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We use high signal-to-noise ratio long-slit spectra in the λλ3500–7000-Å range of three early-type (Hubble type S0) normal, two low-ionization nuclear emission region (LINER) and three Seyfert 2 galaxies to make a comparative study of the variation of the stellar population properties as a function of distance from the nucleus. In order to characterize the stellar population and other continuum sources (e.g. featureless continuum) we have measured equivalent widths (Ws) of six absorption features, continuum colours and their radial variations, and performed spectral population synthesis as a function of distance from the nucleus. The Ws for the normal galaxies are strongest at the nucleus and decrease outwards. The two LINER galaxies show a similar behaviour, but with the nuclear Ws in the blue (λ<4500 Å)≈10–20 per cent smaller. Both normal galaxies and LINERs present a redder nucleus than their surroundings. The three Seyfert 2 galaxies present the opposite behaviour, with most Ws in the blue being smaller at the nucleus, which is also bluer than outside. By combining the results of the present work with those of a previous study, we conclude that, in a sample of 12 Seyfert 2s of Hubble type S0, 10 (≈80 per cent) present nuclear Ws smaller than those of normal galaxies, while at ≈1 kpc from the nucleus this number decreases to 4 (≈30 per cent). Sampling regions of ≈100–200 pc, we conclude that the normal galaxies have nuclear stellar populations dominated by the components with an age of 10 Gyr and metallicity solar or above. The contribution of this component decreases, while that of 1 Gyr increases, with increasing distance from the nucleus. The LINERs present a similar behaviour but have a ≈10–20 per cent larger contribution of the 1-Gyr component at the nucleus. For the Seyfert 2 galaxies, there are additional components: in the three cases studied here, there is a significant contribution from a 100-Myr-old population at the nucleus plus featureless continuum in two cases and also of a 3-Myr-old component in Mrk463E. Outside the nucleus, the stellar population is similar to that of normal galaxies in ESO138-G1, but it has an excess of old metal-poor components in NGC7743 and of young components in Mrk463E, when compared with normal galaxies. The reddening of the continuum is lowest in the normal galaxy nuclei [E(B-V)[sub i]≈0.00] and highest in the Seyfert [E(B-V)[sub i]≈0.20] nuclei. [ABSTRACT FROM AUTHOR]

Mid-infrared images of the Seyfert 1 galaxy Mrk 279 obtained with the ISO satellite are presented together with the results of a one-year monitoring campaign of the 2.5-11.7 micron spectrum. Contemporaneous optical photometric and spectrophotometric observations are also presented. The galaxy appears as a point-like source at the resolution of the ISOCAM instrument, 4-5". The 2.5-11.7 micron average spectrum of the nucleus in Mrk 279 shows a strong power law continuum with a spectral index alpha = -0.80+/-0.05 and weak PAH emission features. The Mrk 279 spectral energy distribution shows a mid-IR bump, which extends from 2 to 15-20 micron . The mid-IR bump is consistent with thermal emission from dust grains at a distance of >= 100 light-days. No significant variations of the mid-IR flux have been detected during our observing campaign, consistent with the relatively low amplitude (~10 % rms) of the optical variability during the campaign. The time delay for the Hbeta line emission in response to the optical continuum variations is 16.7 +5.3/5.6 days, consistent with previous measurements., Astronomy

This paper combines data from the three preceding papers in order to analyze the multi-wave-band variability and spectral energy distribution of the Seyfert I galaxy NGC 4151 during the 1993 December monitoring campaign. The source, which was near its peak historical brightness, showed strong, correlated variability at X-ray, ultraviolet, and optical wavelengths; The strongest variatIons were seen in medium-energy (~1.5 keV) X-rays, with a normalized variability amplitude (NVA) of 24%. Weaker (NVA = 6%) variations (uncorrelated with those at lower energies) were seen at soft gamma ray energies of ~100 keV. No significant variability was seen in softer (0.1-1 keV) X-ray bands. In the ultraviolet/optical regime the NVA decreased from 9% to 1% as the wavelength increased from 1275 to 6900 A. These data do not probe extreme ultraviolet (1200 A to 0.1 keV) or hard X ray (2-50 keV) variability. The phase differences between variations in different bands were consistent with zero lag, with upper limits of <~ 0.15 day between 1275 A and the other ultraviolet bands, <~0.3 day between 1275 A and 1.5 keV, and <~1 day between 1275 and 512 A. These tight limits represent more than an order of magnitude improvement over those determined in previous multi wave band AGN monitoring campaigns. The ultraviolet fluctuation power spectra showed no evidence for periodicity, but were instead well fitted with a very steep, red power law (a <= -2.5). If photons emitted at a "primary" wave band are absorbed by nearby material and "reprocessed" to produce emission at a secondary wave band, causality arguments require that variations in the secondary band follow those in the primary band. The tight interband correlation and limits on the ultraviolet and medium-energy X-ray lags indicate that the reprocessing region is smaller than ~0.15 lt-day in size. After correcting for strong (a factor of ~> 15) line-of-sight absorption, the medium-energy X-ray luminosity variations appear adequate to drive the ultraviolet/optical variations. However the medium-energy X-ray NVA is 2- 4 times that in the ultraviolet, and the single-epoch absorption- corrected X-ray/gamma ray luminosity is only about one third of that of the ultraviolet optical/infrared, suggesting that at most about a third of the total low energy flux could be reprocessed high-energy emission. The strong wavelength dependence of the ultraviolet NVAs is consistent with an origin in an accretion disk, with the variable emission coming from the hotter inner regions and nonvariable emission from the cooler outer regions. These data, when combined with the results of disk fits indicate a boundary between these regions near a radius of order R ~ 0.07 lt-day. No interband lag would be expected, as reprocessing (and thus propagation between regions) need not occur, and the orbital timescale of 1 day is consistent with the observed variability timescale. However, such a model does not immediately explain the good correlation between ultraviolet and X-ray variations., Astronomy

We present the results of an intensive ultraviolet monitoring campaign on the Seyfert I galaxy NGC 4151, as part of an effort to study its short-timescale variability over a broad range in wavelength. The nucleus of NGC 4151 was observed continuously With the International Ultraviolet Explorer for 9.3 days, yielding a pair of LWP and SWP spectra every ~70 minutes, and during 4 hr periods for 4 days Prior to and 5 days after the continuous-monitoring period. The sampling frequency of the observations is an order of magnitude higher than that of any previous UV monitoring campaign on a Seyfert galaxy. The continuum fluxes in bands from 1275 to 2688 A went through four significant and well-defined events of duration 2-3 days during the continuous-monitoring period. We find that the amplitudes of the continuum variations decrease with increasing wavelength, which extends a general trend for this and other Seyfert galaxies to smaller timescales (i.e., a few days). The continuum variations in all the UV bands are simultaneous to within an accuracy of ~0.15 days, providing a strict constraint on continuum models. The emission-line light curves show only one major event during the continuous monitoring (a slow rise followed by a shallow dip) and do not correlate well with continuum light curves over the short duration of the campaign, because the timescale for continuum variations is apparently smaller than the response times of the emission lines., Astronomy

An 8 month monitoring campaign on the Seyfert 1 galaxy Fairall 9 has been conducted with the International Ultraviolet Explorer in an attempt to obtain reliable estimates of continuum-continuum and continuum-emission-line delays for a high-luminosity active galactic nucleus (AGN). While the results of this campaign are more ambiguous than those of previous monitoring campaigns on lower luminosity sources, we find general agreement with the earlier results: (1) there is no measurable lag between ultraviolet continuum bands, and (2) the measured emission-line time lags are very short. It is especially notable that the Lyα + N V emission-line lag is about 1 order of magnitude smaller than determined from a previous campaign by Clavel, Wamsteker, & Glass (1989) when Fairall 9 was in a more luminous state. In other well-monitored sources, specifically NGC 5548 and NGC 3783, the highest ionization lines are found to respond to continuum variations more rapidly than the lower ionization lines, which suggests a radially ionization-stratified broad-line region. In this case, the results are less certain, since none of the emission-line lags are very well determined. The best-determined emission line lag is Lyα + N V, for which we find that the centroid of the continuum-emission-line cross-correlation function is τcent ≈ 14-20 days. We measure a lag τcent lesssim 4 days for He II λ1640; this result is consistent with the ionization-stratification pattern seen in lower luminosity sources, but the relatively large uncertainties in the emission-line lags measured here cannot rule out similar lags for Lyα + N V and He II λ1640 at a high level of significance. We are unable to determine a reliable lag for C IV λ1550, but we note that the profiles of the variable parts of Lyα and C IV λ1550 are not the same, which does not support the hypothesis that the strongest variations in these two lines arise in the same region., Astronomy

High-resolution mid-infrared (MIR) images of the central region of NGC 1068 have been obtained with VISIR, the multi-mode MIR instrument recently installed at the European Southern Observatory Very Large Telescope on Paranal. A map of the emission at 12.8 μm with increased sensitivity over the central arcsec2 area is discussed. It shows a central core (unresolved along the east–west direction) and extended emission which has a spiral pattern similar to that observed in near-infrared images. Patches of MIR emission can be detected up to a distance of 4 arcsec from the core. The deconvolved 12.8-μm map is fully consistent with previous high-resolution MIR observations. It highlights the structure of the extended emission, already seen in the un-deconvolved image, and allows us to identify a set of MIR sources: seven in the north-eastern quadrant and five in the south-western quadrant. The MIR emission map is compared with those obtained at comparable angular resolution in the near-infrared and in [O iii] line emission. The very good correlation between the VISIR map and the Hubble Space Telescope optical map supports the idea that the MIR emission not associated with the torus arises from dust associated with the narrow-line-region clouds. The north–south extension of the MIR core (0.44 arcsec) is then probably simply due to the mixing of the MIR emission from the dusty torus and the MIR emission from narrow-line-region cloud B, located only 0.1 arcsec to the north. [ABSTRACT FROM AUTHOR]