Hubble classification scheme
Catalogs and Atlases of Galaxies
•! In late 1700’s, Messier made a catalog of 109
nebulae so that comet hunters wouldn’t
mistake them for comets!
–! ~40 of these were galaxies, e.g., M31, M51, M101.
–! Most are gaseous nebulae within the Milky Way,
e.g., M42, the Orion Nebula
–! Some are stellar clusters, e.g., M45, the Pleiades
Catalogs and Atlases of Galaxies
•! New General Catalogue (Dreyer 1888)
–! Based on lists of Herschel (5079 objects)
–! Plus some more for total of 7840
–! ~50% are galaxies, catalog includes any non-stellar object
•! Index Catalogue (IC) – (Dreyer 1895, 1898)
–! Additions to the NGC, 6900 more objects
–! See www.ngcic.org for online info
•! Shapley-Ames Catalog (Harvard 1932)
–!
–!
–!
–!
Bright galaxies, mpg < 13.2
Whole-sky coverage, fairly homogenous
1246 galaxies, all in NGC/IC
Revised by Sandage & Tamman in 1981
Catalogs and Atlases of Galaxies
•! Uppsala General Catalog (UGC --Nilson 1973)
–! Based on Palomar Observatory Sky Survey (POSS)
–! Size limited, a > 1 arcmin
–! 13000 objects
•! ESO (European Southern Observatory) Catalog
–! Similar to UGC in southern sky, ! < 30°
–! 18000 objects
•! Morphological Catalog of Galaxies (MCG, VorontsovVel’yaminov et al)
–! Based on POSS plates
–! 32000 objects, -2° < ! <-18°
Catalogs and Atlases of Galaxies
•! Hubble Atlas (Sandage 1961)
–! Present plates used by Hubble in developing classification system
plus explanation of system
•! Atlas of Galaxies Useful for measuring the Cosmological
Distance Scale (Sandage & Bedke 1988)
•! Nearby Galaxies Atlas & Catalog (Tully 1988)
–! V < 3000 km/s
•! Reference Catalog of Bright Galaxies (RC3, deVaucoleurs et al
1991)
–! B < 15.5, 23022 galaxies
–! Also, RC1 (1964, 2599 galaxies) and RC2 (1976, 4364 galaxies)
Catalogs and Atlases of Galaxies
•! Catalogs of sources in x-ray, radio, infrared, etc.
•! More recent galaxy surveys – APM survey, CfA
Redshift Survey, 2dF redshift survey, Sloan Digital
Sky Survey (SDSS)
•! The list is fairly endless!
•! The NASA/IPAC Extragalactic Database (NED) is a
good source of information on galaxies, plus has
many galaxy catalogs on-line: http://
nedwww.ipac.caltech.edu/
•! Carnegie Atlas (Sandage & Bedke 1994) – Images of galaxies in
the Revised Shapley Ames Catalog
Overview of Galaxy Properties
E
Color
S0
Sa
Sb
Sc
Red
Sd Irr
Blue
Stellar Old
Pop.
Old +
Intermediate
Old +
Intermediate +
Young
SFR
zero
low
higher
HI
(gas)
Zero/
low
low
modest
dust
Zero/
low
Dyn.
Bulge/halo
dom.
Higher
highest
Disk dominated, so rotation
Intermediate +
Young
high
high highest
Lower (less
metals)
Overview of Galaxy Properties
•! As a fiducial, the Milky Way
–! Radial Scale Length of 3-4 kpc
–! Blue Luminosity of ~ 1.5 x 1010 L!
–! Absolute blue magnitude, -20.7
–! Total Mass of ~1011 – 1012 M !
•! Depending on how much dark matter there is
Overview of Elliptical Galaxies
•! About 20% of field galaxies are ellipticals
•! Most ellipticals are found in clusters!
•! There are a number of different types of
ellipticals
–! E’s (normal ellipticals)
–! cD’s (massive bright ellipticals at the centers of
galaxy clusters)
–! dE’s (dwarf ellipticals)
–! dSph’s (dwarf spheroidals)
–! Note that these do not form a continuous
sequence, they are structurally, kinematically, and
physically different objects.
The Coma Cluster
Overview of Elliptical Galaxies
•! Measure the size of ellipticals by its effective radius –
radius which encloses half the light
–! For comparison the effective radius in an exponential disk is
1.7 x the scale length
–! Sizes range from few tenths of a kpc (dE’s) to tens of kpc
(cD’s)
–! Absolute magnitudes range from –10 (dSph’s) to –25 (cD’s),
a factor of 106 in luminosity
–! Masses range from 107 M! to 1013 - 1014 M!
Center of the Coma Cluster
cD – NGC 4881
Overview of Spiral Galaxies
Hubble Deep Field
•! About ! of galaxies in the field are spirals
•! Most spirals are found in the field (in groups)
•! Spiral galaxy scale lengths run from ~1 kpc
(dwarfs) to ~50 kpc
•! Absolute magnitudes ranging from –16 to –23,
that’s a factor of ~1000 in luminosity!
•! Masses ranging from 109 to 1012 M!
Hubble Deep Field – zoomed in
Released March 9, 2004
11.3 days of observing!
Panning over the Ultra Deep Field
Spiral Galaxies at different wavelengths
HUDF09 – WFC3/IR, released January 5, 2010
Spiral Galaxies at different wavelengths
Overview of Irregular Galaxies
Spiral Galaxies at different wavelengths
Irregular Galaxies at different wavelengths
•! Make up a few % of the field galaxy
population
•! Generally smaller, sizes of a few kpc
•! Absolute magnitudes of –13 to –20
•! Masses of 108 to 1010 M!
Optical
Near-infrared
Hubble’s Law
Hubble’s Original Data
•! Due to the Big Bang, the universe is
expanding!!
•! This was discovered by Hubble in 1929
–! Discovered a linear relationship: v = H0 x D
•! Where v = recession velocity, H0 = Hubble’s constant,
and D = distance to galaxy in Mpc
•! Hubble found H0 ~ 500 km/s/Mpc !!
•! Is used commonly to find distances to galaxies, especially
to very high redshift galaxies where other distance
indicators don’t work!!
•! z = v/c ="#/#
Galactic Coordinate System
Galaxy Photometry
•! Inverse square law
–! f=L/4$d2, f= flux (energy/unit area/second), L=luminosity
(energy per second), d=distance
•! Astronomers work in magnitudes:
–! m1 – m2 = -2.5 log (f1/f2)
–! m – M = 5 log(d) – 5
•! d = distance in pc
•! Absolute magnitude, M, defined as apparent magnitude object
would have if it were located at 10 pc
•! m – M is also known as the distance modulus
•! Note the solar constants,
–! M! = 4.76, L! =3.826 x 1033 erg/s (bolometric)
Galaxy Photometry
•! We use filters to observe stars and galaxies:
–! V = -2.5 log "0 f#,0 F(#)d# + CV
%
•! Where f#,0 = intrinsic flux hitting atmosphere, F(#) = transmission
in V including filter response, telescope + instrument throughput,
atmospheric transmission
•! Flux is in erg cm-2 s-1 Å-1
–! CV = constant, for Johnson-Cousins set so U-B = B-V =0 for
Vega (note there are lots of other filter systems)
–! Measure colors, B – V = MB – MV
Galaxy Photometry
•! AB system:
–! m(AB) = -2.5 log(f) - 48.60 where
monochromatic flux f is measured in erg
sec^-1 cm^-2 Hz^-1!
–! where the value of the constant is selected
to define m(AB)=V for a flat-spectrum
source. In this system, an object with
constant flux per unit frequency interval has
zero color.
–! Example, the SDSS system u’g’r’i’z
Johnson-Cousins Filter System
Johnson-Cousins Filter System
Band
FWHM(nm)
U
Effective
#(nm)
365
B
445
94
V
551
88
R
658
138
I
806
149
J
1220
213
H
1630
307
K
2190
390
66
SDSS Filter System
Reddening!
•! We know that there is extinction so we need to
account for it:
–! m# – M# = 5 log d - 5+ A#
–! Absorption is color dependent, makes objects redder and
fainter.
–! Reddening also depends on position
–! Can be measured by the color excess and using a standard
extinction curve:
•! E(B-V) = (B-V)obs – (B-V)0
•! A# = R# x E(B-V) –- RB=4.1, RV=3.1, RR=2.3, RI=1.5
–! Or by dust maps…
Color-color diagram
Extinction Curve
A(#)/E(B-V)
Extinction Curve
A(#)/E(B-V)