Measuring the Local Intergalactic Ionizing Background: Do Intergalactic HI Clouds Glow?

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Session 60 -- Intergalactic Medium
Display presentation, Thursday, 2, 1994, 9:20-6:30

[60.05] Measuring the Local Intergalactic Ionizing Background: Do Intergalactic HI Clouds Glow?

Megan Donahue (STSCI), Greg Aldering (UMinn), John T. Stocke (UColo)

We report on our efforts to measure the H$\alpha$ surface brightness of local, intergalactic HI clouds using narrow-band CCD observations obtained with the Burrell-Schmidt on Kitt Peak. These observations are used to set limits on photoionization by the local metagalactic radiation background. Our current detection limits for the Leo Ring and the N4631 group are $\mu_{H\alpha} < 102$ mR, set by the RMS variations in off-cloud regions of these fields. This limit corresponds to a background ionizing intensity of $Jo(13.6 \, {\rm eV})$ $< 1.2 \times 10^{-22}$ erg s$^{-1}$ cm$^{-2}$ sr$^{-1}$ Hz$^{-1}$ or an ionizing flux at the face of the cloud of $5.6 \times 10^4$ phot cm$^{-2}$ s$^{-1}$. These values are consistent with estimates of the local ionizing background from quasar counts, the ``proximity'' effect in local Lyman-$\alpha$ forest clouds, models to reproduce the sharp edges in the HI-profiles of disks, and extrapolations from the far-UV background.

Our limits are determined by how well we can ``flatten'' the night sky, rather than by photon statistics. Flattening is limited by how well we can characterize the systematics of the CCD, by intrinsic spatial variations in the night sky, and by temporal changes in OH line-induced fringing in the CCD. We will give a thorough discussion of our treatment of these limiting factors. In addition, we hope to have improved limits based on new data on these fields and the intergalactic HI cloud in Virgo by the time of the meeting. We will also report on the detection of an extremely low surface brightness tidal tail in the N4631 group.

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