P. W. J. L. Brand

Radiative feedback from an early X-ray background

The first generation of stars (commonly known as population III) are expected to form in lowmass protogalaxies in which molecular hydrogen is the dominant coolant. Radiation from these stars will rapidly build up an extragalactic ultraviolet (UV) background capable of photodissociating H2, and it is widely believed that this background will suppress further star formation in low-mass systems. However, star formation will also produce an extragalactic X-ray background. This X-ray background, by increasing the fractional ionization of protogalactic gas, promotes H2 formation and reduces the effectiveness of ultraviolet feedback. In this paper, we examine which of these backgrounds has the dominant effect. Using a simple model for the growth of the UV and X-ray backgrounds, together with a detailed onedimensional model of protogalactic chemical evolution, we examine the effects of the X-ray backgrounds produced by a number of likely source models. We show that in several cases, the resulting X-ray background is strong enough to offset UV photodissociation in large H2-cooled protogalaxies. On the other hand, small protogalaxies (those with virial temperatures T vir < 2000 K) remain dominated by the UV background in all of the models we examine. We also briefly investigate the effects of the X-ray background upon the thermal and chemical evolution of the diffuse intergalactic medium.

On the photodissociation of H2 by the first stars

The first star formation in the Universe is expected to take place within small protogalaxies, in which the gas is cooled by molecular hydrogen. However, if massive stars form within these protogalaxies, they may suppress further star formation by photodissociating the H2. We examine the importance of this effect by estimating the time-scale on which significant H2 is destroyed. We show that photodissociation is significant in the least massive protogalaxies, but becomes less so as the protogalactic mass increases. We also examine the effects of photodissociation on dense clumps of gas within the protogalaxy. We find that while collapse will be inhibited in low-density clumps, denser ones may survive to form stars.

Ratios of molecular hydrogen line intensities in shocked gas - evidence for cooling zones

Column densities of molecular hydrogen have been calculated from 19 infrared vibration-rotation and pure rotational line intensities measured at peak 1 of the Orion molecular outflow. The run of column density with energy level is similar to a simple coolng zone model of the line-emitting region, but is not well fitted by predictions of C-shock models current in the literature. 20 references.

Detection of H53-alpha emission from M82 - A reliable measure of the ionization rate and its implications

Emission in the hydrogen recombination line H53-alpha has been detected, for the first time, in another galaxy: the nearby starburst system M82. This line is produced primarily by spontaneous emission and provides the most direct, extinction-independent estimate of the ionization rate in the star-forming complex. The line strength implies an ionization rate of 1.1 x 10 to the 54th/s, approximately five times larger than that inferred from Br-alpha observations, and indicates a dust extinction at 4 microns of more than 1 mag. Comparison of the 3.3 mm free-free continuum and H53-alpha fluxes implies an average electron temperature of about 5000 K. Analysis of the line excitation conditions, using the H53-alpha emission in conjunction with that in the far-IR O(2+) and N(2+) forbidden lines, together with the IR luminosity, suggests that only a very restricted range of stellar masses are formed. 41 refs.

Near-IR Fluorescent Molecular Hydrogen Emission from NGC 2023

Spectra from 1 to 2.5 micrometres, at 230-430 spectral resolution, are presented of the fluorescent molecular hydrogen line emission from two locations in the reflection nebula NGC 2023. Over 100 H2 lines can be identified in the spectra, although blending and poor atmospheric transmission mean that reliable level column densities can only be obtained from 35 lines. This latter group includes lines from v = 1-8 and v = 10, spanning an energy range from 6000 to 45,000 K above the ground state These data may be used to constrain models of photodissociation regions and of fluorescent excitation for molecular hydrogen.

High spectral resolution observations of fluorescent molecular hydrogen in molecular clouds

The 1-0 S(1) line of molecular hydrogen has been observed at high spectral resolution in several sources where the emission was suspected of being fluorescent. In NGC 2023, the Orion Bar, and Parsamyan 18, the S(1) line is unresolved, and the line center close to the rest velocity of the ambient molecular cloud. Such behavior is expected for UV-excited line emission. The H2 line widths in molecular clouds thus can serve as diagnostic for shocked and UV-excitation mechanisms. If the lines are broader than several km/s or velocity shifts are observed across a source it is likely that shocks are responsible for the excitation of the gas. 27 refs.

Formation Pumping of Molecular Hydrogen in the Messier 17 Photodissociation Region

We have imaged the emission from the near-infrared v = 1-0 S(1), 1-0 S(7), 2-1 S(1) and 6-4 O(3) lines of molecular hydrogen in the Northern and South Western Bars of M17, together with the hydrogen Bry and Br10 lines. This includes the first emission-line image ever to be obtained of a line from the highly excited v = 6 level of molecular hydrogen. In both Bars, the H 2 emission is generally distributed in clumps along filamentary features. The 1-0 S(1) and 2-1 S(1) images have similar morphologies. Together with their relative line ratios, this supports a fluorescent origin for their emission, within a photodissociation region. The SW-Bar contains a clumpy medium, but in the N-Bar the density is roughly constant. The 1-0 S(7) line image is also similar to the 1-0 S(1) image, but the 6-4 O(3) image is significantly different from it. Since the emission wavelengths of these two lines are similar (1.748 to 1.733 μm), this cannot be due to differential extinction between the v = 6 and the v = 1 lines. We attribute the difference to the pumping of newly formed H 2 into the v = 6, or to a nearby, level. However, this also requires a time-dependent photodissociation region (where molecule formation does not balance dissociation), rather than it to be in steady state, and/or for the formation spectrum to vary with position in the source. If this interpretation of formation pumping of molecular hydrogen is correct, it is the first clear signature from this process to be seen.

H2 fluorescence in HH 7 and DR 21

We present evidence for Lyα pumping of the Lyman band system of molecular hydrogen in Herbig-Haro 7 and the bipolar outflow DR 21. For this study we have measured several vibrational-rotational emission lines of H2 whose energy levels are widely spaced and ranging from 6000 (v = 1) to 25000 Kelvin (v = 4). We show that the near-infrared H2 emission from the shocked gas in HH 7 can be well described by a bow C-type shock. The enhanced emission observed from the higher energy levels (v > 3) can be well modelled by employing the Lyα pumping mechanism.In the DR 21 outflow the multi-line study showed that different physical conditions exist in the eastern and western emission lobes. The higher H2 line ratios measured in the eastern lobe suggests a higher Lyα pump rate which may be locally produced in the fast bowshocks. The FUV radiation field emanating from the central HII regions may in addition be exciting the Lyman and Werner bands of H2 in the molecular lobes.We show that the observed H2 emission can be interpreted in terms of a simple model consisting of a C-type bowshock, which produces the low excitation H2 emission, and a FUV radiation field with enough Lyα line radiation to produce the high excitation H2 emission through fluorescence.

Observations of Shocked [FEII] and H2 Line Profiles in Orion Bullet Wakes

Recent near-IR imaging of the Orion molecular cloud has revealed a complex of dense bullets, visible as [Fell] emitting HH-objects at the tips of H2 wakes, ejected explosively from the cloud core. Having resolved individual bow-shock structures for the first time in this bright source, we have observed [Fell] 1.644μm velocity profiles of selected bullets and H2 1-0 S(1), 2.122μm velocity profiles for a series of positions along and across the corresponding bow-shock wakes. We present observed profiles for the bullet M42 HH1 and its associated wake and compare with theoretical bow-shock models.

Observations of Shocked H2 And [FeII] Line Profiles in Orion Bullet Wakes

New observations of H2 velocity profiles in the Orion bullet wakes are extremely difficult to reconcile with existing steady-state shock models. We have observed [FeII] 1.644µm velocity profiles of selected bullets and H2 1-0 S(1) 2.122µm velocity profiles for a series of positions along and across the corresponding bow-shock wakes. Integrated [FeII] velocity profiles of the brightest bullets are consistent with theoretical bow shock predictions. Observations of broad, singly-peaked H2 1-0 S(1) profiles in the most clearly resolved bullet wakes challenge our understanding of molecular shocks. It may be necessary to model the effects of instabilities and turbulence in the Orion bullet wakes in order to fit our observations.