Galaxies Lecture 6: stellar populations in elliptical galaxies; cusp/core and clues to formation · stellar populations in ellipticals · Implications of old stars in ellipticals · Correlation of cusp/core with luminosity · Clues to elliptical galaxy formation Galaxies 1 Stellar populations - overview · Simple stellar populations of single age and chemical composition are best observed in star clusters. · One can then observe directly how differing stellar ages and chemical compositions determine the integrated properties of a stellar population. · From this it is a small conceptual leap to understand that the spread of stellar population ages and chemical compositions - effectively the galaxy star formation history - are the principle drivers of the observed spectral energy distribution. Galaxies 3 HR diagram: stellar evolution = galaxy evolution 105 GOT Supergiants (1) 10 Dwarfs (V) 103 1 requiere's Giants (111) 102- Luminosity (La) 104 100 10-1- 001 F8 10-2 10-21 0.031 "@White Dwarfs 10-4 40000 20000 1000G 5000 2300 Temperature (K) Galaxies 5 Stellar populations - overview · A galaxy may be thought of as a population of stars. . The distribution of stars can be visualised on the Hertzprung-Russell diagram with axes of either surface temperature versus luminosity or colour versus magnitude. · The spectral energy distribution (colour) of the galaxy may then be computed as the luminosity weighted integral over the spectral energy distribution (colour) of its constituent stars. Galaxies 2 Hertzsprung-Russell diagram from HIPPARCOS satellite - bright, nearby stars. 10 15 05 20 Galaxies 4 Stellar Populations .In this sense, we noted that ellipticals are red and spirals are blue. ·From the above discussion we can now understand that 1) spirals are composed of younger, actively forming stellar populations. 2) ellipticals are composed of predominantly old stars with little ongoing star formation ·Stellar populations in elliptical galaxies are old and simple .Combined with the absence of gas and dust in elliptical galaxies we can make some simple statements regarding their evolutionary history. Galaxies 6 1
Evolved versus 'young' galaxies 30 25 20 1.5 -23 -22 -21 -20 -19 Figure 9. Colour-magnitude diagram for galaxies in the weighted super- clean combined spirals sample. Systems classified as spiral are shown in black, those classified as elliptical in red. 7 [Cll] single burst of star formation 1 Hell flux F2 (arbitrary units) X3646 Bolmet limit [ouI] Hel after 10 Myr 2 Not after 100 Myr 117 He M?b 1 Gyr NOD sky Ho sky 10 Gyr 4000 5000 6000 7000 wavelength (Å) Galaxies 40 RGB 41) :001 10 -2.0 125# 40 LOG IT,) 30 10 17 k+a 9 11 0, 5, 1, 224, 1.5 1.25, 1, 05, 0.25, Lititi of lominceity and surface temperature art the same as in Figure 1."Trawerad sines betwen labeled points nos given in Table IIL and IV. Daled portions of evolucionary padu are estinalas. Stellar populations in E's · The absence of stars of spectral type A or earlier indicates that typical elliptical galaxies have not experienced major bursts of star formation within the last