Effects of Dust on the Observed SEDs of Galaxies

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Effects of Dust on the Observed SEDs of Galaxies

• Andrew Schurer
• INAF/SISSA MAGPOP

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Overview

• Interstellar Dust in Astrophysics
• Why is the study of dust important?
• How can dust reprocessing be effectively
  modelled, e.g. GRASIL
• My Role within this field
• My current work,
• Future research possibilities.

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The Importance of Dust in Astrophysics

• Trumpler (1930) found that distant stars dimmed
  by something in addition to inverse squared law
  dust.
• Recent IR observations (IRAS) have shown how
  important this effect is. In the local universe
  30 of starlight is dust reprocessed.
• Dust absorbs and scatters photons, mostly at UV
  wavelengths, emitting it in the IR

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Dust Models

• Seek to reproduce the SED of an astrophysical
  object including the presence of dust.
• Unrealistic geometry -2D slab model
• Incomplete radiative transfer
• no emission
• Empirical fitting laws used.
• No link to galaxy evolution
• estimate of adsorbed stellar radiation
• many only interested in interpretation of a
  single object
• GRASIL (Silva 1998) - complete radiative transfer
  computation of self-consistent galactic models
  including dust with realistic geometry.
• Calculates SED from radio to far UV
• including PAH, molecular and nebular lines

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GRASIL - geometry

• Galaxies are composed of either a spheroid a disk
  or a combination of both.
• Two main Dust Components - Molecular clouds
  (where stars are born)
  - Interstellar medium
• Also Dust envelope around AGB stars, represents
  shell of expelled material.
• 
  
• 
  Age Dependent
  Extinction

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Chemical Evolution

• Chemical Evolution - Requires history of SF,
  initial mass function, metallically and residual
  gas function.
• Semi Analytic Models e.g. Durham model - Galform
• Monolithic chemical evolution code -che evo
• CHE EVO
• SFR based on a Schmidt law with the possibility
  truncating the SF or of adding a burst of star
  formation.

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Observational Data Sample

• Goldmine data set - Boselli (2003) (MAGPOP
  collaborator)
• Data available for wide range of wavelengths from
  radio to UV, including IR data from ISO CAM/PHOT
  and IRAS
• Optically selected Virgo sample (100 galaxies)
• Galaxies later than SO
• Whole range of morphologies and galaxy densities.
• Virgo serendipitous sample (18 galaxies)
• Representative of a mid-IR selected sample of
  nearby galaxies
• An additional 6 Virgo galaxies observered by
  ISOCAM as part of other projects
• A1367 and Coma clusters samples (contursi 2001)
  (18 galaxies)
• High star forming late time galaxies with
  peculiar morphologies (not a complete sample)
• Additional 3 galaxies in the Coma cluster

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GRASIL Parameters

• Che-Evo
• Star formation efficiency
• Infall Timescale
• Geometry of Galaxy
• For Bulge one scale length
• For Disk two scale lengths
• For a combination disk to bulge ratio
• Other
• Radius and mass of MCs (one independent
  parameter)
• Fraction of residual gas in MCs
• Escape Timescale

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Conclusions

• Shown GRASIL is capable of recreating
  observations of late type galaxies of different
  type.
• Several problems still. Particularly in UV, radio
  and when describing galaxies of earlier type,
• Future Work
• Improve fits, by increasing size of library -
  more chemical evolutions.
• Through an investigation into the parameters,
  typical values can be found.
• Analyse nebula emission lines and check if best
  fit models agree with other observational
  evidence e.g. physical size.
• Detect and correct any further problems found.

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Future Work

• Improved model of PAH bands
• First version of GRASIL based on pre-ISO data,
  (following Xu De Zotti 1989)
• Following release of ISO data PAH treatment
  updated (Vega 2005 following Li Draine 2001).
  ISO able to measure short wavelengths only in
  bright objects e.g. visual reflection nebula.
• PAH suppressed in the MCs to match
  observations.
• IRAC aboard the Spitzer space telescope has
  increased sensitivity and resolution, allowing
  for better treatment of the PAH lines (Draine
  2006).
• More bands and slight changes in existing bands,
  better treatment of NIR continuum.
• GRASIL should be updated to incorporate this.

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• Evolution of Dust Grain Population
• Complex problem, need to explain how dust grains
  are produces and destroyed (Dwek 98, Morgan
  Edmunds 2003).
• The main ways in which dust grains can be
  produced are in the stellar outflow of stars such
  as AGBs and supernova and also by being built up
  in the ISM.
• Processes which lead to their destruction include
  shock waves due to supernova and collisions with
  other grains.
• Not clear which of the processes dominate.
• GRASIL does not include any details of the
  evolution of the grain population. E.g. Birth
  and destruction of grains.
• By attempting to include these processes within
  the GRASIL model it should be possible to make it
  more physically consistent and possibly more
  accurate at high redshift.

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• Modelling embedded clusters.
• Young Massive Clusters - possibly evolutionary
  fore-runners of Globular Clusters (Tagle 2003).
• Two types blue YMCs, for whom the blue
  photosphere of the young stars is directly
  visible and embedded YMCs.
• Observations of embedded YMCs in a starburst
  environment require an angular resolution in the
  MIR which has only become available recently.
• Using new data it is possible to try to model
  embedded YMCs in starburst environments, using a
  dust model such as GRASIL.
• Can improve our understanding of starburst
  phenomena, the formation of globular clusters as
  well as about star formation itself.
• Coupling to SAMs in Munich (MAGPOP node)
• It is essential that any galaxy evolution model
  contains a correct treatment of dust in order to
  compare models to observations.
• Work with Munich to develop a GRASIL black box
  which can be attached to their SAMs.

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Summary

• Interstellar dust is a very important component
  of galaxies and a rigorous treatment must be
  adopted for a wide range of studies including the
  modelling of SEDs and determination of the star
  formation rate.
• GRASIL is a state of the art model which
  incorporates the effects of dust reprocessing
  from the X-Ray to radio wavelengths within a
  realistic galactic geometry.
• It can be used to model a wide range of different
  galaxies with careful treatment of the parameters
  of the model.
• Needs an updated treatment of the PAH lines based
  on the new SPITZER observations.
• It should include a more physical treatment of
  the evolution of the dust component.
• It can be used to study the evolution of Young
  Massive Clusters.
• It could be combined with the Munich SAMs.