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