Effects of Dust on the Observed SEDs of Galaxies

Andrew Schurer

SISSA & MAGPOP

Andrew Schurer• Studied at Nottingham University for four years - Msci in physics

– Attended lecture course on a large range of topics including many in astrophysics.

– Undertook several research projects including a 3 month project on magnetic resonance.

• Currently one year into a PhD at SISSA, the majority of which has been spent attending a comprehensive series of lectures.

– Radiative Processes

– Galaxy Formation

– Observational and Classical Cosmology

– General Relativity

– High energy Astrophysics

– Extragalactic Astrophysics

– Numerical Methods

– Cosmological Perturbations and CMB

– After recombination

• At SISSA attend regular seminars given by internal and external experts as well as students.

MAGPOP• Member of an EU funded research and training network, MAGPOP,

(Multi-wavelength Analysis of Galaxy Populations)

• As part of the MAGPOP network I have attended;– the first MAGPOP network meeting in Toledo where I heard many experts

talk about diverse areas within the topic of galaxies and planned a collaboration with a fellow member of the MAGPOP network

– A MAGPOP workshop in Budapest, where I attended lectures on observations, stellar synthesis and SAMs and participated in a telescope time application project.

• I found these events useful as they have not only improved my knowledge within these areas but also offered an opportunity to meet other researchers within the field.

• In addition these events have given me an opportunity to present my ideas to fellow researchers, improving my communication skills.

My Research

• Objectives of Magpop are to use SEDs of galaxies to extract key physical information about the galaxy and then to use this to constrain theoretical models of galaxy formation.

• In particular my role will be in studying the effect that dust reprocessing has on the SEDs of galaxies

• Dust Reprocessing particularly important in objects with particularly high Star Formation Rates, e.g. Star bursts.

• Dust plays an important role in star formation:

– Dense molecular clouds prevents stellar radiation from penetrating allowing molecules to cool and form stars.

• Importance increases with redshift.

– Detection of a FIR/sub-mm background by the COBE satellite.

– Detection of many highly extincted source at high Z

• Major effect on galaxy evolution theories.

– Initial studies of cosmic Star Formation Rates from optical surveys suggested that SFR peaked at about z =1 or 1.5

– IR surveys revealed that a large proportion of the cosmic SFR had been lost because of dust reprocessing.

– IR surveys suggest that SFR is constant after z=1.

• Dust is important as a diagnostic:

– Its Emission spectrum provides an indication of physical conditions including an estimate of SFR.

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

GRASIL• To try to recreate observations I have been using the dust reprocessing

model GRASIL (Silva 1998), developed by my supervisor Gian Luigi Granato and Laura Silva.

• GRASIL is a code that computes the spectral evolution of stellar systems. These stellar sources are distributed realistically in a model galaxy which also contains dust.

• It performs a complete radiative transfer computation of the effect of the dust on the emission of the stellar sources.

– Calculates SED from radio to far UV

– includes PAH, molecular and nebular lines

• The overall evolution of the galaxy over time is controlled by a separate chemical evolution code which follows the star formation rate, initial mass function, metallically and residual gas function.

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.

Conclusions• This work has increase my own knowledge about dust reprocessing and more

specifically about the model GRASIL.

• What it does

• How to use it

• What free parameters the models has.

• It has also improved my programming skills, particularly in Fortran and IDL

• Shown GRASIL is capable of recreating any late type galaxy.

• Through an investigation into the parameters, typical values have been found.

• Highlighted problems in the radio.

• Future Work

– Improve fits, increase knowledge of parameters.

– Analyse nebula emission lines and check if best fit models agree with other observational evidence.

– Detect and correct any further problems found.

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 (following Li &

Draine 2001). ISO able to measure short wavelengths only in bright objects e.g. visual reflection nebula.

– 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.

• 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.

– 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.

• 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.

Summary• Have now spent one year as part of the MAGPOP scheme living and

working in Trieste.

• From the first year at SISSA have received a thorough training in a broad variety of different topics through lectures and seminars.

• Have attended a MAGPOP conference in Toledo and a workshop in Budapest, where I attended lectures more specific to my particular area of research and met potential collaborators.

• Have now begun my own research into the effect of dust on the SED of galaxies, which is an important objective of MAGPOP.

• I have used GRASIL to recreate the SEDs of galaxies.– Have shown GRASIL can recreate late type galaxies of different type.

– Have learned about dust reprocessing and in particular the GRASIL model

– Have improved my programming skills.

• By giving presentations I have improved my communication skills.

• Plenty of opportunity for future work and future collaborations.