Disentangling Stellar Populations
with 3D Spectroscopy
N. Cardiel1,2, A.J. Cenarro1,3, S. Pedraz1,2, J. Gorgas1, P. SánchezSánchez-Blázquez1
1
Departamento de Astrofísica,
Astrofísica, Universidad Complutense de Madrid, Spain
2
3
Calar Alto Observatory, Almería,
Almería, Spain
UCO/Lick Observatory, Santa Cruz, California, USA
INTRODUCTION
The interpretation of the stellar content of early-type galaxies is one of the main
approaches to unravel their history of formation and evolution. In fact, the analysis
of spectral features in their integrated spectra, and their comparison with stellar
population model predictions, is the key to compare the “real world” with the
theoretical framework. The use of new spectroscopic indicators, like the Calcium
Triplet (CaT), the strongest absorption features in the spectra of relatively old
stellar populations at around 8600 Angstroms, is providing new insights into this
subject.
Here, we present new 3D spectroscopic observations of field early-type galaxies
performed with PMAS, an integral field unit that, in combination with the technique
of Nod and Shuffle, allows an uncommonly accurate removal of the dense sky
line forest in the near-infrared. In this sense we have been able, for the first time,
to measure reliable CaT gradients in this kind of galaxies.
OBSERVING WITH
AN IFU +
NOD & SHUFFLE
Making use of charge
shuffling capabilities
of new CCDs…
DSS Image
Interlaced observations of
target and a separate sky
region with the same
fibers and same pixels!
sky position
16’’x16’’ FOV
Several integrations on
target and sky within each
single exposure.
Acquisition
Image
Acquisition
image
Acquisition
image
target
(x16)
3D SPECTROSCOPY & PMAS
sky
(x16)
Important efforts
devoted to the
development
of
3D spectroscopy
in Europe.
Imaging &
spectroscopic
information
simultaneously
λ
Each readout provides
two sets of 16 x 16
spectra: target and sky.
The intensity variations
of the individual lines of
the OH airglow during
each exposure are thus
properly sampled.
δ
PMAS, the Postdam Multi-Aperture
Spectrophotometer, is an IFU installed at the
Cassegrain focus of the 3.5 m telescope at the
Calar Alto Observatory.
The micro-lens array contains
16x16 1mm square aspherical
silica lenses. Lenses produce
micropupils of the telescope
PMAS at the 3.5m (CAHA)
aperture to be fed into a 16x16
fiber bundle.
The 2D field is re-arranged
into a slit sampling 256
spatial elements. Three
different foreoptics provide
FOV 8x8, 12x12 and 16x16
arcseconds.
Four elliptical galaxies were
observed with PMAS using Nod
& Shuffle. Radial line-strength
gradients of the CaT* were
measured performing different
spatial binnings.
Software tool for the analysis
of PMAS data.
CaT* map in the central
pointing of NGC221.
NGC 221
NGC 507
NGC 5982
NGC 6703
256
spectra
Advantages of 3D spectroscopy
Avoids time dependent variations (seeing, air mass, refraction, weather).
Pointing less crucial.
No slit losses.
Slit width / aperture size `adjustable´ in data reduction.
Spectral resolution independent from spatial sampling & seeing.
RESULTS
galaxy+sky
http://www.aip.de/Euro3D
α
sky
The sky subtraction in the nearIR is much better than with a
typical long-slit.
galaxy
DISCUSSION: THE CALCIUM PUZZLE
Contrary to the rest of metallic lines,
central CaT* indices of field ellipticals
anti-correlate with mass. In addition, the
absolute CaT* values are typically
lower than model predictions. In fact
differences in age and metallicity
exclusively do not explain the locus of
elliptical galaxies: calcium underabundances or changes in the
dwarf/giants ratio (IMF variations)?
See also Saglia et al. (2002),
Falcón et al. (2003); Michielsen et al. (2003)
Model prediction
Vazdekis et al. (2003)
Cenarro et al. (2003)
Cenarro et al. (2003)
DSS charts of the 4 galaxies of our sample. The red square indicates the central
pointing and the blue square (when present) a second pointing off center.
CaT* gradients for our galaxy
sample. The red and green
points
correspond
to
unbinned and binned data in
the central pointing, whereas
the blue point is the averaged
index for the 256 spectra of
the off-center pointing.
The displayed data are
consistent with flat CaT*
gradients in the sampled
galaxy regions.
Central CaT and Ca4227 indices exhibit
significant differences when comparing earlytype galaxies in the Coma Cluster and in the
Field. The answer to the CaT problem is not
overabundances exclusively. On the other
hand, IMF variations predict too high Ca4227
values. Could it be that Field galaxies are (in
the central parts) more metal rich than
galaxies in Coma?
The flatness of the CaT* gradients, in
comparison with the strong gradients found in
another α-element feature like Mg2, suggests
a complex star formation history in field
galaxies. This should be a key piece to solve
the Calcium puzzle.
On the other hand, the measurement of CaT*
gradients in the Coma cluster galaxies are
highly needed to test whether the dichotomy
found in the central regions also applies to the
galaxies as a whole.
field Es
Coma Es
Cenarro et al. (2004)