optical observations of high-latitude clouds

Optical Observations of High-Latitude Clouds

Adolf N. Witt

University of Toledo

Collaborators:

Steve Mandel, Hidden Valley Obs., Soquel, CAThomas G. Dixon, Univ. of Hertfordshire, UKPaul H. Sell, Univ. of ToledoKarl D. Gordon, Univ. of ArizonaUma P. Vijh, STScI

With support from:

NSF Galactic Astronomy ProgramRC Optical SystemsSanta Barbara Instrument Group, Inc.Software BisqueAstrodon

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, L19Led 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 morphologyof 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 102 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

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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

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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.

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Focus: Large-Scale Cloud MorphologyMBM 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 AbsorptionM 81; Image Credit: Tony Hallas

http://www.astrophoto.com/ (with permission)

M81 detail

Linear structures again; nT ~ 106 K cm-3

Same M81 cirrus observed in CO by A. Heithausen2006, A&A, 450, 193

cirrus resolves into small molecular clumps

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Small-area molecular clumps (SAMS) near M81

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Combining optical structure data with IR and radio structure data can extend 2-D angular power spectrum (S. J. Gibson, 2006, SINS)

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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.