Optical jets associated with young stars

Burrows and collaborators have obtained WFPC2 images of a number of young stars that, from their integrated mm emission, are known to have circumstellar material (Burrows et al., 1996). In all cases, this material is visible in scattered light. It can be seen directly how the material is distributed, and its relationship to the associated optical jets and radio outflows is apparent. A particularly appealing case is the circumstellar environment around the young star HH30. In this object (Fig. 4.3.4/1), there is little confusion from an extended envelope, and one can clearly see the accretion disc that is feeding the protostar and the bipolar collimated jets that are shown to be within a few degrees of perpendicular to the disc in 3-D. This beautifully confirms and illustrates the standard paradigm for the formation of new stars. Detailed models of the scattering of the stellar light in this disc has yielded a constraint on the mass distribution within the disc. The disc appears to be pressure-supported and flares in a manner consistent with its being isothermal and accreting at a constant rate.

Figure 4.3.4/1: Hubble Space Telescope image of HH30, a young stellar object clearly showing an accretion disc and an ionised jet.

Reference
Burrows, C.J., Stapelfeldt, K.R., Watson, A.M., Krist, J., Ballester, G.E., Clarke, J.T., Crisp, D., Gallagher, J.S., III, Griffiths, R.E., Hester, J. J., Hoessel, J.G., Holtzman, J.A., Mould, J.R., Scowen, P.A., Trauger, J.T. & Westphal, J.A. (1996). Astrophys. J., in press.

Circumstellar disc of Beta Pictoris

The main sequence star Beta Pictoris has been the subject of intense scrutiny by Burrows and collaborators since it was found by IRAS to be surrounded by circumstellar material (Burrows et al., 1995). Coronagraphic imaging from the ground showed that the material is distributed in a disc and consists of relatively large particles. There is a clear zone within about 30 AU of the star. New HST images obtained by Burrows (Fig. 4.3.4/2) show that the disc is warped. The simplest explanation for the warp is a Jupiter-size planet causing the disc axis to align with the orbital pole of the planet. This process would warp the disc, as the outer disc has not had time to align in the 200 million year lifetime of the star. The images reveal the inner regions of the disc for the first time. It is likely that the material seen is the product of collisions between comet nucleus-like bodies with minimum diameters of around 10 km.

Figure 4.3.4/2: Hubble Space Telescope image of the inner part of the remnant disc surrounding the nearby star beta Pictoris.

Reference
Burrows, C.J., Krist, J., Stapelfeldt, K.R., Ballester, G.E., Casertano, S., Crisp, D., Gallagher, J.S., III, Griffiths, R.E., Hoessel, J.G., Holtzman, J.A., Scowen, P.A., Trauger, J.T. & Westphal, J. A. (1995). BAAS 187, 3205.

Faint galaxies at high redshifts

Using a large number of deep HST images, Casertano, in collaboration with the group led by R.E. Griffiths, has studied the morphology, magnitude, size and colour of faint galaxies (I=20-26 mag) at redshift z~0.5 (Casertano et al., 1995; Im et al., 1995). Faint galaxies appear essentially normal, covering the whole range of morphological types found at low redshift, but the number of irregular galaxies seems to increase at faint magnitudes. The size and morphology distribution indicates that a significant amount of luminosity evolution has occurred in the last few billion years, possibly stronger for low luminosity galaxies. Another result obtained from the distribution of galaxies in the sky is that the fraction of galaxies in pairs does not increase substantially at high redshift, thus indicating that major merging is probably not the mainspring of galaxy evolution (Neuschaefer et al., 1995).

References
Casertano, S., Ratnatunga, K.U., Griffiths, R.E., Im, M., Neuschaefer, L.W., Ostrander, E.J. & Windhorst, R.A. (1995). Astrophys. J. 453, 599.
Neuschaefer, L.W., Ratnatunga, K.U., Griffiths, R.E., Casertano, S. & Im, M. (1995). Astrophys. J. 453, 559.

Nuclear radio sources in elliptical galaxies

Colina continues to uncover the effects of the collision of ellipticals in the onset of nuclear radio sources. The first detailed characterisation of Fanaroff-Riley type I (FR I) radio source host galaxies was undertaken in collaboration with L. de Juan (Univ. Autonoma, Madrid). FR I host galaxies turn out to be mostly ellipticals involved in collisions. The collisions are characterised by:

• a mass ratio (companion to FR I host galaxy) of 0.2
• a projected distance of 23 kpc
• a velocity of 490 km/s
• an age of 40 million years (Colina & de Juan, 1995).

The first deep high resolution image of the hot X-ray emitting gas in a FR I interacting host galaxy was obtained in collaboration with K. Borne (Goddard Space Flight Center) using ROSAT. The image reveals for the first time the presence of X-ray tails and bridge connecting the two galaxies. Also, the hot gas distributions do not peak at the optical centres of the galaxies but are displaced in the same sense as the tidal distensions seen in the stellar component. These features show for the first time the complexity of structure that develops in the hot gas distribution when both hydro-dynamical and tidal forces come into play during collisions between ellipticals (Colina & Borne, 1995).

References
Colina, L. & Borne, K. (1995). Astrophys. J. 454, L101.
Colina, L. & de Juan, L. (1995). Astrophys. J. 448, 548.

Properties of starburst galaxies

Colina continues to investigate the radio and IR properties of compact dusty starburst galaxies. The first complete set of evolutionary models to investigate the temporal evolution of the radio and far-IR properties in compact starbursts was developed in collaboration with D. Perez-Olea (Univ. Autonoma, Madrid). The models (Perez-Olea & Colina, 1995; Colina & Perez-Olea, 1995) take into account for the first time the contribution of young bright radio supernovae. Both instantaneous and continuous bursts are considered for a wide variety of scenarios characterised by different metallicities and initial mass function parameters. The outputs of the models have a direct applicability in the study of dusty, star-forming environments like those present in starburst galaxies and luminous IR galaxies. The X-ray emission in active galaxies, ranging from pure active galactic nuclei (AGN) to pure starburst galaxies, and composite AGN plus starburst galaxies, has been investigated in collaboration with D. Perez-Olea (Univ. Autonoma, Madrid) using own and published ROSAT high resolution images (Perez-Olea & Colina, 1996). Circumnuclear starburst rings can contribute up to 20% of the total soft X-ray emission in composite active galaxies, whereas pure starbursts are well differentiated from pure AGN. For a given optical luminosity, AGN are 100 times more powerful than pure starbursts in X-rays. In collaboration with M. Mas-Hesse (INTA-LAEFF, Madrid), M. Garcia-Vargas (IUE-VILSPA), A. Krabbe (MPE, Munich) and A. Alberdi (INTA-LAEFF, Madrid), L. Colina started an HST snapshot project to image circumnuclear starbursts in the UV. Eight galaxies have so far been observed with the WFPC2 camera, resulting in the detection of several circumnuclear star-forming regions in four of the galaxies. A detailed analysis of the images is in progress.

References
Colina, L. & Perez-Olea, D. (1995). MNRAS 277, 845.
Perez-Olea, D. & Colina, L. (1995). MNRAS 277, 857.
Perez-Olea, D. & Colina, L. (1996). Astrophys. J. 468, 191.

Active Galactic Nuclei

A large research programme on the properties of AGN and the optical counterparts to radio jets has been conducted by a team led by Macchetto. The group included D. Axon, an ESA Fellow, and A. Capetti, as well as several outside collaborators: W.B. Sparks, J. Biretta and S. Baum at STScI, and A. Boksenberg at the Royal Greenwich Observatory (Cambridge). In all these studies, the HST has become one of the most important instruments to investigate the physical nature of AGN. High spatial and spectral resolution observations of a wide variety of sources have been carried-out. These include LINERS, Seyferts I and II, as well as powerful radio sources. The HST results are in broad agreement with the unified theories for AGN. However, these results have also made important modifications to the basic theory, such as the need for additional sources of ionisation (e.g. shocks) to explain the observed filamentary structure in the narrow-line region. In particular, Macchetto, Axon, Capetti and Sparks have carried out medium-band imaging of four Seyfert II galaxies, Mrk 573, Mrk 348, Mrk 3 and Mrk 78, which show extended radio emission, and obtained ground-based Johnson R images of Mrk 3 and Mrk 78 with the William Herschel Telescope. Together, these two data sets allowed them to investigate the relationships between the narrow-line region (NLR) emission structure, the radio structure and the large-scale morphology of the host galaxy (Capetti et al., 1996, and in preparation). In all four objects, there is a close association between the NLR emission-line morphology and that of the radio emission. Furthermore, it appears that the NLR takes a different form depending on the structure of the radio emission. The emission line regions associated with the radio lobes are shell-like or bow shock-like, while those associated with the jets are linear. There is no evidence for an unresolved bright core that could be identified as the active nucleus in any of these galaxies. From this, they conclude that the nucleus must be hidden along the line of sight. In all four objects, an absorption lane crossing the nuclear region has been detected. In Mrk 348, Mrk 573 and Mrk 3, the typical scale height of the dust lane is less than 50 pc, while in Mrk 78, it is 180 pc. They conclude that these dust lanes are associated with the obscuring torus that occults the Seyfert nucleus.

Reference
Capetti, A., Macchetto, D., Sparks, W. B. & Boksenberg, A. (1995). Astrophys. J. 448, 600.

Interstellar medium in elliptical galaxies

As the PI of an ESO Key Programme, D. Macchetto in collaboration with N. Caon (ESA Fellow), M. Pastoriza (Porto Alegre, Brazil), W. B. Sparks (STScI), R. Bender (Munich, Germany) and M. Capaccioli (Napoli, Italy), has studied the properties of the interstellar medium in elliptical galaxies (Caon, Macchetto & Pastoriza, 1995; Macchetto et al., 1996). They carried out a CCD optical imaging survey of the ionised gas in 73 luminous elliptical and lenticular galaxies, selected from the RC3 catalogue to represent a broad variety of X-ray, radio, IR and kinematic properties. For each galaxy they used broad-band and narrow-band images centred at the H alfa and [NII] emission lines to derive the luminosity and distribution of the ionised gas. They found that a large fraction of E (72%) and SO (85%) galaxies contain ionised gas. The gas morphology appears to be rather smooth for most galaxies; however, 12% of the sample galaxies show a very extended filamentary structure. According to the morphology and size of the gas distribution, the galaxies have been classified into three broad groups: small disc (SD), regular extended (RE) and filamentary structure (F). The mean diameter of the emitting region ranges between 1 kpc and 10 kpc; the derived mass of the ionised gas ranges between 10³ and 10⁵M_Sun. A significant correlation between H alfa+[NII] and X-ray luminosities is found for those galaxies (27% of the sample) for which they have detected ionised gas and are also listed as X-ray sources. However, there are relatively strong X-ray emitting galaxies for which they have not detected H alfa+[NII] emission, and objects that show emission lines but are not listed either in the Einstein or ROSAT databases. The distribution of data points and upper limits in this diagram suggests that galaxies with warm gas are also X-ray emitters, while there are X-ray emitters without measurable H alfa+[NII] emission. Similar characteristics are present in the correlation between the IR luminosity in the 12 µm band and L_{H alfa+[NII]}. A strong correlation was also found between the H alfa+[NII] luminosity and the luminosity in the B band inside the region occupied by the line-emitting gas. They use these correlations to discuss the possible mechanisms responsible for the gas ionisation and excitation, analysing in particular the role of the post-Asymptotic Giant Branch (post- AGB) stars and the thermal conduction from the X-ray halo in providing the necessary source of ionisation.

References
Caon, N., Macchetto, D. & Pastoriza, M. (1995). The Interstellar Medium in Elliptical Galaxies, IAU Symposium 171, 26-30 June, University of Heidelberg. Macchetto, D., Pastoriza, M., Caon, N., Sparks, W.B., Giavalisco, M., Bender, R. & Capaccioli, M. (1996). Astron. Astrophys. 120, 463.

Studies of Luminous Blue Variables

Nota has continued her research into large- and small-scale structures in Luminous Blue Variable (LBV) nebulae. Very massive stars (M_ZAMS>25M_Sun )evolve at roughly constant bolometric luminosity throughout their various phases. O stars, WN stars, LBVs and possibly also blue and red supergiants, represent different evolutionary phases of a star with a certain ZAMS (Zero Age Main Sequence) mass. A primary goal in stellar physics is to investigate the correct chronological order of these various phases. However, this goal has not yet been fully achieved. At some stage, O stars with masses of about 20-120M_Sun have to undergo extremely high mass-loss rates and lose several solar masses. This might happen in the very short-lived (=~10⁴ years) evolutionary transition identified as the LBV phase. LBVs are indeed well known to be surrounded by ring nebulae composed of stellar ejecta containing several solar masses (Nota et al., 1995). A simple 2-D hydrodynamic simulation by Nota et al. (1995) has demonstrated that a morphology similar to that of R127 can be easily obtained from such a model. It has also been shown by Garcia-Segura & MacLow (1995) using multi-dimensional hydrodynamic calculations for interacting steady winds, that the succession of massive star winds, which corresponds to the different evolutionary stages of the central star, provides very different morphologies of the circumstellar nebulae for different evolutionary sequences. These can be easily identified both in the large- and small-scale structure of the circumstellar nebulae. In the large-scale structure, the circumstellar shell is swept up by a slow wind, followed by a fast wind e.g. during and after the LBV phase (Garcia-Segura et al., 1995; Nota et al., 1995). Since the occurrence of each type of instability requires specific conditions in the interacting gas components, the detection of small-scale structures in circumstellar nebulae, together with the large scale dynamical behaviour, can yield rigorous conclusions about the winds that formed them. A comparison of high resolution HST images with the predictions of multi- dimensional, hydrodynamical models has been instrumental in establishing the reality of this hypothesis, and has been used to constrain, quantitatively, the wind velocity and mass-loss histories of the star. A recent experiment conducted on recent HST images of the galactic LBV AG Carinae has proved the method is effective and has demonstrated its potential as a diagnostic tool for empirically reconstructing the mass loss history and, thereby, the evolutionary history of the central star.

References
Nota, A., Livio, M., Clampin, M. & Schulte-Ladbeck, R. (1995). Astrophys. J. 448, 788.
Nota, A., Clampin, M., Garcia-Segura, G., Leitherer, C. & Langer, N. (1996). Large and Small Scale Structures in the AG Carinae Nebula from WFPC2/HST Observations. In Science with the Hubble Space Telescope II, (Eds. P. Benvenuti, F.D. Macchetto & E.J. Schreier), STScI, p.398.

The Hubble Constant and the age of the Universe

In the frame of an international collaboration, which includes A. Sandage (OCIW, Pasadena), G.A. Tammann and L. Labhardt (Basel University) and A. Saha (STScI), Macchetto and Panagia have used the Hubble Space Telescope to calibrate Type Ia Supernovae (SNe Ia) as standard candles and, from there, to determine the value of the Hubble constant that measures the local expansion of the Universe. The method is based on determining accurate distances (accuracy better than 10%) to parent galaxies of SNe Ia by discovery and photometry, done with HST, of Cepheid variables. In this way, the absolute luminosity of SNe Ia is measured with high precision (i.e. to within ±0.1 magnitudes). This value is then used to estimate accurate distances to distant SNe Ia, i.e. super-novae that have occurred in galaxies with recession velocities in excess of several thousand km/s for which the effects of peculiar local motions are negligible. Finally, a simple linear regression between velocity and distance provides the value of the local expansion rate, i.e. the Hubble constant H₀. HST has so far provided Cepheid distances to seven Type Ia supernovae in six distinct galaxies, leading to mean absolute magnitudes M_B=-19.53±0.07 and M_V=-19.49±0.07 (Saha et al., 1995, 1996; Sandage et al., 1996; Tammann et al., 1996). Direct comparison of these values with the Hubble diagrams for distant SNe Ia implies a Hubble constant of H₀=57±3 km/s/Mpc. After allowing for systematic effects, such as a dependence of the SN Ia peak brightness on the light curve decline rate and allowance for reddening towards distant SNe Ia, one obtains a maximum value of the Hubble constant of H₀(max)=63±5 km/s/Mpc. This result demonstrates that there is no discrepancy between the age of the Universe and its expansion rate.

References
Saha, A., Sandage, A., Labhardt, L., Schwengeler, H., Tamman, G.A., Panagia, N. & Macchetto, F.D. (1995). Astrophys. J. 438, 8.
Saha, A., Sandage, A., Labhardt, L., Tammann, G.A., Macchetto, F.D. & Panagia, N. (1996). Astrophys. J. 466, 55.
Sandage, A., Saha, A., Tammann, G.A., Labhardt, L., Panagia, N. & Macchetto, F.D. (1996). Astrophys. J. 460, L15.
Tammann, G.A., Labhardt, L., Federspiel, M., Sandage, A., Saha, A., Macchetto, F. D. & Panagia, N. (1995). In Science with the Hubble Space Telescope II, (Eds. P. Benvenuti, F. D. Macchetto & E. J. Schreier), STScI, p.9.

The nuclear region of the spiral galaxy M51

Panagia carried out a detailed study of the dust and star distribution and properties in the nuclear region of M51. The spiral galaxy M51 was observed with the Hubble Space Telescope Wide Field Camera as part of the Supernova Intensive Study (SINS) programme to determine accurate UV (early epoch) and optical (late epoch) magnitudes of supernova 1994I, which occurred about 15 arcsec offset from M51's nucleus.

From an analysis of the colour-magnitude and colour-colour diagrams for the nuclear region of M51, Panagia and collaborators (Panagia et al., 1995, 1996; Scuderi et al., 1996, in preparation) find that the extinction in the nuclear core is patchy (Fig. 4.3.4/3, left panel) and is maximum on the Y-shaped dark lane. They estimate that the total mass in dust grains over the nuclear core (a region 0.83 arcsec in radius) is about 330 M_Sun, which is about 10^-5 the mass in stars (see below). Such a low content of dust (approximately 100 times lower than in the solar neighbourhood) indicates that dust destruction processes have prevailed over formation processes during the lifetime of the nuclear core. The stellar population in the core (<2 arcsec diameter) is much bluer than that in the surrounding bulge (an oval region of 10-15 arcsec). A detailed study of the spectral distributions indicates that, while the bulge population is older than 8 Gyears, the stars in the core have an age of ~450 Myears. The total luminosity of the core, i.e. inside a 0.83 arcsec radius (30 pc for an adopted distance of 7.5 Mpc), is L_tot=~8x10⁷L_Sun, and the total stellar mass in the core is M_tot=~4x10⁷M_Sun. This corresponds to an average stellar density of 350 M_Sun/pc², which is about 3000 higher than in the solar neighbourhood. After allowance for extinction (Fig. 4.3.4/3, right panel), the starlight distribution in the inner nucleus appears to be almost perfectly circular and displays a distinct light peak in the middle. Panagia and collaborators believe that the central point-like source is the very nucleus of M51, which has a size smaller than 2 pc and a luminosity of about 10⁶L_Sun. The occurrence of a 'middle age' starburst about 450 million years ago is the possible effect of the last 'close encounter' of M51 with its dwarf companion NGC 5195.

Figure 4.3.4/3: Hubble Space Telescope image of the inner core (the central 4.6 4.6 arcsec area) of M51 obtained in the visual. The left panel displays the observed image, and the right panel the one corrected for dust extinction.

References
Panagia, N., Capetti, A., Scuderi, S., Lamers, H. & Kirshner, R.P. (1995), BAAS 27, 1345.
Panagia, N., Capetti, A., Scuderi, S., Lamers, H. & Kirshner, R.P. (1996). Dust and Stars in the Nuclear Region of M51. In Cold Dust in Galaxies, (Ed. D. Block), Kluwer Academic Publ., Dordrecht, p.234.

Circumstellar material

Studies of circumstellar material are being carried out by Skinner, in collaboration with M.J. Barlow, R.J. Sylvester, S.K. Dunkin and A.K. Speck (University College London), M. Meixner (University of Illinois), A. Dayal and W.F. Hoffmann (Steward Observatory), J.L. Hora (University of Hawaii), K. Justtanont (Groningen) and A.G.G.M. Tielens (NASA-Ames). In particular, a full radiative transfer model for axisymmetric dust shells has been developed, and applied to the well known protoplanetary nebula AFGL2688 (Skinner et al., 1996). This work implies that the toroidal dust nebula around the star must be optically thick even at wavelengths as long as 10 µm, and shows that the star must have departed the AGB with a tremendous burst of mass loss, or superwind, which was equatorially concentrated. This work forms part of a large survey of post-AGB stars, in which mid-IR (8-24 m) images and spectra have been obtained at UKIRT and the IRTF (Skinner et al., 1995a).

So far, about 60 objects have been observed, and almost half are resolved in the mid-IR images. The resolved images appear to show that every source has an equatorially concentrated superwind; none shows spherical symmetry. This is an interesting reversal of a frequently asked question: why are so many planetary nebulae axisymmetric, and not spherically symmetric?. Images and models have been constructed of a group of five C-rich post-AGB stars displaying a strong spectral feature at 21 m which may be due to polycyclic aromatic hydrocarbons (PAHs). The same model is currently being adapted to model dusty discs around Vega-excess stars. Multi-wavelength observations have been made of a sample of about 20 of these sources, covering the wave- length range from the visible to the mm region (Sylvester et al., 1996). Optically thin dust disc models have been constructed of most of these sources, showing that a very large range of dust grain sizes must be present, from small nm sizes up to the mm range, and possibly larger (Sylvester & Skinner, 1996). For some sources, the optically thin models suggest that the optical depth in the disc plane may be quite large, and so full radiative transfer models are in the process of being constructed. Mid-IR images of one of these sources have been published recently (Skinner et al., 1995b) and further mid-IR images were obtained during June 1996 at the AAT.

References
Skinner, C.J., Meixner, M., Barlow, M. J., Justtanont, K., Arens, J.F. & Jernigan, J.G. (1995a). Astrophys. Space Sci. 224, 383.
Skinner, C.J., Sylvester, R.J., Graham, J.R., Barlow, M. J., Meixner, M., Keto, E., Arens, J.F. & Jernigan, J.G. (1995b). Ap. J. 444, 861-873.
Sylvester, R.J. & Skinner, C. J. (1996). MNRAS 283, 457.
Sylvester, R.J., Skinner, C.J., Barlow, M.J. & Mannings, V. (1996). MNRAS 279, 915.
Skinner, C.J., Meixner, M., Barlow, M.J., Collison, A.J., Justtamont, K., Blanco, P., Pina, R., Ball, J.R., Kato, E., Arens, J.F. & Jernigan, J.G. A&A, in press.

Studies of galaxies

Stiavelli has done a number of studies on fundamental aspects of structure and nature of galaxies: an IR study of the M87 jet and radiolobe, an HST study of the core properties of spiral galaxies, and a study of galaxy formation. For M87, the deepest K band image of the radiolobe was obtained and used to constrain theoretical models of the synchrotron emission. The data showed that the known apparent shift of the radio to optical spectral index in the jet is actually due to a shift in the break frequency of relativistic electrons (Stiavelli, Peletier & Carollo, 1996). Images from an ongoing HST snapshot survey of the bulges of spiral galaxies were analysed. It was found that many early type spiral galaxies are possibly misclassified and do not seem to posses a bright bulge, and that spiral structure often extends down to the centre. A comparison of quantitative properties of the cores of spirals and ellipticals is under way. The study of galaxy formation was carried out both observationally, addressing the evolution with redshift of the fundamental plane of elliptical galaxies, and theoretically, by studying models of hierarchical clustering.

Reference
Stiavelli, M., Peletier, R.F. & Carollo, C.M. (1996). MNRAS, in press.