowing us to confirm previous high-resolution observations made by Junor, Biretta, & Livio, who have indicated that a wide opening angle was seen near the base of the jet. Phase referencing successfully improved our image sensitivity, yielding detections and providing accurate positions for NGC 4278, NGC 4374, and NGC 6166. These sources are point dominated but show suggestions of extended structure in the direction of the large-scale jets. However, higher sensitivity will be required to study their subparsec jet structure.

Radio emission on subparsec scales from the intermediate-mass black hole in NGC 4395

The Seyfert 1 nucleus of NGC 4395 is energized by a black hole of mass 3.6 × 105 M☉, making it one of only two nuclear black holes of intermediate mass, 103-106 M☉, detected in the radio regime. Building on UV and X-ray evidence for outflows from this Seyfert nucleus, the VLBI High Sensitivity Array was used at 1.4 GHz to search for extended structure on scales greater than 5 mas (0.1 pc). Elongated emission was discovered, extending over 15 mas (0.3 pc) and suggesting an outflow on subparsec scales from this intermediate-mass black hole. The Seyfert nucleus is located at the center of an elliptical star cluster, and the elongation position angle of the subparsec radio structure is only 19° from the star clusters minor axis.

Discovery of Twin Relativistic Jets in the Nearby E/S0 Galaxy NGC 3894

VLBI techniques were used to monitor the milliarcsecond-scale continuum structure of the E/S0 galaxy NGC 3894. The resulting VLBI images span 15 yr and provide morphological and spectral evidence for twin, parsec-scale jets emerging from a synchrotron-self-absorbed core. Analysis of the twin-jet kinematics requires that the jets be both mildly relativistic (v ~ 0.3c) and oriented well away from the line of sight (θ ~ 50°). Slow jets could be deflected or disrupted, so evidence for such processes is examined. A large viewing angle could influence the galaxys nuclear properties at other wavebands, and those properties are explored.

The Atlas3D project - XXXI. Nuclear radio emission in nearby early-type galaxies

We present the results of a high-resolution, 5 GHz, Karl G. Jansky Very Large Array study of the nuclear radio emission in a representative subset of the ATLAS^(3D) survey of early-type galaxies (ETGs). We find that 51 ± 4 per cent of the ETGs in our sample contain nuclear radio emission with luminosities as low as 10^(18) W Hz^(−1). Most of the nuclear radio sources have compact (≲25–110 pc) morphologies, although ∼10 per cent display multicomponent core+jet or extended jet/lobe structures. Based on the radio continuum properties, as well as optical emission line diagnostics and the nuclear X-ray properties, we conclude that the majority of the central 5 GHz sources detected in the ATLAS^(3D) galaxies are associated with the presence of an active galactic nucleus (AGN). However, even at subarcsecond spatial resolution, the nuclear radio emission in some cases appears to arise from low-level nuclear star formation rather than an AGN, particularly when molecular gas and a young central stellar population is present. This is in contrast to popular assumptions in the literature that the presence of a compact, unresolved, nuclear radio continuum source universally signifies the presence of an AGN. Additionally, we examine the relationships between the 5 GHz luminosity and various galaxy properties including the molecular gas mass and – for the first time – the global kinematic state. We discuss implications for the growth, triggering, and fuelling of radio AGNs, as well as AGN-driven feedback in the continued evolution of nearby ETGs.