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
MASS ASSEMBLY
V. SMOOTH NO METALS
V. LUMPY, METAL RICH Galaxies – AS 3011 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 late- starters, 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: β γ Lz = Lo(1+ z) , φz = φo(1+ z) β +γ ⇒ Jz = Jo(1+ z) • By constructing two luminosity functions at two redshifts one can constrain β and γ
Luminosity Number € Evolution Evolution logφ logφ
Abs. magnitude Abs. magnitude Galaxies – AS 3011 8
4 Results from the VLT VIRMOS Deep Survey (Ilbert et al 2006)
• Basically no Redshift or little change seen out to z=1 except possible in the blue spheroids ! • More data needed subject rapidly advancing due to new technologies.Galaxies – AS 3011 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 = 5logd + 25 + A + K
K-correction Extinction € correction Galaxies – AS 3011 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 FL F i.e.,L ∝ ABS ⇒ 10−0.4m ∝ 10−0.4M App d 2 d 2 M = m − 5log f − 25 + 2.5logF M = m − 5logd − 25 − A • 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 i = 60o i = 88o (face-on) (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 f(λ) Spectrum at z=0 λ λ1 λ2 2 ∞ dλ Lλ = ∫ f (λ)dλ = ∫ T(λ) f (λ)dλ λ 0 T(λ)=Transfer or filter function 1 • 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 Typical Galaxy f(λ) f1 Spectrum at z=0 λ1 λ2 Typical Galaxy f2 f(λ) Spectrum at z=1 f3 λ1 λ2 (1+z)λ1 (1+z)λ2 • We measure f2 but we want f3 to compare to f1 ∞ ∫ T(λ (1+ z)) f (λ (1+ z))dλ 0 K(z) = ∞ (1+ z) ∫ T(λ) f (λ)dλ Galaxies – AS 3011 0 19 € 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( ) + 5logh − 25 − A − K(z) 50 0.5 V 2 M = −18.3+ 5logh0.5 − (3.13z + 0.24z ) Galaxies – AS 3011 M = −19.3+ 5logh0.5 21 € Galaxies – AS 3011 22 11