What can emission lines tell us? lecture 1 Grażyna Stasińska

some basic literature dealing with the ionized ISM books Physical Processes in the Interstellar Medium, Spitzer, 1978 Astrophysics of Gaseous Nebulae and Active Galactic Nuclei, Second Edition, Osterbrock & Ferland, 2005 Astrophysics of the Diffuse Universe, Dopita & Sutherland, 2003 lectures Stasinska 2002 astro-ph/ « Abundance determinations in HII regions and planetary nebulae » Stasinska 2007 astro-ph « What can emission lines tell us? »

What can emission lines tell us? The mere presence of emission lines indicates the existence of gas eg emission line galaxies contain gas in large amount while galaxies emitting only a continuum with absorption features (such as elliptical galaxies) do not the existence of an ionizing agent (most emission lines come from ionized species) hot star(s) active nucleus (shocks) …

a gallery of nebular spectra

a g the galaxy T e diagnostic

IFU data for the most metal-poor HII galaxy I Zw 18 Kehrig et al 2006 [OIII] image H image

deep UV-FIR spectrum of the high excitation planetary nebula NGC 7027 Zhang et al 2005 observed dereddened

high resolution spectrum of the PN NGC 6153 showing many recombination lines Liu et al 2000

SPITZER IRS spectrum of the PN SMP83 in the LMC Bernard-Salas et al 2004

XMM spectrum of the corona of α Cen Liefke & Schmitt 2006

line displacements tell about radial velocities and allow one to measure redshifts of galaxies dark matter mass in galaxies using PNe as test particles (eg Romanowsky et al 2003) internal motions in zones of line emission (eg line broadening in AGNs) expansion velocities high resolution multislit spectroscopy of the PN NGC 7009 in the [NeIII] line showing expansion of the envelope Wilson 1958

Basic mechanisms in ionized nebulae and emission line production ionisation and recombination processes heating and cooling processes line production mechanisms about radiation transfer in nebulae equilibrium versus out of equilibrium the nebular physicists compendium

ionisation and recombination processes ionization Photoionization Collisions Charge exchange Recombination Radiative recombination Dielectronic recombination Charge exchange

heating and cooling processes Heating Photoionization Collisional ionization cooling Free-free radiation Free-bound radiation Bound-bound radiation

what determines the ionic fractions and the temperature? Ionization Ionic fractions Recombination Heating Electron temperature Cooling Photoionization Collisions Charge exchange Radiative recombination Dielectronic recombination Charge exchange Photoionization (mainly H and He) Collisional ionization Free-free radiation Free-bound radiation Bound-bound radiation (mainly O)

line production mechanisms Recombination followed by cascade (these lines are named with the recombined ion) H lines: Balmer …, Paschen etc … He I lines (He I 5876…) He II lines (He II 4686…) Collisional excitation followed by radiative deexcitation Forbidden lines: [OIII]5007, [NII]6584 Semi-forbidden lines : CIII]1907 … Resonance lines: CIV 1550, NV 1240, OVI 1035, SiIV 1400 Photoexcitation and fluorescence Bowen lines: OIII 3133, 3444 (Bowen 1934) Fe K line (probe of astrophysical black holes Fabian et al 2000) notes Each line can be produced by several processes, but usually only one dominates H Ly is produced both by recombination and by collisional excitation

about radiation transfer in ionized nebulae Lyman continuum photons from the ionizing source ionizing photons produced by the nebula non-ionizing photons

Lyman continuum photons from the ionizing source They suffer geometrical dilution away from the source They suffer line-of-sight absorption (main absorbers H and He) The first ones to be absorbed are the ones with energies close to the ionization threshold ( -3 ) => hardening of the ionizing radiation field in external zones

ionizing photons produced by the nebula those photons are emitted in all directions The on the spot approximation assumes that all ground-state recombination photons are reabsorbed OTS is justified for analytical order of magnitude estimations but computed T e is incorrect by ~1000K-2000K (Gruenwald et als 3D code) The outward only approximation Radial outward only (Ferlands Cloudy) Full outward only (Stasinskas PHOTO) complete treatment Traditional iterative way (Harrington, Rubin) Using Monte-Carlo transfer (Ercolanos Mocassin) resonance iine radiation is locally scattered many times Can be treated with a quasi on the spot approximation (Ferlands Cloudy) Can be treated with an local escape probability approximation Can be treated exactly (Dumonts Titan)

non-ionizing photons (including lines emitted by the nebula) In general they escape freely (the optical thickness of the nebulae is small enough) They can be attenuated by dust absorption Exceptions resonance lines which suffer scattering (eg H Ly ) and may be selectively destroyed by dust FIR lines can suffer self-absorption as abs is larger than for optical lines (Rubin 1968) but turbulent/expansion velocities favour their escape (Abel et al 2003)

equilibrium versus out of equilibrium typical timescales for nebulae with n=10 3 cm -3 Recombination time t rec = 1/( n e )100 yr Cooling time t cool = (nkT e )/ 50 yr Dynamical time t dyn =R/v exp 2x10 4 yr for a single star HII region Stellar evolution time t*5x10 6 yr for HII regions (=t MS ) 1x10 4 yr for PNe (=t PAGB ) Most ionized nebulae are in ionization and thermal equilibrium low density plasmas can be out of equilibrium

The nebular physicists compendium The Stromgren sphere The H luminosity and surface brightness What drives the electron temperature? What determines the ionization structure of a nebula? Why is the gas temperature roughly uniform in photoionized nebulae? Comments on energy losses Other comments on the gas temperature Comments on line intensities

The nebular physicists compendium The Stromgren sphere The H luminosity and surface brightness What drives the electron temperature? What determines the ionization structure of a nebula? Why is the gas temperature roughly uniform in photoionized nebulae? Comments on energy losses Other comments on the gas temperature Comments on line intensities

The nebular physicists compendium The Stromgren sphere The H luminosity and surface brightness What drives the electron temperature? What determines the ionization structure of a nebula? Why is the gas temperature roughly uniform in photoionized nebulae? Comments on energy losses Other comments on the gas temperature Comments on line intensities

Q H [ph/sec] M* [M ] M ion [M ] (n=10 2 )(n=10 4 ) PN3x O7 star3x starburst3x the Strömgren radius In a homogeneous medium of density n and filling factor, the radius R S up to which gas is fully ionized is obtained from Q H = 4/3 R S 3 n 2 B (H) => R S [cm] = 9720 (Q H n )1/3 nb: The transition region between ionized and neutral gas is usually small: maximum nebular mass that can be ionized M ion = 4/3 R S 3 n m H => M ion [M ] = 5 x Q H n -1

The nebular physicists compendium The Stromgren sphere The H luminosity and surface brightness What drives the electron temperature? What determines the ionization structure of a nebula? Why is the gas temperature roughly uniform in photoionized nebulae? Comments on energy losses Other comments on the gas temperature Comments on line intensities

density bounded case: L H = M neb n (H )/ m H L H is independent of Q H ionization bounded case: L H = Q H (H )/ B (H) L H is a measure of Q H H luminosity: L H = 4/3 R 3 n 2 (H ) H surface brightness:S H = L H / (4 R 2 ) ionization bounded case: S H = A (Q H n 4 2 ) 1/3 narrow slit spectra are of better quality for denser nebulae density bounded case: S H = B (M neb n 5 2 ) 1/3 narrow slit spectra are of better quality for denser nebulae

The nebular physicists compendium The Stromgren sphere The H luminosity and surface brightness What drives the electron temperature? What determines the ionization structure of a nebula? Why is the gas temperature roughly uniform in photoionized nebulae? Comments on energy losses Other comments on the gas temperature Comments on line intensities

energy gains : G = i, j n i j i j where the i j are the gains per ion (photoionization and collisional ionization) energy losses : L = i, j n i j i j where the i j the losses per ion (recombination and collisional excitation followed by photon emission) net energy gain : dE / dt = G - L If thermal equilibrium is achieved, the temperature is determined by: G = L

The nebular physicists compendium The Stromgren sphere The H luminosity and surface brightness What drives the electron temperature? What determines the ionization structure of a nebula? Why is the gas temperature roughly uniform in photoionized nebulae? Comments on energy losses Other comments on the gas temperature Comments on line intensities

ionization equilibrium equation between n i and n i+1, at a fraction f of the Stromgren radius R S n i 4 J i d = n i+1 n e i where the mean intensity of the radiation field in photons s -1 is 4 J = Q H g (T*) / r 2 => expression of the ionization state as a function of Q H, n, f,T*: n i+1 / n i = (Q H n 2 ) 1/3 f -2 g(T*) ionization parameter definition: U = Q H / (4 R 2 n c) quivalent expression: e U = A (Q H n 2 ) 1/3 the ionization state is fully defined by the product Q H n 2 once T* is specified the average ionization is higher for larger Q H and larger n in a given nebula, regions of higher n are more recombined

The nebular physicists compendium The Stromgren sphere The H luminosity and surface brightness What drives the electron temperature? What determines the ionization structure of a nebula? Why is the gas temperature roughly uniform in photoionized nebulae? Comments on energy losses Other comments on the gas temperature Comments on line intensities

Energy gained by photoionization of H at a distance r from the source G = n(H°) 4 J (h -h °) d erg cm -3 s -1 ] Ionization equilibrium equation of H at distance r n(H°) J d = n(H + ) n e B (H) Substituting: G = n(H + ) n e B (H) with = 4 J (h -h °) d 4 J d is the mean energy gain per absorbed photon A T* Energy gains due to photoionization of H are independent of distance to the star proportional to the star temperature

The nebular physicists compendium The Stromgren sphere The H luminosity and surface brightness What drives the electron temperature? What determines the ionization structure of a nebula? Why is the gas temperature roughly uniform in photoionized nebulae? Comments on energy losses Other comments on the gas temperature Comments on line intensities

The most important cooling process is collisionally excited line radiation For a given ion in a two-level approximation, the cooling rate is given by L coll = n 2 A 21 h 21 erg cm -3 s -1 ] where n 2 results from the equilibrium equation of levels 1 and 2 : n 1 n e q 12 = n 2 (A 21 + n e q 21 ) In the limit of small n e one has L coll = n 1 n e q 12 h 21 where q 12 is the collisional excitation rate q 12 = (1,2) / 1 T e -0.5 exp (-E 12 /kT e ) note for « normal abundances » the most important cooling ion is O ++ H and He have too high excitation potentials to be excited at normal temperatures Cooling by collisional line excitation is more important for abundant ions lines corresponding to large levels that can be easily attained at the temperature of the medium

The nebular physicists compendium The Stromgren sphere The H luminosity and surface brightness What drives the electron temperature? What determines the ionization structure of a nebula? Why is the gas temperature roughly uniform in photoionized nebulae? Comments on energy losses Other comments on the gas temperature Comments on line intensities

Spatial variations of T e are mostly determined by the mean energy of the absorbed photons the populations of the main cooling ions are generally small except at high metallicities in the O ++ zone cooling is very efficient through emission of [OIII]52, 88 lines which have very low excitation potentials in the O + zone the cooling efficiency is smaller (O + has no low-lying levels) photoionization models showing the effect of metallicity Stasinska Z < Z __ Z > Z General dependence of T e with the defining properties of the nebulae for a given T*, T e as Z for a given Z, T e as T* for given T*, ionization state and Z, T e if n above n crit TeTe

The nebular physicists compendium The Stromgren sphere The H luminosity and surface brightness What drives the electron temperature? What determines the ionization structure of a nebula? Why is the gas temperature roughly uniform in photoionized nebulae? Comments on energy losses Other comments on the gas temperature Comments on line intensities

Temperature dependence of emission lines Collisionally excited lines (CEL) I CEL = n 1 (X) n e (1,2) / 1 T e -0.5 exp (-E 12 /kT e ) h 21 Recombination lines (RL) I RL = n(X) n e T e - ( with 1) Temperature dependence of line ratios Ratios RL / RL are almost independent of T e Ratios CEL(IR) / RL almost independent of T e Ratios CEL(opt or UV) / CEL(opt or UV) usually depend on T e Ratios CEL(opt) / RL strongly depend on T e

temperature dependence of emission lines The example of lines emitted by O ++

vthe Orion Nebula oDell

volume visualization of the Orion Nebula oDell

a new view of the Orion Nebula images resolved in velocity and ionization Garcia-Diaz & Henney 2006