Thom et al. (2008), ApJ

Transcription

1 Star S674 along the same LOS as Complex C Star S441 along the same LOS as Complex C Thom et al. (2008), ApJ

2 Distances to HVCs From spectroscopy of high Galactic latitude stars at small angular separations in the sky. Stellar spectrum should have few absorption lines to avoid confusion (ideally early-type stars i.e., O, B, A) Early type stars extremely rare at d > 1 kpc for b > 10 degrees. Several relatively bright (+11 < m < +16) RR Lyrae stars in the Galactic Halo. But RR Lyrae have strong absorption lines Best probes are Blue Horizontal Branch (BHB) stars (though not many are known)

3 Structure Within the Disc Mapping the distances to H I clouds, CO complexes, H II regions, and OB associations CO complexes (aka molecular clouds) : extinction measurements to individual stars along the same line of sight as molecular clouds. The distances to the stars will be known. Use foreground and background stars and bracket the distance to the molecular cloud. H II regions : use the L σ relation for H II regio

4 L sigma relation for H II Regions Terlevich & Melnick 1981

5 For further reading: Melnick, Terlevich & Moles 1988, MNRAS, 235, 297 Giant H II regions as distance indicators Chavez et al. 2014, MNRAS, 442, 3565, The L-sigma relation for massive bursts of star formation

6 Structure Within the Disc Mapping the distances to H I clouds, CO complexes, H II regions, and OB associations Positions of molecular clouds outside the solar circle Positions of molecular clouds inside the solar circl Dame et al. 1986

7 Structure Within the Disc Sun The distribution of H II regions and GMCs with arm models fitted to the distribution. Galactic centre Galactic longitude is shown around the inside perimeter Hou et al. 2009

8 Structure Within the Disc

9 van de Hulst, Muller, & Oort cm emission

10 Kwee, Muller, & Westerhout cm emission

11 Dame & Thaddeus, 2011, ApJ HI and CO observations. Before that some hint from 21 cm emission studies by Kwee et al. 1954

12 Characterization of the disk, bulge and bar through observations of star light in the IR ( ): Blitz & Spergel (1991) : evidence for a bar at the Galactic center through 24 micron observations and modeling the emission with a stellar bar. Robin, Creze & Mohan (1992) : addressed the question how far does the stellar Galactic disk go? - based on observations of stars towards the Galactic anti-center. Says that Galactic stellar radius doesn t extend more than 14 kpc. Benjamin et al. (2005): GLIMPSE survey generated a huge point source catalogue using which the authors said (a) galaxy has an exponential disk (b) with radial scale length of 3.9 +/- 0.6 kpc, and (c) a bar with half length of R = 4.4 +/- 0.5 kpc at the center ( long bar ).

13 Sun Orion spur crosses the Perseus arm, linking the Sagittarius and Outer arms (aka Cygnus arm)

14 Where are we in the Galaxy? How are stars, gas and dust distributed in the Galaxy? Harlow Shapley : Globular Clusters and the Structure of the Galactic System (1918, PASP, Vol. 30) Harlow Shapley ( )

15 M3 globular cluster

16 M13 globular cluster

17 M13 globular cluster. The cepheid V2 is arrowed near its minimum (left) and maximum (right) brightness. Light curve of the star V2

18 Stellar Pulsations as Distance Indicators Period Luminosity Relationship

19 The horizontal line at z = 0 kpc corresponds to the mid-plane of the Galaxy Top panel: The spatial distribution of the Milky Way globular clusters as measured by Shapley (1918). Lower Panel: The spatial distribution from more recent data. W.E.Harris s Saas-Fee Lectures on Globular Cluster Systems (2000)

20 Where are we in the Galaxy? Estimates on the distance to the Galactic center from other groups Ghez et al. (2008) Gillessen et al. (2009) R o = 8.4 ± 0.4 kpc R o = 8.33 ± 0.35 kpc Inferences from mapping the distribution of globular clusters : There is a morphological center to the Galaxy We are not at the center R o = 8.2 ± 1.0 kpc