Transcript Aging nearby spiral galaxies using H

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 starforming 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
 Hα : UV output decreases over Myr
» UV relatively constant
» Hα caused by stars M > 10M:
· High temperature = UV photons
· large mass = short lifetime
Red: UV (1500Ǻ) Blue: Optical H-α (6563Ǻ) image of M51 (GALEX)
 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
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
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
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-α
» Different indices depending on mass
» High mass stars hottest → most luminous → easiest to
observe → distribution best understood
 Salpeter (1955): canonical IMF: α = 2.35
 Kroupa (1997): accounts for underabundance of
low-mass stars in Salpeter α=
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
»
»
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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