arXiv subjects

J. R. Walsh

Publications and source records attributed to J. R. Walsh.

64 records · Page 4Linked to original sources

arXiv1999

Acquisition and analysis of adaptive optics imaging polarimetry data

The process of data taking, reduction and calibration of near-infrared imaging polarimetry data taken with the ESO Adaptive Optics System ADONIS is described. The ADONIS polarimetric facility is provided by a rotating wire grid polarizer. Images were taken at increments of 22.5 degrees of polarizer rotation from 0 to 180 degrees, over-sampling the polarization curve but allowing the effects of photometric variations to be assessed. Several strategies to remove the detector signature are described. The instrumental polarization was determined, by observations of stars of negligible polarization, to be 1.7% at J, H and K bands. The lack of availability of unpolarized standard stars in the IR, in particular which are not too bright as to saturate current IR detectors, is highlighted. The process of making polarization maps is described. Experiments at restoring polarimetry data, in order to reach diffraction limited polarization, are outlined, with particular reference to data on the Homunculus reflection nebula around Eta Carinae.

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arXiv1999

Spectra of planetary nebulae in NGC 5128 (Centaurus-A)

Low dispersion spectra have been obtained of five planetary nebulae in the elliptical galaxy NGC 5128 (Centaurus-A) from the catalogue of Hui et al. (1993). The planetary nebulae (PN) cover a range of galactocentric radius from 7.9 to 17.7 arcmin (8 to 18Kpc). The spectra display typical emission lines of H I, He I, He II, [O III], [N II] and [S II] and appear very similar to high excitation planetary nebulae in the Galaxy. This implies that, from a stellar evolution viewpoint, there should be no peculiar effects introduced by considering the bright cut-off of the PN luminosity function for distance estimation. In particular the brightest PN detected in NGC 5128 is not spectroscopically unusual. One of the PN shows relatively strong He II and [N II] lines and the derived N/O ratio indicates that it may be a Type I nebula, considered to arise from a high mass progenitor star. Determinations of the oxygen abundance of the five PN shows a mean value 0.5 dex below solar. Given that NGC 5128 is an elliptical galaxy with a presumably metal rich stellar content, the low metallicities of the PN are unexpected, although a similar situation has been observed in the bulge of M 31.

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arXiv1999

Bok Globules in the LMC

We report the discovery of small, isolated dust clouds in the Large Magellanic Cloud, which are excellent candidates for counterparts to the Bok globules observed in the Galaxy. We detect these small clouds silhoutted against diffuse H-alpha emission, based on parallel imaging with the WFPC-2 on HST. The clouds we identify as Bok globule candidates have typical sizes of approximately one arcsecond, corresponding to about 0.25 parsec linear diameter at the distance of the LMC. We derive lower limits to the optical depth within the dark clouds, and masses assuming that the clouds have density distributions similar to Galactic Bok globules. The sizes and estimated masses for LMC globules are comparable to those estimated for Galactic globules. An extend sample of such objects would be excellent targets for high-resolution infrared and millimeter observations to study low-mass star formation in such clouds in low-metallicity environments, and where the distance is well known.

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arXiv1997

The STIS Parallel Survey: Introduction and First Results

The installation of the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST) allows for the first time two-dimensional optical and ultraviolet slitless spectroscopy of faint objects from space. The STIS Parallel Survey (SPS) routinely obtains broad band images and slitless spectra of random fields in parallel with HST observations using other instruments. The SPS is designed to study a wide variety of astrophysical phenomena, including the rate of star formation in galaxies at intermediate to high redshift through the detection of emission-line galaxies. We present the first results of the SPS, which demonstrate the capability of STIS slitless spectroscopy to detect and identify high-redshift galaxies.

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arXiv1997

Detection of WR stars in the metal-poor starburst galaxy IZw 18

Wolf-Rayet stars (WR) have been detected in the NW region of the metal-poor starburst galaxy IZw 18. The integrated luminosity and FWHM of the bumps at 4650 A and 5808 A are consistent with the presence of a few individual stars of WC4 or WC5 type. Evolutionary synthesis models predict few WRs in this galaxy, but only of WN type. The presence of WC stars at such low metallicity could however be explained by high mass loss rates, which would constrain the IMF upper mass cut-off in IZw 18 to be higher than 80 Msol or alternatively favor a binary channel for WR formation. WC stars could also explain the strong and narrow HeII 4686 A emission line which peaks co-spatially with the WR bump emission, as suggested by Schaerer (1996). This detection shows that WR stars, even of WC type, are formed at metallicities below 1/40th solar.

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arXiv1997

Chemical Abundances of Planetary Nebulae in the Sagittarius Dwarf Elliptical Galaxy

Spectrophotometry and imaging of the two planetary nebulae He2-436 and Wray16-423, recently discovered to be in the Sagittarius dwarf elliptical galaxy, are presented. Wray16-423 is a high excitation planetary nebula (PN) with a hot central star. In contrast He2-436 is a high density PN with a cooler central star and evidence of local dust, the extinction exceeding that for Wray16-423 by E(B-V)=0.28. The extinction to Wray16-423, (E(B-V)=0.14), is consistent with the extinction to the Sagittarius (Sgr) Dwarf. Both PN show Wolf-Rayet features in their spectra, although the lines are weak in Wray16-423. Images in [O III] and H-alpha+[N II], although affected by poor seeing, yield a diameter of 1.2'' for Wray16-423 after deconvolution; He~2-436 was unresolved. He2-436 has a luminosity about twice that of Wray16-423 and its size and high density suggest a younger PN. In order to reconcile the differing luminosity and nebular properties of the two PN with similar age progenitor stars, it is suggested that they are on He burning tracks The abundance pattern is very similar in both nebulae and shows an oxygen depletion of -0.4 dex with respect to the mean O abundance of Galactic PN and [O/H] = -0.6. The Sgr PN progenitor stars are representative of the higher metallicity tail of the Sgr population. The pattern of abundance depletion is similar to that in the only other PN in a dwarf galaxy companion of the Milky Way, that in Fornax, for which new spectra are presented. However the abundances are larger than for Galactic halo PN suggesting a later formation age. The O abundance of the Sgr galaxy deduced from its PN, shows similarities with that of dwarf ellipticals around M31, suggesting that this galaxy was a dwarf elliptical before its interaction with the Milky Way.

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arXiv1997

The Abundance Gradient of NGC 1365: Evidence for a Recently Formed Bar in an Archetype Barred Spiral Galaxy?

Emission-line optical spectrophotometry for 55 H II regions in the prominent southern barred spiral galaxy NGC 1365 is presented. Nebular diagnostic diagrams such as [N II]/[O II] and [S II]/[O II] versus [O II]/{O III] show that the H II regions of the barred galaxy have the same range of physical conditions as found in non-barred late-type galaxies. Extinction is moderately high across the disc and there is evidence for a slight trend of extinction with galactocentric distance; the logarithmic extinction at H-beta falls from about c = 1.2 in the centre to 0.6 -- 0.8 in the outer regions. The global O/H distribution has a moderate gradient of about -0.5 dex/rho0 (about -0.02 dex/kpc) consistent with the known trend between the slope of the abundance gradient and the strength of the bar. A break is seen in the O/H gradient just beyond the -4/1 resonance, the gradient being moderately steep at about -0.8 dex/rho0 (-0.05 dex/kpc) inside this resonance, and flat beyond rho/rho0 > 0.55. The abundance distribution is compared with another barred spiral galaxy, NGC 3359, and with that of two well-sampled normal spiral galaxies, NGC 2997 and M 101. The possibility that the bar formed recently in NGC 1365 is considered. The difficulties encountered in doing spectrophotometry with fibre optics are discussed and shown not to be insurmountable.

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arXiv1997

The O/H Distribution in the Transition Magellanic Galaxy NGC 1313

Multi-fibre emission-line spectrophotometry of 33 H II regions and 3 diffuse interstellar medium positions are presented for the barred Magellanic galaxy NGC 1313. The H II regions show a fairly narrow range of thermal conditions characteristic of high excitation nebular gas. Electron temperature was directly determined in four of the H II regions. The global O/H abundance distribution appears very flat across the disk at 12 + log O/H = 8.4 +/- 0.1, with possibly the bar regions having abundances higher by 0.2 dex than the outer disc. NGC 1313 is the highest mass barred galaxy known not to have any radial abundance gradient. The key role of the bar on the abundance distribution in disc galaxies is revisited.

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arXiv1996

Two planetary nebulae in the Sagittarius Dwarf Galaxy

Two planetary nebulae are shown to belong to the Sagittarius Dwarf Galaxy, on the basis of their radial velocities. This is only the second dwarf spheroidal galaxy, after Fornax, found to contain planetary nebulae. Their existence confirms that this galaxy is at least as massive as the Fornax dwarf spheroidal which has a single planetary nebula, and suggests a mass of a few times 10**7 solar masses. The two planetary nebulae are located along the major axis of the galaxy, near the base of the tidal tail. There is a further candidate, situated at a very large distance along the direction of the tidal tail, for which no velocity measurement is available. The location of the planetary nebulae and globular clusters of the Sagittarius Dwarf Galaxy suggests that a significant fraction of its mass is contained within the tidal tail.

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arXiv1996

Highly supersonic motions within the outer features of the Eta Carinae nebulosity

Spatially resolved H$α$ and [N{\sc ii}] line profiles have been obtained over striking features in the outer regions of the Eta Carinae nebula. The highly irregular outer `shell' of low ionization, [N{\sc ii}] bright, knots has been shown to exhibit radial velocities between -1200 and +300 km/s with respect to the systemic radial velocity, over its perimeter. Furthermore, several knots have been found which appear to emit only in the H$α$ line with radial velocities up to -1450 km/s. However, the most intriguing features are a narrow `spike' that projects through this outer shell and a faint `arc' of emission that extends well beyond it. The `spike', which exhibits a change of radial velocity along its length, could be a narrow collimated jet with an outflow velocity of > 1000 km/s. In one interpretation the `arc' is modelled by a conical outflow and mechanisms for generating [N{\sc ii}] emission from highly supersonic gas are also discussed.

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