Aging nearby spiral galaxies using Halpha to UV flux ratios: Effect of model parameters

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Aging nearby spiral galaxies using H-alpha to UV
flux ratios Effect of model parameters

• Francesca von Braun-Bates

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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

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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-a line

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Importance of the flux ratio

• Ha UV output decreases over Myr
• UV relatively constant
• Ha caused by stars M gt 10M?
• High temperature UV photons
• large mass short lifetime

• 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

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Modelling star forming regions

• 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

• Leitherer et al. Starburst99 program
• Simulates evolution of single GMC
• Input plausible parameters for nearby spirals
• Outputs photometry spectral data

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Metallicity

• 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

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SalpeterKroupa
Initial Mass Function

• 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-a
• Different indices depending on mass
• High mass stars hottest ? most luminous ? easiest
  to observe ? distribution best understood
• (1955) canonical IMF a 2.35
• (1997) accounts for
  underabundance of low-mass stars in Salpeter a

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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

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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

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Results
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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 lt2Myr
• Further discrimination requires independent age
  data

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Further Research

• Age maps
• Narrow escape fraction uncertainty currently
  0-50 (!)
• Use truncated IMF reduces stars gt30M ?

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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 PHYS1500 17/09/2007
• Initial mass function Chabrier G. 2003 Galactic
  Stellar and Substellar Initial Mass Function
  Publications of the Astronomical Society of the
  Pacific, vol. 115 pp.763-795
• Evolutionary track various citation of Maeder
  Maynet