Central Stellar Populations of S0 Galaxies in The Fornax Cluster

0.16 MB·English

by  A. G. Bedregal

Proceedings Title IAU Symposium
Proceedings IAU Symposium No. 241, 2007 (cid:13)c 2007International AstronomicalUnion
A.C. Editor, B.D. Editor & C.E. Editor, eds. DOI:00.0000/X000000000000000X
Central Stellar Populations of S0 Galaxies in
the Fornax Cluster
A.G. Bedregal1, A. Arag´on-Salamanca1, M.R. Merrifield1 and N.
Cardiel2
1School of Physics and Astronomy,Univ.of Nottingham, Nottingham, NG7 2RD,U.K.
2Departamento deAstrof´ısica, Facultad de F´ısicas, Univ.Complutense deMadrid, Spain
Abstract. Based on FORS2-VLTlong-slit spectroscopy, theanalysis of the central absorption
line indices of 9 S0 galaxies in the Fornax Cluster is presented. Central indices correlate with
7 central velocity dispersions (σ0) as observed in ellipticals (E). However, the stellar population
0 propertiesoftheseS0sindicatesthattheobservedtrendsareproducedbyrelativedifferencesin
0 ageandα-elementabundancesandnotinmetallicity([Fe/H])aspreviousstudieshavefoundinE
2 galaxies.Theobservedscatterinthelineindicesversusσ0relationscanbepartiallyexplainedby
n therotationally-supportednatureofmanyofthesesystems.Thepresenceoftighterlineindices
a vs.maximum(circular)rotationalvelocity(VMAX)relationsconfirmsthisstatement.Itwasalso
J confirmedthatthedynamicalmassisthedrivingphysicalpropertyofallthesecorrelationsand

0 in our Fornax S0sit has to be estimated assuming rotational support.
3
Keywords. galaxies: elliptical and lenticular, stellar content,kinematics and dynamics

2
v
0
1. The Index∗ vs. log(σ ) Relations
3 0
8 In Figure 1 (left), different central line indices are plotted against σ0 for our sample
1
(Bedregal et al. 2006). Bright and faint S0s lay in two separate clumps, each one in
0
7 opposite extremes of the σ0 range. When models from Bruzual & Charlot (2003) are
0 used to estimate stellar population parameters, the observed trends seem to be driven
/ by age and α-element relative abundances. This is opposite to what is usually found in
h
E, where [Fe/H] is an important driverof the correlations.By parametrisingline indices
p
- usingBruzual & Charlot (2003)modelswefoundthatagecanexplaintheobservedslopes
o
of Hβ-log(σ0) and Fe-log(σ0) relations,while additional differences in α-element relative
r
t abundances would be required to explain the Mg-log(σ0) trends. On the other hand,
as [Fe/H] by itself cannot explain any of the observed Index∗-log(σ0) relations.
:
v
i
X 2. The Index∗ vs. log(V ) Relations:
MAX
ar In Figure 1 (right) we present the Index∗ vs. log(VMAX) relations for our sample of
S0s. Clear trends appear in all the panels. It is interesting to notice that the standard
deviation of the linear fits performed is smaller for almost all the indices in comparison
to its counterpart from the Index∗-log(σ0) fits. Assuming that mass is the fundamental
physical parameter governing the properties of these galaxies, the improvement in the
fits may be interpreted in the sense that VMAX is a better estimator of the dynamical
mass than σ0. To test this hypothesis, the mass of these galaxies was parametrised as
a function of σ and VMAX in order to compare how central indices correlate with both
possible descriptions. Rd and Re are the disk scale and effective radius of the bulge
respectivelly and the mean σ was estimated within Re (see Figure 2).
Clearly,both σ and VMAX aretracing the massof these systems.However,when mass
1
2 Bedregal et al.
Figure1.Lineindicesvs.σ0(left)andVMAX(right)for9S0sintheFornaxCluster.Continuous
lines:bestfitstothedata.Dashedlines:bestfitsfoundbyKuntschner(2000) forEsinFornax.
Triangle: Fornax A, a merger remnant. Two dots with arrows: pressure supported systems, for
which thedeprojected azimuthal velocity was used as a lower limit of VMAX.
Figure2. Lineindicesvs.dynamicalmassestimatedfromσ(left)andfromVMAX(right).Open
symbols:twopressuresupportedsystems(arrowsasFig1).Openstars:dynamicalmassofthese
two galaxies estimated from σ by assuming an isothermal sphere in hydrostaticequilibrium.
isestimatedusingσ (Figure2,leftpanel),itisunderestimatedinrotationallysupported
systems with respect to pressure supported ones. On the other hand, the right panel of
Figure 2 shows that galaxiesfollow a commonsequence when VMAX is used. The tighter
trends in this figure agree with the idea that VMAX is a better tracer of dynamical mass
than σ for this sample of galaxies.
References
BedregalA.G,Arag´on-SalamancaA,MerrifieldM.R,Milvang-JensenB,2006,MNRAS,371,1912
Bruzual G., Charlot S., 2003, MNRAS,244,1000
KuntschnerH., 2000, MNRAS,315,184