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Cedric Lacey

Multi-wavelength modelling of galaxy evolution: Lecture 1: Computing galaxy SEDs from the UV to the radio. Cedric Lacey. Outline. Stars Stellar evolution Stellar spectra Integrated spectra of stellar populations Dust Extinction Emission Radio Thermal radio emission

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Cedric Lacey

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  1. Multi-wavelength modelling of galaxy evolution:Lecture 1: Computing galaxy SEDs from the UV to the radio Cedric Lacey Foz de Iguacu

  2. Outline • Stars • Stellar evolution • Stellar spectra • Integrated spectra of stellar populations • Dust • Extinction • Emission • Radio • Thermal radio emission • Radio synchrotron emission Foz de Iguacu

  3. Stars Foz de Iguacu

  4. Modelling the light from Stellar Populations • Stellar evolution tracks • Stellar spectra • Stellar initial mass function (IMF) • Star formation history (SFH) • Chemical enrichment history Ingredients: Foz de Iguacu

  5. Simple Stellar population (SSP) • Set of stars all with same age  and initial metallicity Z • Stellar evolution models give evolution of stars in mass, L, Teff, g as function of  , Z & initial mass m Foz de Iguacu

  6. Stellar Evolution Tracks Low mass High mass Girardi et al 2000 Foz de Iguacu

  7. Simple Stellar population (SSP) • Set of stars all with same age  and metallicity Z • Stellar evolution models give evolution of stars in mass, L, Teff, g as function of  , Z & initial mass m • Assign spectrum to each star as fn of L, Teff, g & Z • Sum spectra from all stars in HR diagram to get spectrum of SSP Foz de Iguacu

  8. Stellar Spectra • Use library of observed stellar spectra • more accurate • but only includes stars can observe easily, e.g. lack stars with low Z & high m, also v.high Z stars • Use theoretical stellar atmosphere models • can cover all Teff, g, Z & /Fe • but models do not reproduce all features of observed spectra Foz de Iguacu

  9. Stellar Initial Mass Function (IMF) • IMF specifies relative number of stars of different initial masses m • (m)dm = no of stars in mass range (m,m+dm) • Usually assume power law over some range of mass Foz de Iguacu

  10. What IMF? • Conventional choice is Salpeter (1955) IMF: x=1.35 for 0.1<m<100 Mo • However, in solar neighbourhood, IMF is much flatter than this for m<1Mo, and probably a bit steeper for m>1Mo • Better fit is Kennicutt (1983) (or similar): x=0.4 for 0.1<m<1 Mo x=1.5 for 1<m<100 Mo • But IMF in other environments might be different! Foz de Iguacu

  11. Spectral Energy Distribution (SED) of an SSP Bruzual & Charlot 1993 • SEDs shown for different ages in Gyr) • Salpeter IMF assumed • SED dominated by high-m stars at young ages & low-m stars at old ages Foz de Iguacu

  12. Composite Stellar Population (CSP) • In general, have mixture of ages & metallicities Z: t,Z) dt dZ = mass of stars formed in time (t,t+dt) with metallicities in range (Z,Z+dZ) • In single-zone chemical evolution model, there is one-to-one relation Z(t) • But in multi-zone model, or where galaxies form by mergers, there is distribution of Z even at fixed t Foz de Iguacu

  13. SED of composite population • where is SED of population with single mass m, age metallicity Z Foz de Iguacu

  14. Example SEDs of CSPs for different star formation histories const SFR exp decaying SFR Foz de Iguacu Bruzual & Charlot 1993

  15. Galaxy SEDs along present-day Hubble sequence • average broad-band UV- near-IR SEDs for galaxies of different Hubble types can be quite well fit with simple SFHs, e.g. • Single burst long in past • Const SFR • Exponentially-decaying SFR Foz de Iguacu

  16. Fit of model to observed galaxy SEDs Elliptical Irregular Foz de Iguacu Bruzual & Charlot 1993

  17. Fit of model to observed SEDs along Hubble sequence E - Sc galaxies Foz de Iguacu Bruzual & Charlot 1993

  18. However, this does not prove that real galaxies all had such simple star formation histories! • Broad-band SEDs given by integral over wide range of stellar mass & age • insensitive to details of SFH • Small-scale spectral features (absorption features & spectral breaks) more sensitive to particular types of star • Analysis of these features implies more complicated SFHs for many galaxies, e.g. recent bursts Foz de Iguacu

  19. What about effects of ISM? • Dust • Absorbs & scatters light from stars in UV & optical • Absorbed energy re-emitted in IR & sub-mm • Gas ionized by stars (in HII regions) • Emission lines in UV, optical & IR • Thermal bremsstrahlung emission in radio • Relativistic electrons accelerated in supernova remnants (SNRs) • Emit synchrotron radiation in radio when move in galactic magnetic field Foz de Iguacu

  20. Dust Foz de Iguacu

  21. Observational constraints on dust properties • Extinction & reflection of starlight • Infra-red emission • Interstellar abundances & depletions • X-ray scattering & absorption • Polarization of starlight Foz de Iguacu

  22. Extinction curve of local ISM Analytical representations of measured extinction curves along different lines of sight Average extinction curve for local diffuse ISM Fitzpatrick 2004 Foz de Iguacu

  23. IR emission from local diffuse ISM Dust heated by local diffuse interstellar radiation field Foz de Iguacu

  24. Element depletions in local ISM Gas-phase abundances in local ISM more depleted for higher condensation temperature Foz de Iguacu

  25. What are dust grains made of? Main clue is spectral features in absorption or emission • Graphite: - bump in extinction curve at 2175 A - can be explained as electronic transition in carbon in graphite-like structure - could either be small graphite grains or large polycyclic aromatic hydrocarbon molecules (PAHs) Foz de Iguacu

  26. What are dust grains made of? (2) • PAHs (polyclyclic aromatic hydrocarbon molecules) - strong emission bands at 3-13 m - explained as vibrational (stretching & bending) modes of PAH molecules Foz de Iguacu

  27. What are dust grains made of? (3) • Silicates: - strong absorption features at 9.7 & 18 mm - implies silicates, e.g. Mg2 SiO4 - absence of fine structure => amorphous (glassy) Foz de Iguacu

  28. Optical extinction curve requires range of grain sizes ~ 0.01-0.1 m - wavelength dependence constrains size distribution - suggests power-law similar to • Mid-IR emission (~ 3-20 m) requires v.small grains ~ 0.001-0.01 m (10-100 A) - such grains heated to T much higher than equilibrium value by single photons • Overall constraint on grain chemical compositions from element depletions in ISM Foz de Iguacu

  29. Scattering & Absorption cross-section for spherical grain • Scattering/absn efficiency: • Q = a^2) • Q ~ const for a • Q falls rapidly for a • absorption dominates at a Foz de Iguacu

  30. Emission from dust grains • Large grains • Small grains • PAH molecules Foz de Iguacu

  31. Large Grains (>100 Å), don’t cool in the time between absorption of two photons, so reach thermal equilibrium with the interstellar radiation field. • Tis determined by solving the energy balance equation: Angle averaged In Absorption Emission Note that equilibrium grain T depends weakly on radiation field & on grain size Foz de Iguacu

  32. Indeed, absorption occurs mainly in optical–UV where Q,a»1, while emission is in IR where Q,a ~ - with ~1.5-2. Thus, as an order of magnitude we get e.g. to double T would require an increase of U by 60! The SED of optically thin dust emission is relatively stable. Foz de Iguacu

  33. 2. Small grains(<100 Å) fluctuate in temperature between two photons and a probability distribution P(T)dT to find a grain between T and T+dThas to be computed (e.g. Guhathakurta & Draine 1989, Siebenmorgen et al. 1992). Once this is done: replacing Foz de Iguacu

  34. Temperature fluctuations of small grains Draine 2005 Foz de Iguacu

  35. Effect of temperature fluctuations Predicted spectrum PredictedP(T) With fluctuations Neglecting fluctuations Foz de Iguacu

  36. 100Å 30Å 60Å 9Å Contribution of different size grains to local diffuse ISM emission Foz de Iguacu

  37. PAHs (polyciclyc aromatic hydrocarbons) are a family of very stable planar molecules, based on benzene ring which has an aromatic bond in which a  orbital is shared in the chain. Foz de Iguacu

  38. “Unidentified Infrared Bands” commonly interpreted by C-C and C-H vibration modes, due to the absorption of a single uv photon, in large planar Polycyclic Aromatic Hydrocarbons (PAHs) molecules, with size ~ 10 Å and containing ~ 50–100 C atoms. Foz de Iguacu

  39. PAH vibrational spectra resembles those of emission bands in many astrophysical objects Observed mid-IR spectra require mixture of PAHs, but mixture seems to be similar in different objects where PAHs seen However, there is evidence that in denser environments and stronger UV field intensities the PAHs may be depleted. Foz de Iguacu

  40. Effects of geometry on dust extinction & emission Foz de Iguacu

  41. Geometrical effects (2) • Dust is in 2-phase medium: dense molecular clouds (MCs) & diffuse ISM • Stars also not smoothly distributed: stars form inside MCs & then escape or disperse parent clouds after few Myr Foz de Iguacu

  42. Geometrical effects (3) Consequences: • Stellar age-dependent dust extinction (since young stars in dustiest regions) • Dust column density effectively wavelength dependent (since younger stars emit at shorter wavelengths) - has implications for attenuation law • Different radiative heating of dust grains in different environments - large dust grains heated to higher T in star-forming clouds than in diffuse ISM Foz de Iguacu

  43. Computing effects of dust on galaxy SED • Need to compute radiative transfer of starlight through dust distribution, allowing for clumping of stars & gas • Need to compute temperatures of dust grains of different types & sizes in different radiative environments • A code which does this is GRASIL (Silva, Granato & Bressan & Danese1998) Foz de Iguacu

  44. Example SEDs computed using GRASIL fit to observed galaxies (Silva et al 1998) M100 (spiral) M82 (starburst) tesc = 3 Myr tesc = 10 Myr Foz de Iguacu

  45. Important parameters in GRASIL dust model • tesc = timescale for stars to escape from parent clouds • net UV attenuation v. sensitive to this in star-forming galaxies • cloud = dust optical depth in molecular clouds • mid-IR dust emission sensitive to this, since clouds opt thick in mid-IR Foz de Iguacu

  46. Radio Foz de Iguacu

  47. Contributions to radio emission from normal galaxies • Thermal emission - from ionized gas in HII regions • Non-thermal emission • from relativistic electrons accelerated in supernova remnant (SNR) shock waves • emit synchrotron radiation when orbit in magnetic field Foz de Iguacu

  48. Thermal Radio emission (Condon 92) Thermal • free-free (bresstrahlung) emission from ionized gas in HII regions • Mainly sensitive to Q(H) = H-ionizing photon luminosity • This is easy to obtain from integrated stellar SED • Weak dependence on metallicity through equilibrium Te • Thermal radio emission begins promptly when O stars form • The radio slope is flat in Lvs  Foz de Iguacu

  49. Non Thermal Radio emission • FIR/Radio correlation for normal (including starburst) galaxies suggests link with SF • thought to be Synchrotronradiation from relativistic electrons accelerated in shock waves of SNRs produced by Type II supernovae • implies link to SNII rate • contributions from both individual SNRs & electrons in general ISM • but general ISM dominates • slope depends on energy spectrum of relativistic electrons, observationally find: • so non-thermal dominates over thermal at low (large ) Foz de Iguacu

  50. Non Thermal Radio emission (2) • total non-thermal energy emitted per SN not known theoretically • - in principle depends on both efficiency of shock acceleration of electrons and on strength of B-field • but tightness of radio-FIR correlation in galaxies of differerent types (normal & starburst) seems to require that NT energy per SN nearly constant • - therefore calibrate relation on values for our Galaxy: Foz de Iguacu

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