arXiv subjects

J. R. Walsh

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

At least 37 records · Page 2Linked to original sources

arXiv2011

The Expansion Proper Motions of the Planetary Nebula NGC 6302 from HST imaging

Planetary nebulae expand on time scales of 10^3-10^4 yr. For nearby objects, their expansion can be detected within years to decades. The pattern of expansion probes the internal velocity field and provides clues to the nebula ejection mechanism. In the case of non-symmetric nebulae, and bipolar nebulae in particular, it can also provide information on the development of the morphology. We have measured the expansion proper motions in NGC 6302 from two epochs of HST imaging, separated by 9.43 years. This is used to determine the expansion age and the structure of the velocity field. We use HST images in the [N II] 6583Å filter from HST WF/PC2 and WFC3. The proper motions were obtained for a set of 200 individual tiles within 90" of the central star. The velocity field shows a characteristic linear increase of velocity with radial distance (a so-called Hubble flow). It agrees well with a previous determination by Meaburn et al. (2008), made in a lobe further from the star, which was based on a much longer time span. The pattern of proper motion vectors is mostly radial and the origin is close to the position of the central star directly detected by Szyszka et al. (2009). The results show that the lobes of NGC 6302 were ejected during a brief event 2250 \pm 35yr ago. In the inner regions there is evidence for a subsequent acceleration of the gas by an additional 9.2 km/s, possibly related to the onset of ionization. The dense and massive molecular torus was ejected over 5000yr, ending about 2900yr ago. The lobes were ejected after a short interlude (the 'jet lag') of \sim 600 yr during a brief event. The torus and lobes orig- inate from separate mass-loss events with different physical processes. The delay between the cessation of equatorial mass loss and the ejection of the lobes provides an important constraint for explaining the final mass-loss stages of the progenitor stellar system.

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arXiv2011

The Hubble Legacy Archive ACS Grism Data

A public release of slitless spectra, obtained with ACS/WFC and the G800L grism, is presented. Spectra were automatically extracted in a uniform way from 153 archival fields (or "associations") distributed across the two Galactic caps, covering all observations to 2008. The ACS G800L grism provides a wavelength range of 0.55-1.00 μ$m, with a dispersion of $40 \ Å/ pixel$ and a resolution of $\sim 80\ Å$ for point-like sources. The ACS G800L images and matched direct images were reduced with an automatic pipeline that handles all steps from archive retrieval, alignment and astrometric calibration, direct image combination, catalogue generation, spectral extraction and collection of metadata. The large number of extracted spectra (73,581) demanded automatic methods for quality control and an automated classification algorithm was trained on the visual inspection of several thousand spectra. The final sample of quality controlled spectra includes 47,919 datasets (65% of the total number of extracted spectra) for $32,149$ unique objects, with a median $i_{\rm AB}$-band magnitude of 23.7, reaching 26.5 AB for the faintest objects. Each released dataset contains science-ready 1D and 2D spectra, as well as multi-band image cutouts of corresponding sources and a useful preview page summarising the direct and slitless data, astrometric and photometric parameters. In order to characterize the slitless spectra, emission-line flux and equivalent width sensitivity of the ACS data were compared with public ground-based spectra in the GOODS-South field. An example list of emission line galaxies with two or more identified lines is also included, covering the redshift range $0.2-4.6$.

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arXiv2010

Chemical abundances in the protoplanetary disk LV2 (Orion): clues to the causes of the abundance anomaly in HII regions

Optical integral field spectroscopy of the archetype protoplanetary disk LV2 in the Orion Nebula is presented, taken with the VLT FLAMES/Argus fibre array. The detection of recombination lines of CII and OII from this class of objects is reported, and the lines are utilized as abundance diagnostics. The study is complemented with the analysis of HST Faint Object Spectrograph ultraviolet and optical spectra of the target contained within the Argus field of view. By subtracting the local nebula background the intrinsic spectrum of the proplyd is obtained and its elemental composition is derived for the first time. The proplyd is found to be overabundant in carbon, oxygen and neon compared to the Orion Nebula and the sun. The simultaneous coverage over LV2 of the CIII] 1908-A and [OIII] 5007-A collisionally excited lines (CELs) and CII and OII recombination lines (RLs) has enabled us to measure the abundances of C++ and O++ for LV2 with both sets of lines. The two methods yield consistent results for the intrinsic proplyd spectrum, but not for the proplyd spectrum contaminated by the generic nebula spectrum, thus providing one example where the long-standing abundance anomaly plaguing metallicity studies of HII regions has been resolved. These results would indicate that the standard forbidden-line methods used in the derivation of light metal abundances in HII regions in our own and other galaxies underestimate the true gas metallicity.

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arXiv2010

Hubble Space Telescope WFC3 Early Release Science: Emission-Line Galaxies from Infrared Grism Observations

We present grism spectra of emission-line galaxies (ELGs) from 0.6-1.6 microns from the Wide Field Camera 3 on the Hubble Space Telescope. These new infrared grism data augment previous optical Advanced Camera for Surveys G800L 0.6-0.95 micron grism data in GOODS-South from the PEARS program, extending the wavelength covereage well past the G800L red cutoff. The ERS grism field was observed at a depth of 2 orbits per grism, yielding spectra of hundreds of faint objects, a subset of which are presented here. ELGs are studied via the Ha, [OIII], and [OII] emission lines detected in the redshift ranges 0.2 2.

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arXiv2010

A study of the interplay between ionized gas and star clusters in the central region of NGC 5253 with 2D spectroscopy

ABRIDGED: A detailed 2D study of the central region of NGC5253 has been performed to characterize the stellar and ionized gas structure as well as the extinction distribution, physical properties and kinematics of the ionized gas in the central ~210pc x 130pc. We utilized optical integral field spectroscopy (IFS) data obtained with FLAMES. A detailed extinction map for the ionized gas in NGC5253 shows that the largest extinction is associated with the prominent Giant HII region. There is an offset of ~0.5" between the peak of the optical continuum and the extinction peak in agreement with findings in the infrared. We found that stars suffer less extinction than gas by a factor of 0.33. The [SII]l6717/[SII]l6731 map shows an electron density (N_e) gradient declining from the peak of emission in Ha (790cm^-3) outwards, while the argon line ratio traces areas with $N_e~4200 - 6200cm^(-3). The area polluted with extra nitrogen, as deduced from the excess [NII]/Ha, extends up to distances of 3.3" (~60pc) from the maximum pollution, which is offset by ~1.5" from the peak of continuum emission. Wolf-Rayet features are distributed in an irregular pattern over a larger area (~100pc x 100pc) and associated with young stellar clusters. We measured He^+ abundances over most of the field of view and values of He^++/H^+<~0.0005 in localized areas which do not coincide, in general, with the areas presenting W-R emission or extra nitrogen. The line profiles are complex. Up to three emission components were needed to reproduce them. One of them, associated with the giant HII region, presents supersonic widths and [NII] and [SII] emission lines shifted up to 40km/s with respect to Ha. Similarly, one of the narrow components presents offsets in the [NII] line of <~20km/s. This is the first time that maps with such velocity offsets for a starburst galaxy have been presented.

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arXiv2009

Empirical H-alpha emitter count predictions for dark energy surveys

Future galaxy redshift surveys aim to measure cosmological quantities from the galaxy power spectrum. A prime example is the detection of baryonic acoustic oscillations (BAOs), providing a standard ruler to measure the dark energy equation of state, w(z), to high precision. The strongest practical limitation for these experiments is how quickly accurate redshifts can be measured for sufficient galaxies to map the large-scale structure. A promising strategy is to target emission-line (i.e. star-forming) galaxies at high-redshift (z~0.5-2); not only is the space density of this population increasing out to z~2, but also emission-lines provide an efficient method of redshift determination. Motivated by the prospect of future dark energy surveys targeting H-alpha emitters at near-infrared wavelengths (i.e. z>0.5), we use the latest empirical data to model the evolution of the H-alpha luminosity function out to z~2, and thus provide predictions for the abundance of H-alpha emitters for practical limiting fluxes. We caution that the estimates presented in this work must be tempered by an efficiency factor, epsilon, giving the redshift success rate from these potential targets. For a range of practical efficiencies and limiting fluxes, we provide an estimate of nP_{0.2}, where n is the 3D galaxy number density and P_{0.2} is the galaxy power spectrum evaluated at k=0.2h/Mpc. Ideal surveys must provide nP_{0.2}>1 in order to balance shot-noise and cosmic variance errors. We show that a realistic emission-line survey (epsilon=0.5) could achieve nP_{0.2}=1 out to z~1.5 with a limiting flux of 10^{-16} erg/s/cm^{-2}. If the limiting flux is a factor 5 brighter, then this goal can only be achieved out to z~0.5, highlighting the importance of survey depth and efficiency in cosmological redshift surveys.

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arXiv2009

Detection of the Central Star of the Planetary Nebula NGC 6302

NGC 6302 is one of the highest ionization planetary nebulae known and shows emission from species with ionization potential >300eV. The temperature of the central star must be >200,000K to photoionize the nebula, and has been suggested to be up to ~ 400,000K. On account of the dense dust and molecular disc, the central star has not convincingly been directly imaged until now. NGC 6302 was imaged in six narrow band filters by Wide Field Camera 3 on HST as part of the Servicing Mission 4 Early Release Observations. The central star is directly detected for the first time, and is situated at the nebula centre on the foreground side of the tilted equatorial disc. The magnitudes of the central star have been reliably measured in two filters(F469N and F673N). Assuming a hot black body, the reddening has been measured from the (4688-6766Å) colour and a value of c=3.1, A_v=6.6 mag determined. A G-K main sequence binary companion can be excluded. The position of the star on the HR diagram suggests a fairly massive PN central star of about 0.64,M_sun close to the white dwarf cooling track. A fit to the evolutionary tracks for (T,L,t)=(200,000K, 2000L_sun, 2200yr), where t is the nebular age, is obtained; however the luminosity and temperature remain uncertain. The model tracks predict that the star is rapidly evolving, and fading at a rate of almost 1 % per year. Future observations could test this prediction.

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arXiv2009

A VLT VIMOS study of the anomalous BCD Mrk 996: mapping the ionised gas kinematics and abundances

A study of the blue compact dwarf (BCD) galaxy Mrk 996 based on high resolution optical VLT VIMOS integral field unit spectroscopy is presented. Mrk 996 displays multi-component line emission, with most line profiles consisting of a narrow, central Gaussian with an underlying broad component. The broad HI Balmer component splits into two separate broad components inside a 1".5 radius from the nucleus; these are attributed to a two-armed mini-spiral. The rotation curve of Mrk 996 derived from the Hαnarrow component yields a total mass of 5x10^8 Msol within a radius of 3 kpc. The high excitation energy, high critical density [O III] 4363 and [N II] 5755 lines are only detected from the inner region and exist purely in broad component form, implying unusual excitation conditions. Surface brightness, radial velocity, and FWHM maps for several emission components are presented. A separate physical analysis of the broad and narrow emission line regions is undertaken. We derive an upper limit of 10,000 K for the electron temperature of the narrow line gas, together with an electron density of 170 cm^-3, typical of normal H II regions. For the broad line component, we estimate a temperature of 11,000 K and an electron density of 10^7 cm^-3. The broad line emission regions show a N/H enrichment factor of ~20 relative to the narrow line regions, but no He/H, O/H, S/H, or Ar/H enrichment is inferred. The dominant line excitation mechanism is photoionisation by the ~3000 WR stars and ~150,000 O-type stars estimated to be present in the core. This is indeed a peculiar BCD, with extremely dense zones of gas in the core, through which stellar outflows and possible shock fronts permeate contributing to the excitation of the broad line emission. [Abridged]

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arXiv2009

Spectrophotometrically Identified stars in the PEARS-N and PEARS-S fields

Deep ACS slitless grism observations and identification of stellar sources are presented within the Great Observatories Origins Deep Survey (GOODS) North and South fields which were obtained in the Probing Evolution And Reionization Spectroscopically (PEARS) program. It is demonstrated that even low resolution spectra can be a very powerful means to identify stars in the field, especially low mass stars with stellar types M0 and later. The PEARS fields lay within the larger GOODS fields, and we used new, deeper images to further refine the selection of stars in the PEARS field, down to a magnitude of mz = 25 using a newly developed stellarity parameter. The total number of stars with reliable spectroscopic and morphological identification was 95 and 108 in the north and south fields respectively. The sample of spectroscopically identified stars allows constraints to be set on the thickness of the Galactic thin disk as well as contributions from a thick disk and a halo component. We derive a thin disk scale height, as traced by the population of M4 to M9 dwarfs along two independent lines of sight, of h_thin = 370 +60/-65 pc. When including the more massive M0 to M4 dwarf population, we derive h_thin = 300 +/- 70pc. In both cases, we observe that we must include a combination of thick and halo components in our models in order to account for the observed numbers of faint dwarfs. The required thick disk scale height is typically h_thick=1000 pc and the acceptable relative stellar densities of the thin disk to thick disk and the thin disk to halo components are in the range of 0.00025<f_halo<0.0005 and 0.05<f_thick<0.08 and are somewhat dependent on whether the more massive M0 to M4 dwarfs are included in our sample.

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arXiv2008

Integral field spectroscopy of protoplanetary disks in Orion with VLT FLAMES

We discuss integral field spectroscopy of proplyds in M42 using the FLAMES Argus unit and report the first detection of recombination lines of C II and O II from the archetypical Laques-Vidal-2 object. These lines can provide important new diagnostics of the physical conditions in proplyds. We also draw attention to the future capabilities of the MUSE spectrograph in relation to similar studies.

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arXiv2008

Integral field spectroscopy of planetary nebulae: mapping the line diagnostics and hydrogen-poor zones with VLT FLAMES

(Abridged) Results from the first dedicated study of Galactic PNe by means of optical integral field spectroscopy with the VLT FLAMES Argus IFU are presented. Three typical Galactic-disk PNe have been mapped with the 11.5''x7.2'' Argus array: two dimensional spectral maps of NGC 5882, 6153 and 7009 with 297 spatial pixels per target were obtained at sub-arcsec resolutions and 297 spectra per target were obtained in the 396.4-507.8 nm range. Spatially resolved maps of emission lines and of nebular physical properties were produced. The abundances of helium and of doubly ionized carbon and oxygen were derived from optical recombination lines (ORLs), while those of O^2+ were also derived from the collisionally excited lines (CELs). The abundance discrepancy problem was investigated by mapping the ratio of ORL/CEL abundances for O^2+ (the abundance discrepancy factor; ADF) across the face of the PNe. The ADF varies between targets and also with position within the targets attaining values of ~40 in the case of NGC 6153. Correlations of the ADF with geometric distance from the nucleus, as well as with [O III] electron temperature, plasma ionization state and other physical properties are established. Very small values of the temperature fluctuation parameter in the plane of the sky are found in all cases. It is argued that these results provide further evidence for the existence in typical PNe of a distinct nebular component consisting of hydrogen-deficient plasma. The zones containing this posited component appear as undulations in the C II and C II ORL abundance diagnostics of about 2 spatial pixels across; any associated structures should have physical sizes of less than ~1000 astronomical units. We propose that circumstellar disks, Abell 30-type knots, or Helix-type cometary globules may be involved.

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arXiv2006

Unravelling the chemical inhomogeneity of PNe with VLT FLAMES integral-field unit spectroscopy

Recent weak emission-line long-slit surveys and modelling studies of PNe have convincingly argued in favour of the existence of an unknown component in the planetary nebula plasma consisting of cold, hydrogen-deficient gas, as an explanation for the long-standing recombination-line versus forbidden-line temperature and abundance discrepancy problems. Here we describe the rationale and initial results from a detailed spectroscopic study of three Galactic PNe undertaken with the VLT FLAMES integral-field unit spectrograph, which advances our knowledge about the small-scale physical properties, chemical abundances and velocity structure of these objects across a two-dimensional field of view, and opens up for exploration an uncharted territory in the study and modelling of PNe and photoionized nebulae in general.

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arXiv2006

The Planetary Nebula population of the Sagittarius Dwarf Spheroidal Galaxy

The identification of two new Planetary Nebulae in the Sagittarius Dwarf Spheroidal Galaxy (Sgr) is presented. This brings the total number to four. The first, StWr 2-21, belongs to the main body of Sgr. The second, the halo PN BoBn 1, has a location, distance and velocity in agreement with the leading tidal tail of Sgr. We estimate that 10 per cent of the Galactic halo consists of Sgr debris. The specific frequency of PNe indicates a total luminosity of Sgr, including its tidal tails, of M_V=-14.1. StWr 2-21 shows a high abundance of [O/H]=-0.23, which confirms the high-metallicity population in Sgr uncovered by Bonaficio et al. (2004). The steep metallicity--age gradient in Sgr is due to ISM removal during the Galactic plane passages, ISM reformation due to stellar mass loss, and possibly accretion of metal-enriched gas from our Galaxy. The ISM re-formation rate of Sgr, from stellar mass loss, is 5 X 10^-4 M_sun yr^-1, amounting to ~10^6 M_sun per orbital period. HST images reveal well-developed bipolar morphologies, and provide clear detections of the central stars. All three stars with deep spectra show WR-lines, suggesting that the progenitor mass and metallicity determines whether a PN central star develops a WR spectrum. One Sgr PN belongs to the class of IR-[WC] stars. Expansion velocities are determined for three nebulae. Comparison with hydrodynamical models indicates an initial density profile of rho ~ r^-3. This is evidence for increasing mass-loss rates on the AGB. Peak mass-loss rates are indicated of ~ 10^-4 M_sun yr^-1. The IR-[WC] PN, He 2-436, provides the sole direct detection of dust in a dwarf spheroidal galaxy, to date.

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arXiv2006

Planetary nebulae with emission-line central stars

The kinematic structure of a sample of planetary nebulae, consisting of 23 [WR] central stars, 21 weak emission line stars (wels) and 57 non-emission line central stars, is studied. The [WR] stars are shown to be surrounded by turbulent nebulae, a characteristic shared by some wels but almost completely absent from the non-emission line stars. The fraction of objects showing turbulence for non-emission-line stars, wels and [WR] stars is 7%, 24% and 91%, respectively. The [WR] stars show a distinct IRAS 12-micron excess, indicative of small dust grains, which is not found for wels. The [WR]-star nebulae are on average more centrally condensed than those of other stars. On the age-temperature diagram, the wels are located on tracks of both high and low stellar mass, while [WR] stars trace a narrow range of intermediate masses. Emission-line stars are not found on the cooling track. One group of wels may form a sequence wels--[WO] stars with increasing temperature. For the other groups both the wels and the [WR] stars appear to represent several, independent evolutionary tracks. We find a discontinuity in the [WR] stellar temperature distribution and suggest different evolutionary sequences above and below the temperature gap. One group of cool [WR] stars has no counterpart among any other group of PNe and may represent binary evolution. A prime factor distinguishing wels and [WR] stars appears to be stellar luminosity. We find no evidence for an increase of nebular expansion velocity with time.

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arXiv2005

Recent developments of the ACS spectral extraction software aXe

The software package aXe provides comprehensive spectral extraction facilities for all the slitless modes of the ACS, covering the Wide Field Channel (WFC) grism, the High Resolution Channel (HRC) grism and prism and the Solar Blind Channel (SBC) prisms. The latest developments to the package apply to all ACS slitless modes leading to improved spectral extraction. Many thousands of spectra may be present on a single deep ACS WFC G800L image such that overlap of spectra is a significant nuisance. Two methods of estimating the contamination of any given spectrum by its near neighbours have been developed: one is based on the catalogue of objects on the direct image; another uses the flux information on multi-filter direct images. An improvement to the extracted spectra can also result from weighted extraction and the Horne optimal extraction algorithm has been implemented in aXe. A demonstrated improvement in signal-to-noise can be achieved. These new features are available in aXe-1.5 with the STSDAS 3.4 release.

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arXiv2005

Calibration of ACS Prism Slitless Spectroscopy Modes

The Advanced Camera for Surveys is equipped with three prisms in the Solar Blind (SBC) and High Resolution (HRC) Channels, which together cover the 1150 - 3500 A range, albeit at highly non-uniform spectral resolution. We present new wavelength- and flux calibrations of the SBC (PR110L and PR130L) and HRC (PR200L) prisms, based on calibration observations obtained in Cycle 13. The calibration products are available to users via the ST-ECF/aXe web pages, and can be used directly with the aXe package. We discuss our calibration strategy and some caveats specific to slitless prism spectroscopy.

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arXiv2005

Slitless spectroscopy with the Advanced Camera for Surveys

The Advanced Camera for Surveys (ACS) enables low resolution slitless spectroscopic imaging in the three channels. The most-used modes are grism imaging with the WFC and the HRC at a resolution of 40 and 24 A/pixel, respectively. In the far UV there are two prisms for the SBC and a prism for the HRC in the near-UV. An overview of the slitless spectroscopic modes of the ACS is presented together with the advantages of slitless spectroscopy from space. The methods and strategies developed to establish and maintain the wavelength and flux calibration for the different channels are outlined. Since many slitless spectra are recorded on one deep exposure, pipeline science quality extraction of spectra is a necessity. To reduce ACS slitless data, the aXe spectral extraction software has been developed at the ST-ECF. aXe was designed to extract large numbers of ACS slitless spectra in an unsupervised way based on an input catalogue derived from a companion direct image. In order to handle dithered slitless spectra, drizzle, well-known in the imaging domain, has been applied. For ACS grism images, the aXedrizzle technique resamples 2D spectra from individual images to deep, rectified images before extracting the 1D spectra. aXe also provides tools for visual assessment of the extracted spectra and examples are presented.

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arXiv2005

Evidence for a Massive Post-Starburst Galaxy at z ~ 6.5

We present results from a search for high-redshift J--band ``dropout'' galaxies in the portion of the GOODS southern field that is covered by extremely deep imaging from the Hubble Ultradeep Field (HUDF).Using observations at optical, near-infrared and mid-infrared wavelengths from the Hubble and Spitzer Space Telescopes and the ESO-VLT, we search for very massive galaxies at high redshifts and find one particularly remarkable candidate. Its spectral energy distribution is consistent with a galaxy at z ~ 6.5 and a stellar mass of 6x10e11 M(sun) (for a Salpeter IMF). We interpret a prominent photometric break between the near-infrared and Spitzer bandpasses as the 3646A Balmer discontinuity. The best-fitting models have low reddening and ages of several hundred Myr, placing the formation of the bulk of the stars at z > 9. Alternative models of dusty galaxies at z ~ 2.5 are possible but provide significantly poorer fits. The object is detected with Spitzer at 24 micron. This emission originats from an obscured active nucleus or star formation. We present optical and near-infrared spectroscopy which has, thus far, failed to detect any spectral features. This helps limit the solution in which the galaxy is a starburst or active galaxy at z ~ 2.5. If the high-redshift interpretation is correct, this object would be an example of a galaxy that formed by a process strongly resembling traditional models of monolithic collapse, in a way which a very large mass of stars formed within a remarkably short period of time, at very high redshift.

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