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The ABC of dEs
First results of the MAGPOP-ITP
Dolf MichielsenCentre for Astronomy & Particle TheorySchool for Physics & AstronomyUniversity of Nottingham
IntroductionDwarf elliptical galaxies (dEs)
• MB > -18 mag
• gas- and dust-less (or -poor)• exponential surface brightness profiles• very difficult to observe!
IntroductionDwarf elliptical galaxies (dEs)
• MB > -18 mag
• gas- and dust-less (or -poor)• exponential surface brightness profiles• most abundant type of galaxy in nearby clusters• in hierarchical structure formation,
dwarf galaxy-size DM halos are the first to form dEs probe early universe?
• In the last years reasonable samples became available (e.g. with kinematic information)
Introduction
L-σ relation for dEs (Faber-Jackson)
De Rijcke, Michielsen et al., 2005
Introduction
Formation and evolution of dEs
• Isolated formation– SN-driven galactic winds
Introduction
Formation and evolution of dEs
• Isolated formation– SN-driven galactic winds
• Impact of environment– Ram pressure stripping
Introduction
Formation and evolution of dEs
• Isolated formation– SN-driven galactic winds
• Impact of environment– Ram pressure stripping– Harassment
Introduction
Formation and evolution of dEs– Harassment
Moore et al., 1998
Introduction
Formation and evolution of dEs• Isolated formation
– SN-driven galactic winds
• Impact of environment– Ram pressure stripping– Harassment
• Evidence for transformation– Disk/spiral structures in dEs– Ionized/neutral gas in dEs
Introduction
Transformation: distribution of dEdis in Virgo
Lisker et al., 2006
MAGPOP-ITP
MAGPOP• European Marie-Curie Training Network
(including IAC) • Aim: study galaxy formation and evolution• Focus on population synthesis through
multiwavelength observations
MAGPOP-ITP
MAGPOP
International Time Programme (ITP)• 5% of telescope time on Canary telescopes is
dedicated to large, international programmes• “Star formation history of dwarf galaxies”
(PI: Peletier)• Survey of dwarf galaxies in Virgo and field• Optical/NIR imaging/spectroscopy of dIrrs/dEs
MAGPOP-ITP
• Sample selection– UV / optical imaging from GALEX / SDSS available– Virgo: complete sample
• dEs : mB > 16 mag – 43 objects
• dIrrs: mB > 15.5 mag – 50 objects
– Field: • dEs ~15 objects
difficult to find dEs in field – faint + residual star formation• dIrrs ~25 objects
– Archival data from HST and GOLDMine
MAGPOP-ITP
• 70 nights allocated on 4 large telescopes on La Palma (48 + 12)
• dIrr part almost completed
• dE part suffered from bad weather– 5/6 nights on NOT with ALFOSC– 2/3 nights on WHT with WYFFOS– ½/5 nights on WHT with ISIS– 0/6 nights on NOT with NOTCAM
NOT observations
• 25 dEs: 18 cluster (Virgo) and 7 field• Long-slit 3500 – 6100 A @ 7.8 A (FWHM)• S/N = 20 – 40 (per resolution element)
Field dEs
Virgo dEs
Lick indices
• Break age-metallicity degeneration of optical colors
• Use age and metallicity sensitive lines– Age:
H-balmer series
– Metallicity:Mg, Fe lines
Lick indices
SSP models
Single-ageSingle-metallicityPopulation
or
SimpleStellarPopulation
Ages and Metallicities
• [MgFe] versus Hβ for field and cluster dEs
FieldM32
Virgo ClusterCancer
α/Fe abundance
Measures star formation history• α-elements produced in SNII on rapid timescales
(α-elements : O, Si, Ca, Mg,…)• Fe mainly produced in SNIa on long timescales• SSP models constructed with solar abundance
resulting from continuous star formation• Normal Es form in a rapid burst, and exhibit
α/Fe overabundance
α/Fe abundance
• Mgb versus <Fe> for field and cluster dEs
Virgo ClusterCancer
FieldM32
Normal Es: SAURON
• dEs versus Es: age/met and α/Fe
SAURON EsOur dEs
Consequences
• dEs are not just small Es• dEs are not simple, old, metalpoor objects
as predicted by naive interpretation of hierarchical merging scenario
• Like Local Group dSphs, dEs have a prolonged star formation history
• Need to try and disentangle old underlying stellar population from dominating (in light) young population
Special case VCC 21
SDSS g-band HST/ACS F450 band
Special case VCC 21• Nuclei younger than host
• Right nucleus more metal-rich + solar abundance
• Blue-core dE (Lisker et al., 2006)
Conclusions
• Field and cluster dEs have the same properties (but sample problems : very few field dEs)
• (Mean, luminosity-weighted) ages and metallicities : dEs are younger than normal EsdEs are less metal-rich than normal Es
• dEs show α/Fe underabundance whereas Es show α/Fe overabundance
• Galactic winds combined with prolonged star formation can explain the low metallicities and underabundance
• dEs probably are a mixed population of ‘original’ dEs + transformed dIrrs
• Intermediate-type dE/dIrrs : galaxies in transition?
Invitation
There is a lot of data to analyse…
Anyone interested in working with the MAGPOP-ITP data contact– Alexandre Vazdekis– Javier Cenarro– Marc Balcells