The Three Types of Galaxy Disk
A study of the surface-brightness profiles of more than 100 S0–Sb galaxies points to a strong
connection between bars and the structure of galaxy disks. We find three profile types: pure exponential (Type I); profiles which are steeper beyond a “break radius” (Type II or “truncations”;
Freeman 1970); and profiles which become shallower beyond a break radius (Type III or “antitruncations”; Erwin et al. 2005). Most Type II profiles in S0–Sb galaxies are probably related
to the outer Lindblad resonance of bars, and do not represent truncations due to star-formation
thresholds (common in late-type spirals). Type III profiles are mostly changes in disk structure,
not excess light from spheroids. They may represent side-effects of minor, gas-rich mergers, and
are strongly anti-correlated with bar strength: the weaker the bar, the more likely the galaxy has
a Type III profile.
Type I
Type II
Type III
(NGC 4245)
(NGC 936)
(NGC 4371)
outer extent of bar
break
break
sky uncertainty limit
Combining results from Pohlen & Trujillo (2006), Erwin et al. (2008), and Gutiérrez et al. (2010), the
frequency of disk profile types over most of the Hubble sequence (S0–Sd) is: Type I = 20%; Type II =
52%; Type III = 38% (including galaxies with inner Type II profiles). Type II profiles are much more
common in barred galaxies; Type III profiles show the opposite behavior. In late-type spirals, Type II
profiles may be mostly due to star formation thresholds; but in S0–Sb disks, Type II profiles are usually
associated with outer rings in barred galaxies and may be a result of bar-driven secular evolution.
References:
• Erwin et al. 2005, ApJL, 626, L81
• Erwin et al. 2008, AJ, 135, 20
• Freeman 1970, ApJ, 160, 811
• Gutiérrez et al. 2010, in prep
• Pohlen & Trujillo 2006, A&A, 454, 759
S0
Fraction
The fraction of disk profile types also shows
strong trends with Hubble type (right): in
particular, Type II profiles (“truncations”) are
much more common in late-type spirals, while
Type I (single-exponential) profiles are most
common in S0 and early-type spirals.
Sa
Sb
Sc
Type III
Type I
Type II
Hubble Type T
P. Erwin, L. Gutiérrez, J.E. Beckman, M. Pohlen
Sd
Sm