The Outermost Regions of Galactic Disks

Ken FreemanRSAA, 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) = 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)

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 ...

NGC 4565

Truncated disk

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

The truncation of

M33's disk

(Ferguson et al 2003)

M33 is a puredisk 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 distributionOuter contour 2 x 1019 cm-2

star count limit

Interpretations of the truncation radius

? the radius associated with the maximum angular momentumof 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 300Bland-Hawthorn, Vlajic, Freeman, Draine, astroph/503488

Similar to M33In Scl group,distance 2.1 Mpc

Deep GMOS images0".6 arcsec seeing2.2 hours per field

stellar photometry complete to r = 27 mag

r band, 2 GMOS imaging fields

3 completeness at r = 27

• r-band star counts

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

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

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

M83

ATCA + Parkes HI data

Koribalski et al optical image (Malin)

60 '

HI removed from disk of M83 by interaction with smallneighbor NGC 5264 ? Where will it settle ?

M83

Deep image: MalinHI

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 thisouter disk confirms that it is rotating almost as rapidlyas the inner disk (Ibata et al 2005) and has a velocitydispersion 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 outerdisk 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