Spectroscopic studies of Spectroscopic studies of dE galaxies: dE galaxies:

past, present, and futurepast, present, and futureIgor Chilingarian

(Observatoire de Paris - LERMA)

Collaborators: Veronique Cayatte (ObsPM LUTH, France)Florence Durret (IAP, France) Olga Sil’chenko (SAI MSU, Russia)Christophe Adami (LAM, France)Laurent Chemin (ObsPM GEPI, France)Philippe Prugniel (CRAL, France)Victor Afanasiev (SAO RAS, Russia)

Image credits: MASTER

dE galaxies: historydE galaxies: history

M31 satellites M31 satellites (Baade 1944)(Baade 1944)

– NGC205: prototypical dENGC205: prototypical dE– M32: prototypical cEM32: prototypical cE

dE’s are the numerically dominant dE’s are the numerically dominant population in the nearby Universepopulation in the nearby Universe

dE galaxies: structural propertiesdE galaxies: structural propertiesDwarf (MDwarf (MBB>-18.0 and/or >-18.0 and/or <100 km/s)<100 km/s)

EllipticalEllipticalDiffuse (low surface brightness, shallower light profiles Diffuse (low surface brightness, shallower light profiles compared to E’s, n=1…2)compared to E’s, n=1…2)Form a separate sequence on the Kormendy diagram Form a separate sequence on the Kormendy diagram

Some of them contain ISM Some of them contain ISM ((De De Rijcke et al. 2003; Rijcke et al. 2003; Michielsen et al., 2004), although most of dE’s are Michielsen et al., 2004), although most of dE’s are completely lacking of itcompletely lacking of it

Many of them (brighter ones) exhibit embedded Many of them (brighter ones) exhibit embedded structures (Jerjen et al., 2000, Barazza et al. 2002, structures (Jerjen et al., 2000, Barazza et al. 2002, Graham et al. 2002, Lisker et al. 2006)Graham et al. 2002, Lisker et al. 2006)

E’s, bulges, cE’s

E’s, bulges, cE’s

E’s, bulges, cE’s

E’s, bulges, cE’s

dE’sdE’s

dE’sdE’s

Not easy objects for spectral studies due to low surface brightness, low

dE’s: what is their origin?dE’s: what is their origin?

Internal agents:Internal agents:– Collapse + feedback of star formationCollapse + feedback of star formation (Dekel & Silk, 1986, (Dekel & Silk, 1986,

Mori et al. 1997)Mori et al. 1997)

Environment:Environment:– Ram pressure stripping of S, dIm in clusters (Mori & Ram pressure stripping of S, dIm in clusters (Mori &

Burkert, 2000) and groups (Marcolini et al. 2003)Burkert, 2000) and groups (Marcolini et al. 2003)– Gravitational (e.g. tidal) harassment (Moore et al., 1996)Gravitational (e.g. tidal) harassment (Moore et al., 1996)

How to chose the scenario?

Answer: study them spectroscopically!Answer: study them spectroscopically!

Internal kinematicsInternal kinematics can be used to build dynamical models can be used to build dynamical models and study the mass distribution in dE’s. It also may keep and study the mass distribution in dE’s. It also may keep track of violent phenomena (e.g. mergers) if they happened track of violent phenomena (e.g. mergers) if they happened recentlyrecently

Stellar populationStellar population keeps a fossil record of star formation in keeps a fossil record of star formation in a galaxy, and this information can be extracted from spectra a galaxy, and this information can be extracted from spectra integrated along a line of sightintegrated along a line of sight

Existing spectroscopic surveys are not very appropriate to Existing spectroscopic surveys are not very appropriate to study dE’s – e.g. for SDSS the nearby ones (Virgo) are study dE’s – e.g. for SDSS the nearby ones (Virgo) are usually too extended, more distant ones (Coma) are too usually too extended, more distant ones (Coma) are too faint. Indeed, some studies have been completed faint. Indeed, some studies have been completed successfully (e.g. Lisker et al. 2006)successfully (e.g. Lisker et al. 2006)

CRUCIAL POINTS:

To be successful in the spectral studies of dwarf elliptical galaxies you need:

1. Good calibrations and accurate data reduction

2. Good calibrations and accurate data reduction

3. Good calibrations and accurate data reduction

4. Apart from this it might be helpful to have an efficient spectrograph attached to a large telescope

Spectral studies of dE’s in the past: Spectral studies of dE’s in the past: Bender & Nieto 1990: the first attempt to obtain internal Bender & Nieto 1990: the first attempt to obtain internal kinematics of dE’s. They DO NOT rotate (~10 galaxies)kinematics of dE’s. They DO NOT rotate (~10 galaxies)

Simien & Prugniel 2002: Most of them DO rotate (several Simien & Prugniel 2002: Most of them DO rotate (several dozens of galaxies), although some of them do not dozens of galaxies), although some of them do not (Geha et al. 2002); more data (Van Zee et al. 2004a)(Geha et al. 2002); more data (Van Zee et al. 2004a)

Some of them exhibit kinematically-decoupled coresSome of them exhibit kinematically-decoupled cores (De (De Rijcke et al. 2004, Geha et al. 2005, Thomas et al. 2006)Rijcke et al. 2004, Geha et al. 2005, Thomas et al. 2006)

Dynamical modelling (De Rijcke et al. 2006) suggests Dynamical modelling (De Rijcke et al. 2006) suggests that brighter dE’s contain only about 50% of dark matterthat brighter dE’s contain only about 50% of dark matter

Usually metallicities are slightly subsolar ([Fe/H] ~ -0.Usually metallicities are slightly subsolar ([Fe/H] ~ -0.44)), , mean ages are aboutmean ages are about 5 5 Gyr Gyr (Geha et al. 2003, Van Zee (Geha et al. 2003, Van Zee et al. 2004b)et al. 2004b)

What we do: fitting integrated light spectraWhat we do: fitting integrated light spectra• Classical approachClassical approach

• Measure internal kinematics by deconvolving with a single Measure internal kinematics by deconvolving with a single spectrum of a star observed with the same set-up. Better, use a spectrum of a star observed with the same set-up. Better, use a combination of templates (optimal-template fitting).combination of templates (optimal-template fitting).

• Evaluate the stellar population by the mean of line strength indicesEvaluate the stellar population by the mean of line strength indices• Low-resolution (10 A), LickLow-resolution (10 A), Lick• High-resolution with High-resolution with correction correction

• CaveatCaveat• Template mismatch: limitation to the precision of kinematicsTemplate mismatch: limitation to the precision of kinematics• Metallicity – velocity dispersion degeneracyMetallicity – velocity dispersion degeneracy• Do not make optimal use of the information contained in the Do not make optimal use of the information contained in the

spectrumspectrum

• New approachNew approach• Simultaneous fitting of kinematics and stellar populations using Simultaneous fitting of kinematics and stellar populations using

high-resolution models (PEGASE.HR, Le Borgne et al. 2004)high-resolution models (PEGASE.HR, Le Borgne et al. 2004)

Applying this technique to the spectroscopic observations of dE

galaxies

How and what we observedHow and what we observed

Detailed studies of a small sample of Detailed studies of a small sample of nearby objects at a distance of Virgo using nearby objects at a distance of Virgo using IFU spectroscopy with the Russian 6-m IFU spectroscopy with the Russian 6-m telescopetelescope

Spectroscopy of larger sample of more Spectroscopy of larger sample of more distant dE’s (~130 Mpc) using VLT distant dE’s (~130 Mpc) using VLT FLAMES/GiraffeFLAMES/Giraffe

MPFS SpectrographMPFS Spectrograph

BTA SAO RAS

R=1300…1800

=4200…5600 A

IC 3653IC 3653 ( (rather compactrather compact))MMBB = -16. = -16.88 = 70 km/s= 70 km/s [Fe/H] = 0.[Fe/H] = 0.00 Age =5 GyAge =5 Gy

Presence of embedded disc revealed by analysis of kinematics is confirmed by the HST ACS imagery

Existence of this structure supports N-body modelling (Mastropietro et al. 2005), showing that even after serious morphological transformation of dwarf galaxies in a cluster, discs will not be completely destroyed.

Chilingarian et al., 2007, MNRAS

ICIC 783 (spiral 783 (spiral dEdE))

• Rotation in the inner regionRotation in the inner region

• Young nucleusYoung nucleus ( (33 GyrGyr))

• Low metallicityLow metallicity

• Two consequent crossing of the cluster Two consequent crossing of the cluster centre?centre?

MMBB = -16. = -16.33 = 30 km/s= 30 km/s

ageagecntcnt=3.3 Gyr=3.3 Gyr [Fe/H][Fe/H]cntcnt=-0.35=-0.35 ageageoutout=13 Gyr=13 Gyr [Fe/H][Fe/H]outout=-0.8=-0.8

Chilingarian et al., 2007, AstL

IC 3468 (barred dE)IC 3468 (barred dE)

• Kinematical axis is turned by Kinematical axis is turned by 35-40 35-40 degrees off the degrees off the photometric onephotometric one. . HST images reveal faint warped structure, HST images reveal faint warped structure, corresponding to this orientationcorresponding to this orientation

• Young extended embedded structure (disc)Young extended embedded structure (disc)

• Velocity dispersion map shows a dip, corresponding to this Velocity dispersion map shows a dip, corresponding to this “disc”“disc” ( (“bar”“bar” according toaccording to Barazza et al. 2002)Barazza et al. 2002)

MMBB = -16. = -16.77 = 40 km/s= 40 km/s

ageagecntcnt=5.3 Gyr=5.3 Gyr [Fe/H][Fe/H]cntcnt=-0.4=-0.4 ageageoutout=8.6 Gyr=8.6 Gyr [Fe/H][Fe/H]outout=-0.6=-0.6

Chilingarian et al., 2007, AstL

IC 3509 (IC 3509 (classicalclassical dE)dE)

• Was chosen as a “prototypical” dE: without bar, disc, spiralsWas chosen as a “prototypical” dE: without bar, disc, spirals

• Rotation along two perpendicular directions. Kinematical Rotation along two perpendicular directions. Kinematical appearance looks very similarly to giant E NGC5982, where it appearance looks very similarly to giant E NGC5982, where it was explained as a projection of orbits in a 3-axial potential was explained as a projection of orbits in a 3-axial potential (without need for embedded structure)(without need for embedded structure)

• Rather youngRather young (4 (4 GyrGyr)) metal-richmetal-rich ( ([Fe/H]=0) core[Fe/H]=0) core

MMBB = -16. = -16.22 = = 550 km/s0 km/s

ageagecntcnt==44.1.1 Gyr Gyr [Fe/H][Fe/H]cntcnt=-0.0=-0.0 ageageoutout=7.8 Gyr=7.8 Gyr [Fe/H][Fe/H]outout=-0.4=-0.4

Chilingarian et al., 2007, AstL

Sample of galaxies Sample of galaxies in the Abell 496 clusterin the Abell 496 cluster

Chilingarian et al., submitted to A&AChilingarian et al., submitted to A&A

NearbyNearby ( (z=9800 km/sz=9800 km/s) ) relativelyrelatively poor poor ((richnessrichness class class 1) 1) clustercluster

Deep multicolour imagery of the whole Deep multicolour imagery of the whole clustercluster and deep multi-object spectroscopy and deep multi-object spectroscopy of a sample of dwarf galaxiesof a sample of dwarf galaxies

DataData

ImageryImagery (u’, g’, r’, i’) using MegaCam @ CFHT(u’, g’, r’, i’) using MegaCam @ CFHT

Spectroscopy: 3 fields with Giraffe in MEDUSASpectroscopy: 3 fields with Giraffe in MEDUSA mode (2mode (2 GTO nights) GTO nights)

48 cluster members48 cluster members

First observations of a wide sample of dE’s in a cluster at such a high (R=6000) spectral resolution

What signal-to-noise ratio do we need to What signal-to-noise ratio do we need to constraint stellar populations?constraint stellar populations?

40 per pixel?40 per pixel?

=73±1 km/s

What signal-to-noise ratio do we need to What signal-to-noise ratio do we need to constraint stellar populations?constraint stellar populations?

Maybe 15 per pixel?Maybe 15 per pixel?

=38±2 km/s

What signal-to-noise ratio do we need to What signal-to-noise ratio do we need to constraint stellar populations?constraint stellar populations?

With the resolution of FLAMES we can do With the resolution of FLAMES we can do something with SNR as low as 5 per pixelsomething with SNR as low as 5 per pixel

=30±3 km/s

Data analysisAmong 110 objects 48 are cluster members; 46 of them we Among 110 objects 48 are cluster members; 46 of them we can fit well. 36 dEs/dS0s, 2 SBs, 10 low-luminosity E/S0/Sacan fit well. 36 dEs/dS0s, 2 SBs, 10 low-luminosity E/S0/Sa

For 46 galaxies we obtained:For 46 galaxies we obtained:– Accurate internal kinematics Accurate internal kinematics scaling relations scaling relations– [Mg/Fe] abundance ratios by measuring Lick indices[Mg/Fe] abundance ratios by measuring Lick indices– Estimations of age and metallicity for the luminosity-Estimations of age and metallicity for the luminosity-

weighted population (single starburst)weighted population (single starburst)Mapping relations between stellar populations and velocity Mapping relations between stellar populations and velocity dispersions in the low-dispersions in the low- regime regime

Colour maps for 48 galaxiesColour maps for 48 galaxies

Faber-Jackson RelationFaber-Jackson Relation

Lick indices, ages, metallicitiesLick indices, ages, metallicities

Principal results on Virgo dEsPrincipal results on Virgo dEsEmbedded discs – early type progenitorsEmbedded discs – early type progenitors

Evolutionary decoupled cores – ram pressureEvolutionary decoupled cores – ram pressure

Principal results on Abell496 dEsPrincipal results on Abell496 dEs[Mg/Fe]=0, therefore the starburst events in dE [Mg/Fe]=0, therefore the starburst events in dE galaxies must have lasted at least 1-2 Gyr to galaxies must have lasted at least 1-2 Gyr to explain observed iron enrichmentexplain observed iron enrichment

Only brighter galaxies exhibit embedded Only brighter galaxies exhibit embedded structuresstructures

Many galaxies exhibit red and blue coresMany galaxies exhibit red and blue cores

Main conclusion: “external” channel of dE Main conclusion: “external” channel of dE formation (ram pressure stripping + formation (ram pressure stripping + harassment) is a plausible scenario, while SN harassment) is a plausible scenario, while SN winds are much less probablewinds are much less probable

dE’s: future studiesdE’s: future studies

Obtaining extremely high S/N spectra of nearby Obtaining extremely high S/N spectra of nearby dEs in order to recover star formation histories in dEs in order to recover star formation histories in details – details – feasiblefeasible, but difficult to convince TACs, but difficult to convince TACs

New multi-object (possibly falcon-style multi-IFU) New multi-object (possibly falcon-style multi-IFU) spectrographs with higher efficiency than present spectrographs with higher efficiency than present ones will slightly improve the situation and allow to ones will slightly improve the situation and allow to go down to Mgo down to MBB=-14 – =-14 – feasiblefeasible

Observing yet star-forming dE progenitors at Observing yet star-forming dE progenitors at z=0.4…0.8 – z=0.4…0.8 – feasiblefeasible, although difficult, ELT task, although difficult, ELT task

Need to be 3 mag deeper than SDSS to perform Need to be 3 mag deeper than SDSS to perform massive studies of brighter dEs (-18<Mmassive studies of brighter dEs (-18<MBB<-15) in <-15) in

the nearby Universe (z<0.1) – the nearby Universe (z<0.1) – questionablequestionable

dE’s: future studiesdE’s: future studies

Need for IFU’s with large fields of view and Need for IFU’s with large fields of view and largelarge spaxels (like PMAS-PPAK but on 8m class spaxels (like PMAS-PPAK but on 8m class telescopes) – no need for high spatial resolution, telescopes) – no need for high spatial resolution, however spectral resolution should be however spectral resolution should be sufficiently high – sufficiently high – questionablequestionable

Observing spectroscopically extremely low-Observing spectroscopically extremely low-surface brightness dSph galaxies (“dark matter surface brightness dSph galaxies (“dark matter worlds”). Sky background coming from the worlds”). Sky background coming from the interplanetary dust becomes important at this interplanetary dust becomes important at this level, so need to go out of the Solar system level, so need to go out of the Solar system plane – plane – unfeasibleunfeasible

SummarySummarySpectroscopic studies of dE galaxies are Spectroscopic studies of dE galaxies are crucial for understanding their formation crucial for understanding their formation and evolution, which is still a missing block and evolution, which is still a missing block in our understanding of galaxy formationin our understanding of galaxy formation

What is already done for brighter dE What is already done for brighter dE galaxies is major achievements in this galaxies is major achievements in this field, but there is still need to go toward field, but there is still need to go toward lower masses/luminosities. We still do not lower masses/luminosities. We still do not know anything about (1) dark matter know anything about (1) dark matter content and (2) star formation histories of content and (2) star formation histories of fainter dE’sfainter dE’s