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Spectroscopic studies of dE galaxies: past, present, and future

Spectroscopic studies of dE galaxies: past, present, and future. Igor 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)

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Spectroscopic studies of dE galaxies: past, present, and future

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  1. Spectroscopic studies of dE galaxies: past, present, and future Igor 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)

  2. dE galaxies: history M31 satellites (Baade 1944) • NGC205: prototypical dE • M32: prototypical cE dE’s are the numerically dominant population in the nearby Universe Image credits: MASTER

  3. dE galaxies: structural properties E’s, bulges, cE’s • Dwarf (MB>-18.0 and/or s<100 km/s) • Elliptical • Diffuse (low surface brightness, shallower light profiles compared to E’s, n=1…2) • Form a separate sequence on the Kormendy diagram • Some of them contain ISM (De Rijcke et al. 2003; Michielsen et al., 2004), although most of dE’s are completely lacking of it • Many of them (brighter ones) exhibit embedded structures (Jerjen et al., 2000, Barazza et al. 2002, Graham et al. 2002, Lisker et al. 2006) dE’s dE’s E’s, bulges, cE’s Not easy objects for spectral studies due to low surface brightness, low s

  4. dE’s: what is their origin? • Internal agents: • Collapse + feedback of star formation(Dekel & Silk, 1986, Mori et al. 1997) • Environment: • Ram pressure stripping of S, dIm in clusters (Mori & Burkert, 2000) and groups (Marcolini et al. 2003) • Gravitational (e.g. tidal) harassment (Moore et al., 1996) How to chose the scenario?

  5. Answer: study them spectroscopically! Internal kinematics can be used to build dynamical models and study the mass distribution in dE’s. It also may keep track of violent phenomena (e.g. mergers) if they happened recently Stellar population keeps a fossil record of star formation in a galaxy, and this information can be extracted from spectra integrated along a line of sight Existing spectroscopic surveys are not very appropriate to study dE’s – e.g. for SDSS the nearby ones (Virgo) are usually too extended, more distant ones (Coma) are too faint. Indeed, some studies have been completed successfully (e.g. Lisker et al. 2006) • CRUCIAL POINTS: • To be successful in the spectral studies of dwarf elliptical galaxies you need: • Good calibrations and accurate data reduction • Good calibrations and accurate data reduction • Good calibrations and accurate data reduction • Apart from this it might be helpful to have an efficient spectrograph attached to a large telescope

  6. Spectral studies of dE’s in the past: • Bender & Nieto 1990: the first attempt to obtain internal kinematics of dE’s. They DO NOT rotate (~10 galaxies) • Simien & Prugniel 2002: Most of them DO rotate (several dozens of galaxies), although some of them do not (Geha et al. 2002); more data (Van Zee et al. 2004a) • Some of them exhibit kinematically-decoupled cores(De Rijcke et al. 2004, Geha et al. 2005, Thomas et al. 2006) • Dynamical modelling (De Rijcke et al. 2006) suggests that brighter dE’s contain only about 50% of dark matter • Usually metallicities are slightly subsolar ([Fe/H] ~ -0.4), mean ages are about 5 Gyr (Geha et al. 2003, Van Zee et al. 2004b)

  7. What we do: fitting integrated light spectra • Classical approach • Measure internal kinematics by deconvolving with a single spectrum of a star observed with the same set-up. Better, use a combination of templates (optimal-template fitting). • Evaluate the stellar population by the mean of line strength indices • Low-resolution (10 A), Lick • High-resolution with  correction • Caveat • Template mismatch: limitation to the precision of kinematics • Metallicity – velocity dispersion degeneracy • Do not make optimal use of the information contained in the spectrum • New approach • Simultaneous fitting of kinematics and stellar populations using high-resolution models (PEGASE.HR, Le Borgne et al. 2004) Applying this technique to the spectroscopic observations of dE galaxies

  8. How and what we observed • Detailed studies of a small sample of nearby objects at a distance of Virgo using IFU spectroscopy with the Russian 6-m telescope • Spectroscopy of larger sample of more distant dE’s (~130 Mpc) using VLT FLAMES/Giraffe

  9. MPFS Spectrograph BTA SAO RAS R=1300…1800 l=4200…5600 A

  10. IC 3653 (rather compact) 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

  11. Chilingarian et al., 2007, AstL IC 783 (spiraldE) • Rotation in the inner region • Young nucleus (3Gyr) • Low metallicity • Two consequent crossing of the cluster centre?

  12. Chilingarian et al., 2007, AstL IC 3468 (barred dE) • Kinematical axis is turned by 35-40 degrees off the photometric one. HST images reveal faint warped structure, corresponding to this orientation • Young extended embedded structure (disc) • Velocity dispersion map shows a dip, corresponding to this “disc” (“bar”according toBarazza et al. 2002)

  13. Chilingarian et al., 2007, AstL IC 3509 (classicaldE) • Was chosen as a “prototypical” dE: without bar, disc, spirals • Rotation along two perpendicular directions. Kinematical appearance looks very similarly to giant E NGC5982, where it was explained as a projection of orbits in a 3-axial potential (without need for embedded structure) • Rather young (4 Gyr)metal-rich ([Fe/H]=0) core

  14. Sample of galaxies in the Abell 496 clusterChilingarian et al., submitted to A&A • Nearby (z=9800 km/s) relativelypoor (richnessclass 1) cluster • Deep multicolour imagery of the whole clusterand deep multi-object spectroscopy of a sample of dwarf galaxies

  15. Data • Imagery(u’, g’, r’, i’) using MegaCam @ CFHT • Spectroscopy: 3 fields with Giraffe in MEDUSAmode (2GTO nights) • 48 cluster members First observations of a wide sample of dE’s in a cluster at such a high (R=6000) spectral resolution

  16. What signal-to-noise ratio do we need to constraint stellar populations? 40 per pixel? s=73±1 km/s

  17. What signal-to-noise ratio do we need to constraint stellar populations? Maybe 15 per pixel? s=38±2 km/s

  18. What signal-to-noise ratio do we need to constraint stellar populations? With the resolution of FLAMES we can do something with SNR as low as 5 per pixel s=30±3 km/s

  19. Data analysis • Among 110 objects 48 are cluster members; 46 of them we can fit well. 36 dEs/dS0s, 2 SBs, 10 low-luminosity E/S0/Sa • For 46 galaxies we obtained: • Accurate internal kinematics  scaling relations • [Mg/Fe] abundance ratios by measuring Lick indices • Estimations of age and metallicity for the luminosity-weighted population (single starburst) • Mapping relations between stellar populations and velocity dispersions in the low-s regime • Colour maps for 48 galaxies

  20. Faber-Jackson Relation

  21. Lick indices, ages, metallicities

  22. Principal results on Virgo dEs • Embedded discs – early type progenitors • Evolutionary decoupled cores – ram pressure Principal results on Abell496 dEs • [Mg/Fe]=0, therefore the starburst events in dE galaxies must have lasted at least 1-2 Gyr to explain observed iron enrichment • Only brighter galaxies exhibit embedded structures • Many galaxies exhibit red and blue cores Main conclusion: “external” channel of dE formation (ram pressure stripping + harassment) is a plausible scenario, while SN winds are much less probable

  23. dE’s: future studies • Obtaining extremely high S/N spectra of nearby dEs in order to recover star formation histories in details – feasible, but difficult to convince TACs • New multi-object (possibly falcon-style multi-IFU) spectrographs with higher efficiency than present ones will slightly improve the situation and allow to go down to MB=-14 – feasible • Observing yet star-forming dE progenitors at z=0.4…0.8 – feasible, although difficult, ELT task • Need to be 3 mag deeper than SDSS to perform massive studies of brighter dEs (-18<MB<-15) in the nearby Universe (z<0.1) – questionable

  24. dE’s: future studies • Need for IFU’s with large fields of view and large spaxels (like PMAS-PPAK but on 8m class telescopes) – no need for high spatial resolution, however spectral resolution should be sufficiently high – questionable • Observing spectroscopically extremely low-surface brightness dSph galaxies (“dark matter worlds”). Sky background coming from the interplanetary dust becomes important at this level, so need to go out of the Solar system plane – unfeasible

  25. Summary • Spectroscopic studies of dE galaxies are crucial for understanding their formation and evolution, which is still a missing block in our understanding of galaxy formation • What is already done for brighter dE galaxies is major achievements in this field, but there is still need to go toward lower masses/luminosities. We still do not know anything about (1) dark matter content and (2) star formation histories of fainter dE’s

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