Class #14/15 14/16 October 2008

Transcription

1 Class #14/15 14/16 October 2008

2 Thursday, Oct 23 in class You ll be given equations and constants Bring a calculator, paper Closed book/notes Topics Stellar evolution/hr-diagram/manipulate the IMF ISM phases/detection techniques Distribution of stars in MWG and other spirals How do we measure the MWG s rotation curve? Relationships between velocity, mass, potential Tracers of star formation Basic differences between spirals and ellipticals (including spectra) Reading Chapters: 1,2,3,5 Lectures

3 Basic Properties of Elliptical Galaxies Stellar Populations in Ellipticals Stellar Kinematics

4 Morelli et al 2008

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7 Formation did all ellipticals form via mergers of gas-rich disk galaxies? Are central cusps evidence of central black holes? What is the role of cannibalism and the formation of extended stellar halos? What is the origin of gaseous shells around some ellipticals? What is the true shape of elliptical galaxies?

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9 Abell Cluster Coma Cluster

10 Surface photometry I(r)=I e exp{-7.67[(r/r e ) 1/4-1]} r 1/4 law R e = effective radius at which ½ of light is emitted Comparable to bulges of disk galaxies Classification: E0-E7 describing increase in flattening Stellar populations: old, metal rich Environment: dense, usually in clusters (morphology-density relationship)

11 Deviations from basic structure Centers in cd galaxies the surface brightness turns up in the center. Halos some have extended halos (excess light at large radii) What distinguishes ellipticals from spirals? Ratio of V ROT /σ V ROT is the circular velocity σ is the velocity dispersion In MWG V ROT ~220 km s -1, σ~50 km s -1 ; it s the opposite in ellipticals Ellipticals are kinematically dominated by the velocity dispersion rather than the circular velocity Presence of large amounts of cool gas

12 Basic colors redder than spirals Radial color gradient Metallicity gradient: d[fe/h]/dlog 10 r = Based on Mg, Fe lines spectrum (compare with a spiral) Look like G or F stars; prominent Mg lines Need high resolution spectra over the 400.0nm nm range + stellar absorption line models Age-abundance degeneracy Can use Hβ line strengths as age indicator

13 Trager et al Age spread: 2-12 Gyr Nearly solar metallicity ([Z/H]) Slightly subsolar [Fe/H] Yields a high [Mg/Fe] ratio Other fits: old population with frosting of younger stars Bottom line: good fraction of old stars, but how old and real fraction is not well known.

14 Trager et al. 2000

15 Cool gas Usually outside of main galaxy Associated with shells Disks out to large radii (exception: dwarf ellipticals with nuclear clouds) No correlation between presence of cool gas and any other property of the galaxy Ionized gas not much Hot gas lots

16 L X goes as L B (empirical result) ρ gas goes as r -3/2 M gas ~ M o Origin of hot gas Reservoir External Internal Mass return from AGB stars ~1.5 x M o yr -1 L o -1 Heated by SNe, thermal motion of stars at km s -1

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19 Measuring intrinsic shape/potential 1. Surface photometry mass distribution potential (Φ, M(r) model) orbit library superposition of orbits iterate 2. Surface photometry + resolved velocity fields and velocity dispersion maps You measure of line-of-sight velocity distribution v los = F(v los )v los dv los Then, σ 2 = (v los -v los ) 2 F(v los ) dv los This accounts for the streaming and random motion of stars.

20 LOSVD Assume a gaussian (don t we always?) and this makes things easier. F exp[-(v los -v los ) 2 /2σ los2 ] exp -b (b = (1/2)w 2 ) This all yields a symmetric line profile. Remember we re sampling a specific absorption feature (e.g. a Mg II line) But life isn t all that symmetric, so we need some way to describe deviations from gaussian Gauss-Hermite series which combine a gaussian with a polynomial F goes as e -k [1 + k=3n h k H k (w)] (this is a polynomial of order k, with some coefficient h)

21 1. Describe stellar kinematics with four terms: v, σ 2, h 3, h Third term: h 3 (2w 3-3w)/(3 1/2 ) 2. Fourth term: h 4 (4w 4-12w 2 +1)/(24 1/2 ) 2. h 3 measures skewness or deviation from symmetry. Large positive h 3 represents a secondary bump at v > v so that the peak of the line is now < v 3. h 4 measures kurtosis or symmetric departures from a gaussian. Large h 4 yields a boxy profile centered on v

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25 Main lines are Hβ, Mg II, Fe I (generally in blue part of the spectrum Case study: NGC 1399 (cd in Fornax) V ~ km s -1 σ 2 ~ km s -1 h 3 ~0.02, h 4 ~0.02 Next step: pick a potential that will fit observed kinematics and surface brightness profile

26 e.g.: Φ(R,Z)=(1/2)v 02 ln(r c2 +R 2 ) Maximize the things you know (stars) Use four key kinematic parameters Fit weirdness (e.g. steep rise in velocity dispersion in the center by including a black hole see SOS program) Alternatively, use tracers at large radii (PN and GCs) to obtain overall velocity

27 M/L ratio increases with R M/r = 5 x M 0 kpc -1 M/L ~ in some cases Kinematics dominated by a dark halo beyond 1-2 R e flat σ vs R curves (just like spirals) (further confirmation of dark halo comes from power law like distribution of hot x-ray emitting gas)

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