The Ionization State of the Halo Planetary Nebula NGC 2438

NGC 2438 is a classical multiple shell or halo planetary nebula (PN). Its central star and the main nebula are well studied. Also it was target of various hydrodynamic simulations (Corradi et al. 2000). This initiated a discussion whether the haloes are mainly containing recombined gas (Schoenberner et al. 2002), or if they are still ionized (Armsdorfer et al. 2003). An analysis of narrow-band images and long slit spectra at multiple slit positions was done to obtain a deeper look on morphological details and the properties of the outer shell and halo. For this work there was data available from ESO (direct imaging and long slit spectroscopy) and from SAAO (spectroscopic observations using a small slit - scanning over the whole nebula). Using temperature measurements from emission lines resulted in an electron temperature which clearly indicates a fully ionized stage. Additionally measurements of the electron density suggest a variation of the filling factor.Comment: 3 pages, 1 figure, to appear in proceedings of the IAU Symposium 283: "Planetary Nebulae: An Eye to the Future", Eds.: A. Manchado, L. Stanghellini and D. Schoenberne

Ionisation Feedback in Star and Cluster Formation Simulations

Feedback from photoionisation may dominate on parsec scales in massive star-forming regions. Such feedback may inhibit or enhance the star formation efficiency and sustain or even drive turbulence in the parent molecular cloud. Photoionisation feedback may also provide a mechanism for the rapid expulsion of gas from young clusters' potentials, often invoked as the main cause of 'infant mortality'. There is currently no agreement, however, with regards to the efficiency of this process and how environment may affect the direction (positive or negative) in which it proceeds. The study of the photoionisation process as part of hydrodynamical simulations is key to understanding these issues, however, due to the computational demand of the problem, crude approximations for the radiation transfer are often employed. We will briefly review some of the most commonly used approximations and discuss their major drawbacks. We will then present the results of detailed tests carried out using the detailed photoionisation code MOCASSIN and the SPH+ionisation code iVINE code, aimed at understanding the error introduced by the simplified photoionisation algorithms. This is particularly relevant as a number of new codes have recently been developed along those lines. We will finally propose a new approach that should allow to efficiently and self-consistently treat the photoionisation problem for complex radiation and density fields.Comment: Invited review presented at the IAU Symposium 270: Computational Star Formation held in Barcelona (May 31st- June 4th 2010) - Refereed paper version; 8 Pages, 4 Figure

Spectral classification of emission-line galaxies

A revised method of classification of narrow line active galaxies and H II region-like galaxies is proposed. It involves the line ratios (O III) lambda 5007/H beta, (N II) lambda 6583/H alpha, (S II) (lambda lambda 6716 = 6731)/H alpha, and (O I) lambda 6300/H alpha. These line ratios take full advantage of the physical distinction between the two types of objects and minimize the effects of reddening correction and errors in the flux calibration. Large sets of internally consistent data are used including new previously unpublished measurements. Prediction of recent photoionization models by power law spectra and by hot stars are compared with the observations. The classification is based on the observational data interpreted on the basis of these models

Coronal Line Emission from NLS1s

We discuss the optical coronal line spectra observed for a sample of 19 Narrow-Line Seyfert 1 galaxies. We find no correlation between the coronal line strength and the soft X-ray power-law index derived from ROSAT PSPC data. There is a trend for broader coronal lines to have larger equivalent widths. In addition, a strong trend is found between line width and velocity relative to the NLR. This trend is interpreted in terms of a decelerating outflow, originating close to the nucleus.Comment: Contributed talk presented at the Joint MPE,AIP,ESO workshop on NLS1s, Bad Honnef, Dec. 1999, to appear in New Astronomy Reviews; also available at http://wave.xray.mpe.mpg.de/conferences/nls1-worksho

Off-Axis Energy Variability of AGNs: a New Paradigm for Broad-Line- and Continuum-Emitting Regions

The general picture of how thermal AGNs work has become clearer in recent years but major observational puzzles threaten to undermine this picture. These puzzles include AGNs with extremely asymmetric emission line profiles, inconsistent multi-wavelength variability, rapid apparent changes in the sizes of emitting regions and in the direction of gas flow, a curious insensitivity of gas in some narrow velocity ranges to changes in the ionizing continuum, and differing dependencies of polarization on gas velocity. It is proposed that all these puzzles can readily be explained by off-axis variability, and that there is no need to invoke exotic explanations such as binary supermassive black holes or recoiling black holes.Comment: Invited talk given at 8th SCSLS. Accepted for publication in Baltic Astronomy, Vol. 20. 8 pages, 8 figure

The galaxy-wide distributions of mean electron density in the HII regions of M51 and NGC 4449

Using ACS-HST images to yield continuum subtracted photometric maps in H\alpha of the Sbc galaxy M51 and the dwarf irregular galaxy NGC 4449, we produced extensive (over 2000 regions for M51, over 200 regions for NGC4449) catalogues of parameters of their HII regions: their H\alpha luminosities, equivalent radii and coordinates with respect to the galaxy centers. From these data we derived, for each region, its mean luminosity weighted electron density, , determined from the H\alpha luminosity and the radius, R, of the region. Plotting these densities against the radii of the regions we find excellent fits for varying as R^{-1/2}. This relatively simple relation has not, as far as we know, been predicted from models of HII region structure, and should be useful in constraining future models. Plotting the densities against the galactocentric radii, r, of the regions we find good exponential fits, with scale lengths of close to 10 kpc for both galaxies. These values are comparable to the scale lengths of the HI column densities for both galaxies, although their optical structures, related to their stellar components are very different. This result indicates that to a first approximation the HII regions can be considered in pressure equilibrium with their surroundings. We also plot the electron density of the HII regions across the spiral arms of M51, showing an envelope which peaks along the ridge lines of the arms.Comment: Accepted for publication in ApJ Letters. 5 pages, 5 figures. Formatted with emulateap

Helium line detections from ELDWIM at 1.4 GHz

Helium line observations towards 11 Galactic positions using Westerbork Synthesis Radio Telescope(WSRT) have been reported. These observations were made towards nearby positions where already hydrogen lines were detected at sufficiently high intensity($\geq$50mK) at 1.4 GHz. This approach gave a fair chance for the detection of helium line as well, keeping in mind the relative abundance(10%) of helium with respect to hydrogen. Care was also taken to avoid the presence of HII regions along the line of sight so that the line emission originates from the extended diffuse low density ionized component, ELDWIM of the Galaxy. The observations have resulted in the detection of helium line towards 5 positions out of 11 with signal to noise ratio(snr) $>$ 4$\sigma$. An attempt has been made to associate detection/non-detection of helium line to the presence of surrounding HII regions. A weighting scheme that accounts for nearby($<$ 500pc) HII regions, their distances and other factors produces favourable results. It is seen from this weighting scheme that a higher weight favours the detection of helium line while lower weight is associated with non-detection. The idea is to correlate ionization of ELDWIM with the surrounding HII regions.Comment: 15 pages, 6 figures, 3 tables, Published with minor changes in The Astronomical Journa

Detection of infalling hydrogen in transfer between the interacting galaxies NGC 5426 and NGC 5427

Using velocity tagging we have detected hydrogen from NGC 5426 falling onto its interacting partner NGC 5427. Our observations, with the GHaFaS Fabry-Perot spectrometer, produced maps of the two galaxies in Halpha surface brightness and radial velocity. We found emission with the range of velocities associated with NGC 5426 along lines of sight apparently emanating from NGC 5427, superposed on the velocity map of the latter. After excluding instrumental effects we assign the anomalous emission to gas pulled from NGC 5426 during its passage close to NGC 5427. Its distribution, more intense between the arms and just outside the disk of NGC 5427, and weak, or absent, in the arms, suggests that the infalling gas is behind the disk., ionized by Lyman continuum photons escaping from NGC 5427. Modeling this, we estimate the distances of these gas clouds- behind the plane: a few hundred pc to a few kpc. We also estimate the mass of the infalling (ionized plus neutral) gas, finding an infall rate of 10 solar masses per year, consistent with the high measured SFR across the disk of NGC 5427 and with the detected circumnuclear galactic wind.Comment: 10 pages, 4 figures. Accepted for publication in ApJ

The contribution of Narrow-Line Seyfert 1 galaxies to the soft X-ray background

The ROSAT Ultradeep HRI survey in the Lockman Hole contains a complete sample of 91 X-ray sources with fluxes in the 0.5-2 keV band larger than 1.2 times 10e-15 erg cm-2 s-1, where over about 75 per cent of the sources are quasars or Seyfert galaxies. During the course of our optical identification work, we have obtained optical spectra of 67 narrow emission line galaxies (NELG), which are physically not associated with the X-ray sources. We have derived the equivalent width (EW) and the full width at half maximum (FWHM) for the most prominent emission lines of 41 quasars and Seyfert galaxies taken from the ROSAT Deep Survey (RDS), which has a flux limit of 5.5 times 10e-15 erg cm-2 s-1 in the 0.5-2.0 keV band. Furthermore we have obtained the EW and FWHM values of the field NELGs. Here we present the spectroscopic discrimination between RDS Seyfert galaxies and field galaxies (NELG). The analysis of the emission lines has revealed that a single object out of 69 spectroscopically identified AGN fits the optical criteria of Narrow-Line Seyfert 1 galaxies (NLS1). This may indicate that NLS1 contribute only marginally to the soft X-ray background, but we can not exclude a possible larger contribution.Comment: Invited talk presented at the Joint MPE,AIP,ESO workshop on NLS1s, Bad Honnef, Dec. 1999, to appear in New Astronomy Reviews; also available at http://wave.xray.mpe.mpg.de/conferences/nls1-worksho

Broad Components in Optical Emission Lines from the Ultra-Luminous X-ray Source NGC 5408 X-1

High-resolution optical spectra of the ultraluminous X-ray source NGC 5408 X-1 show a broad component with a width of ~750 km/s in the HeII and Hbeta lines in addition to the narrow component observed in these lines and [O III]. Reanalysis of moderate-resolution spectra shows a similar broad component in the HeII line. The broad component likely originates in the ULX system itself, probably in the accretion disk. The central wavelength of the broad HeII line is shifted by 252 \pm 47 km/s between the two observations. If this shift represents motion of the compact object, then its mass is less than ~1800 M_sun.Comment: 5 pages, 3 figures, accepted for publication in ApJ Letter