Joss Bland-Hawthorn

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 which escape the Galactic disk. We have modeled the production and transport of ionizing photons through an opaque interstellar medium. Normalization to the Stream detections requires f?6%, which is 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? 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? observations.

Warm Gas and Ionizing Photons in the Local Group

Several lines of argument suggest that a large fraction of the baryons in the universe may be in the form of warm (T ~ 105-107 K) gas. In particular, loose groups of galaxies may contain substantial reservoirs of such gas. Observations of the cosmic microwave background by COBE place only weak constraints on such an intragroup medium within the Local Group. The idea of a Local Group corona dates back at least 40 years (Kahn & Woltjer). Here we show that gas at T ~ (2-3) × 106 K (the approximate virial temperature of the Local Group)—extremely difficult to observe directly—can in principle radiate a large enough flux of ionizing photons to produce detectable Hα emission from embedded neutral clouds. However, additional constraints on the corona—the most stringent being pulsar dispersion measures toward the Magellanic Clouds, and the timing mass—rule out an intragroup medium whose ionizing flux dominates over the cosmic background or the major Local Group galaxies. A cosmologically significant coronal gas mass could remain invisible to Hα observations. More massive galaxy groups could contain extensive coronae which are important for the baryon mass and produce a strong, local ionizing flux.

Ionizing Photons and Extreme-Ultraviolet Excesses in Clusters of Galaxies

Observations with the Extreme Ultraviolet Explorer satellite are purported to show extreme-ultraviolet (EUV) and soft X-ray excesses in several clusters of galaxies. If interpreted as thermal emission, this would imply the presence of warm (T ~ 106 K) gas in these clusters with a mass comparable to that of gas at coronal temperatures. If true, this would have profound implications for our understanding of galaxy clusters and the distribution of baryons in the universe. Here we show that because of the large ionizing photon emissivities of gas at such low temperatures, the ionizing photon fluxes seen by disk galaxies in the observed clusters can be very large, resulting in minimum emission measures from neutral gas in such disks as high as 100 cm-6 pc. This result is essentially independent of the mechanism actually responsible for producing the alleged EUV excesses. The predicted emission measures in Abell 1795 (z = 0.063) are about an order of magnitude larger than seen in the Reynolds layer of the Galaxy, providing a straightforward observational test of the reality of the EUV excess. New tunable filter Hα images and Wide Field Planetary Camera images from the Hubble Space Telescope archive do not support the existence of the claimed EUV excess.

Opening the dynamic infrared sky

While optical and radio transient surveys have enjoyed a renaissance over the past decade, the dynamic infrared sky remains virtually unexplored from the ground. The infrared is a powerful tool for probing transient events in dusty regions that have high optical extinction, and for detecting the coolest of stars that are bright only at these wavelengths. The fundamental roadblocks in studying the infrared time-domain have been the overwhelmingly bright sky background (250 times brighter than optical) and the narrow field-of-view of infrared cameras (largest is VISTA at 0.6 sq deg). To address these challenges, Palomar Gattini-IR is currently under construction at Palomar Observatory and we propose a further low risk, economical, and agile instrument to be located at Siding Spring Observatory, as well as further instruments which will be located at the high polar regions to take advantage of the low thermal sky emission, particularly in the 2.5 micron region.

Feeding the IGM Through Galaxy Interactions

The Magellanic Stream is IGM fuel which originates from the interaction of the Large and Small Magellanic Clouds (LMC & SMC) with each other and the Galaxy. The Stream trails the Clouds for over 100° and serves as a probe of the mass and density of the Galactic halo. Figure 1 shows the column density distribution of the Magellanic System (N HI > 2 x 1018 cm-2) using HVC reduced HIPASS data (-500 > V LSR < 500 km s-1; excluding ± 90 km s-1; see Putman et al. 2002a). The Stream extends over 700 km s-1, or 400 km s-1 in terms of a Galactic reference frame, from l,b ∼ 290°, -45° to l, b ∼ 90°, -35°. Small clumps of Magellanic debris follow the length of the Stream in position and velocity. The Leading Arm (the HI clouds on the opposite side of the Magellanic Clouds to the Magellanic Stream) shows that the interaction between the Clouds and the Galaxy is predominantly a tidal one; however, these tidal features are most likely being shaped by a Galactic halo medium. The entire system contains 1.2 x 109 M⊙ of neutral hydrogen (at an average distance of 55 kpc), or ∼l/3 the Hi mass of the Galaxy.