arXiv subjects

J. Bland-Hawthorn

Publications and source records attributed to J. Bland-Hawthorn.

At least 379 records · Page 21Linked to original sources

arXiv1998

The Galactic Halo Ionizing Field and H-alpha Distances to HVCs

There has been much debate in recent decades as to what fraction of ionizing photons from star forming regions in the Galactic disk escape into the halo. The recent detection of the Magellanic Stream in optical line emission at the CTIO 4m and the AAT 3.9m telescopes may now provide the strongest evidence that at least some of the radiation escapes the disk completely. While the distance to the Magellanic Stream is uncertain, the observed H-alpha emission is most plausibly explained by photoionization due to hot, young stars. Our model requires that the mean Lyman-limit opacity perpendicular to the disk is tau \approx 2.8 (f_esc = 6%), assuming the covering fraction of resolved clouds is close to unity. Within the context of this model, it now becomes possible to determine distances to high velocity clouds, and the 3D orientation of the Magellanic Stream. Here, we discuss complications of the model (e.g., porosity, topology), future tests, ongoing improvements, and the importance of H-alpha limb brightening from surface ionization. More speculatively, we propose a direct experiment for locating an HVC in 6-dimensional phase space above the Galactic plane.

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arXiv1998

Warm Gas and Ionizing Photons in the Local Group

We investigate how much warm (T \sim 10^6 K) gas may exist in a Local Group corona, and, in particular, whether such a corona could be detected indirectly through its ionizing radiation. We calculate the non-LTE cooling of warm, low-density gas and the resulting ionizing photon emissivities for a range of metallicities. In principle a corona can produce a photon flux which significantly exceeds the cosmic ionizing background. However, the additional constraints which can be imposed on intragroup gas rule out models which produce detectable ionizing photon fluxes at the Milky Way's distance from the Local Group barycenter.

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arXiv1998

Ionizing Photon Sources within the Local Group

We review the possible sources of ionizing photons within the Local Group. Throughout most of the LG volume, the cosmic background radiation will dominate, but locally (e.g., within d\sim 100 kpc of the Galactic disk) stellar ionizing photons escaping from galaxies may dominate. The magnitude of the cosmic ionizing background should be determined in the very near future by observations of H alpha emission from the outer neutral hydrogen disks of late-type spirals. The detection of the Magellanic Stream in H alpha suggests that a few percent of the ionizing photons produced in the Galaxy escape the disk. Neither a warm Local Group corona nor decay photons from a neutrino halo can explain the Stream emission.

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arXiv1998

The Escape of Ionizing Photons from the Galaxy

The Magellanic Stream and several high velocity clouds have now been detected in optical line emission. The observed emission measures and kinematics are most plausibly explained by photoionization due to hot, young stars in the Galactic disk. The highly favorable orientation of the Stream allows an unambiguous determination of the fraction of ionizing photons, F_esc, which escape the disk. We have modelled the production and transport of ionizing photons through an opaque interstellar medium. Normalization to the Stream detections requires F_esc = 6%, in reasonable agreement with the flux required to ionize the Reynolds layer. Neither shock heating nor emission within a hot Galactic corona can be important in producing the observed H-alpha emission. If such a large escape fraction is typical of L_* galaxies, star-forming systems dominate the extragalactic ionizing background. Within the context of this model, both the three-dimensional orientation of the Stream and the distances to high-velocity clouds can be determined by sensitive H-alpha observations.

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arXiv1998

The Smith Cloud: HI associated with the Sgr dwarf?

The Smith high velocity cloud (V(LSR) = 98 kms) has been observed at two locations in the emission lines [OIII]5007, [NII]6548 and H-alpha. Both the [NII] and H-alpha profiles show bright cores due to the Reynolds layer, and red wings with emission extending to V(LSR) = 130 kms. This is the first simultaneous detection of two emission lines towards a high velocity cloud, allowing us to form the ratio of these line profiles as a function of LSR velocity. At both cloud positions, we see a clear distinction between emission at the cloud velocity, and the Reynolds layer emission (V(LSR) = 0). The [NII]/H-alpha ratio (=0.25) for the Reynolds layer is typical of the warm ionised medium. At the cloud velocity, this ratio is enhanced by a factor of 3-4 compared to emission at rest with respect to the LSR. A moderately deep upper limit at [OIII] (0.12R at 3-sigma) was derived from our data. If the emission arises from dilute photoionisation from hot young stars, the highly enhanced [NII]/H-alpha ratio, the [OIII] non-detection and weak H-alpha emission (0.24-0.30R) suggest that the Smith Cloud is 26+/-4 kpc from the Sun, at a Galactocentric radius of 20+/-4 kpc. This value assumes that the emission arises from an optically thick slab, with a covering fraction of unity as seen by the ionizing photons, whose orientation is either (a) parallel to the Galactic disk, or (b) such as to maximize the received flux from the disk. The estimated mass and size of the cloud are 4x10^6 Msun and 6 kpc. We discuss a possible association with the much larger Sgr dwarf, at a galactocentric radius of 16+/-2 kpc, which lies within 35 degrees (~12 kpc) of the Smith Cloud.

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arXiv1998

Near and Mid Infrared Imaging Polarimetry of NGC1068

We present the results of a series of observations of the near- and mid-infrared polarisation properties of the Seyfert 2 galaxy NGC1068. Our data agree well with previously published results in showing the need for a separate polarisation mechanism in the near infrared apart from scattering. We find that the predictions of a simple model in which this component arises through absorptive dichroism due to aligned grains within the extended warm (~400K) dust fits the data reasonably if the obscured background source is itself due to dust emission (at temperature >1000K). By considering the change of polarisation with wavelength we show that the extinction to this hot dust region is in the range AV=20-40. Consideration of the observed data then leads us to the conclusion that if viewed face-on, NGC1068 would have a strong near-infrared excess similar to Seyfert 1 galaxies. Comparison with other independent measures of the extinction to the active nucleus itself lead us to the conclusion that the hot dust must provide screening equivalent to at least AV=40, and possibly much higher. We speculate that this component alone may be the `classical' torus discussed in terms of the unified model, and the more extensive mid-infrared emission may arise from circumnuclear molecular cloud material, and dust in the ionisation cones.

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arXiv1998

The Metagalactic Ionizing Field in the Local Group

We discuss the sources which are likely to dominate the ionizing field throughout the Local Group. In terms of the limiting flux to produce detectable H-alpha emission (4-10 x 10**3 phot/cm**2/s), the four dominant galaxies (M31, Galaxy, M33, LMC) have spheres of influence which occupy a small fraction (<10%) of the Local Volume. There are at least two possible sources of ionization whose influence could be far more pervasive: (i) a cosmic background of ionizing photons; (ii) a pervasive warm plasma throughout the Local Group. The COBE FIRAS sky temperature measurements permit a wide variety of plasmas with detectable ionizing fields. It has been suggested (Blitz et al. 1996; Spergel et al. 1996; Sembach et al. 1995, 1998) that a substantial fraction of high velocity clouds are external to the Galaxy but within the Local Group. Deep H-alpha detections are the crucial test of these claims and, indeed, provide a test bed for the putative Local Group corona.

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arXiv1997

Optical rotation curves beyond the HI cut-off in spirals

In the best observed spiral galaxies, the HI is observed to decline slowly with radius for up to 90% of the maximum extent, before experiencing a rapid truncation. Early models predicted that the outer parts of spirals would be fully ionized by a metagalactic UV field at column depths of roughly 10^{19} atoms cm^{-2}. Weak optical line emission has now been observed beyond the HI edge in a number of cases and, furthermore, this warm gas has been used to trace the dark halo far beyond the HI cut-off. But the observed emission measures (40-90 mR) are higher than expected for a cosmic origin. A more likely explanation is ionization of the warped outer disk by the blue stellar population at smaller radius. There are many implications of ionized warped edges, in particular, severe HI warps may be greatly under represented in current HI surveys. We briefly discuss the implications of this model for the Galactic warp.

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arXiv1997

Where do the disks of spiral galaxies end?

In spiral galaxies, the HI surface density declines with increasing radius to a point where it is seen to truncate dramatically in the best observed cases. It was anticipated that if the ambient radiation field is sufficiently strong, there exists a maximum radius beyond which the cold gas is unable to support itself against ionization. We have now succeeded in detecting ionized gas beyond the observed HI disk in spirals. Here, we report on our findings for the Sculptor galaxy NGC 253. The HI disks in Sculptor galaxies extend to only about 1.2 R_25 although we have detected ionized gas to the limits of our survey out to 1.4 R_25. This has important ramifications for spiral galaxies in that it now becomes possible to trace the gravitational potential beyond where the HI disk ends. The detections confirm that the rotation curve continues to rise in NGC 253, as it appears to do for other Sculptor galaxies from the HI measurements, but there is a hint that the rotation curve may fall abruptly not far beyond the edge of the HI disk. If this is correct, then it suggests that the dark halo of NGC 253 may be truncated near the HI edge, and provides further support for the link between dark matter and HI. The line ratios are anomalous with [NII]6548/H-alpha ratios close to unity. While metallicities at these large radii are uncertain, such enhanced ratios compared to solar-abundance HII regions ([NII]/H-alpha = 0.05-0.2) are likely to require selective heating of the electron population without further ionization of N^++. We discuss the most likely sources of ionization and heating, and the possible role of refractory element depletion (e.g. Ca, Si, Fe) onto dust grains.

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arXiv1997

The Asymmetric Wind in M82

We have obtained detailed imaging Fabry-Perot observations of the nearby galaxy M82, in order to understand the physical association between the high-velocity outflow and the starburst nucleus. The observed velocities of the emitting gas in M82 reveal a bipolar outflow of material, originating from the bright starburst regions in the galaxy's inner disk, but misaligned with respect to the galaxy spin axis. The deprojected outflow velocity increases with radius from 525 to 655 km/s. Spectral lines show double components in the centers of the outflowing lobes, with the H-alpha line split by ~300 km/s over a region almost a kiloparsec in size. The filaments are not simple surfaces of revolution, nor is the emission distributed evenly over the surfaces. We model these lobes as a composite of cylindrical and conical structures, collimated in the inner ~500 pc but expanding at a larger opening angle of ~25 degrees beyond that radius. We compare our kinematic model with simulations of starburst-driven winds in which disk material surrounding the source is entrained by the wind. The data also reveal a remarkably low [NII]/H-alpha ratio in the region of the outflow, indicating that photoionization by the nuclear starburst may play a significant role in the excitation of the optical filament gas, particularly near the nucleus.

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arXiv1997

TTF: a flexible approach to narrowband imaging

The Taurus Tunable Filter (TTF) is a tunable narrowband interference filter covering wavelengths from 6300 A to the sensitivity drop-off of conventional CCDs (9600 A), although a blue `arm' (3700-6500 A) is to be added by the end of 1997. The TTF offers monochromatic imaging at the cassegrain foci of both the Anglo-Australian and William Herschel Telescopes, with an adjustable passband of between 6 and 60 A. In addition, frequency switching with the TTF can be synchronized to movement of charge (charge shuffling) on the CCD which has many important applications in astrophysics. Here we review different modes of TTF. We maintain a web site at http://msowww.anu.edu.au/~dhj/ttf.html describing all aspects of TTF and its operation. In addition to general use, the instrument is available in AAT service time. Details can be found at http://www.aao.gov.au/local/www/jmc/service/service.html.

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arXiv1997

Rugate filters for OH-suppressed imaging at near-infrared wavelengths

Ground-based observations at near-infrared wavelengths are severely affected by atmospheric OH bands. Many authors have recognized the potential gains in sensitivity from suppressing these features. Dispersive instruments show some promise but are both expensive and complicated to build. OH suppression filters using single or periodic notches have the advantage of simplicity but significant gains have not yet been realised. Rugate filters (with graded index inhomogeneous coatings) offer key advantages for astronomical imaging. It is possible to produce a transmission profile comprising a series of irregular and sharply defined bandpasses. We demonstrate through numerical simulation that it should be possible to achieve 95% suppression of the OH features in the J photometric band, while retaining roughly half of the spectral coverage. This would lead to extraordinary gains in sensitivity even for observations of continuum sources. In addition, these filters allow much longer exposures before the detector saturates on the sky background. (I and z-band filters can also be envisaged.) In 1" seeing, a J-band rugate filter used in conjunction with a 4m telescope would detect a J=23 continuum source at 5.0-sigma in a single 10 min exposure. In comparison, a conventional J filter requires multiple exposures for a 10 minute integration time and achieves only a 2.5-sigma detection. For emission line sources, the rugate filter has an even bigger advantage over conventional filters, with a fourfold increase in signal:noise ratio possible in certain instances. Astrophysical studies which could benefit from rugate filters are searches for very low mass stars and galaxy evolution out to z=3.

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arXiv1997

Kinematics and mass modelling of NGC 1068

We present the kinematics of the ionized gas over the inner 140 arcsec (10 kpc) of NGC 1068 from observations with the HIFI Fabry-Perot interferometer. There is clear evidence for density wave streaming and bar-driven streaming across the field, with bi-symmetric arms that penetrate to within 200 pc of the nucleus. CO maps show linear structures along (although slightly offset from) the bar consistent with a strong shock. Along the spiral arms which encircle the bar, the HII regions lie downstream of the CO gas in the rest frame of the bar, as do the dust lanes, only if the gas outruns the stellar bar. As a first step towards understanding the details of the gas kinematics, and attempting to determine the mass inflow rate towards the nucleus, we build a mass model for the central disk constrained by near-infrared images. We plan to use this model as gravitational background potential for hydro-dynamical simulations of the gas response to the bar. Comparing these with the data presented should enable us to constrain various quantities such as pattern speed, stellar mass-to-light ratio, central mass concentration, and gas fueling rate.

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arXiv1997

Strict Limits on the Ionizing Luminosity in NGC 1068 from Jet-axis Molecular Clouds

The radio jet axis of NGC 1068 is characterised by energetic activity from x-ray to radio wavelengths. Detailed kinematic and polarization studies have shown that this activity is confined to bipolar cones centered on the AGN which intersect the plane of the disk. Thus, molecular clouds at 1 kpc distance along this axis are an important probe of the nuclear ionizing luminosity and spectrum. Extended MIR emission coincident with the clouds is reasonably understood by dust heated to high temperatures by the nuclear radiation field. This model predicts that the nuclear spectrum is quasar-like (power law + blue excess) with a luminosity 2-5 times higher than inferred by Pier et al. Consequently, there is little or no polyaromatic hydrocarbon (PAH) emission associated with the radio-axis molecular clouds. We review this model in the light of new observations. A multi-waveband collage is included to illustrate the possible orientations of the double cones to our line of sight and the galaxian plane.

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arXiv1997

OSSE Observations of the Ultraluminous Infrared Galaxies Arp 220, Mrk 273 and Mrk 231

We report results of soft gamma-ray observations of the ultraluminous infrared galaxies Arp 220, Mrk 273, and Mrk 231 in order to test whether the infrared radiation from these sources originates from buried active galactic nuclei (AGNs). Only upper limits are measured, implying that the emergent soft gamma-ray luminosities are 1-2 orders of magnitude smaller than the infrared luminosities. Monte Carlo simulations of radiation transport through tori are used to infer the minimum column densities $N_H$ required to block transmission of soft gamma-rays from a buried AGN, assuming that spectra of AGNs in such sources are similar to those of radio-quiet quasars. Lack of measured gamma-ray emission provides no supporting evidence for the existence of buried AGNs in these galaxies, but is consistent with an origin of the infrared luminosity from starburst activity.

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arXiv1997

Classical Spectroscopy

Tunable filters are set to revolutionize many aspects of experimental astrophysics, particularly for applications in observational cosmology. After a summary of the fundamentals of classical spectroscopy, we present a review of the current technologies on offer. For extragalactic studies, small-gap etalons are the preferred option in terms of cost, photometric performance and throughput (e.g. the TAURUS Tunable Filter; http://www.aao.gov.au/).

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arXiv1997

The Galactic Halo Ionizing Field

There has been much debate in recent decades as to what fraction of ionizing photons from star forming regions in the Galactic disk escape into the halo. The recent detection of the Magellanic Stream in optical line emission at the CTIO 4m and the AAT 3.9m telescopes may now provide the strongest evidence that at least some of the radiation escapes the disk completely. We present a simple model to demonstrate that, while the distance to the Magellanic Stream is uncertain, the observed emission measures (${\cal E}_m \approx 0.5 - 1$ cm$^{-6}$ pc) are most plausibly explained by photoionization due to hot, young stars. This model requires that the mean Lyman-limit opacity perpendicular to the disk is $\tauLL \approx 3$, and the covering fraction of the resolved clouds is close to unity. Alternative sources (e.g. shock, halo, LMC or metagalactic radiation) contribute negligible ionizing flux.

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arXiv1997

Ionized Hydrogen at Large Galactocentric Distances

We summarize recent attempts to detect warm ionized gas at large galactocentric distances. This includes searching for gas at the edges of spirals, in between cluster galaxies, towards extragalactic HI clouds, and towards high velocity clouds and the Magellanic Stream in the Galaxy. With the exception of extragalactic HI clouds, all of these experiments have proved successful. Within each class, we have only observed a handful of objects. It is premature to assess what fraction of the missing baryonic mass fraction might be in the form of ionized gas. But, in most cases, the detections provide a useful constraint on the ambient ionizing flux, and in the case of spiral edges, can even trace dark matter haloes out to radii beyond the reach of radio telescopes.

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