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The Outermost Regions of Galactic Disks

The Outermost Regions of Galactic Disks. NGC 6946: WSRT, Tom Oosterloo. Ken Freeman RSAA, ANU. MNRF Symposium 7.6.05. 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

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The Outermost Regions of Galactic Disks

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  1. The Outermost Regions of Galactic Disks NGC 6946: WSRT, Tom Oosterloo Ken Freeman RSAA, ANU MNRF Symposium 7.6.05

  2. Disks have a roughly exponential light distribution in R and z I(R,z) = Io exp (-R/h) exp (-z/hz) out to R = (3 to 5) h, then often truncated truncation quantified first by van der Kruit & Searle (1981, 1982)

  3. Reasons for the form of the observed radial light distribution are not well understood. Favored ideas include • Origin of radialexponential 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 ...

  4. NGC 4565 Truncated disk

  5. What is the origin of this disk truncation - very common Kregel et al (2001) find Rmax /hR = 3.6 ± 0.6 for 34 edge-on disk galaxies

  6. The truncation of M33's disk M33 is a pure disk galaxy in the Local Group (Ferguson et al 2003)

  7. V~31 mag arcsec -2 Disk Truncation M33 Surface Brightness Profile: i-band surface photometry out to R ~ 35' profile extended to R ~ 60' using star counts sharp decrease in surface brightness beyond 5 scalelengths.. cf. van der Kruit's (1982) disk edges: ~3-5 scalelengths, then abrupt truncation (also Pohlen et al 2002) Ferguson et al 2005

  8. M33 HI distribution Outer contour 2 x 1019 cm-2 star count limit Corbelli et al 1989

  9. 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

  10. ? 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)

  11. 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

  12. ? 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 ? truncation of proto-disk by encounters (cf accretion disks : Clarke & Pringle 1993) - unlikely because pure disks show truncation Truncation of disks is probably related to the formation of the disks: remains an interesting problem

  13. 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

  14. r band, 2 GMOS imaging fields

  15. 3 completeness at r = 27

  16. NGC 300: new deep r-band counts from Gemini GMOS (Bland-Hawthorn, KCF et al): exponential disk goes for at least 10 scale lengths without truncation • r-band star counts

  17. Outer stars are very unlikely to belong to the bulge/spheroid of NGC 300 (if there is such a thing)

  18. our outer field NGC 300: HI (Puche et al 1990)

  19. M83 60 ' ATCA + Parkes HI data Koribalski et al optical image (Malin) HI removed from disk of M83 by interaction with small neighbor NGC 5264 ? Where will it settle ?

  20. M83 Deep image: Malin HI

  21. M83 GALEX (Thilker et al 2004) Star formation in the outermost disk of M83

  22. The Outer Disk of M31 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)

  23. 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

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