Duccio Macchetto

The Fading Optical Counterpart of GRB 970228, 6 Months and 1 Year Later

We report on observations of the fading optical counterpart of the gamma-ray burst GRB 970228, made with the Hubble Space Telescope (HST) and the Keck I telescope. The gamma-ray burst (GRB) was observed approximately 6 months after outburst, on 1997 September 4, using the HST/STIS CCD, and approximately 1 year after outburst, on 1998 February 24, using HST/NICMOS, and on 1998 April 4 using the NIRC on Keck. The unresolved counterpart is detected by STIS at V=28.0 ± 0.25, consistent with a continued power-law decline with exponent -1.10 ± 0.05. The counterpart is located within, but near the edge of, a faint extended source with diameter ~08 and integrated magnitude V=25.8 ± 0.25. A reanalysis of HST and New Technology Telescope observations performed shortly after the burst shows no evidence of proper motion of the point source or fading of the extended emission. Although the optical transient is not detected in the NICMOS images (H≥25.3), the extended source is visible and has a total magnitude H=23.3 ± 0.1. The Keck observations find K=22.8 ± 0.3. Comparison with observations obtained shortly after outburst suggests that the nebular luminosity has also been stable in the infrared. We find that several distinct and independent means of deriving the foreground extinction in the direction of GRB 970228 all agree with AV=0.75 ± 0.2. After adjusting for this Galactic extinction, we find that the size of the observed extended emission is consistent with that of galaxies of comparable magnitude found in the Hubble Deep Field (HDF) and other deep HST images. Only 2% of the sky is covered by galaxies of similar or greater surface brightness. We therefore conclude that the extended source observed about GRB 970228 is almost certainly its host galaxy. Additionally, we find that independent of assumed redshift, the host is significantly bluer than typical nearby blue dwarf irregulars. With the caveat that the presently available infrared observations of the HDF are only fully complete to a limit about one-half magnitude brighter than the host, we find that the extinction-corrected V-H and V-K colors of the host are as blue as any galaxy of comparable or brighter magnitude in the HDF. Taken in concert with recent observations of GRB 970508, GRB 971214, and GRB 980703 our work suggests that all four GRBs with spectroscopic identification or deep multicolor broadband imaging of the host lie in rapidly star-forming galaxies.

Unveiling the Active Nucleus of Centaurus A

We report new HST WFPC2 and NICMOS observations of the center of the nearest radio galaxy Centaurus A (NGC 5128) and discuss their implications for our understanding of the active nucleus and jet. We detect the active nucleus in the near–IR (K and H) and, for the first time, in the optical (I and V), deriving the spectral energy distribution of the nucleus from the radio to X-rays. The optical and part of the near-IR emission can be explained by the extrapolation of the X-ray power law reddened by AV ∼ 14, a value consistent with other independent estimates. The 20pc-scale nuclear disk discovered by Schreier et al. (1998) is detected in the [FeII]λ1.64� m line and presents a morphology similar to that observed in Paα with a [FeII]/Paα ratio typical of low ionization Seyfert galaxies and LINERs. NICMOS 3 Paα observations in a 50 ′′ ×50 ′′ circumnuclear region suggest enhanced star formation (∼ 0.3M⊙ yr −1 ) at the edges of the putative bar seen with ISO, perhaps due to shocks driven into the gas. The light profile, reconstructed from V, H and K observations, shows that Centaurus A has a core profile with a resolved break at ∼ 4 ′′ and suggests a black–hole mass of ∼ 10 9 M⊙. A linear blue structure aligned with the radio/X–ray jet may indicate a channel of relatively low reddening in which dust has been swept away by the jet.

A survey of the interstellar medium in early-type galaxies. III. Stellar and gas kinematics

We present gaseous and stellar kinematics for 14 gas-rich early-type galaxies. Half of the sample exhibit irregular gaseous velocity profiles; gas/star counter-rotation is visible in five galaxies. We also find five counter-rotating stellar cores, while five more galaxies display inner components kinematically decoupled from the main stellar body. We interpret our results as an indication that the ionized gas is of external origin, is generally not in equilibrium, and may have been acquired recently. The merging or accretion events that brought the gas into the galaxy have also affected the stellar kinematics.

Is There Really a Black Hole at the Center of NGC 4041? Constraints from Gas Kinematics

We present Space Telescope Imaging Spectrograph spectra of the Sbc spiral galaxy NGC 4041, which were used to map the velocity field of the gas in its nuclear region. We detect the presence of a compact (r 04 40 pc), high surface brightness, rotating nuclear disk cospatial with a nuclear star cluster. The disk is characterized by a rotation curve with a peak-to-peak amplitude of ~40 km s-1 and is systematically blueshifted by ~10-20 km s-1 with respect to the galaxy systemic velocity. With the standard assumption of constant mass-to-light ratio and with the nuclear disk inclination taken from the outer disk, we find that a dark point mass of (1) × 107 M☉ is needed to reproduce the observed rotation curve. However, the observed blueshift suggests the possibility that the nuclear disk could be dynamically decoupled. Following this line of reasoning, we relax the standard assumptions and find that the kinematical data can be accounted for by the stellar mass provided that either the central mass-to-light ratio is increased by a factor of ~2 or the inclination is allowed to vary. This model results in a 3 σ upper limit of 6 × 106 M☉ on the mass of any nuclear black hole (BH). Overall, our analysis only allows us to set an upper limit of 2 × 107 M☉ on the mass of the nuclear BH. If this upper limit is taken in conjunction with an estimated bulge B magnitude of -17.7 and with a central stellar velocity dispersion of 95 km s-1, then these results are not inconsistent with both the MBH-Lsph and the MBH-σ* correlations. Constraints on BH masses in spiral galaxies of types as late as Sbc are still very scarce; therefore, the present result adds an important new data point to our understanding of BH demography.

The supermassive black hole in Centaurus A: a benchmark for gas kinematical measurements

We present new HST Space Telescope Imaging Spectrograph observations of the nearby radio galaxy NGC 5128 (Centaurus A). The bright emission line with longest wavelength accessible from HST, [S III]λ9533 A, was used to study the kinematics of the ionized gas in the nuclear region with a 0.

Supermassive black hole mass measurements for NGC 1300 and 2748 based on Hubble Space Telescope emission-line gas kinematics

We present Space Telescope Imaging Spectrograph emission-line spectra of the central regions of the spiral galaxies NGC 1300 and 2748. From the derived kinematics of the nuclear gas we have found evidence for central supermassive black holes in both galaxies. The estimated masses of the black holes in NGC 1300 and 2748 are (6.6 +6.3) × 10 7 and (4.4 +3.5) × 10 7 M� , respectively (both at the 95 per cent confidence level). These two black hole mass estimates contribute to the poorly sampled low-mass end of the nuclear black hole mass spectrum. Ke yw ords: black hole physics ‐ galaxies: individual: NGC 1300 ‐ galaxies: individual: NGC 2748 ‐ galaxies: kinematics and dynamics ‐ galaxies: nuclei ‐ galaxies: spiral.

Supermassive black holes in the Sbc spiral galaxies NGC 3310, NGC 4303 and NGC 4258 ,

We have undertaken an HST Space Telescope Imaging Spectrograph survey of 54 late type spiral galaxies to study the scalin g relations between black holes and their host spheroids at the low mass end. Our aim is to measure black hole masses or to set upper limits for a sizeable sample of spiral galaxies. In this paper we present new Space Telescope Imaging Spectrograph (STIS) observations of three spiral galaxies, NGC 4303, NGC 3310 and NGC 4258. The bright optical emission lines Hα λ 6564A, [NII]λλ 6549, 6585A and [SII]λλ 6718, 6732A were used to study the kinematics of the ionized gas in the nuclear region of each galaxy with a∼ 0.07 ′′ spatial resolution. Our STIS data for NGC 4258 were analyzed in conjunction with archival ones to compare the gas kinematical estimate of the black hole mass with the accurate value from H20-maser observations. In NGC 3310, the observed gas kinematics is well matched by a circularly rotating disk model but we are only able to set an upper limit to the BH mass which, taking into account the allowed disk inclinations, varies in the range 5.0× 10 6 − 4.2× 10 7 M⊙ at the 95% confidence level. In NGC 4303 the kinematical data require the presence of a BH with mass MBH = (5.0) +0.87

The supermassive black hole in the Seyfert 2 galaxy NGC 5252

We present results from HST/STIS long-slit spectroscopy of the gas motions in the nuclear region of the Seyfert 2 galaxy NGC 5252. The observed velocity field is consistent with gas in regular rotation with superposed localized patches of disturbed gas. The dynamics of the circumnuclear gas can be accurately reproduced by adding to the stellar mass component a compact dark mass of MBH = 0.95 (−0.45; +1.45) ×10 9 M ⊙ , very likely a supermassive black hole (BH). Contrarily to results obtained in similar studies rotational broadening is sufficient to reproduce also the behaviour of line widths. The BH mass estimated for NGC 5252 is in good agreement with the correlation between MBH and bulge mass. The comparison with the MBH vsc relationship is less stringent (mostly due to the relatively large error inc); NGC 5252 is located above the best fit line by between 0.3 and 1.2 dex, i.e. 1 - 4 times the dispersion of the correlation. Both the galaxys and BH mass of NGC 5252 are substantially larger than those usually estimated for Seyfert galaxies but, on the other hand, they are typical of radio-quiet quasars. Combining the determined BH mass with the hard X-ray luminosity, we estimate that NGC 5252 is emitting at a fraction ∼ 0.005 of LEdd. In this sense, this active nucleus appears to be a quasar relic, now probably accreting at a low rate, rather than a low black hole mass counterpart of a QSO.We present results from HST/STIS long-slit spectroscopy of the gas motions in the nuclear region of the Seyfert 2 galaxy NGC 5252. The observed velocity field is consistent with gas in regular rotation with superposed localized patches of disturbed gas. The dynamics of the circumnuclear gas can be accurately reproduced by adding to the stellar mass component a compact dark mass of MBH = 0.95 (-0.45;+1.45) 10E9 M(sun), very likely a supermassive black hole. Contrarily to results obtained in similar studies rotational broadening is sufficient to reproduce also the behaviour of line widths. The MBH estimated for NGC 5252 is in good agreement with the correlation between MBH and bulge mass. The comparison with the MBH vs sigma relationship is less stringent (mostly due to the relatively large error in sigma); NGC 5252 is located above the best fit line by between 0.3 and 1.2 dex, i.e. 1 - 4 times the dispersion of the correlation. Both the galaxys and MBH of NGC 5252 are substantially larger than those usually estimated for Seyfert galaxies but, on the other hand, they are typical of radio-quiet quasars. Combining the determined MBH with the hard X-ray luminosity, we estimate that NGC 5252 is emitting at a fraction ~ 0.005 of L(Edd). In this sense, this active nucleus appears to be a quasar relic, now probably accreting at a low rate, rather than a low black hole mass counterpart of a QSO.

Hubble Space Telescope Near-Infrared Snapshot Survey of 3CR Radio Source Counterparts at Low Redshift

We present newly acquired images of the near-infrared counterpart of 3CR radio sources. All the sources were selected to have a redshift of less than 0.3 to allow us to obtain the highest spatial resolution. The observations were carried out as a snapshot program using the Near-Infrared Camera and Multiobject Spectrograph (NICMOS) on-board the Hubble Space Telescope (HST). In this paper we describe 69 radio galaxies observed for the first time with NICMOS during HST cycle 13. All the objects presented here are elliptical galaxies. However, each of them has unique characteristics such as close companions, dust lanes, unresolved nuclei, arclike features, globular clusters, and jets clearly visible from the images or with basic galaxy subtraction.

Infrared emission from the nearby cool core cluster abell 2597

We observed the brightest central galaxy (BCG) in the nearby (z = 0.0821) cool core galaxy cluster Abell 2597 with the IRAC and MIPS instruments on board the Spitzer Space Telescope. The BCG was clearly detected in all Spitzer bandpasses, including the 70 and 160 μm wave bands. We report aperture photometry of the BCG. The spectral energy distribution exhibits a clear excess in the far-IR over a Rayleigh-Jeans stellar tail, indicating a star formation rate of ~4-5 M☉ yr-1, consistent with the estimates from the UV and its Hα luminosity. This large far-IR luminosity is consistent with that of a starburst or a luminous infrared galaxy, but together with a very massive and old population of stars that dominate the energy output of the galaxy. If the dust is at one temperature, the ratio of 70 to 160 μm fluxes indicates that the dust emitting mid-IR in this source is somewhat hotter than the dust emitting mid-IR in two BCGs at higher redshift (z ~ 0.2-0.3) and higher far-IR luminosities observed earlier by Spitzer in clusters Abell 1835 and Zwicky 3146.