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Lecture 12: Galaxy Evolution • An empirically driven subject: – The Mass versus Age plot of all surveys • Completing the local census: – New dwarf galaxies in the local group – The dwarf galaxy problem • Comparative evolution: – Luminosity function evolution • Luminosity evolution • Number evolution • Practicalities – K-correction – Dust Galaxies – AS 3011 1 MA V. SMOOTH NO METALS Galaxies – AS 3011 V. LUMPY, METAL RICH SS AS SE MB LY 2 1 The Mass-Age plot 1. Completing the local census 2. Comparative studies Galaxies – AS 3011 3 Dwarf galaxies • Dwarf galaxies are a crucial part of the galaxy evolution puzzle but we know very little about them. • Main theory (see later) proposes that galaxies built-up from smaller units through repeated merging. • Numerical simulations typically predict several thousand dark matter haloes in the local group. • ~ 55 Local Group galaxies known. • ~ 1 new Local Group dwarf galaxy discovered every 18 months. • Very wide range of properties = a combination of latestarters, relics, debris and stunted systems. • Space-density extremely poorly constrained, I.e., important to appreciate that our current backyard census is woefully incomplete. Galaxies – AS 3011 4 2 2 New Local Group galaxies discovered recently… • • • • Bootes Mv=-5.7 mag µo=28.1 mag/sq arcsec Belokurov et al (2006) • • • • Canes Venatici MV=-7.9 mag µo=27.8 mag/sq arcsec Zuker et al (2006) Galaxies – AS 3011 5 The Luminosity-Surface Brightness Plane Galaxies – AS 3011 6 3 Comparative studies • Many comparisons are possible, e.g., – Profile shapes – Gas, dust, plasma and stellar content – Fundamental plane and Faber-Jackson relation – Tully-Fischer relation – Star-formation rates – Line indices, metallicity and colours – Morphologies and luminosity-size relations – Overall and component luminosity functions • Main issues are sample selection bias and demonstrating that a comparison of the high and low z samples is valid, comprehensive and complete. Galaxies – AS 3011 7 The evolution of the galaxy luminosity function ? • Typically allow luminosity and number-density to evolve according too: Lz = Lo (1+ z) β , φ z = φ o (1+ z)γ ⇒ J z = J o (1+ z) β +γ • By constructing two luminosity functions at two redshifts one can constrain β and γ Luminosity Evolution € logφ Abs. magnitude Galaxies – AS 3011 Number Evolution logφ Abs. magnitude 8 4 Results from the VLT VIRMOS Deep Survey (Ilbert et al 2006) • Basically no or little change seen out to z=1 except possible in the blue spheroids ! • More data needed subject rapidly advancing due to new Galaxies – AS 3011 technologies. Redshift 9 Evolutionary forms • Pure luminosity evolution (brighter in past) • Pure number evolution (more or less in past) • Hierarchical merging (more but fainter in past) • Downsizing (big things form first then fade) • Upsizing (little things form first then fade) • Early monolithic collapse (everything formed at high-z, no change over low z) Galaxies – AS 3011 10 5 E.g., if galaxies are evolving such that β=0.5 and γ=-0.3 what kind of evolution is this and what is the relative luminosity density at z=1 ? • L(z=1)=1.41L(z=0), I.e., galaxies brighter in past • φ(z=1)=0.81φ(z=1), I.e., fewer galaxies in past • 20% of galaxies have formed below z=1, the galaxies have either faded or the new formers been of lower luminosity (downsizing). • J(z=1)=1.15J(z=0) • The luminosity density of the Universe has decreased since z=1. Galaxies – AS 3011 11 Practicalities • 2 effects which can dramatically change a galaxies magnitude ! – Extinction • Caused by attenuation of flux by intervening medium, e.g., dust – K-correction • Important over cosmological distances where observed wavelength is substantially different to emmitted wavelength m − M = 5log d + 25 + A + K € Galaxies – AS 3011 Extinction correction K-correction 12 6 Practicalities: Attenuation • E.g., Dust (either in our galaxy, the target galaxy or some inbetween IGM). • Flux received less than it should be by some factor F (0<F<1). FLABS F ⇒ 10−0.4 m ∝ 2 10−0.4 M 2 d d M = m − 5log f − 25 + 2.5log F M = m − 5log d − 25 − A i.e.,LApp ∝ • Av=-2.5log(F)=Attenuation in units of V mag (λ dependent) Galaxies – AS 3011 13 € Practicalities: Attenuation • Extinction, if not corrected for can give low values for H0 (I.e., distance are overestimated, H0=v/d) • Where can extinction occur ? HERE HERE HERE HERE HERE • Big problem, typically dust is more transparent at longer wavelength as dust grains are very small. • Major advantage of the near-IR now opening up. Galaxies – AS 3011 14 7 NO DUST i = 0o (face-on) i = 60o i = 88o (edge-on) WITH DUST (B band) Galaxies – AS 3011 15 NGC891 Galaxies – AS 3011 16 8 Impact of dust on global B LF Luminosity density doubles, I.e., only 50% of photons escape ~0.8mag Galaxies – AS 3011 17 Practicalities: K-corrections • Normally we observe through filters, I.e., over some dλ: Typical Galaxy Spectrum at z=0 f(λ) λ1 dλ λ2 T(λ)=Transfer or filter function λ2 Lλ = ∫ λ1 ∞ f ( λ)dλ = ∫ T(λ) f (λ)dλ 0 • As we look towards galaxies at higher redshift through the same filter (or (T(λ)) we are actually receiving flux from shorter wavelengths at a slower € rate. If we know the spectral shape we can correct for this using the K-correction. Galaxies – AS 3011 18 9 Practicalities: K-correction f(λ) Typical Galaxy Spectrum at z=0 f1 λ1 λ2 f2 f(λ) f3 Typical Galaxy Spectrum at z=1 λ2 (1+z)λ1 (1+z)λ2 λ1 • We measure f2 but we want f3 to compare to f1 ∞ ∫ T(λ (1+ z)) f (λ (1+ z))dλ K(z) = Galaxies – AS 3011 0 ∞ (1+ z) ∫ T( λ ) f ( λ )dλ 19 0 € Practicalities: K-correction E/S0 dM Sabc Sd/Irr Redshift Galaxies – AS 3011 20 10 Practicalities: K-correction • Typically K(z) is given as: K(z)=az+bz2 • Where a and b are derived for different types Type a b E/S0 3.13 0.24 Sabc 2.63 -0.107 (V-band values) Sd/Irr 0.62 0.14 • E.g., an elliptical galaxy at z=0.3 has an apparent magnitude of mV=23 mag, assuming Av=0 mag what is its absolute magnitude ? cz M = m − 5log( ) + 5log h0.5 − 25 − AV − K(z) 50 M = −18.3 + 5log h0.5 − (3.13z + 0.24z 2 ) Galaxies – AS 3011 M = −19.3 + 5log h0.5 21 € Galaxies – AS 3011 22 11