Photometric Properties of Spiral Galaxies

1
Photometric Properties of Spiral Galaxies

• Bulges
• Luminosity profiles fit r1/4 or r1/n laws
• Structure appears similar to Es, except bulges
  are more flattened (though bulges can be quite
  different from Es dynamically)

NGC 7331 Sb galaxy R-band isophotes

• Disks
• Many are well-represented by an exponential
  profile
• I(R) Ioe-R/Rd (Freeman 1970)

Disk scale length
Central surface brightness (Id in BM)
2
NGC 7331
(Rd)
(?R)

• Bulge dominates in center and again at very large
  radii (if bulge obeyed r1/4 to large R)
• Disk dominates at intermediate radii
• Rd 1 - 10 kpc (I-band 20 longer in B-band -
  why?)
• Disk in many spirals appear to end at some Rmax
  around 10 to 30 kpc or (3-5Rd)

3
(van der Kruit 1978)

• Io (B-band) 21.5 ? 0.5 (or ?oB)
• Freemans Law (1970) - found that almost all
  spirals had disk surface brightness around this
  value
• Partly a selection effect since low-surface
  brightness (LSB) galaxies are harder to identify
• Many LSB disks identified since
• extreme case - Malin 1 (Io 25.5 and Rd55 kpc!)

4
Ursa Major galaxy group
Open circles I(0)gt19.5

• Spirals get bluer and fainter along the sequence
  SO ? Sd
• S0 color like K giant stars most young, blue
  stars absent
• Later types have more young stars

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Disks - Vertical Distribution of Starlight

• Disks are puffed up by vertical motions of stars
• Observations of edge-on disks (and MW stars) show
  the luminosity density is approximated by

j(R,z) joe-R/Rdsech2(z/2zo) for RltRmax

Scale height (sometimes ze which is 2zo) van der
Kruit and Searle (1981,1982)

• At face-on inclination, obeys exponential SB law
• At large z, j(z) joexp(-z/zo) in SB ?
  I(R,z) I(R)exp(-z/zo)
• Theoretical distribution of a self-gravitating
  sheet (see BT)
• Disks fit well with typical Rd and Rmax values
  and zo 350 pc
• Scale height is constant with radius R does
  this seem odd?

6

• Examine disk dynamics (board derivation) to
  understand implications of zo?zo(R)
• ?Vz2? 2?G?zo
• If zo is constant with R, and ? decreases with
  increasing R, ?Vz2? must also decrease with
  increasing R. Why does Vz decrease with radius ?
• Disk is continually heated by random acceleration
  of disk stars by GMCs
• Number of GMCs decrease with radius

Alternatively, could be explained by including
mass density of atomic and molecular gas ( fact
that disks do show some flaring or increased
scale height with R)
(Narayan Jog 2002)
7

• Scale height varies strongly with stellar type
• zo 100 pc for young stars
• zo 400 pc for older stars
• In addition to the main disk, there is evidence
  for a thick disk in some galaxies (including our
  own) with zo1 kpc
• Mostly older stars
• Formed either through puffing up of disk stars
  (e.g. via minor merger?)

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Homework Assignment 1 Galaxy classification SB
Profile fitting Choose one galaxy, extract an
azimuthally averaged surface brightness profile,
calibrate counts to surface brightness units, and
fit the bulge and disk to r1/4 and exponential
functions, respectively. Derive a) effective
radius and surface brightness for the bulge (Ie
and Re) b) scale length and central surface
brightness for the disk (Rd and I0) c) bulge/disk
luminosity ratio