Arecibo Survey(s) of the Magellanic Stream: Past & Future

1. Arecibo Survey(s) of the Magellanic Stream: Past & Future Snezana Stanimirovic (UC Berkeley) • Recent Arecibo HI observations of the Magellanic Stream • Results and Motivation for Future Surveys • Future Surveys of the Magellanic Stream with ALFA • Baseline Removal & Standing Wave Concerns People involved: J. Dickey (Univ. of Minnesota), M. Krco (Colgate Univ.), A. Brooks (Columbia Univ.), A. Hedden (Univ. of Arizona)

2. HI Observational Perspective Parkes Multibeam data, 15 arcmin Putman et al. (2002) MS V MS VI Arecibo data, 3 arcmin Stanimirovic et al. (2002) Old Parkes data, 15 arcmin Mathewson & Ford (1984)

3. Recent Arecibo (305 m) HI Observations • Hidden Beast of a Mapping Machine!!! • All spatial scales ! • Good spatial resolution, 3-4 arcmin. • Great sensitivity •  Allows fast mapping • On-the-fly mapping of many small maps (10’x10’): drive in RA (4’’/s), step in Dec (2’) and record data every 30 sec. • B=6.25 MHz, N=1024, Δv ~ 1 km/s, final θ ~ 4 arcmin. • Data processing performed completely in IDL. • Calibration, baseline removal by fitting a polynomial. • Maps combined during the gridding process using AO_Gridzilla. • Philosophy: Calibrate quickly, make maps & follow interesting features! Maps produced almost in real time!

4. The AO_Gridzilla Strikes !

5. MS VI: Morphology & Velocity Field Velocity Channels Broad velocity profiles DEC • Broad line profiles. • Compression fronts, head-tail morphology, velocity gradients. •  Blending of clumps & their interaction with the surrounding medium. RA

6. Main Results: • Complex and interesting morphology found far away from the Magellanic Clouds.Bow-shock-like features, head-tail morphology, interesting velocity gradients. • Magellanic Stream as a thermometer for the outer Halo: • We have investigated confinement mechanism of several isolated Stream clumps. • Clumps are not in free expansion. They can not be gravitationally stable unless large amounts of dark matter are involved. • Clump properties are the easiest explained with external pressure confinement by the hot Galactic Halo gas. • Upper limit on the Halo density can be estimated. • Fully sampled, large-scale survey of the northern Stream will find many more clumps and allow building of a 2-D distribution of the Galactic Halo density.

7. Clumps Must be Confined by the Hot Galactic Halo gas ! nh ~ 10 -3 cm-3 at z ~ 15 kpc nh~ 3x10-4 cm-3 at z ~ 45 kpc

8. Future GALFA Survey(s) of the Magellanic Stream: an example • To survey whole northern part of the Stream visible from Arecibo: • (l=90 +/-10 deg, b=-55 to -20 deg) ~ 800 square degrees • B ~ 6 MHz • N ~ 2k • S ~ 0.1 K (or less) • scanning in RA with rate of 12 “/sec and stepping in Dec by 4.5’ requires 75 min per square degree or 1200—1400 hours for the whole survey. • As lines are broad a good technique for baseline removal is essential.

9. Arecibo Baselines: an example Multiple standing waves whose cancellation is a real challenge.

10. Best Technique for Baseline Removal ? • Position switching: - using nearby reference spectrum, e.g. Lizano et al. (1988) - using ‘empty’ edge spectra - difficult when sources are very extended • Polynomial fitting --- hard when profiles are broad • Frequency switching --- currently not possible due to complex spectral standing waves - Briggs et al. (1997) were able to characterize standing waves pre- upgrade and successfully remove them from drift-scan data - standing wave serious characterization hasn’t been done after the upgrade - Salter & Ghosh (2001): successfully characterized standing waves due to a point source in the main beam - Stanimirovic & Salter (2002): explored freq. switching and signal recovery algorithms Serious concern and requires serious investigation.