Magne&c fields in HVCs

Transcription

1 Magne&c fields in HVCs Alex S Hill Naomi McClure- Griffiths, Bob Benjamin, Ann Mao, Jay Lockman, Bryan Gaensler, Greg Madsen CSIRO ASTRONOMY & SPACE SCEINCE

2 ARI 19 July :54 Cloud life&me N HI (10 18 cm 2 ) Velocity (km s 1 ) (GSR) N HI and velocity Declination (J2000) h 58 m 56 m 54 m Galactic latitude ( ) Figure h 00 m 6 h 58 m 56 m 54 m Right ascension (J2000) Galactic longitude ( ) Left: Head-tail HI halo clouds with the outer contour at N HI = cm 2 (15.5-arcmin resolution; Putman, Saul & Putman, Mets 2011). Right: Peek, A HI high& velocity Joung cloud that (2012 shows signatures ARA&A) of interaction with the halo medium from the edge features (4-arcmin resolution; Peek et al. 2007). Putman, Saul, & Mets (2011 MNRAS) Peek et al (2007 ApJ) cloud (the fraction escaping normal to the disk is f esc 6% in this model) (Bland-Hawthorn et al. 1998; Bland-Hawthorn & Maloney 1999, 2002; Bland-Hawthorn & Putman 2001; Putman et al. 2003a). Most of the distances derived from Hα observations of HVCs have been consistent with their direct distance constraints using halo stars. Clouds without direct distance constraints (in particular 2 Magne(c the CHVCs) fields in HVCs are also Alex usually S Hilldetected in Hα emission, which is consistent with them being within 40 kpc of the Galactic disk (Tufte et al. 2002, Putman et al. 2003a). The striking Figure 5. Distance D 0 and timescale τ 0 within which a cloud of given mass will lose its total H i content. Each track represents the time sequence of one model, with each point indicating D 0 or τ 0 of the largest fragment (see Equations (3) and (4) and text). Thus, the disruption rate dm/dt increases over time. Colors denote the cloud velocities, showing that faster clouds are shorter-lived. Symbol sizes indicate model times, with large symbols for early times and small ones for late times. Note that D 0 is not a distance above the plane, but a disruption length scale. Heitsch & Putman (2009 ApJ)

3 Magne&c fields Hydrodynamics: HVCs should lose much of their neutral H before reaching the disk MagnePc fields could stabilise clouds Few numerical simulapons (Konz et al 2002, SanPllan et al 2004) or observapons to date Measure rotapon measure using Faraday rotapon RM n e B ds 3 Magne(c fields in HVCs Alex S Hill

4 o. 2, 2009 Rota&on measure ROTATION MEASURE IMAGE OF THE SKY Figure 3. Plot of 37,543 RM values over the sky north of δ = 40. Red circles are positive rotation measure and blue circles are negative. The size of the circle scales linearly with magnitude of rotation measure. Faraday rotapon towards extragalacpc point sources from NVSS, p > 0.4 mjy Taylor et al (2009 ApJ) Magne(c fields in HVCs Alex S Hill

5 HVC HVC in Leading Arm of Magellanic System Head- tail morphology H I RM > +σ RM < - σ RM < σ McClure- Griffiths et al (2010 ApJ) 5 Magne(c fields in HVCs Alex S Hill

6 HVC EsPmate magnepc field WHAM Hα: EM < 0.09 pc cm with assumppons, N(H + ) cm N(H I) > cm - 2 RM n e B L implies B +6 μg (towards observer) N(H I) (10 19 cm - 2 ) H I RM HVC > +σ RM HVC < - σ RM HVC < σ McClure- Griffiths et al (2010 ApJ) 6 Magne(c fields in HVCs Alex S Hill

7 Smith Cloud H I v GSR = +247 km/s (v LSR = +100 km/s at head) 7 Magne(c fields in HVCs Alex S Hill

8 Smith Cloud H I v GSR = +247 km/s (v LSR = +100 km/s at head) RM > 0 RM < 0 8 Magne(c fields in HVCs Alex S Hill

9 Decelerated gas v LSR +100 km/s +40 km/s 9 Magne(c fields in HVCs Alex S Hill

10 Decelerated gas v LSR +100 km/s +40 km/s 10 Magne(c fields in HVCs Alex S Hill

11 Decelerated gas v LSR +100 km/s +40 km/s 11 Magne(c fields in HVCs Alex S Hill

12 Hα, H I, and RMs +40 km/s H I +100 km/s H I +40 km/s WHAM- NSS Hα (grayscale) 12 Magne(c fields in HVCs Alex S Hill

13 Hα, H I, and RMs +40 km/s H I +100 km/s H I +40 km/s WHAM- NSS Hα (grayscale) RM HVC > 0 RM HVC < 0 13 Magne(c fields in HVCs Alex S Hill

14 Hα, H I, and RMs +40 km/s H I +100 km/s H I +40 km/s WHAM- NSS Hα (grayscale) RM HVC > 0 RM HVC < 0 n e s EM L H + B = RM 0.81n e L H + RM 0.81(L H +EM) 1/2 14 Magne(c fields in HVCs Alex S Hill

15 Hα, H I, and RMs +40 km/s H I +100 km/s H I +40 km/s WHAM- NSS Hα (grayscale) RM HVC > 0 RM HVC < 0 n e s EM L H + B = RM 0.81n e L H + RM 0.81(L H +EM) 1/2 RM = 95 rad m - 2 (σ =23 rad m - 2 ) EM = 1.1 pc cm - 6 (σ=0.7 pc cm - 6 ) B +6 μg (towards observer) 15 Magne(c fields in HVCs Alex S Hill

16 Conclusions RM signature evident in two HVCs Correlated with H I emission from HVC in Leading Arm - non- detecpon of Hα Correlated with decelerated H I and Hα in Smith Cloud - not correlated with (smooth) Hα at Smith Cloud velocity lower limit on B of 6 μg in each case - B ~ 4 μg sufficient to balance ram pressure for Leading Arm HVC These head- tail HVCs are in very different environments Real data for MHD simulapons? 16 Magne(c fields in HVCs Alex S Hill

17 Thank you Alex S Hill OCE Postdoctoral Fellow p e alex.hill@csiro.au CSIRO ASTRONOMY & SPACE SCIENCE