In Figure 5.5 we show various diagnostic diagrams, using optical and near-IR indices. On panel (a) we repeat Figure 5.2 for guidance. We have omitted NGC 3607 as dis- cussed in the previous section. The colour and symbol coding in this figure is the same as in the previous plots, i.e. red triangles represent the field early-type galaxies, black circles Fornax cluster members. The dashed lines in panels (c), (d) and (f) show the least-squares linear fits to the old Fornax galaxies only.
Some of these diagrams can be used to separate quiescent (old) galaxies from those with signatures of recent star formation (filled symbols). In panels (c) and (d) we plot the dependence of the near-IR indicesDCOand Na I on the optical metallicity indicator
[MgFe]’. In theDCO vs. [MgFe]’ diagram the old Fornax galaxies follow a clear corre-
lation, the younger galaxies have generally higherDCO values as compared with their
older counterparts with similar [MgFe]’. The field galaxies show a larger spread and follow to some extent the relation for the old Fornax galaxies. The Na I vs. [MgFe]’ diagram is a better tool to distinguish young from old galaxies. Silva et al. (2008) have shown this plot for the Fornax galaxies only, here we add our field sample. Young and intermediate age field early-type galaxies are well separated from the relation valid for the old galaxies.
In the next two panels we show the dependence of the optical age indicator Hβ
from near-IR indices (panels (e) and (f)). Both of them show the separation between young and old galaxies clearly. Old field galaxies follow the relation for the Fornax old galaxies. However, the behaviour of theDCOindex is more complicated at younger
ages. Field galaxies with strong Hβhave both weak and strong DCO. This may be due
to the fact that DCO index depends strongly on the metallicity when the age is around
Figure 5.5: Various index-index diagnostic diagrams. Panel (a) is repeating Figure 5.2 for guidance. Colour and symbol coding for the galaxies is the same as in previous figures. The dashed lines in panels (c), (d) and (f) show the linear least-squares fits to the old Fornax galaxies.
Figure 5.6: Optical and near-IR indices for old Fornax (open circles) and field (red triangles) early-type galaxies vs. their metallicity, derived from Figure 5.2. The solid lines in the top plots represent the weighted linear fit to all data points.
vs. [MgFe]’ diagram).
The resemblance of panel (f) with panel (a) is remarkable. The Na I index tracks the metallicity of the old Fornax galaxies, as suggested earlier, while the Hβ strength decreases mildly. Field galaxies follow this relation as well. The four intermediate age galaxies are clearly separated from the other two age groups. Young Fornax and field galaxies are also well distinct from the older ones. Therefor similar plots, based on these two indices, promise to be good age-metallicity diagnostic diagrams, when better stellar population models in the near-IR become available. The differentiation in age and metallicity looks not as complex as it is, when using Hβvs. [MgFe]’.
This argument is further supported in Figure 5.6, where we show the dependence of optical and near-IR line strengths from the metallicity of the galaxies, which we have derived from Figure 5.2 (see Table 5.3). The top panel shows the optical metallicity indi- cator [MgFe]’ for old Fornax (open circles) and field (red triangles) early-type galaxies. The solid line represents the weighted linear fit to the data and has an equation:
[MgFe]’=3.279±0.218+1.931±0.853 [Z/H] (5.1) For the middle panel we have used the near-IR Na I index. The correlation between this index and the metallicity is very good. The weighted linear fit has an equation:
Na I=2.644±0.315+5.841±1.260 [Z/H] (5.2) The bottom panel shows that theDCOindex tends to saturate for [Z/H]>0.2.
5.5
Comparisons with stellar population models
In Figure 5.7 we show a comparison of our sample of field early-type galaxies, together with Fornax cluster members, with the current SSP models of Maraston (2005) for the DCO index. As discussed in Section 4.4, these models do not give a good description
of the DCO index at sub-solar metallicities and intermediate age, but for older ages,
solar and higher metallicities they may be describing the data well. The DCO index
values, measured from the theoretical models, also have to be interpreted with caution (see the discussion in Section 4.4.2). This index definition is not suitable to measure theDCO index on that low spectral resolution SEDs (∼200ÅinFW H M). However, the
expected error is of the order of∼3 % of the index value and is not going to influence our findings. We will use the DCO index values of the theoretical SEDs as an estimate
of the general trends of dependence from age and metallicity. When higher spectral resolutions models become available the following comparisons should be re-maid to obtain more quantitative estimates.
In the top panel we have marked with a black rectangle the locus of theDCO index
values for old Fornax early-type galaxies. The age range has been chosen according to the optical age-metallicity diagram, presented in Figure 5.2 and extends between 8 and 14 Gyr. The data agree reasonably well with the models for super-solar metallicity. With an open circle and a square we have indicated NGC 1380, the only Fornax galaxy in the intermediate age range. With solid black circles we have indicated the three
Figure 5.7: Comparison of the predictions of the stellar populations models of Maraston (2005) with Fornax cluster (top panel) and field galaxies (bottom panel). Dark blue lines indicate different metallicities. The region, surrounded by a black rect- angle is the locus of DCO index values for old Fornax cluster galaxies. The region,
surrounded by red rectangle (bottom panel) shows the extent of the DCO index in old
field galaxies (8 – 14 Gyr). The dashed red rectangle shows theDCOindex in intermedi-
ate age (3 – 7 Gyr) galaxies. Filled symbols indicate the two young galaxies. The light blue shaded regions show our own calibration of the DCO index from Chapter 4 for
metallicities indicated in the boxes. The horizontal orange line indicates theDCOindex
galaxies in the Fornax cluster, which have signatures of a younger population accord- ing to Figure 5.2. For the two galaxies at ∼2 Gyr, NGC 1316 and NGC 1344, the DCO index agrees with the models for metallicity of [Z/H]∼0.35 (the derived metallicities for these galaxies from Figure 5.2 is about 0.6). However, the youngest Fornax clus- ter galaxy in the sample, NGC 1375, has a lowerDCO index as compared to the model
predictions for the optical indices expected metallicity of solar or twice solar.
In the bottom panel we show the same models, this time together with data of our field early-type galaxies sample. The red rectangle shows the location of the old field galaxies and extends between 8 and 14 Gyr. The red dashed rectangle indicated the location of the DCO index values in the intermediate age galaxies, with ages between
3 and 7 Gyr. The red filled triangles mark the two galaxies with a contribution from a young population, visible in Figure 5.2. Our sample of old field galaxies shows a simi- lar behaviour as the Fornax cluster galaxies with a larger spread in theDCO index. The
galaxy at∼2 Gyr, NGC 2549, exhibits an increase in DCO, consistent with the models, while the same index, measured in the youngest galaxy, NGC 3489, has a lower value. In Figure 5.7 we have also included our observations of globular clusters (GCs) in the LMC, presented in Section 4.4, and indicated their DCO index values with blue
shaded regions. The metallicities are noted inside these regions. There we have seen the same trends as with the early-type galaxies in this chapter: the youngest objects in our samples with ages∼1 Gyr have a too lowDCO index value, compared to the mod- els. In Section 4.4 we have discussed, as a possible reason for this disagreement, the carbon stars, which are the major contributor to theK-band light in intermediate age clusters. Their DCO index is very much dependant on the metallicity (see Figure 4.19
and discussion in Section 4.4.2). In that section we reached the conclusion that mod- els, calibrated with carbon stars with improper metallicity may be the reason for the disagreement with the data, however other factors, than metallicity, may also play a role. With the GCs we tested the case of half-solar metallicity. Here we see a similar behaviour in early-type galaxies which presumably have higher metallicities. There are two possible explanations. First, the most important contributors to the luminosity weighted DCO index in these very young galaxies (∼1 Gyr, according to SSP models)
are stars, which have formed from metal poor gas. Second, the current carbon star sam- ple from the Milky Way (Lanc¸on & Mouhcine 2002) may be not enough to establish the spectral properties of carbon stars with different metallicities, using assumptions about the stellar evolution. The LMC clusters with age∼1 Gyr haveDCO∼1.12±0.01, while in NGC 1375 it is 1.19±0.004. This difference may be explained with the higher metal- licity of the young stars, which favours less carbon star production and brings theDCO
index up. In order to confirm this speculation there is a need for a larger sample of near-IR spectra of AGB stars (carbon and oxygen rich) to be able to derive a more con- sistent picture of this very important stellar evolutionary phase and its contribution to the integrated properties of globular clusters and galaxies.
In Figure 5.8 we show another near-IR index as a function of the age of the galaxies: the Na I index. Like in Figure 5.7, we indicated our calibration of this index regarding age and metallicity, derived from our own work on LMC GCs, presented in Chapter 4 with blue shaded regions (metallicity values are given in the regions). From Figures 5.3 and 5.5 and their discussion in the text we know that in old Fornax cluster galaxies
Figure 5.8: Na I index in Fornax and field early-type galaxies, plotted vs. their age. Colours, symbols and ages as in Figure 5.7. With blue shaded regions we have shown the values of Na I index for different metallicities (as noted in each region), derived from our own work on LMC globular clusters (Chapter 4) for sub-solar metallicity. The Na I index as a function of the metallicity of old Fornax early-type galaxies is shown with an arrow, extending between [Z/H]=+0.0 and +0.5. The horizontal orange solid and dashed lines indicate the Na I index and its error for Maffei I.
the Na I index is proportional to the metallicity. This we have indicated with an arrow, showing the metallicity range between [Z/H]=+0.0 and +0.5.
Na I in Fornax cluster galaxies (top panel) is consistent with the LMC calibration for the galaxies at∼2 Gyr, after we take into account their higher metallicity, as compared to the LMC. However, our sample of globular clusters and galaxies is limited to distinct groups of sub-solar and above solar metallicities and we do not have a clear picture of the development of the near-IR indices at metallicities in between. In field galaxies (bottom panel) we have used the same symbols to indicate the young, intermediate age and old galaxies, like in Figure 5.7 Na I tends to increase with decreasing age, which is consistent with our observations of LMC globular clusters, discussed in Section 4.5, but scaled to the higher metallicity of the galaxies.
In the Figures 5.7 and 5.8 we have indicated theDCOand Na I index value of Maffei I
with orange solid lines. The 1σerror bar is indicated only for the Na I index (dashed orange line), because for theDCO index it is rather small (0.003) and overlaps with the
index value. Given that there are no literature measurements of the optical indices we cannot infer an estimate of the age and the metallicity of this galaxy only from our near-IR spectra and will have to wait until more detailed near-IR stellar population models become available.
In Figure 5.9 we show a diagnostic diagram based on a combination between opti- cal and near-IR indices. SSP models for the optical Hβindex are taken from Thomas et al. (2003). TheDCO index values have been computed from the model SEDs of Maraston
(2005) for Kroupa IMF. Clearly a better choice for this diagram would be the near-IR Na I index vs. Hβ, as already discussed in Section 5.4. However, the measurement of the Na I index from the current near-IR theoretical SEDs is practically impossible, due to their very low spectral resolution. As in previous figures we overplotted the in- dex values for the field and Fornax cluster early-type galaxies. Old galaxies follow the trends of decreasing Hβ and increasing DCO, expressed by the solid lines of constant
age and increasing metallicity. The four intermediate age galaxies follow a similar trend, shifted towards younger age. The interpretation of the indices for the galaxies with optical signatures of a younger population is more difficult, due to the complex behaviour of the near-IR DCO index. In this figure the data points describing the old
and intermediate age galaxies are located outside of the model predictions. However, this might be due to the expected weakness of the DCO model index values, as dis-
cussed at the beginning of the section, due to the very low spectral resolution of the theoretical SEDs. The arrow at the bottom left corner of the plot gives an estimate of the correction factor, that has to be added to the models to simulate theDCO index at
the spectral resolution of the data. When this correction factor is taken into account, the agreement between the data and the models is very good.
5.6
Conclusions
In this chapter we have discussed the near-IR central index properties of a sample of early-type galaxies in low density environments, throughout the text referred as to field galaxies. We compared them with literature studies of Fornax cluster galaxies and we
Figure 5.9: Combination of optical and near-IR SSP models. Hβindex predictions are from Thomas et al. (2003). The DCO index predictions are computed from the SEDs of
Maraston (2005). Solid lines represent constant age, dashed lines constant metallicity. Field and Fornax cluster galaxies are overplotted as on previous plots. The arrow at the bottom left corner indicates the correction factor to be added to the models to simulate theDCOindex at the spectral resolution of the data.
explored the similarities and the differences with a variety of methods. We divided the two galaxy samples into three sub-groups, based on their luminosity weighted, SSP- equivalent age, as derived from optical spectral indices. The group of young galaxies consists of five objects with ages≤3 Gyr. We have four intermediate age galaxies with ages between 3 and 7 Gyr. The rest of the samples (eleven galaxies in total) we call old galaxies.
Index-σscaling relations in the optical and the near-IR.Near-IR index-σrelations correlate with the optical ones. Both, the Na I andDCO index, correlate with the central
velocity dispersion for old field and Fornax cluster galaxies, although field galaxies exhibit a larger scatter. We consider this scatter is due to the lower quality of the field galaxies data. Here we compare spectra obtained with a 2 m class telescope and an early near-IR detector (the field sample is observed with the 2.1 m KPNO telescope) with an 8 m class telescope and more advanced instrument (the Fornax sample has been acquired with VLT/ISAAC). Silva et al. (2008) pointed out that the Na I index is strongly correlated with the σ in old Fornax cluster galaxies. Here we confirm this result by adding data for old field early type galaxies. Galaxies with clear signatures of recent star formation do not follow the mainDCO and Na I index-σrelations for old
galaxies.
NaI and DCO indices and the metallicity. The near-IR Na I index shows a good
correlation with the optical metallicity indicator [MgFe]’ for old Fornax galaxies. Thus it shows that it is significantly influenced by the metallicity of the population at old ages. We derive an empirical calibration for the Na I index as function of metallicity. The DCO index also increases with increasing metallicity, although with a shallower
slope, and agrees roughly with the SSP models of Maraston (2005).
NaI and DCO indices and the age. Field and Fornax cluster galaxies with optical
signatures of a recent star formation are clearly distinguished in several diagnostic diagrams. Na I tends to increase with decreasing age in early-type galaxies, as well as in LMC globular clusters. The DCO index exhibits a more complex behaviour. It
increases with decreasing age in galaxies with age up to ∼2 Gyr and then weakens for galaxies at∼1 Gyr luminosity weighted and SSP equivalent age. Such trends are consistent with the presence of TP-AGB stars in the galaxies. We have seen the same in Chapter 4 for globular clusters in the LMC, which have sub-solar metallicity.
Comparisons with stellar population models. In this chapter we have tested the above solar metallicity regime of stellar populations. Current near-IR SSP models agree with theDCOindex for old early-type galaxies, but a correction factor to account for the
poor spectral resolution of the models have to be considered. There are signatures that the models overestimate this index at very young ages, however there are also other possible explanations. This observation implies the need for a better understanding of the most important contributors to the near-IR light in young and intermediate age globular clusters and galaxies, i.e. AGB stars. This understanding is of paramount importance to the study of high redshift galaxies, close to the time of their formation. The near-IR Na I index reacts stronger to the presence of AGB stars at that stage, as compared toDCO. For older ages it tracks the metallicity of the population. Thus we
consider the combination between the optical Hβindex and the near-IR Na I index as a potentially good age-metallicity indicator once more sophisticated stellar population
Chapter 6
Central
K-band kinematics and line
strength maps of NGC 1399
based on M. Lyubenova, H. Kuntschner, and D. R. Silva
Astronomy & Astrophysics, 2008, 485, 425
6.1
Introduction
The centres of giant early-type galaxies are interesting laboratories for exploring im-