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Theoretical Properties of Lyα Cooling Radiation and Its Impact on Gas Dynamics

This presentation delves into the theoretical properties of Lyman-alpha (Lyα) cooling radiation, emphasizing its significance in gas cooling processes in astrophysical environments. Key topics include the interplay between gas temperature and cooling mechanisms, observable attributes of Lyα emission, and the implications for gas collapse dynamics. It also explores the outcomes of Monte Carlo simulations designed to analyze the emergent Lyα spectrum, along with a discussion on the frequency-dependent surface brightness profile and its relation to gas motion. The impact of intergalactic medium on observed spectra is also evaluated.

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Theoretical Properties of Lyα Cooling Radiation and Its Impact on Gas Dynamics

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  1. Theoretical Properties of Ly Cooling Radiation’Mark Dijkstra (CfA) Collaborators: Z. Haiman, M.Spaans & A. Lidz Texas Symposium, Melbourne

  2. ‘Theoretical Properties of Ly Cooling Radiation’Mark Dijkstra (CfA) Collaborators: Z. Haiman & M.Spaans . motivation Texas Symposium, Melbourne

  3. ‘Theoretical Properties of Ly Cooling Radiation’Mark Dijkstra (CfA) Collaborators: Z. Haiman, M.Spaans & Lidz Outline of talk • Gas cooling & Ly emission • Observable properties of Ly cooling emission (DHS 06a,b) • Observable properties of continuum cooling radiation (D, submitted to MNRAS) Texas Symposium, Melbourne

  4. Gas cooling Primordial Cooling curve T < 6.e4 K: H cooling dominates Thoul & Weinberg ‘95 Texas Symposium, Melbourne

  5. The Impact of Cooling on Gas Collapse • Adiabatic collapse: Gas shell virializes at r=rmax/2 And heated to virial T of halo • Turn on cooling mechanism tcool<<tdyn Gas cools to 1e4 K rapidly Thoul & Weinberg ‘95 Texas Symposium, Melbourne

  6. The Impact of Cooling on Gas Collapse. • Gas collapses at T=1e4 K (no virial shock, for M<Mcrit) • Cooling @ T=1.e4 K is dominated by collisional excitation of H • Collisions to 2p and 2s states of H • 2p 1s: Ly cooling • 2s 1s: 2- emission Thoul & Weinberg ‘95 Texas Symposium, Melbourne

  7. Gas cooling. • Gas cooling is dominated by Ly emission • Spatially extends up to ~100 kpc. • Luminosities in the range L=1e42-1e44 erg/s (Haiman et al ‘00, Fardal et al ‘01, Yang et al ‘06) Yang et al ‘06 Texas Symposium, Melbourne

  8. Lyman Alpha ‘Blobs’ • Observed Spatially extended Ly emission up to ~ 100 kpc. • Several tens have been discovered at z=3-5. (e.g. Matsuda et al, 2004; Saito et al, 2006/2007) • Luminosities ~ 1e42-1e44 erg/s • Powered by cooling radiation? • Alternatives: • Obscured starburst/AGN • Shock heating by superwinds. • Fluoresence (next talk) Steidel et al. (2000) Texas Symposium, Melbourne

  9. Ly Cooling Radiation: Properties • Observational signatures of cooling radiation? • Cooling clouds are optically thick to Ly-> radiative transfer (RT). Well studied problem (> 60 years) HARD Texas Symposium, Melbourne

  10. Ly Transfer • A simple problem: a Ly source inside a uniform static neutral H cloud. • Calculate emerging spectrum Harrington ‘73, Neufeld ‘90, DHS06a • Generally no analytic solution can be found: • Monte-Carlo. Texas Symposium, Melbourne

  11. Ly Transfer • Calculate Ly transfer through series of models representing cooling clouds • Goal: To extract basic properties of Ly cooling radiation • Use Monte-Carlo: follow individual photons through the collapsing cloud. • The code is reliable. Texas Symposium, Melbourne

  12. Ly Transfer • Cute: deuterium • N_H=2e19 (static) Texas Symposium, Melbourne

  13. Ly Cooling Radiation: Properties • Use Monte-Carlo method to calculate emerging Ly spectrum+surface brightness profiles. • Result 1: Radiative Transfer of Ly through collapsing (optically thick) gas results in a blueshift of the line. The opposite is true for outflows. • Frequency off-set of Ly-line constrains gas motion. Frequency-> Surface brightness Radius Texas Symposium, Melbourne Data: Smith & Jarvis, 2007

  14. Ly Cooling Radiation: Properties • What if one can’t tell whether there is an off-set? • Ly cooling radiation has frequency dependent surface brightness profile: Red: reddest 15% of Ly Blue: bluest 15% of Lya Texas Symposium, Melbourne

  15. Ly Cooling Radiation: Properties • Why a frequency dependent surface brightness profile? Texas Symposium, Melbourne

  16. Ly Cooling Radiation: Properties • Caution: Spectra shown are affected by IGM. The impact of the IGM is non-trivial (e.g. Santos ‘04; D, Wyithe & Lidz ‘07) Texas Symposium, Melbourne

  17. Ly Cooling Radiation: Properties • Has cooling radiation from cold accretion been seen? • Perhaps (e.g. talk by M.Rauch) • Saito et al’07 find asymmetric Ly profiles with enhanced blue emission. Texas Symposium, Melbourne

  18. Ly Cooling Radiation: Properties • Cooling radiation seen? Wilman et al ‘05 Texas Symposium, Melbourne

  19. Cooling Radiation: Properties Part II (=short) Texas Symposium, Melbourne

  20. 2- Cooling Radiation: Properties • 1 collisional excitation 1s2p is accompanied by 0.6 excitations 1s2s • 2s1s++, 2 photons have combined energy of 10.2 eV. • Results in continuum emission redward of Ly-a. The spectrum of this emission has been calculated by Spitzer & Greenstein (‘51) Texas Symposium, Melbourne

  21. 2- Cooling Radiation: Properties • How weak is continuum? Prominence of Ly line relative to continuum is quantified by the equivalent width (EW) EW=1000-1500 Å Emitted restframe EW However IGM opaque to Ly; Observed restframe EW EW< 200 Å Texas Symposium, Melbourne

  22. 2- Cooling Radiation: Properties • Shape of continuum can also ‘betray’ cooling Cooling powered or resonant scattering or recombination emission? Texas Symposium, Melbourne

  23. ‘Theoretical Properties of Ly-a Cooling Radiation’Mark Dijkstra (CfA) Collaborators: Z. Haiman & M.Spaans Summary • Gas cooling is accompanied by copious Ly emission (especially the ‘cold’ mode). • Observational signposts of this emission are: • Intrinsic blueshift of line • Steepening of surface brightness towards bluer Ly wavelength • Faint continuum redward of Ly with weird spectrum. • Caution: Absence does not immediately rule out cooling radiation. • Currently, no convincing observational evidence exists, but d/dt(almost convincing) > 0. Texas Symposium, Melbourne

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