The Outermost Regions of Galactic Disks Ken Freeman RSAA, ANU MNRF Symposium 7.6.05 NGC 6946: WSRT, Tom Oosterloo
Disks have a roughly exponential light distribution in R and z I(R,z) = I o exp (-R/h) exp (-z/h z ) out to R = (3 to 5) h, then often truncated truncation quantified first by van der Kruit & Searle (1981, 1982)
Reasons for the form of the observed radial light distribution are not well understood. Favored ideas include Origin of radial exponential disk: collapse of a torqued gas cloud within dark halo -> exponential gas disk in place before or during star formation gas in disk is radially redistributed by viscous torques associated with star formation -> exponential disk Now look at the radial truncation of the disk...
What is the origin of this disk truncation - very common Kregel et al (2001) find R max /h R = 3.6 ± 0.6 for 34 edge-on disk galaxies
The truncation of M33's disk (Ferguson et al 2003) M33 is a pure disk galaxy in the Local Group
M33 Surface Brightness Profile: i-band surface photometry out to R ~ 35' profile extended to R ~ 60' using star counts Disk Truncation cf. van der Kruit's (1982) disk edges: ~3-5 scalelengths, then abrupt truncation (also Pohlen et al 2002) V~31 mag arcsec -2 Ferguson et al 2005 sharp decrease in surface brightness beyond 5 scalelengths..
Corbelli et al 1989 M33 HI distribution Outer contour 2 x 10 19 cm -2 star count limit
Interpretations of the truncation radius ? the radius associated with the maximum angular momentum of the disk baryons in the proto-galaxy - unlikely - many disks have HI out far beyond the truncation radius. NGC 6946: the HI extends far beyond the stellar disk
? the radius where the gas density goes below the critical value for star formation (Kennicutt 1989) - star formation regulated by disk stability ? the radius to which the disk has grown today - unlikely ! The outer disk IS younger but still typically many Gyr old ( eg Bell & de Jong 2000, Ferguson et al 2003)
Stellar Content of the Outer Disk of M33 looks like an intermediate/old, fairly metal-poor ([Fe/H]~ -1.2) population dominating the outer disk of M33 Ferguson et al 2003
Truncation of disks is probably related to the formation of the disks: remains an interesting problem ? truncation of proto-disk by encounters (cf accretion disks : Clarke & Pringle 1993) - unlikely because pure disks show truncation ? star formation of a star-forming gas disk on viscous timescales can drive the resulting stellar disk towards an exponential profile. The outward angular momentum transport from this viscous evolution will lead to a maximum value of angular momentum in the stellar disk
Gemini observations of the outer disk of NGC 300 Bland-Hawthorn, Vlajic, Freeman, Draine, astroph/503488 Similar to M33 In Scl group, distance 2.1 Mpc Deep GMOS images 0".6 arcsec seeing 2.2 hours per field stellar photometry complete to r = 27 mag
M83 GALEX (Thilker et al 2004) Star formation in the outermost disk of M83
New evidence that the disk of M31 goes out to > 50 kpc = 10 scalelengths. Kinematics of red giants in this outer disk confirms that it is rotating almost as rapidly as the inner disk (Ibata et al 2005) and has a velocity dispersion of only about 30 km s -1 Its mean metallicity [Fe/H] ~ -0.9 Ibata et al ague that it formed from accretion of many small subgalactic structures. Its kinematics indicate that it probably came into M31 in mainly gaseous form, rather than stellar. As in all of these systems (NGC 300, M33, M31), the outer disk is probably a fairly old structure (many Gyr) The Outer Disk of M31
Summary disks typically have the exponential structure in R and z, and many are truncated radially at 3 to 5 scalelengths some disks (NGC 300, M31) extend much further, out to at least 10 scalelengths the radial exponential structure and radial truncation of galactic disks (when it occurs) are still not well understood gas accretion is likely to be important for determining the outermost structure of galactic disks