1. Neutral Atomic Hydrogen Gas at Forbidden Velocities in the Galactic Plane Ji-hyun Kang NAIC Seoul National University Supervisor :Bon-Chul Koo 213 th AAS

2. Faint protruding wing-like features beyond the velocity boundaries. Smoothly extended from disk emission, different from HVC. Sites where some violent events expel their kinetic energies into the ISM. Forbidden-Velocity Wings HI 21cm (l,v) diagram at b=-0°.5 using LDS HVC

3. Possibility as missing old SNRs known Galactic SNRs : 265 (Green's 2006 catalog) SN rate x lifetime ~ 1/50yr x 10 5-6 yrs = 2 x 10 3-4  # of the expected >> # of the observed –Most of them are not visible due to background emission in radio –Old SNRs are faint in radio/X-ray Fast-expanding HI shells of old SNRs V exp = 100~200 km/s when t = (3~8) x 10 4 yr  FVWs could be missing old SNRs??? W44 Koo & Heiles(1995)

4. Catalogue of FVWs Kang & Koo 2007 ApJS, 173, 85 LDS/SGPS HI surveys (|b| 20 km/s, size < 2° 87 FVWs are identified.

5. Distribution of FVWs l~150°, two peaks at l=80°, 210° b~2°, FWHM~13° >> FWHM of SNRs or HII regions (~1°) SNRs (6), galaxies (4), high-velocity clouds (3) 85% are not coincident with known objects

6. Comparison with Expected SNRs SNRs expected at FV (Koo & Kang 2004) –Assumption : 2-dimentional disk with WNM Their distributions look different. –Wrong assumption in SNR model?  ISM filled with tenuous hot gas? –Selection effects of FVWs? –SNR may not majority of FVWs. Distribution of expected SNRs in the Galactic Plane

7. High-Resolution HI Observations 2004 Feb -2006 June : 4 session –Arecibo 305-m radio telescope ( 3'.4 ) Arecibo L-band Feed Array (ALFA) –Green Bank 100-m telescope ( 9 ' ) + (1.4 GHz radio continuum) –22 FVWs (~30% of unknown nature) are observed.

8. Results : Images of 22 FVWs High-resolution HI observations reveal various features of FVWs 1°

9. Classification Channel image / position-velocity diagram Shell-type (12-13/22) –shell structures in images or small scale wing-like feature in (p,v) diagram –shell –type FVWs could be part of expanding shells Cloud-type (9-10/22) –isolated peak in (p,v) images 50km/s 40km/s

10. Shell-Type FVWs About 60% (12-13/22) are shell-type Expanding :5, Stationary :4, Irregular :3  part of expanding shells! Significant fractions seem to be expanding shells! FVWs could be good candidates for old SNRs Most shell-type FVWs do not have OB stars inside Most do not have related emission at other frequencies 3 FVWs with OB stars : future study required FVW 99.5-7.5 HI intensity as a function of V LSR

11. Shell-Type FVWs About 60% (12-13/22) are shell-type Expanding :5, Stationary :4, Irregular :3  part of expanding shells! Significant fractions seem to be expanding shells! FVWs could be good candidates for old SNRs Most shell-type FVWs do not have OB stars inside Most do not have related emission at other frequencies 3 FVWs with OB stars : future study required FVW 99.5-7.5 HI intensity as a function of V LSR (p,v) diagram of FVW190.2+1.2 (p,v) diagram of toy shell model V exp = 100 km/s V dispersion = 10km/s Only local background emission was considered

12. Expanding Shell-like FVWs E k ~ 10 48 -10 50 ergs, V exp ~ 20 - 110 km/s

13. FVW173.0+1.5 : ‘Missing’ SNR? Positional correlation between HI emission and continuum emission Spectral index (F~ν -α ) (Effelsberg 2695 MHz, WENSS 327 MHz) –α =0.42-0.52  Nonthermal!, close to typical shell type SNRs (α~0.5) If in the Perseus Arm (~2 kpc) –HI mass : 960 Mo (uncertainty~40%) @15~45 km/s –V sys ~ -10 km/s, V exp ~55 km/s, energy : 5e+49 erg, size : 90 x 120pc Blue: 15 – 25 km/s Green: 25 – 35 km/s Red : 35 – 45 km/s

14. FVW173.0+1.5 : ‘Missing’ SNR? Positional correlation between HI emission and continuum emission Spectral index (F~ν -α ) (Effelsberg 2695 MHz, WENSS 327 MHz) –α =0.42-0.52  Nonthermal!, close to typical shell type SNRs (α~0.5) If in the Perseus Arm (~2 kpc) –HI mass : 960 Mo (uncertainty~40%) @15~45 km/s –V sys ~ -10 km/s, V exp ~55 km/s, energy : 5e+49 erg, size : 90 x 120pc Blue: 15 – 25 km/s Green: 25 – 35 km/s Red : 35 – 45 km/s Contours : Effelsberg 2695MHz

15. Cloud-Type FVWs About 40% (9-10/22) are cloud-type.  Filamentary in images, related emission at lower velocities : interaction with Galactic disk? wide (20-25km/s) : 3  HVCs/IVCs narrow (4-10km/s) : 7  halo clouds (e.g., Lockman 2002) Origins of halo clouds ? (e.g., Stanimirovic et al. 2006) Shell fragmentation Accretion of external clouds Galactic fountain, thermal condensation in turbulent flow

16. Summary FVWs are high-velocity features protruding from their surroundings in large-scale (l,v) diagrams of HI data in the Galactic plane. Their high-velocities imply that some dynamical phenomena associated. We identified 87 FVWs in low-resolution (36’) HI survey data. We compared them with catalogs of SNRs, galaxies, and HVCs, and 85% of FVWs are not associated with them. 22 FVWs (30% of total FVWs with unknown nature) have been observed at high resolution in the 21-cm emission line. 60% seem to be expanding shells. Many of them do not have early type stars inside, nor related emission at other frequencies. These FVWs are good candidates for old Galactic SNRs invisible in radio. A careful study of FVW statistics may provide a clue on the nature of the Galactic ISM. 40% seem to be cloud complexes and appear to be halo clouds or HVCs. The origin of these anomalous velocity clouds is not known, and we hope our samples could provide a clue on it, too.