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This study of the activity in the nuclei of normal galaxies began as an investigation into the spread in the observed properties of normal galaxies used as standard candles in measurements of departures from the Hubble flow (Hart & Davies 1982). Extensive studies of the integrated properties of normal galaxies show the influence on these properties of quiescent star formation and embedded starburst-type activity (Staveley-Smith & Davies, 1987,1988). Normal galaxies have a significant level of nuclear activity, although it is not strongly correlated with their integrated properties. This investigation of nuclear activity is based on a sample of the 100 brightest nearby Sbc galaxies, a sample unbiassed in its selection of activity.
We have used the TAURUS technique to study the morphology and velocity field of spatially extended emission–line gas in a number of active galaxies. This paper describes our results for NGC 1275 and NGC 4151. The emission line maps of NGC 1275 are the first scientific data from the new 4.2-m William Herschel Telescope, and show that the morphology of the foreground galaxy falling into the Perseus cluster bears a close relationship to that of NGC 1275 itself, implying that the two galaxies are physically associated.
In the case of NGC 4151, we find the emission line region is elongated along the same axis as the nuclear radio source. This cannot be due to emission-line gas being accelerated by radio ejecta, since the emission-line gas rotates with the galactic disk. We suggest that the emission-line gas is being ionized by a highly anisotropic nuclear continuum source, UV photons escaping from the nucleus preferentially along the radio axis.
Although initial, low resolution radio measurements by deBruyn and Wilson (1978), suggested that the radio emission in Seyfert nuclei was in approximate pressure equilibrium with thermal gas in the Narrow Line Region (NLR), higher resolution radio measurements using the VLA and MERLIN (eg Unger et al. 1985) showed that the radio components were at significantly higher pressures than the NLR thermal gas. This is even more apparent in the very long baseline interferometry (VLBI) observations using the European Network (EVN). Observations of NGC4151 (Harrison et al.1986) and NGC7674 (Unger et al. 1988) have already been described, together with a summary of work in progress by Pedlar et al. 1987. Several Seyfert nuclei have components in which relativistic particle/magnetic field pressures as high as 10−7 dynes/cm2 are infered. These observations are also consistent with the collimated ejection of either radio emitting ‘plasmons’ or beams from the optical nucleus.
The fraction of Seyfert galaxies containing linear radio sources is quite substantial and may be more than 50% (Ulvestad and Wilson 1984). About 20 of the well resolved Seyfert galaxies show double or triple structure on the arcsec scale. In this respect these objects are very reminiscent of the classical radio galaxies, but they are typically smaller than these by several orders of magnitude, both with regard to power ∼ 1039–1041 erg/s and linear extent kpc (Wilson, this volume). They are basically confined in the wider nuclear environment. The central small scale () radio sources in Seyfert galaxies are very weak, mJy at 6 cm wavelength, and have therefore hardly been observed with VLBI (see, e.g. Preuss 1984).
We have observed SgrA at 332 MHz (92 cm) with a resolution of 12 arcsec (0.6 pc) using the four configurations of the VIA. These results illustrate the dramatic and almost unique variation of radio spectral index within the central 3–4 arcmin of the galactic center. SgrA East is a non-thermal shell source that could be a supernova remnant or a very low-luminosity example of a radio component associated with the active nucleus of a spiral galaxy. The most dramatic aspect of the new 332 MHz observations is the appearance of the the SgrA West spiral features in absorption against SgrA East. Based on these results, SgrA East is situated behind SgrA West, the center of the galaxy. The halo is in front of or surrounds the former sources. The HII regions to the east of SgrA East (1 = −0°.02, b = −0°.07) are probably associated with the 50 km/s molecular cloud. The 7 arcmin halo (20 pc) has a non-thermal spectrum with turn-over below 1 GHz.
We report on recent MERLIN, VLA and VLBI observations of the compact radio sources in the nearby starburst M82, with angular resolutions ranging from arcseconds to milliarcseconds. The spectral properties of the compact sources have been investigated by 15 GHz VLA-Pie Town observations which show that 16 of the less luminous compact sources are, in fact, HII regions. However the steep non-thermal spectrum, parsec size and small variability of the remaining sources is consistent with their being supernova remnants. Several show clear shell structures at MERLIN resolution (~50 mas) and 5 have been resolved further using VLBI. Measurements of the most compact source (41.95+575) shows an expansion velocity of ~ 2000 km s-1, and one of the best defined SNR shells (43.31+592) shows an expansion velocity of ~ 104 km s-1. Recent VLBI and MERLIN measurements confirm this expansion velocity and show little evidence for deceleration. We comment on the discrepancy between this measured expansion velocity and the low expansion velocities predicted theoretically for remnants in M82.
An 8 day MERLIN deep integration at 5GHz of the central region of the starburst galaxy M82 has been used to investigate the radio structure of a number of supernova remnants in unprecedented detail revealing new shells and partial shell structures for the first time. In addition, by comparing the new deep 2002 image with an astrometrically aligned image from 36 hours of data taken in 1992, it has been possible to directly measure the expansion velocities of 4 of the most compact remnants in M82. For the two most compact remnants, 41.95+575 and 43.31+592, expansion velocities of 2800 ± 300 km s-1 and 8750 ± 400kms-1have been derived. These confirm and refine the measured expansion velocities which have been derived from VLBI multi-epoch studies. For remnants 43.18+583 and 44.01+596, expansion velocities of 10500 ± 750km s-1 and 2400 ± 250 kms-1have been measured for the first time. In addition, the peak of the radio emission for SNR 45.17+612 has moved between the two epochs implying velocities around 7500km s-1. The relatively compact remnants in M82 are thus found to be expanding over a wide range of velocities which appear unrelated to their size. The new 2002 map is the most sensitive high-resolution image yet made of M82, achieving an rms noise level of 17μJy beam-1. This establishes a first epoch for subsequent deep studies of expansion velocities for many SNR within M82.
The major contending scenarios capable of explaining various aspects of the Seyfert phenomenon in AGNs are (i) the Super-massive Black-Hole model and (ii) the Starburst model. Detailed optical emission-line and radio images of Seyfert nuclei, and their mutual correlations, provide important clues in evaluating the claims of each. Using the EVN at λ 18 cm, we have mapped four Seyfert galaxies, Mkn 1, 3, 231 and 463 at a resolution of ∼ 25 mas. The maps, and comparisons with images at other wavelengths, will be presented elsewhere (Ghosh et al. 1993, in preparation). Here, we present parameters derived from elliptical-Gaussian fits to all discernible components (Table 1).
Increasingly, the evidence from optical and infra–red wavebands suggests that the difference between Seyfert 1 and Seyfert 2 nuclei is due largely to orientation effects rather than intrinsic differences between the two classes, but the evidence from radio observations has been less clear–cut. We have observed the CfA Seyfert Sample at 8.4 GHz using the VLA in A– and C–configurations. At this frequency our A–array maps have a resolution of 0.25″ – much higher than those achieved in previous surveys – whilst the 3″ C–array beam is ideal for measuring the total radio flux of the active nucleus. The 1–sigma noise in both sets of observations was 70 μJy.
We present VLA C and D array neutral hydrogen observations of the Seyfert galaxy NCG 3227. The neutral gas is highly disturbed due to the interaction with the nearby elliptical, NGC 3226 and D array observations (with resolution of 40″ and velocity resolution of 40 kms−1) revealed 2 extensive plumes stretching 88 kpc south and 44kpc north of NGC3227. Complex dynamics were observed in the disk, which subsequent C array observations (resolution of 19″ and 20 kms−1) resolved into two components: the galactic disk, which is not significantly disrupted by the interaction and is undergoing predominantly solid body rotation out to a radius of 7 kpc, and an anomalous velocity cloud (with a velocity of 170 kms−1 above systemic) which appears to be associated with the northern plume.
The central regions of many nearby active galaxies are often heavily obscured by dust and gas and are visible at only the far infrared (FIR) and radio wavelengths. High-resolution radio observations provide an invaluable tool for clarifying the dominant power source in an active galaxy which could be due to either an intense burst of star formation or an active galactic nucleus (AGN). An AGN is normally assumed to be powered by a supermassive black hole with an accretion disk, where the black hole forms as the end product of stellar evolution. There have been suggestions linking both these forms of activity to galaxy interactions, but the conditions that might lead to the formation of an AGN and how it is fuelled are not well understood.
Radio emission from Seyfert nuclei appears to be intimately related to narrow line region (NLR) of ionised gas. Both regions have an extent of a few hundred parsecs corresponding to typically a few arcsec, and are only marginally resolved by ground based observations. HST and adaptive optics are giving optical images of the ionised gas with angular resolutions of order 0.1″. It is essential that high quality radio images are available with similar resolution so that models relating the two regions can be tested. The extended MERLIN is ideally suited for this task. It has angular resolutions of 0.05″ at 5GHz and 0.13″ at 1.5GHz and sensitivities of a few 10s of μJy. In this contribution we shall summarise the results on two objects.
A number of unique non-thermal filamentary structures, which are all roughly perpendicular to the galactic plane have been discovered in the vicinity of the Galactic centre (see Yusef-Zadeh 1989 and references there in). On the basis of their uniformity, polarization angles, rigidity, and pressure balance with the surrounding medium, it has been hypothesized that a strong polodial field of milligauss strength pervades the inner 50 pc of the Galaxy (Morris and Yusef-Zadeh 1989 and Morris in this volume). We present here a single wide-field image, at λ90cm, which shows all the filamentary structures in this region and show that these observations are consistent with the above hypothesis.
We present new VLA observations of IRAS 0421+0400 at 1.4 and 5 GHz, providing dual frequency information and higher resolution than previously available. We find extremely accurate alignment of the central double with the closest (though not brightest) feature in the southern kiloparsec hotspot region. There is a tight relation between the symmetric radio structure and emission line gas in the hotspot regions of this Seyfert 2 type galaxy.
We have made multi-wavelength observations of the young planetary nebula NGC 7027. The C-band continuum radio map shown in Figure 1 was obtained from the combination of MERLIN and VLA1 observations (M. Bryce et al., 1996). This map has the highest spatial resolution of any radio images of this object to date and has been compared with the HST archive picture of NGC7027, taken using WFPC2 with a 400 å wide F547N filter, to reveal the differences in the emission at the disparate wavelengths. The two maps, while being superficially different, show emission regions that are similar in size. The contours of radio brightness often coincide with prominent optical features.
Markarian 6, a Seyfert Galaxy of type 1.5, is one of several Seyfert nuclei to be observed with MERLIN at 6 and 18 cm as part of a project to investigate the radio structures of these objects on sub–arcsecond scales (corresponding to scales of a few tens of parsecs at typical distances). The angular resolution of MERLIN at 6 cm is equivalent to that of the HST, making the radio images ideal for comparison with HST images of the optical Narrow–Line Region (NLR). In this paper we briefly discuss the results of our MERLIN observations of Markarian 6, along with a 6–cm Westerbork Synthesis Radio Telescope (WSRT) map of the arcsecond (kpc) scale radio emission. The data is discussed in more detail by Kukula et al. (1996) and Baum et al. (1993).
A new radio image of the core of the starburst galaxy M82 is presented. A pair of probable galactic chimneys are evident. Such structures have been conjectured to result when a giant shell of ionized gas breaks out of the dense plane of a galaxy and ejects hot gas into the halo. In M 82, the starburst activity drives a galactic superwind and theoretical models of this process require a knowledge of the kinematics of the hot gas in the core of the galaxy. The results presented here imply that the ejection of material into the base of the superwind does not occur smoothly over the starburst region; instead, very localised venting of the hot gas appears to be taking place.
At 5 GHz, about 20 point sources were detected earlier by Glendenning & Kronberg (1986) in the central 800 pc of NGC 2146. Our observations with higher sensitivity and resolution made with MERLIN and the VLA confirms the detection of 18 sources, and resolves 7 of them (Fig.1). Additional 1.6-GHz MERLIN observations disclose 9 sources coincident in position with those detected at 5 GHz, which allows us to derive their spectral indices α51.6 (Sv ∼ να). Only 3 sources have indices (α < 0) consistent with synchrotron emission from supernova remnants or radio supernovae, while the others have very steep inverted spectra (α > 0).
We present sensitive, high-resolution λ21-cm VLBA+VLA observations of the radio jet and nuclear HI absorption in NGC 4151. The 25-mas (1.6-pc) resolution continuum image reveals a highly collimated radio jet, underlying the discrete components seen previously with MERLIN and the VLA. Spatially and kinematically complex HI absorption is detected against the whole 3-pc extent of the continuum component predicted by Ulvestad et al. to contain the AGN. Instead, we suggest the component against which the absorption is detected is part of the eastern counterjet, ruling it out as the location for the AGN.
VLA A-array 21cm atomic hydrogen (HI) absorption observed against the central region of the starburst galaxy M82 and MERLIN HI absorption measurements which show HI absorption against 26 of the supernova remnants are discussed. These observations have been compared with the molecular (CO) and ionised ([NeII]) gas distributions and used to constrain the dynamics and structure of the ionised, neutral and molecular gas in this starburst.