Download The Outermost Regions of Galactic Disks

The Outermost Regions of Galactic Disks
NGC 6946: WSRT, Tom Oosterloo
MNRF Symposium 7.6.05
Ken Freeman
RSAA, ANU
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
M33 is a pure
disk galaxy in
the Local Group
(Ferguson et al 2003)
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..
V~31 mag arcsec -2
cf. van der Kruit's (1982) disk edges: ~3-5 scalelengths, then abrupt
truncation (also Pohlen et al 2002)
Ferguson et al 2005
M33 HI distribution
Outer contour 2 x 1019 cm-2
star count limit
Corbelli et al 1989
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
? 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
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
r band, 2 GMOS imaging fields
3 completeness at r = 27
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
Outer stars are very unlikely to belong to the bulge/spheroid
of NGC 300 (if there is such a thing)
our outer field
NGC 300: HI (Puche et al 1990)
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 ?
M83
HI
Deep image: Malin
M83 GALEX
(Thilker et al 2004)
Star formation in the outermost disk of M83
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)
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