Lecture 12: Age, Metalicity, and Observations

Transcription

1 Lecture 12: Age, Metalicity, and Observations Main sequence lifetime: M ' MS = M ( ) L L.. *1 ' ( ) yr L L max R diagram giants Abundance Measurements Star spectra: absorption lines Gas spectra: emission lines Galaxy spectra: both Metal-rich/poor stars: stronger/weaker metal lines relative to. II region spectra Stellar spectra MS = L ' L ( ) * 3 4 yr (since L M 4, M L 1/ 4 ). Main sequence B-V L max (top of main sequence) gives age t. Lab measurements: Unique signature (pattern of wavelengths and strengths of lines) for each element. igh-resolution Spectra Measure line strengths (equivalent widths) for individual elements. Equivalent Width measures the strength (not the width) of a line. EW is width of a 100 deep line with same area. 1

2 Abundance Measurements! Spectra Line strengths (equivalent widths)! Astrophysics Stellar atmosphere models! Physics Laboratory calibrations Abundances: (Full details of this process are part of other courses) ', etc. Bracket Notation Bracket notation for abundance of a star relative to the Sun: ' ( log 10 ) ( n() n() ), = log 10 / *( n() n() ).. - And similarly for other, e.g. relative to : Star with solar abundance: Twice solar abundance: alf solar abundance: ) n(), ) n(),. 0 log 10. * n() - * n() -. / O ', C ',... ' = 0.0 atoms of atoms of ' = log 10( 2) = 0.3 ' = log ( 1/2) = ( Metallicity vs Abundance Metallicity (by mass): Z = n() 4 n(e) X = n() n() 4 n(e) Z X = f n(mg) n() f Abundance (by number): Mg ) * ' ( log 10 n(mg) Z Z = X f *. X. f. n(mg) n(), ) n(mg),. / log * n() -. * ) Z, ) Z, = log 10. / log 10. * f X - * f X -. ) Z = log 0 f. X., 10 * Z. f 0 X. 0 - Primordial: X p = 0.75, Y p = 0.25, Z p = 0.00 Solar: X. = 0.70, Y. = 0.28, Z. = 0.02 * 10 [ Mg ] 10 [ Mg ] Some Key Observational Results Gas consumption: Z = log(µ) for Z < More gas used --> higher metallicity. Radius: Z 1/distance from galaxy centre Near centre of galaxy: Shorter orbit period--> More passes thru µ 1 spiral shocks --> More star generations --> µ lower --> Z higher. (Also, more infall of IGM on outskirts.) Luminosity: 0.3 Z! L B Small (low luminosity) galaxies have lower metallicities. Dwarf irregulars: form late (young galaxies), have low Z because µ is still high. Dwarf ellipticals: SN ejecta expel gas from the galaxy, making µ low without increasing Z. Z 2

3 Primordial e/ measurement Emission lines from II regions in low-metalicity galaxies. Measure abundance ratios: e/, O/, N/, Stellar nucleosynthesis increases e along with metal abundances. Find Y p by extrapolating to zero metal abundance. A spectroscopic sequence Old Young Metallicity gradient in MW α-element enhancement v radius in MW MW metallicity gradient implies an inside-out formation process for the MW 3

4 α-element = easy multiples of e, byproducts of normal SN (Type II) Galaxies with high α must ave formed fast before Many SnIa s injected the non-α elements into the ISM α-element enhancement Less Metals in Small Galaxies 0.3 Z! L B MW bulge formed fast and early, before the disc. Zoccali et al (2006) faint ----> bright Mass-metallicity relation Low mass galaxies have lower metalicity, why? - Younger - More gas loss - igher infall - IMF variation (IGIMF) Low mass galaxies igh mass galaxies The BPT diagnostic diagram Baldwin, Philips Terlevich Known as BPT plot. Used to separate star-forming galaxies from weak AGN (Seyferts/LINERS) WY: AGN are a more energetic process than SF and are able to sustain high energy states longer leading to a distinct line ratio mix Normal Star-forming galaxies Seyfert s Transition region 4

5 Star-formation rate Star-formation can be measured in many ways: FUV flux Optical colours e.g., (g-r) α, [OII], [OIII] FIR flux See review by Kennicutt (1998), ARAA ere we look at one: The α recombination line. FUV rad from recent star-form ionises cloud which recombined and cascades Only stars with M>10M o with lifetimes <20Myr produce FUV SFR(M yr 1 ) = L()(ergs 1 ) = Q( o )(s 1 ) Q( o ) is the ionising photon luminosity constants are derived from evol. synthesis models (e.g., Kennicutt 1982) OI Example: To measure L(α) we need to measure the equivalent width of α and multiple the meantensity of the continuum at the mid-point of the line. α SFR(M! yr 1 ) = L(!)(ergs 1 ) Cosmic star-formation history Star-formation has declined 10-fold since a redshift of 1 (~half age of Universe). Most stellar mass formed early SFR Stellar Mass The fundamental parameters that seem to determine observed metallicity are mass and SFR. This forms the fundamental metallicity relation (FMR). Despite extremely complex underlying physics, the relation exists out to z=2.5 and in a huge range of galaxies/ environments. Stellar Mass 5

6 M31: Andromeda in Ultraviolet Light UV light traces hot young stars, current star formation. More near Galaxy Centres Ellipticals (NGC 3115) Gas depleted, hence no current star formation the inner disk. Spirals (M100) Brief Review Main events in the evolution of the Universe: The Big Bang (inflation of a bubble of false vacuum) Symmetry breaking à matter/anti-matter ratio Quark antiquark annihilation à photon/baryon ratio The quark soup à heavy quark decay Quark-adron phase transition and neutron decay à n/p ratio Big Bang nucleosynthesis à primordial abundances X p = 0.75 Y p = 0.25 Z p = 0.0 Matter-Radiation equality R ~ t 1/2 à R ~ t 2/3 Recombination/decoupling à the Cosmic Microwave Background CMB ripples (ΔT/T~10-5 at z=1100) seed galaxy formation Galaxy formation and chemical evolution of galaxies Main events in the chemical evolution of galaxies: Galaxy formation à Jeans Mass Ellipticals Initial mass and angular momentum, plus mergers. Spirals à Star formation history S(t), gas fraction µ(t) Irregulars Star formation à α = efficiency of star formation The IMF (e.g., Salpeter IMF power-law with slope -7/3) First stars (Population III) from gas with no. Stellar nucleosynthesis à up to Supernovae (e.g. SN 1987A) à beyond r, p, and s processes white dwarfs (M < 8 Msun) or black holes, neutron stars (M > 8 Msun). Galaxy enrichment models: (e.g. Z = - ρ ln(µ), yield ρ ) Metal abundances rise à X = 0.70 Y = 0.28 Z = 0.02 (solar abundances) Gas with à Stars with Planets à Life! 6

7 Fini All enrich at approx same rate as (e.g. to a factor of 2 over a factor of 30 enrichment). Some elements (Mg,O,Si,Ca,Ti,Al) formed early, up to ~2 times faster than in metal-poor stars. [X i /] vs [/] Lowest metal abundance seen stars: [/] ~ -4 7