Aging nearby spiral galaxies using H-alpha to UV flux ratios: Effect of model parameters Francesca von Braun-Bates

Star formation in spiral galaxies  Formation occurs in spiral arms: » Pressure waves change density of gas clouds: · Compression triggers protostar formation · Meanwhile spiral density wave keeps moving » Young blue stars evolve fastest: · Go supernova in very short time · So pressure wave hasn't moved far if star still shining  Therefore blue stars trace star- forming regions

The H-alpha and UV fluxes  UV emitted by all stars » Planck curve: blackbody radiates at all wavelengths » Flux = total radiation integrated over area  Hydrogen-alpha wavelength caused by ionisation: » Interstellar medium 10% He, 90% H (by no. of particles) » HI region + Lyman photon = HII region » Free e - recombine & fall through energy transitions » 3→2 transition emits 6563Å = H- α line

Importance of the flux ratio  Taking ratio of individual “pixels” of galaxy images indicates age of star-forming region »...but this depends on how the ratio decreases... » So run lots of possible scenarios and compare Red: UV (1500Ǻ) Blue: Optical H- α (6563Ǻ) image of M51 (GALEX)  H α : UV output decreases over Myr » UV relatively constant  Hα caused by stars M > 10M  : · High temperature = UV photons · large mass = short lifetime

Modelling star forming regions  Leitherer et al. “Starburst99” program: » Simulates evolution of single GMC » Input plausible parameters for nearby spirals » Outputs photometry & spectral data  Evolutionary synthesis models: » Combine theories of physical stellar proerties: mass loss, spectral output, plasma/gas dynamics &c. » Different options to cover most types of conditions: user-chosen » Outputs projected observable data

 Proportion of star made from “metals” » Big Bang cosmology forms H, He in early universe » All heavier elements formed in stars  metals  Negligible change over model lifetime (Leitherer 97) » Metals returned to ISM by supernovae · Few SNe within 50Myr · Only returned to local region Metallicity

 IMF: total number of stars of a certain mass range initially created per unit volume » “Determines the evolution, surface brightness, chemical enrichment, and baryonic content of galaxies” (Chabrier)  Simple power law: dN/dm m -α » Different indices depending on mass » High mass stars hottest → most luminous → easiest to observe → distribution best understood  (1955): canonical IMF: α = 2.35  (1997) : accounts for underabundance of low-mass stars in Salpeter α= Salpeter Kroupa Initial Mass Function

Evolutionary Mass-Loss Tracks  “Stellar thermostat” : pressure vs gravity » Large star = weak surface gravity → outer layers loosely held → puff off as star ages  Path on H-R diagram forms “mass loss track”: » Mass, luminosity, temperature changes over star's lifetime  2 main models about precise behaviour of star: » Geneva track » Padova track

Converting data to flux ratio  SB99 does not directly output flux ratio » Must be inferred from simulated observable data... »...converted to a standard set of units »...then normalised to match directly-measured real data

Results

Conclusions  Model results consistent = insensitive to parameters  Ages reliable  Zero-age flux ratio: » Discriminator between models » Eliminate extreme models?  Flux ratio calibration: » Very sensitive to zero-age flux: currently assume youngest stars <2Myr  Further discrimination requires independent age data

Further Research  Age maps » Narrow escape fraction uncertainty: currently 0-50% (!) » Use truncated IMF: reduces stars >30M 

Acknowledgements and References  Supervisor: Dr. John O'Byrne  Based on Hons. Thesis by and advice from Madhura Killedar  References (except where cited): » Flux ratio, SB99, converting to flux: Killedar, M. 2006; Mapping ages by determining the H-alpha to UV flux ratio ; Sydney University » Metallicity: Murphy, T. 2007; Galactic Recycling lecture for PHYS /09/2007 » Initial mass function: Chabrier G. 2003; Galactic Stellar and Substellar Initial Mass Function; Publications of the Astronomical Society of the Pacific, vol. 115 pp » Evolutionary track: various citation of Maeder & Maynet