Observational Data

Origin of Elliptical Galaxies
Intensive Lecture @ Kyoto University
12-14 December 2005
Nobuo Arimoto (NAOJ, Tokyo)
Cen A
Hubble Sequence
Brightness of Galaxies
Sandage et al. (1985) AJ 90, 1759
Virgo, Coma, Fornax銀河団
Elliptical Galaxies
Binney & Tremaine (1987)
Galactic Dynamics (Princeton)
Elliptical galaxies are smooth, features population II systems containing little
or no gas or dust. The fraction of bright galaxies that are elliptical is a
function of the local density, ranging from about 10% in low-density regions
to 40% in dense clusters of galaxies. The isophotes (contours of surface
brightness) are approximately concentric ellipse, with axis ratio b/a ranging
from 1 to 0.3. Elliptical galaxies are denoted by the symbols E0, E1, etc, where
the brightest isophotes of a galaxy of type En have axis ratio b/a=1-n/10.
The ellipticity is ε=1-b/a. Thus the most elongated elliptical galaxies are
type E7. Since we see only the projected brightness distribution it is
Impossible to determine directly whether elliptical galaxies are axisymmetric
or triaxial.
Elliptical Galaxies
Binney & Tremaine (1987)
Galactic Dynamics (Princeton)
The surface brightness of an elliptical galaxy falls off smoothly with radius.
Often the outermost parts of a galaxy are undetectable against the
background night-sky brightness. The surface-brightness profiles of most
elliptical galaxies can be fit by the R 1/4 or de Vaucouleurs (1948) law.
I(R) = I(0) exp(-kR1/4) = Ie exp{-7.67[(R/Re)1/4-1]},
where the effective radius Re is the radius of the isophote containing half
of the total luminosity and Ie is the surface brightness at Re.
The effective radius is typically 3h -1kpc for bright ellipticals and is
smaller for fainter galaxies (Kormendy 1977).
Because galaxies do not generally have a sharp outer edge, it is conventional
to specify their total spatial extent by the Holmberg radius, the radius
of the isophote corresponding to surface brightness 26.5 mag (arcsec) -2
in the B-band. This is roughly 1%-2% of sky brightness and is the lowest
surface brightness that can in general be reliably measured.
Effective Radius vs Absolute MB
Sandage & Perelmuter (1990) ApJ 361, 1
暗い楕円銀河ほど
有効半径は小さい
暗い楕円銀河ほど表面輝度が一定の線よりも有効半径が大きい方にずれる。
Elliptical Galaxies
Binney & Tremaine (1987)
Galactic Dynamics (Princeton)
The luminosity of elliptical galaxies range over a factor of 10 7.
The luminosity function φ(L) describes the relative numbers of galaxies of
different luminosities, and is defined so that φ(L)dL is the number of
galaxies in the luminosity interval [L, L+dL] in a representative unit
volume of the Universe. A conventional analytic approximation to
Φ(L) is Schechter’s law,
where n*=1.2×10-2 h3Mpc-3, α=-1.25, and L*=1.0×1010h-2Lo
in visual band (Kirshner et al. 1983). Note that according to Schechter’s
Law,
diverges as L→0; of course, the total law must fail at sufficiently low
luminosities, but the divergence is an accurate reflection of the larger
number of faint galaxies found at the limits of detectability.
Schechter’s LF
Sandage, Binggeli & Tammann (1985) AJ 90, 1759
Elliptical Galaxies
Binney & Tremaine (1987)
Galactic Dynamics (Princeton)
Most giant elliptical galaxies exhibit little or no rotation, even those with
highly elongated isophotes. Their stars have random velocities along the
line of sight whose root mean square dispersion σ p can be measured from
the Doppler broadening of spectral lines. The velocity dispersion in the
inner few kiloparsecs is correlated with luminosity according to the
Faber-Jackson law,
km s-1
Faber & Jackson (1976) ApJ 204, 668
楕円銀河についての記述はこれだ
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