The mass-metallicity relation

126 Chapter 4. Star-formation and chemical history in the large-scale structure of the Hercules Supercluster

Figure 4.21: The same as in Figure4.16usingM_B,cor magnitude and(g−i)_corcolor, corrected for extinction.

4.4. Chemical history of low-mass galaxies in the Hercules Supercluster 127

Figure 4.22: 12 +log(O/H) vs. galaxy stellar mass for our MS galaxies. The black solid line indicates the linear t to all MS galaxies. Galaxies at distancesR≤R₂₀₀ in each cluster are marked with colors: A2151 (blue), A2152 (yellow), A2147 (red).

The error bars in the upper left corner indicate the adopted errors of 0.1 dex in metallicity and 0.2 dex in mass. Cyan points representLee et al. (2006) sample of local Volume dIrr galaxies and the cyan line is the t given byLee et al. (2006).

In order to explore the possible imprint of the cluster environment on the MZR for the three clusters of the HSC, in Figure 4.23 we give the residuals log(O/H)- log(O/H)fitfrom the MZR t for all MS galaxies as a function of local galaxy density (Σ_4,5, upper panel) and galaxy stellar mass (lower panel). We mark the galaxies in the cluster cores with blue (A2151), yellow (A2152), and red (A2147) colors.

In these two plots we see that galaxies inside R ≤ R₂₀₀ regions show higher local densities (as expected), span the whole range of masses, and we nd more of these core galaxies in the part of the positive residuals. We also observe higher positive residuals with decreasing stellar mass for A2151 and A2147 cluster core galaxies.

For A2152 there are only two galaxies in our MS sample with R ≤R₂₀₀ and have metallicities very close to the general MZR t.

A2151 having enough MS galaxies located at distances R ≤ R₂₀₀ (the blue points in Figure4.23), permits to explore the dierences in metallicity as a function of cluster-centric distance and local galaxy density. In the upper panel of Figure 4.24, we plot the mean dierence log(O/H)-log(O/H)fitas a function of the distance from the cluster center for the A2151 MS galaxies, in bins⁴ of 0.2R₂₀₀, and the error bars represent the standard error of the mean. The point sizes are proportional to the number of galaxies included in each bin, and we observe that at ∼ R₂₀₀

4The last bin atR >1.6R200 is a bit wider, to increase statistics.

128 Chapter 4. Star-formation and chemical history in the large-scale structure of the Hercules Supercluster

Figure 4.23: The residuals log(O/H)-log(O/H)fitfrom the MZR t for all MS galaxies as a function of local galaxy density (Σ_4,5, upper panel) and galaxy stellar mass (lower panel). We mark the galaxies in the cluster cores with blue (A2151), yellow (A2152), and red (A2147) colors.

Figure 4.24: Upper panel: the mean log(O/H)-log(O/H)fit as a function of the distance from the cluster center in bins of 0.2R₂₀₀ for A2151 MS galaxies. Lower panel: the same as a function of local galaxy densityΣ_4,5. The error bars represent the standard error of the mean value and the point sizes are proportional to the number of galaxies included in each bin.

4.4. Chemical history of low-mass galaxies in the Hercules Supercluster 129

Figure 4.25: Log (N/O) vs. galaxy stellar mass for the MS galaxies in the HSC.

Galaxies atR≤R₂₀₀ are marked with colors: A2151 (blue), A2152 (yellow), A2147 (red).

there is an increment in the number of galaxies observed, reecting the population of newcomers in the cluster environment, that we have already discussed in the previous sections and in Chapter 3.

At distancesR≤R₂₀₀we observe a marginal trend of increasing O/H compared to the expected values from the MZR t, as we move toward the cluster core. At the lowest distances to the cluster center R ≤ 0.2R₂₀₀, the mean O/H increment reaches ∼ 0.1 dex. This is in agreement with the result obtained in Section 3.8, where we have found that dwarf/irregular galaxies located at the highest densities inside the cluster R₂₀₀ region, were located at the upper part of the general MZR.

In the lower panel of Figure 4.24, we plot the mean O/H-O/Hfit as a function of local galaxy density, in bins⁵ of 0.2 dex in log Σ_4,5. Here again the error bars represent the standard error of the mean and the point sizes are proportional to the number of galaxies included in each bin. We see that there appears to be a marginal average increment in galaxy metallicity at local galaxy densities logΣ_4,5 >1.8(see also Section 3.7), but there does not appear a clear general trend.

It is known that galaxy N/O ratio shows a complex behavior, due to the pri- mary/secondary nature of nitrogen (see Section 1.3.1) and its delayed delivery to the ISM & 250 Myr, as compared to oxygen, which is typically released after ∼10 Myr in Type II SNe. Additionally, the N/O ratio has a dierent response than O/H in eects of gas interchange with the environment, e.g. gas infall and outows

5The rst and last density bin are slightly wider to include all A2151 MS galaxies and increase statistics.

130 Chapter 4. Star-formation and chemical history in the large-scale structure of the Hercules Supercluster

Figure 4.26: Upper panel: the mean log(N/O)-log(N/O)fit as a function of the distance from the cluster center in bins of 0.2R₂₀₀ for A2151 MS galaxies. Lower panel: the same as a function of local galaxy densityΣ_4,5. The error bars represent the standard error of the mean and the point sizes are proportional to the number of galaxies included in each bin.

(Edmunds 1990;Köppen & Hensler 2005;van Zee & Haynes 2006;Pérez-Montero &

Contini 2009). Therefore, tracing the N/O ratio in galaxies can give an important piece of information regarding galaxy chemical evolution.

In Figure 4.25 we plot the N/O ratio vs. galaxy stellar mass, nding a well dened correlation. The points in colors represent galaxies in the cluster cores as previously, and we see that these cluster core galaxies do not show any evident trend on this plane. We perform a linear t to the plot in Figure4.25, and in Figure4.26 and we explore the residuals from the t for A2151 MS galaxies, as we did previously for the MZR. We plot the mean of the dierence log(N/O)-log(N/O)fit as a function of the distance to the cluster center (top) and as a function of local galaxy density (bottom). We observe the same trends as in Figure 4.24; a slight enhancement of the N/O ratio in galaxies in the more central parts of the cluster, as compared to the expected value form the t. This enhancement is also veried at local galaxy densities Σ_4,5 ∼1.8.

The trends observed in O/H and N/O in A2151 low-mass SF galaxies could be interpreted by the galaxy surviving scenario proposed in Chapter 3 (Section3.7):

only the more massive (and thus more metallic) of these galaxies have been able to survive (as SF galaxies) until reaching the cluster core. The star formation of the lower-mass galaxies would get faster switched o by the cluster environment, before the crossing of the cluster center. In Chapter 7 we will discuss another possible

4.5. Summary 131