Semester: 2002A	Gemini reference: Not Available	Partner reference: GS-2001B-03
Observing mode: queue	Instruments(s): NIRI, GMOS North	Partner ranking: 8
PI time requested: 15.0 hours	PI total from all partners: 30.0 hours (joint proposals)
PI future time requested: 0.0 nights	PI minimum time requested: 8.0 hours
NTAC recommended time: 10.0 hours	NTAC minimum recommended: 10.0 hours
Proposal submitted to: Gemini Staff	...also to...Canada, United States (joint proposals)

Science Justification

A number of studies of molecular hydrogen and its excitation in galaxies have been made over the last decade (e.g. Joseph et al. 1984; Puxley, Hawarden & Mountain 1988, 1990; Moorwood & Oliva 1990; Kawara & Taniguchi 1993; van der Werf et al. 1993). Early indications from low spatial and spectral resolution H2 line ratios that the gas was always shock excited were challenged by parallel theoretical developments showing that in dense, warm photo-dissociation regions heated by UV-photons the low-lying (and brightest) transitions could be thermalised. Moreover, Puxley et al. (1990) argued that given the observed Br-gamma recombination line emission, the copious non-ionizing UV photons would be expected to produce substantial H2 emission when absorbed by molecular gas. Indeed, the different dependences of H and H2 emission on incident UV flux provided a diagnostic of the relative spatial distributions of exciting stars and gas. Despite these arguments, however, it remains widespread in the current literature that H2 in galaxies is shock excited (by supernova remnants and stellar outflows), although these studies are invariably without detection or reliable analysis of the critical higher level line fluxes.

One of the principal difficulties in determining accurate fluxes for H2 lines above the lowest v=1-0 transitions is the strength of the underlying stellar continuum and the presence of metal absorption features in the cool stellar atmospheres. Even with very high S/N data, local variations in stellar population and element abundance produces systematic features after subtraction of empirical or theoretical stellar templates. This restricts the detectability in galaxies with strong continua to transitions brighter than those required to definitively determine the H2 excitation.

The wider astrophysical context for studying H2 emission from galaxies is in understanding the interaction between massive stars and their environment (e.g. the relative importance of the excitation mechanisms, the effect of the formation of massive star clusters on the local ISM and its feedback into the star-formation process), the distribution of stars and interstellar material and the subsequent evolution of the starburst. Paradoxically, within our own galaxy it can be difficult to establish, say, the H and H2 emission and excitation mechanisms for a single star-forming complex. For example, although the peak H2 flux from shocks is 100 times that from the diffuse component in Orion, only by integrating over a 7 x 9 arcmin2 region is it revealed that the shocked and UV-excited luminosities are equal (Burton & Puxley 1990; Usuda et al. 1996). Observation of a galaxy provides such a census of the entire emission in a single measurement.

During an IR spectroscopic study of several Wolf-Rayet galaxies, four lines of H2 were serendipitously detected in the nearby dwarf galaxy NGC5253 (Lumsden, Puxley & Doherty 1994) : 1-0 S(1), 1-0 S(0) and marginally 3-2 S(3) and 2-1 S(1). This observation represented the first suggestive evidence for the dominance of UV-photon excitation of H2 in at least some galaxies, now strongly supported by the spectrum of NGC5461 in M101 (Fig. 1).

Proposed Observations

The NGC5253 and NGC5461 observations raise several interesting questions: Is UV-excitation of H2 actually a general phenomenon in galaxies? What are the relative fractions of UV- and thermally-excited gas? What does the UV-excited component and hydrogen recombination line emission imply about the physical relationship of sources in the emitting region e.g. are they consistent with the models described by Puxley, Hawarden & Mountain (1988, 1990)?

To address these questions we require measurements of the H2 line excitations sufficient to determine the contributions from UV irradiated low and high density gas, and from shocks. Hence we propose observations of a sample of weak continuum, dwarf galaxies. All of the galaxies have been detected in the 1-0 S(1) H2 transition (Doherty, Puxley, Lumsden & Doyon 1995). The weakness of the continua provides two benefits (i) a relatively large H2-to-continuum ration and (ii) weak underlying stellar absorption features.

Technical Justification

The expected continuum flux density in NGC5253 is 0.5 x 10-15 W/m2/um (K ~ 14.7) assuming approximately uniform surface brightness in scaling from our previous 3 x 3 arcsec^2 measurement.

Similar previous observations of the rest of the sample gives expected continuum flux densities of 0.5 - 2 x 10-15 W/m2/um (K ~13.3-14.8) and 1-0 S(1) line / continuum ratios of 0.5-1.0. The galaxies with weaker continua have larger line/continuum ratios and thus we expect similar integration times.

The total request to observe the four targets, allowing 1 hour per target for calibration and acquisition, and 75% efficiency on long integrations is 15 hours.

Observation Details

Resources

• Gemini North
  • Imaging (NIRI)
    • Camera
      • f/6 (0.12 arcsec)
    • Filter
      • Broad-Band
        • J (1.25 um)
        • H (1.65 um)
        • K (2.20 um)
      • Narrow-Band
        • H-continuum (1.570 um)
        • J-continuum (1.207 um)
        • K-continuum (2.090 um)
  • GMOS Spec1 (GMOS North)
    • Focal Plane Unit
      • Longslit 0.50 arcsec
      • Longslit 0.75 arcsec
    • Disperser
      • Mirror
    • Filter
      • g_G0301
      • r_G0303
      • OG515_G0306

Observing Conditions

Name	Image Quality	Sky Background	Water Vapor	Cloud Cover
Global Default	Any	Any	Any	Any
Best conditions	20%	20%	20%	20%

Scheduling Information:

Optimal dates:

Impossible dates: 2001/5/15-2001/6/13
Reason: SPIE meeting

Allocation Committee Comments

NTAC Comment

Additional Information

Keywords: Emission lines, Starburst galaxies

Publications: Publications:

• Puxley, Doyon & Ward (1996). The spatial distribution of stellar CO absorption in M83, ApJ, 476, 120.
• Puxley (1997). "Multi-wavelength hydrogen recombination lines in the compact HII region K3-50a" , Star-formation with ISO, in press."
• Harrison, Puxley, Brand & Russel. "Molecular hydrogen and the ortho/para ration in NGC253", MNRAS, in press.
• Puxley & Skinner (1996). "Search for IR positronium emission from the great annihilator", CTIO/ESO conf. on the Galactic Centre, in press.
• Lumsden & Puxley (1996). "Near IR spectroscopy of the ultracompact HII region G45.12". MNRAS, 281, 493.

Proposal Contents