F. Duccio Macchetto

Detection of compact ultraviolet nuclear emission in liner galaxies

Low-ionization nuclear emission-line regions (LINERs), which exist in a large fraction of galaxies, may be the least luminous manifestation of quasar activity. As such, they may make possible the study of the active galactic nucleus (AGN) phenomenon in the nearest galaxies. The nature of LINERs has, however, remained controversial because an AGN-like nonstellar continuum source has not been directly observed in them. We report the detection of bright (greater than or approximately = 2 x 10(exp -16) ergs/s/sq cm/A), unresolved (full width at half maximum (FWHM) less than or approximately = 0.1 sec) point sources of UV (approximately 2300 A) emission in the nuclei of nine nearby galaxies. The galaxies were imaged using the Faint Object Camera (FOC) on the Hubble Space Telescope (HST), and seven of them are from a complete sample of 110 nearby galaxies that was observed with HST. Ground-based optical spectroscopy reveals that five of the nuclei are LINERs, three are starburst nuclei, and one is a Seyfert nucleus. The observed UV flux in each of the five LINERs implies an ionizing flux that is sufficient to account to the observed emission lines through photoionization. The detection of a strong UV continuum in the LINERs argues against shock excitation as the source of the observed emission lines, and supports the idea that photoionization excites the lines in at least some objects of this class. We have analyzed ground-based spectra for most of the northern-hemisphere galaxies in the HST sample and find that 26 of them are LINERs, among which only the above five LINERs have a detected nuclear UV source.

Optical and Radio Polarimetry of the M87 Jet at 02 Resolution

We discuss optical (HST/WFPC2 F555W) and radio (15 GHz VLA) polarimetry observations of the M87 jet taken during 1994-1995. The angular resolution of both of these observations is ∼ 0.2 ′′ , which at the distance of M87 corresponds to 15 pc. Many knot regions are very highly polarized (∼ 40 −50%, approaching the theoretical maximum for optically thin synchrotron radiation), suggesting highly ordered magnetic fields. High degrees of polarization are also observed in interknot regions. The optical and radio polarization maps share many similarities, and in both, the magnetic field is largely parallel to the jet, except in the “shock-like” knot regions (parts of HST-1, A, and C), where it becomes perpendicular to the jet. We do observe significant differences between the radio and optical polarized structures, particularly for bright knots in the inner jet, giving us important insight into the radial structure of the jet. Unlike in the radio, the optical magnetic field position angle becomes perpendicular to the jet at the upstream ends of knots HST-1, D, E and F. Moreover, the optical polarization appears to decrease markedly at the position of the flux maxima in these knots. In contrast, the magnetic field position angle observed in the radio remains parallel to the jet in most of these regions, and the decreases in radio polarization are smaller. More minor differences are seen in other jet regions. Many of the differences between optical and radio polarimetry results can be explained in terms of a model whereby shocks occur in the jet interior, where higher-energy electrons are concentrated and dominate both polarized and unpolarized emissions in the optical, while the radio maps show strong contributions from lower-energy electrons in regions with B parallel, near the jet surface.

Hubble Space Telescope Observations of Globular Clusters in M87 and an Estimate of H0

Hubble Space Telescope observations of over 1000 globular clusters in the central region of M87 have been made using the Wide Field and Planetary Camera 2. The limiting magnitude in V is 26 mag, more than 2 mag beyond the turnover of the luminosity function. The distribution is well fitted by a Gaussian profile with a mean of m -->V0 = 23.72 ± 0.06 mag and a width of 1.40 ± 0.06 mag. Assuming a value of M -->V0 = -7.4 ± 0.25 mag for the globular cluster luminosity function results in a distance modulus of (m - M) = 31.12 mag to the Virgo Cluster. Using a value of ΔComa-Virgo(m - M) = 3.71 ± 0.10 mag and adopting a velocity for the Coma Cluster of 7188 km s-1 result in a value for the Hubble constant of H0 = 78 ± 11 km s-1 Mpc-1. The V - I color distribution is bimodal, with peaks at V - I = 0.95 mag and 1.20 mag. The mean size of the clusters is Reff = 3 pc, with a scatter of 1 pc.

The Hubble Space Telescope View of LINER Nuclei: Evidence for a Dual Population?*

We study a complete, distance-limited sample of 25 LINERs, 21 of which have been imaged with the Hubble Space Telescope. In nine objects we detect an unresolved nucleus. To study their physical properties, we compare the radio and optical properties of the nuclei of LINERs with those of other samples of local active galactic nuclei (AGNs), namely, Seyfert galaxies and low-luminosity radio galaxies (LLRGs). Our results show that the LINER population is not homogeneous, as there are two subclasses: (1) the first class is similar to the LLRG class, as it extends the population of radio-loud nuclei to lower luminosities; (2) the second is similar to Seyfert galaxies and extends the properties of radio-quiet nuclei toward the lowest luminosities. The objects are optimally discriminated in the plane formed by the black hole mass versus nuclear radio loudness: all radio-loud LINERs have MBH 108 M☉, while Seyfert galaxies and radio-quiet LINERs have MBH 108 M☉. The different nature of the various classes of local AGNs are best understood when the fraction of the Eddington luminosity they irradiate, Lo/LEdd, is plotted against the nuclear radio-loudness parameter: Seyfert galaxies are associated with relatively high radiative efficiencies Lo/LEdd 10-4 (and high accretion rates onto low-mass black holes); LLRGs are associated with low radiative efficiencies (and low accretion rates onto high-mass black holes); all LINERs have low radiative efficiency (and accretion rates) and can be radio-loud or radio-quiet, depending on their black hole mass.

Month - timescale optical variability in the M87 jet

A previously inconspicuous knot in the M87 jet has undergone a dramatic outburst and now exceeds the nucleus in optical and X-ray luminosity. Monitoring of M87 with the Hubble Space Telescope and Chandra X-Ray Observatory during 2002-2003, has found month-timescale optical variability in both the nucleus and HST-1, a knot in the jet 082 from the nucleus. We discuss the behavior of the variability timescales as well as spectral energy distribution of both components. In the nucleus, we see nearly energy-independent variability behavior. Knot HST-1, however, displays weak energy dependence in both X-ray and optical bands but with nearly comparable rise/decay timescales at 220 nm and 0.5 keV. The flaring region of HST-1 appears stationary over 8 months of monitoring. We consider various emission models to explain the variability of both components. The flares we see are similar to those seen in blazars, albeit on longer timescales, and so could, if viewed at smaller angles, explain the extreme variability properties of those objects.

Face-on dust disks in galaxies with optical jets

The presence of optical synchrotron jets in radio galaxies is relatively rare. Here we show that of the nearest five FR I 3CR radio galaxies showing optical jets, four show evidence for almost circular, presumably face-on, dust disks. This is strong support for the twofold idea that (1) jets emerge close to perpendicular to inner gas disks and (2) optical nonthermal synchrotron emission is seen only when the jet points toward the observer. The implied critical angle to the line of sight is approximately 30°-40°; i.e., if the angle of the jet to the line of sight is less than about 40° we see an optical jet. The corresponding relativistic γ factor is ≈1.5, which is consistent with current observations of jet proper motion that show a range up to γ ~ 6 for M87. The relatively low speeds implied by γ ≈ 1.5 may be due to a global deceleration of the jet as in unified theories or else to stratification within the jet. Unresolved nuclei are common in the optical. Their luminosities are also consistent with the beaming concept when compared to inclination inferred from the dust lanes. The disk sizes are typically several hundred parsecs to kiloparsec size. The galaxy with an optical jet that does not show a face-on disk, M87, instead has more complex radial dust and ionized gas filaments.

Ultraviolet Hubble Space Telescope Snapshot Survey of 3CR Radio Source Counterparts at Low Redshift

We present ultraviolet images of 27 3CR radio galaxies with redshifts z <0.1 that have been imaged with the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope (HST). The observations employed the NUV-MAMA and broad-band filters with peak sensitivity at 2200u We find that the UV luminosities show approximately a factor of 10 to 100 higher dispersion than the optical. We compare the UV morphologies with optical V - and R-band WFPC2 snapshot survey images. We have found dramatic, complex and extended ultraviolet emission from radio galaxies even at zero redshift. We find a diverse range of UV morphologies, some completely divergent from their visual morphology, which are reminiscent of the chaotic high-z radio galaxies structures seen in rest-frame UV. The UV morphologies show regions of star formation, jets, and possible scattered AGN continuum. The UV emission is generally not aligned with the radio structure. We also detect the diffuse UV emission of the host galaxy. We propose that these are the same physical phenomena as observed at high redshift, but on a smaller spatial scale.

Observations of grb 970228 and grb 970508, and the neutron-star merger model

We present the discovery observations for the optical counterpart of the g-ray burster GRB 970508 and discuss its light curve in the context of the fireball model. We analyze the HST data for this object and conclude that any underlying galaxy must be fainter than . We also present a detailed analysis of the HST images of R 5 25.5 GRB 970228, from which a proper motion of the optical counterpart was claimed by Caraveo et al., and conclude that, within the uncertainties, there is no proper motion. We examine several aspects of the neutron star merger model for g-ray bursts. In particular, we use this model to predict the redshift distribution of g-ray bursters, and, adopting a recent determination of the cosmic star formation history, we show that the predicted log N‐log P distribution is consistent with that observed for GRBs. Subject headings: cosmology: observations — gamma rays: bursts

Bulges and Disklike Components in the Host Galaxies of Low-Redshift 3CR Sources: A Near-Infrared View of Their Radial Brightness Profiles

We analyze the near-infrared luminosity profiles and photometric parameters of the host galaxies of 3CR radio sources with z < 0.3, to investigate their physical nature. Our sample includes 82 galaxies, of which 22 (27%) are FR Is and 60 (73%) are FR IIs. Using near-infrared data taken both with NICMOS on board the Hubble Space Telescope and from the ground with the Telescopio Nazionale Galileo, we find that luminosity profiles are very well described by a single S?rsic law in 52% of the cases and that for the remaining objects (48%) it is necessary to include an exponential profile, which indicates the presence of a second disklike component. The average bulge-to-disklike components luminosity ratio for the galaxies is (b/e) ~ 1.1. The analysis of the photometric parameters of the subsamples indicates that FR Is and FR IIs show rather similar bulges in terms of effective surface magnitude, effective radius, and S?rsic index. On the other hand, the disklike components in FR I and FR II hosts show, on average, different properties. Central surface magnitudes are dimmer and scale lengths are greater by a factor of 2 in FR Is when compared to FR IIs. We also estimate the black hole mass associated with each galaxy using two different methods that claim tight correlations of the black hole mass (MBH) with the infrared bulge luminosity (Lbulge) and with the S?rsic index (n). Our data indicate that masses obtained through these two methods show a high dispersion and that MBH obtained through Lbulge are systematically higher (by a factor of ~3) than those obtained using n. This result may reflect the fact that for our sample galaxies we do not find any correlation between Lbulge and n.

What Do the Hubble Space Telescope and Chandra Tell Us about the Jet and the Nuclear Region of the Radio Galaxy 3C 270

The Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph ultraviolet image of the FR I radio galaxy 3C 270 shows the presence of a jetlike structure emerging from the position of the nucleus. This feature, which represents the first jetlike component ever detected in the UV in a radio galaxy with jets lying almost on the plane of the sky, has the same position angle as the jet in the radio and X-ray images. We propose two different scenarios for the origin of the emission: (1) nonthermal synchrotron from a mildly relativistic component of the jet and (2) scattered light from the nucleus, where a BL Lac source may be hosted. Either of these pictures would have important consequences for the active galactic nuclei unification schemes and for our knowledge of the jet structure. In the Chandra image, a faint counterjet is also present. From a comparative analysis of the HST images and Chandra X-ray spectrum, we find that the nucleus is only moderately obscured. The obscuring structure might well reside in the geometrically thin dark disk observed on large scales. This fits the scenario in which a standard geometrically and optically thick torus is not present in FR I radio galaxies.