Crucial information about the star formation history is encoded in
the chemical composition of stars. The key observables are mean
(luminosity-weighted) metallicities, abundance ratios, and the run
of these quantities with 
.
It has long been known that the
central regions of nearby massive galaxies are metal-rich (e.g.,
Spinrad & Taylor
1971).
It was only far more recently that reliable
metallicity estimates became possible for massive ETGs at cosmological
distances, showing that they have comparably high metallicities, (e.g.,
Spinrad et al. 1997,
Jørgensen et
al. 2005,
Schiavon et al. 2006),
implying a relatively rapid early chemical enrichment. A
more detailed knowledge of the history of star formation requires
accurate estimates of detailed abundance patterns in a range of
redshifts, but work on abundance ratios has so far been restricted
to relatively nearby samples (but see
Jørgensen et
al. 2005
and
Kelson et al. 2006).
The Mg I 517-518 nm doublet has been the chief metallicity indicator
in early studies of the chemical composition of ETGs from integrated
light, due to its strength and location in a spectral region where
astronomical detectors were very sensitive. Early studies found
evidence for an overabundance of Mg relative to Fe in ETGs (e.g.,
Peterson 1976).
When Mg and Fe lines were finally compared with
models of stellar population synthesis, it was found that
[Mg/Fe] 2
is above solar in the centers of massive ETGs
(Worthey et al. 1992).
This was arguably one of the most influential results
in the history of the field, and it has guided theoretical work to
this day. Further studies showed that Mg enhancement is correlated
with central velocity dispersion
() and metallicity (e.g.,
Jørgensen 1999,
Trager et al. 2000,
Thomas et al. 2005,
Schiavon 2007,
Smith et al. 2009).
At least three scenarios have been invoked
to explain this finding, all involving the balance between Mg
enrichment by SN II and Fe enrichment by SN Ia: short star formation
timescale, top-heavy IMF, and selective winds
(Faber et al. 1992).
One popular interpretation of the data invokes the existence of a
relation between star formation timescale and galaxy mass (e.g.,
Thomas et al. 2005).
The data, however, did not allow one to rule
out scenarios based on selective winds or IMF variations.
Some exciting new results have been presented in a series of papers,
by G. Graves and collaborators. Analyzing a large sample of stacked
SDSS 3
spectra, they mapped the spectroscopic ages, [Fe/H], [Mg/H], and [Mg/Fe]
of ETGs onto the fundamental plane (FP) with particular attention to how
these parameters are distributed along and across the FP
(Graves et al. 2009,
Graves & Faber 2010,
Graves et al. 2010).
Besides recovering the well known trends of age and abundances with
, Graves
et al. showed that, at fixed
, star formation
histories of ETGs correlate strongly with galaxy surface
brightness. Galaxies with higher surface brightness have lower
spectroscopic ages, higher [Fe/H] and [Mg/H], but lower [Mg/Fe].
Graves & Faber
(2010)
contend that the thickness of the FP is accounted for by departures of a
constant dynamical-mass-to-light ratio (Mdyn /
L), which may be
due to variations in either dark-matter fraction or IMF, and not
to effects due to passive evolution of the stellar populations. At
fixed Mdyn, galaxies with higher surface brightness
(located "above" the FP) have a higher surface stellar density and therefore
are characterized by either a lower dark matter fraction or by a
bottom-heavier IMF. They formed stars during a longer timescale,
so that their metallicities are higher, but both their spectroscopic
ages and [Mg/Fe]s are lower than their low-surface-brightness
counterparts. Graves et al. propose a scenario where star formation
in galaxies with same Mdyn was truncated at different
times, with longer/shorter star formation timescales resulting in
higher/lower stellar surface mass density and surface brightness,
higher/lower metallicity, lower/higher [Mg/Fe], and younger/older
spectroscopic ages. In short, these new results single-handedly explain the
thickness of the FP and establish possible (and testable) correlations
between the star formation histories of ETGs and such measurable
quantities as dark-matter fraction and the shape of the low-mass
end of the stellar IMF. Further progress will be determined by
observational tests of these predictions, as well as more sophisticated
chemodynamical modeling of ETG formation.
Past studies suggest that Ca does not behave like Mg, with [Ca/Fe]
being possibly solar (or lower) and not correlated with
.
Trager et al. (1998)
found the Lick/IDS Ca4227 index to be essentially
independent of .
Accordingly,
Thomas et al. (2003)
concluded that [Ca/Fe] in their sample galaxies was also essentially
constant with .
Saglia et al. (2002),
on the other hand, found the Ca II triplet (CaT, 849, 855, 862 nm) to be
mildly decreasing with .
Vazdekis et al. (2003)
and
Cenarro et al. (2004)
compared new single stellar population synthesis models for the CaT with
data for field
and Coma galaxies, again finding very low [Ca/Fe]. While
difficult to understand, given that both Ca and Mg are
elements
manufactured in similar (though not identical) nucleosynthetic
sites, the implications of these results are potentially important,
giving theorists ample room for a wide range of speculations.
The unexpected behavior of Ca seems to be instead most likely caused
by difficulties in the interpretation of the measurements, particularly
because the two Ca indices employed in these studies do not respond
to Ca abundance variations in a clean fashion.
Prochaska et al. (2005)
showed that the Ca4227 index is severely affected by a CN
bandhead which contaminates the blue pseudocontinuum of the index,
making it lower. Because CN is strongly correlated with
(Trager et al. 1998),
the effect is stronger for higher
galaxies,
offsetting any dependence of the Ca line strength itself on
, thus making the index
-independent. Prochaska
et al. demonstrated
this by defining a new index, Ca4227r, which is
less affected by CN contamination. They showed that Ca4227r is
as strongly correlated with
as Mg b.
While the issue of the slope of the
Ca4227- relation is
seemingly resolved, models that account for the effect of CN on the
Ca4227 index still indicate [Ca/Fe] ~ 0 in massive ETGs
(Schiavon 2007,
Graves et al. 2007).
At face value, this confirms the abundance
ratios found by previous studies. However, there may be non-negligible
systematics in the Ca abundances derived by application of the
Schiavon (2007)
models. They are affected by uncertainties in age,
and in the abundances of Fe, C, and N. They are also affected by
uncertainties in the way models account for the contamination of
Ca4227 by CN. So the matter should be considered far from settled.
Regarding the results based on CaT, one should bear in mind that
the integrated spectra of metal-rich stellar populations in the CaT
region is dominated by M giants
(Schiavon & Barbuy
1999),
and that
fact has implications for both the zero point and the slope of the
[Ca/Fe]- relation. First
let us consider the zero point.
The stellar libraries employed in the models used to analyze CaT
data in the past contain hardly any M giants with known metallicity,
let alone known [Ca/Fe]
(Cenarro et al. 2001a).
Therefore, [Ca/Fe]
in the models themselves is uncertain, which obviously makes it
very hard for one to infer reliable [Ca/Fe] from comparison of those
models with the data. As regards the slope of the
CaT-
relation, we recall that the CaT lines are located in a region where
opacity in the spectra of M giants is dominated by TiO lines. While
the definition of the CaT* index employed in these
studies is partly meant to account for TiO contamination
(Cenarro et al. 2001b),
the index has not been shown to be immune to variations in
[Ti/Fe] 4 which may be
important, given that there is evidence that Ti is enhanced in ETGs
(Milone et al. 2000).
Regarding the negative slope of the CaT* ~
relation,
that could be due to the effect of TiO opacity on the pseudocontinuum,
because: 1) TiO is well correlated with
(Trager et al. 1998),
and 2) TiO lines are more sensitive to metallicity than CaT lines
(Schiavon et al. 2000,
Schiavon & Barbuy
1999,
Jørgensen et
al. 1992).
Finally, CN contamination of the CaT indices may also be important
(Erdelyi-Mendes &
Barbuy 1991).
In summary, we suggest that Ca abundances are far from well known in ETGs, and there is no compelling motivation to resort to extreme scenarios to account for the numbers currently available in the literature. More work is needed to produce reliable [Ca/Fe] measurements in ETGs.
While the behavior of C- and N-sensitive indices such as Lick
CN1,
CN2, G4300 and C24668 in ETG spectra has been well
documented for over a decade, it was only after the
Schiavon (2007)
models and their implementation in EZ_Ages
(Graves & Schiavon
2008)
that these indices could be intepreted in terms of [N/Fe] and [C/Fe]
(see also
Kelson et al. 2006).
Both abundance ratios are found to
be super-solar and correlate strongly with
and metallicity
(Schiavon 2007,
Graves et al. 2007,
Smith et al. 2009).
This result has been called into question recently by
Toloba et al. (2009),
who found no correlation between the strength of the near-UV NH3360
feature and in a sample
of nearby galaxies. They argue
that the NH3360 band is a clearer indicator of N abundance than the
Lick CN features used by EZ_Ages, because the latter are also
dependent on C abundance. The absence of a slope in the
NH3360-
relation may be explained by the presence of metal-poor stars, whose
contribution to the integrated light is highest in the UV. In fact,
multiple stellar population models show that the inclusion of a
small fraction of a metal-poor population flattens the
NH3360-
relation, even in the presence of a
[N/Fe]- correlation
(G. Worthey, 2010, private communication).
The existence of a steep slope in the
[N/Fe]- relation,
if confirmed, is an important result, as it may indicate secondary
enrichment of N by stars ranging from 4-8
M
(Chiappini et al. 2003).
Because these stars last for ~ 108 years, the
presence of a secondary-enrichment signature in the chemical
composition of stars in ETGs may constrain the lower limit for the
duration of star formation in the systems that formed the stars
that live today in those galaxies,
(Schiavon 2007)
and, perhaps most importantly, their characteristic masses. The
increasing evidence for the presence of multiple stellar populations in
globular clusters
(Piotto 2009)
may be an important clue in this regard. It
has been long known that there is a marked spread in N and C
abundances in globular cluster stars (e.g.,
Smith & Norris 1982,
Cannon et al. 1998,
Carretta et al. 2005),
which is roughly consistent with enrichment by intermediate mass stars
going through the AGB phase
(Ventura & D'Antona
2008).
The presence of such CN inhomogeneities seems to be a function of both
cluster mass and environment
(Martell & Smith
2009),
as predicted by recent models
(Conroy & Spergel
2010).
One may reasonably speculate that the evidence above indicates that
the stars we see in nearby ETGs were formed in the precursors of
today's Galactic globular clusters. In that scenario, the signature
of secondary N enrichment we see in ETGs today would have been
established in those early systems, before they merged to form the
massive, dynamically hot galaxies we see today. The Galactic halo
can be used as a resolved proxy to test this scenario. The recent
identification of CN bimodality in a sample of halo field stars by
Martell & Grebel
(2010)
argues for a similar process in operation
during the formation of the Galactic halo. Because CN-strong stars
have almost certainly been formed in globular clusters (or their
precursors in the distant past), their presence in the halo field
is evidence of the early dissolution of those systems in the formation
of the Galactic halo. Martell & Grebel estimate that as much as
50% of the halo mass may have been contributed by globular clusters
and their precursors. If a similar process was responsible for the
assembly of stellar mass in ETGs, one might wonder whether a similar
fraction of the total mass would have been contributed by the
globular cluster precursors. Could the slope of the [N/Fe] vs.
[Fe/H] or be used to
constrain that number? What were the
characteristic masses of those systems? Would it be possible to
construct chemodynamical evolution models for those low-mass systems
that are capable of reproducing all the abundance ratios measured
in today's ETGs?
Inclusion of C and N in abundance analyses of ETGs is bringing
interesting new insights on their star formation histories, which
could potentially even lead to a reinterpretation of the data on [Mg/Fe].
Smith et al. (2009)
analyzed a large data set for galaxies
from the Coma cluster and Shapley supercluster, spanning a very
wide range of . They
determined the abundances of several
elements using EZ_Ages, then performed biparametric fits to the
relation between [Mg,Ca,C,N/Fe] and both [Fe/H] and
, thus
disentangling the dependence of abundance ratios on these two
variables. Smith et al. found that both [Mg/Fe] and [Ca/Fe] decrease
with [Fe/H], whereas [N/Fe] and [C/Fe] do not correlate with it.
They suggest that the run of [Mg/Fe] and [Ca/Fe] with [Fe/H] indicates
a short time scale for star formation. They consider that the lack
of correlations of [N/Fe] and [C/Fe] with [Fe/H] is expected since,
unlike Mg and Ca, C and N are contributed by low(er) mass stars,
so that these elements should scale with Fe, not with Mg and Ca.
Interestingly, on the other hand, all abundance ratios show a strong
correlation with . Smith
et al. contend that this result
is difficult to interpret in terms of a simple dependence of star
formation timescale on galaxy mass (e.g.,
Thomas et al. 2005),
because that would preclude a correlation between [C/Fe] and [N/Fe] with
. Clearly, more work is
needed to clarify this issue.
2 More often than not, this result is
phrased in the literature in terms of an overabundance of
elements
relative to iron. Despite the many theoretical reasons in favor
of the assumption that all
elements should vary in
tandem, there is so far no firm evidence that any
element
other than Mg is enhanced in ETGs, except perhaps for Ti
(Milone et al. 2000).
Back.
3 Sloan Digital Sky Survey Back.
4 TiO lines are very sensitive to variations in the abundance of Ti. Back.