In this section, we ask whether bulge-dominated galaxies with M∗ > 1010M lie on the
same Hi scaling relations as the general population of galaxies with similar stellar masses. We have shown in the previous chapter (§3.3) the global Hi scaling relations for all the targets, underlying how they are important for describing galaxy properties and constrain- ing evolutionary models. Our goal is now to explicitly determine whether the presence of the bulge plays a role in regulating the rate at which gas is coverted into stars, for example by stabilizing the Hi disk and preventing its collapse (e.g., Martig et al. 2009). In order to test the role of the bulge in a clean way, we must account for the fact that the physical properties of galaxies are strongly correlated. If one selects a sub-sample of bulge-dominated galaxies from the parentsample A, one will automatically select a sample of galaxies with higher stellar masses, higher stellar mass surface densities, and redder colours. It is therefore important to understand whether or not bulge-dominated galaxies differ in Hi content from the parent sample at fixed values of these parameters.
Our results are presented in Figure 4.3, where we show the Hi scaling relations as a function of colour and structural parameters. In all the plots, blue circles are the average gas fractions obtained from stacking of B-D sample targets. The red lines are the fits to the mean Hi gas fraction relations obtained for sample A; in the case of σ, not considered in the previous chapter, we report the actual data points. Errors are evaluated using the
bootstrap method (§3.2.4). Finally, triangles indicate the upper limits evaluated in the case of a stack that yields a non-detection.
The average gas content of bulge-dominated objects decreases weakly with increasing values of M? and central velocity dispersions, and more strongly with increasing µ? and
colours. Our main result is that the average Higas fractions of B-D galaxies are significantly lower (by approximately a factor of 2) than those of the parent sample at a given value of stellar mass. A similar, but weaker reduction in the average Hi gas fraction is seen for the B-D sample when it is plotted as a function of the central velocity dispersion (corrected for aperture effects, see §2.4) of the bulge. However, the relation between gas fraction and stellar mass surface density and NUV−r colour appears to be insensitive to the morphological cut1. In their paper, Catinella et al. (2010) showed that a linear
combination of stellar mass surface density and NUV−r colour provided an excellent way to “predict” the Hi content of star-forming galaxies more massive than 1010M
. Here we
show that this conclusion holds true independent of the bulge-to-disk ratio of the galaxy.
1For completeness, we report all the values of the points in the plots, for the different cuts and as a function of the different parameters, in Table 4.1.
4.3 Hi study of a complete sample of early-type galaxies 59
Figure 4.3: The dependence of the average Hi gas fraction on stellar mass M? (a), central
velocity dispersionσ(b), stellar mass surface densityµ?(c) and NUV−rcolour (d) for theB-D
sample(blue symbols). The relations found forsample Aare shown in red for comparison (the fit for the M? relation, the actual points for σ). Upside-down triangles indicate upper limits in the case of a non-detection. The numbers written in the panels indicate the numbers of objects co-added in each bin (Tot), and the percentage of them directly detected by ALFALFA inside the parenthesis (%Det). Gray dots showsample Agalaxies with ALFALFA detections. We have also applied more stringent cuts to the B-D sample, as explained in the text: cyan circles represent a sample withC>3 (52% of the original ETG sample); black circles are for a sample with b/a>0.6 (or inclination lower than 55◦ - 68% of the original sample). The values plotted for eachETG sample are reported in Table 4.1
We have tested that our result is robust if we define the B-D sample using more stringent cuts on concentration index (C > 3; cyan dots in Figure 4.3) or on the axis ratio of the galaxy (b/a > 0.6, which implies inclination lower than ∼55◦; black circles). These cuts reduce the early-type galaxies by 50% and 30% respectively. Nevertheless, results shown in Figure 4.3 demonstrate that the average Hi gas fractions of these systems still lie on the same relations when plotted as a function of stellar mass surface density and NUV-r
colour.
Last, we measure how the average gas fractions of galaxies in the B-D sample vary as a function of position in the two-dimensional plane of colour versus stellar mass density µ∗,
since these are the two main parameters driving the gas content in massive galaxies. In Figure 4.4 we plot the B-D targets on the µ∗-(NUV−r) plane. Bulge-dominated galaxies
are mainly found on the red sequence, but there is a minority population with bluer colours. We adaptively bin the sample in two dimensions by recursively dividing the plane into axis- aligned rectangles. We stop dividing a region when a further split would lower the S/N below the detection threshold of 6.5. Figure 4.4 (bottom panel) shows the final binning used. In each bin, the measured gas fraction (expressed as a percentage of the stellar mass) is reported. In Figure 4.4 (top panel) we colour-code the (NUV−r)−µ∗ plane according
to gas fraction. The Hi content decreases going from left to right (towards increasing stellar mass surface density) and from bottom to top (towards redder colours). The most significant variation is clearly along the colour direction.