Optical Observations of High- Latitude Clouds Adolf N. Witt University of Toledo

Collaborators: Steve Mandel, Hidden Valley Obs., Soquel, CA Thomas G. Dixon, Univ. of Hertfordshire, UK Paul H. Sell, Univ. of Toledo Karl D. Gordon, Univ. of Arizona Uma P. Vijh, STScI With support from: NSF Galactic Astronomy Program RC Optical Systems Santa Barbara Instrument Group, Inc. Software Bisque Astrodon

First optical studies of HLCs by Allan Sandage, 1976, AJ, 81, 954, with M81

“Re-discovery” of HLCs by IRAS, F. J. Low et al. 1984, ApJ, 278, L19 Led to new designation as “IR Cirrus” Followed by detailed studies at mm-wavelengths (CO) by Magnani et al.

High-Latitude Clouds are ideal for studying the morphology of the diffuse interstellar medium. Nearby, ~ 100 pc Individual “clouds” seen in relative isolation Little line-of-sight confusion Free of star-formation effects Wide range of optical depths Atomic as well as molecular gas Typical cloud “sizes” can be measured directly We can see actual “clouds”

Advantages of Optical Imaging Approach Combines high spatial resolution (arc sec) with large field of view Sensitive to column density of dust ---> total gas column density Insensitive to dust temperature Independent of atomic or molecular state of the gas as long as dust-to-gas ratio is constant Offers ideal conditions for study of dust luminescence Direct evidence for small-scale structure on few 10 2 AU scale Low cost Disadvantages ( No such thing as a free lunch!!) Low surface brightness (few % of that of the dark night sky)

Instrumentation Remotely operated, self-guided small telescopes, equipped with CCD cameras. Detection of diffuse, extended sources depends only on f-ratio, not aperture. Location: New Mexico Skies Altitude ~7300 ft, near Cloudcroft, NM Primary Goal: Determination of the optical SEDs of HLCs

Optical Bandpasses Special BGRIHa filter set during Phase1 of our program 15-band BATC filter set during Phase 2 of our program. These filters allow the determination of a detailed cloud SED while avoiding the strongest emission features of the permanent airglow from the Earth atmosphere.

What makes a high-latitude cloud shine? Scattering of the Galactic interstellar radiation field by dust. Photoluminescence by nanoparticles, primarily Extended Red Emission excited by far-UV photons from the ISRF. H-alpha in emission, mostly from Galactic HII regions, scattered by dust in the HLCs. This is extended red emission ---->

How important is ERE? about 30% of total surface brightness of HLCs near 600 nm at intermediate latitudes ERE is relatively important, because dust scattering is not very efficient at high galactic latitudes.

Here’s why: Illumination of HLCs comes mainly from the Milky Way Most of the light is scattered with scattering angles ~90 0 HG phase function for forward- scattering grains ---> highly inefficient at ~ 90 0 ERE is emitted isotropically, no dependence on direction of incoming illuminating radiation; depends only on density of UV radiation.

Focus: Large-Scale Cloud Morphology MBM 12

MBM 30

MBM 32

Focus: Small-Scale Structure in MBM 12 Linear structures ~ 500 AU wide

MBM 12 Sharp edges

MBM 12

MBM 30

Focus: Small-Scale Structure in Absorption M 81; Image Credit: Tony Hallas (with permission)

M81 detail Linear structures again; nT ~ 10 6 K cm -3

Same M81 cirrus observed in CO by A. Heithausen 2006, A&A, 450, 193 cirrus resolves into small molecular clumps

Small-area molecular clumps (SAMS) near M81

Combining optical structure data with IR and radio structure data can extend 2-D angular power spectrum (S. J. Gibson, 2006, SINS) We are collaborating with Steven Gibson on the analysis of our data.

Conclusions Optical imaging data of high-latitude clouds reveal ISM morphology and small-scale structures over 3 orders of magnitude of linear scale Cloud morphology: Clumpy cores embedded in low-density envelope Small-scale structure: linear strands of high-density gas, ~500 AU Under special conditions (e.g. M81) structure in Galactic ISM can be observed in absorption. Illumination geometry of high-latitude clouds makes them ideal test beds for studies of extended red emission (ERE) in the diffuse ISM.