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

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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 Silchenko (SAI MSU,
Russia) Christophe Adami (LAM, France) Laurent
Chemin (ObsPM GEPI, France) Philippe Prugniel
(CRAL, France) Victor Afanasiev (SAO RAS, Russia)
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dE galaxies history

• M31 satellites (Baade 1944)
• NGC205 prototypical dE
• M32 prototypical cE

dEs are the numerically dominant population in
the nearby Universe
Image credits MASTER
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dE galaxies structural properties
Es, bulges, cEs

• Dwarf (MBgt-18.0 and/or slt100 km/s)
• Elliptical
• Diffuse (low surface brightness, shallower light
  profiles compared to Es, n12)
• 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 dEs
  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)

dEs
dEs
Es, bulges, cEs
Not easy objects for spectral studies due to low
surface brightness, low s
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dEs 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?
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Answer study them spectroscopically!

• Internal kinematics can be used to build
  dynamical models and study the mass distribution
  in dEs. 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 dEs 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

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Spectral studies of dEs in the past

• Bender Nieto 1990 the first attempt to obtain
  internal kinematics of dEs. 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 dEs 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)

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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
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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
  dEs (130 Mpc) using VLT FLAMES/Giraffe

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MPFS Spectrograph
BTA SAO RAS
R13001800 l42005600 A
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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
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IC 783 (spiral dE)
Chilingarian et al., 2007, AstL

• Rotation in the inner region
• Young nucleus (3 Gyr)
• Low metallicity
• Two consequent crossing of the cluster centre?

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IC 3468 (barred dE)
Chilingarian et al., 2007, AstL

• 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 to
  Barazza et al. 2002)

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IC 3509 (classical dE)
Chilingarian et al., 2007, AstL

• 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/H0) core

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Sample of galaxies in the Abell 496
clusterChilingarian et al., submitted to AA

• Nearby (z9800 km/s) relatively poor (richness
  class 1) cluster
• Deep multicolour imagery of the whole cluster and
  deep multi-object spectroscopy of a sample of
  dwarf galaxies

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Data

• Imagery (u, g, r, i) using MegaCam _at_ CFHT
• Spectroscopy 3 fields with Giraffe in MEDUSA
  mode (2 GTO nights)
• 48 cluster members

First observations of a wide sample of dEs in a
cluster at such a high (R6000) spectral
resolution
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What signal-to-noise ratio do we need to
constraint stellar populations?

• 40 per pixel?

s731 km/s
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What signal-to-noise ratio do we need to
constraint stellar populations?

• Maybe 15 per pixel?

s382 km/s
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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

s303 km/s
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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

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Faber-Jackson Relation
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Lick indices, ages, metallicities
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• Principal results on Virgo dEs
• Embedded discs early type progenitors
• Evolutionary decoupled cores ram pressure
• Principal results on Abell496 dEs
• Mg/Fe0, 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

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dEs 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
  z0.40.8 feasible, although difficult, ELT
  task
• Need to be 3 mag deeper than SDSS to perform
  massive studies of brighter dEs (-18ltMBlt-15) in
  the nearby Universe (zlt0.1) questionable

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dEs future studies

• Need for IFUs 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

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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 dEs