THE MORPHOLOGICAL TRANSFORMATION AND THE QUENCHING OF GALAXIES

Our parent galaxy sample is taken from Wang et al. (2018b). They select all galaxies in the main galaxy sample of the New York University Value-added Galaxy Catalog (NYU-VAGC; Blanton et al. (2005b)) of the SDSS DR7 (Abazajian et al., 2009), with (1) r-band apparent magnitudes $r\leq$ 17.72; (2) redshift completeness $C\geq$0.7; (3) within the reconstruction region of ELUCID simulation (see Wang et al. (2016) for details); (4) the stellar mass $M_{*}>10^{8.5}\ h^{-2}M_{\sun}$. The selection criteria (1) and (2) ensure that most of the selected galaxies are contained in the group catalog of Yang et al. (2007) (with extension to DR7). Criterion (3) guarantees that galaxies can be identified with reliable large-scale environments. Criterion (4) is to minimize the completeness problem for less massive galaxies (see Chen et al. (2019) for more details). The reconstruction region is restricted to the redshift range of $0.01<z<0.12$, where groups with $\lg M_{halo}\gtrsim 12$ are complete. We remove the galaxies with small fraction of their virial volumes contained within the survey boundary ($0<f_{edge}\leq 0.6$). Our final sample contains $\sim$ 0.3 million galaxies. For these galaxies, stellar masses $M_{*}$, in units of $h^{-2}M_{\sun}$, are computed using the relations between the stellar mass-to-light ratio and the (g-r) color as given in Bell et al. (2003), adopting an initial mass function (IMF) according to Kroupa (2001). We refer to Yang et al. (2007) for details.

In this study, we probe the following environmental factors that can impact the morphological transformation and quenching of galaxies: on small scales: the halo mass $M_{halo}$, the halo centric radius $R_{p}/r_{180}$ and the third nearest neighbor distances ($d_{3nn}$); and on large scales: the cosmic web types.

We first calculate the fraction of subsample galaxies out of the total population for all galaxies, in different cosmic web environments. Table 1 shows the fractions of the four types of galaxies out of all galaxies in the four large-scale structures. We also show these fractions in Figure 4. For early types, the fraction of qEs out of all galaxies in all large-scale structures is 11.6%, much higher than the fraction of sEs out of all galaxies(4.1%). The statistics of early types is therefore dominated by qEs, which results in the notion that early types are always thought to be red and passive. The overall fractions of sLs and qLs out of all galaxies in all large-scale environments are 67.2% and 17.0%. Late types are dominated by the star-forming subsample. Thus, they are usually thought to be actively star-forming.

The morphology-density relation can be seen. The fractions of early types out of all cluster galaxies decreases from 23.7% all the way through filaments and sheets, and get to 8.2%, which is the fraction of early types out of all void galaxies; the fraction of late type galaxies out of all cluster galaxies gradually increases from 76.3% to 91.8%, which is the fraction of late type galaxies out of all void galaxies. Table 1 also shows the SFR-density relation. The fraction of quenched galaxies out of all void galaxies increases from 11.2% to 49.6%, which is the fraction of quenched galaxies out of all galaxies in clusters. It’s interesting to note that the star formation properties change much more dramatically than morphology as environment changes (Figure 4).

In early types, the sE fraction ($f_{sE}$) is almost constant in all four large-scale structures ($\sim 4\%$). However, the qE fraction $f_{qE}$ decreases strongly from clusters to voids. In late types, $f_{sL}$ increase while $f_{qL}$ decrease significantly from clusters to voids.

With all these statistics with respect to the large scale environments, we will discuss the environmental effects on the morphological transformation and the quenching with more details in Section 3.2 and Section 4.

There is a degeneracy between the large-scale environments and the in-situ and small-scale environments. Galaxies in clusters, the densest large-scale environments, tend to have more massive central galaxies, be hosted by the most massive halos (Figure 2) and have the smallest separations with their neighbors (smallest $d_{3nn}$ values). Galaxies in voids, the most under-dense large-scale environments, tend to have low mass central galaxies, and the least massive halos (Figure 2) and farthest away from their neighbors (largest $d_{3nn}$ values). In order to disentangle the degeneracy between the large- and small- scale environments, it is therefore reasonable to study the large-scale environmental effects with the small-scale environmental parameters controlled.

We study the in-situ and small-scale environments with three different parameters ($M_{*}$, $M_{halo}$ and $d_{3nn}$) in Figure 5 and Figure 6. Each of these parameters represents an in-situ or small-scale environment at different scale: $M_{*}$ represents the internal physical processes on the galaxy-scale, $M_{halo}$ is a gravity bounded halo-scale parameter; $d_{3nn}$ is the distance to the third nearest bright neighbor (see Section 2.2.1 for more details of the definition).

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  morphological transformation

We first study the morphological transformation of galaxies in different cosmic web environments. The resulting early type galaxy fractions are shown in Fig. 5. Among the star-forming MS galaxies (the left panels), the fraction of early type galaxies increase as a function of $M_{*}$, an in-situ environmental factor. The fraction does not show strong dependence on small-scale environments, $M_{halo}$ and $d_{3nn}$. In addition, the galaxies in different cosmic web environments show quite consistent behaviors. That is, for the star-forming main sequence galaxies, the morphological transformation, which only depends on $M_{*}$, is an in-situ process.

For the quenched galaxies (the right panels), quite similar to the star-forming galaxies, there is a strong dependence of early type fraction on the in-situ parameter, $M_{*}$. There are no small-scale and large-scale environmental dependencies, again indicating that the morphological transformation is an in-situ process. On the other hand, we also notice that, with the same stellar mass $M_{*}$, the quenched galaxies have overall higher early type fractions than those of star-forming galaxies. This difference indicates that the quenching and morphological transformation are correlated.

Bamford et al. (2009) studied the early-type fraction as a function of local density for galaxies in various stellar mass bins in the left panel of their Figure 12. They found that the stellar mass is very important for the early type fractions, with the highest mass bin of early type fraction ($>$60%) three times higher than that of the lowest mass bin ($<$20%). This is consistent with what we found in the top panels of Figure 5. We will discuss more about the comparison between our results and theirs in Section 4.3.

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  quenching

Next, we show the quenched fraction of galaxies in different cosmic web environments in Figure 6. Among the early type galaxies (left panels), the quenched fraction varies as a function of $M_{*}$ and $M_{halo}$. While the dependence on $d_{3nn}$ is not very significant. More importantly, we see that the $M_{halo}$ dependence of the quenched fractions is quite consistent for galaxies in different cosmic web environments. Although we see that the quenched fractions as a function of $M_{*}$ and $d_{3nn}$ are very different for galaxies in different cosmic web environments, these large scale environmental dependencies could be caused by the correlation between the halo mass and cosmic web types. Shown in the right panels of Figure 6 are results for the late type galaxies. The general trends, i.e., the $M_{*}$, $M_{halo}$ and $d_{3nn}$ dependencies are quite similar to those of early type galaxies. Quantitatively, we see that the late type galaxies have overall lower quenched fractions, again indicating that the quenching and morphological transformation are dependant. These behaviors suggest that the quenching of galaxies is possibly only impacted by in-situ and small-scale halo environments, not large-scale environments, in very good agreement with those findings obtained in Wang et al. (2018b).

Let’s get back to the first panel of Figure 6. If we attribute the large-scale environmental dependence to the halo mass dependence, then the significant differences seen in the quenched fraction of low mass galaxies in different cosmic web environments suggests that for low mass galaxies, quenching is mainly driven by halo environments. The higher quenching fraction in denser cosmic web structures for less massive galaxies is the result of the high quenching ratio of satellites in more massive halos ($M_{halo}\gtrsim 10^{13.0}\>h^{-1}\rm M_{\odot}$). We will re-visit this problem in Section 4.2 with Figure 10. While for massive galaxies, the quenching fraction is consistent in all four cosmic web structures, which suggests to be stellar mass quenching dominated.

We first study the morphological transformation from late types to early types on the $\lg M_{halo}-\lg M_{*}$ map in Figure 7 for star forming main sequence and quenched galaxies separately. Bins in Figure 7 are color coded by the early type fraction ($f_{E}$). The upper panel is for the fraction of the early type galaxies out of all MS galaxies ($f_{E}(s)$), and the lower panel is for the fraction of early type galaxies out of all quenched galaxies ($f_{E}(q)$). We also split the centrals and the satellites in the first and second columns, and the third column is the (cen$+$sat) sample.

We find that for both MS and the quenched population, bins with similar early type fraction (similar color coding) are roughly vertically distributed, especially for the satellites (middle column). For the centrals (left column), since the stellar masses and halo masses are in tight correlation separately, it is hard to disentangle their contribution on the $\lg M_{halo}-\lg M_{*}$ map. This suggests that the morphological transformation is mostly regulated by stellar masses and almost independent from the halo masses. In addition, as we have separate galaxies into sub-samples of the star-forming main sequence and the quenched galaxies, we see that the quenched galaxies have overall much higher fractions of early type galaxies.

In addition to the early type fractions, we probe another two parameters that are associated with the morphological transformation of galaxies, i.e., the Hubble types and the bulge-to-total light ratios. Figure 8 shows the distributions of all galaxies on the $\lg M_{halo}-\lg M_{*}$ map. In the left panel, bins are color coded by their median Hubble types T calculated by Equation 2. In the right panel, bins are colored by their median bulge-to-total light ratio, (B/T)s. These two panels together give the information of how significantly the stellar mass and how negligibly halo mass affect the morphological transformation. We note that although the changing of color in the bulge-to-total light ratio panel is not that purely vertical as the Hubble types T, in both panels, the color changes mainly in the stellar mass direction. This confirms our result in Section 3.2 that the morphological transformation is mainly regulated by the stellar mass.

Finally, we check the dependence on another important halo-scale parameter – the halo-centric radius $R_{p}/r_{180}$, which is the distance to the center of the halo of the group where the galaxy is located. We show in Figure 9 the median Hubble types T and the median bulge-to-total light ratio of galaxies on a $R_{p}/r_{180}-\lg M_{*}$ map. Bins with similar T and similar B/T are both distributed vertically. This suggests that the relative position to the center of the halos is also not important in the morphological transformation. These all suggest that the morphological transformation of galaxies is solely an in-situ process.

In Figure 10, we plot the fraction of quenched galaxies among the early type galaxies (upper panel) and among the late type galaxies (lower panel). Again, the $M_{*}$ and the $M_{halo}$ for centrals show tight correlation, and it is hard to tell the dominant parameter to control the changes of the quenched fraction. For satellites (middle panel), the color changes more dramatically in the vertical direction in more massive halos ($M_{halo}\lesssim 10^{13}\ M_{\sun}$) and more dramatically in the horizontal direction at higher stellar masses ($M_{*}\gtrsim 10^{10.5}\ M_{\sun}$). Our results suggest that the quenching of massive galaxies is more of an in-situ process, while the quenching of satellites with lower stellar masses in halos with $M_{halo}\gtrsim 10^{13.0}\>h^{-1}\rm M_{\odot}$ are dominated by their halo environments.

We show the distribution of the fraction of the quenched population ($F_{q}$) on the $R_{p}/r_{180}-\lg M_{*}$ map for the early types (left) and the late types (right) in Figure 11. Bins with similar color coding are mainly vertically distributed and slightly horizontally distributed for galaxies with small distances to the center of the halo ($R_{p}/r_{180}<0.2$).

Our result that quenching is mainly controlled by the stellar mass at high stellar masses, and by the halo mass at low stellar masses (Figure 6 and Figure 10) agrees with the model raised by Peng et al. (2010). They established a simple model that mass quenching dominates in the more massive ranges ($M_{*}>10^{10.6}\ M_{\sun}$ at z=0), and environmental quenching dominates in the less massive ranges ($M_{*}<10^{10.6}\ M_{\sun}$ at z=0). Wang et al. (2018b) further found that the halo mass is the prime environmental parameter for quenching. Our results are consistent with their result that the quenching for galaxies with $M_{*}\lesssim 10^{10.5}\ M_{\sun}$ is probably a halo-scale physical process instead of those on larger environmental scales. The high quenching ratio in denser environments for the less massive galaxies are mostly caused by the less massive satellites in massive halos (Figure 10, middle panel).

Peng et al. (2010) focused on the quenching of SF in their paper. Our paper supplies the supplemental morphology information to their study. Comparing the upper panel with the lower panel of Figure 10, we see an overall more significant halo quenching effect in early type galaxies than in late type galaxies. This might be attributed to their lacking of cold gas or earlier accretion into the massive host halos.

From our result, we think the observed morphology-density relation may actually originate from the different $M_{*}$ distributions in environments with different densities. Denser environments are more abundant in more massive galaxies and less dense environments are more abundant in less massive galaxies (e.g. Constantin et al., 2008; Hoyle et al., 2012; Liu et al., 2015). As shown in our paper, morphology transformation is more regulated by the stellar mass, therefore, denser environments statistically include more early type galaxies and less dense environments are of more later type galaxies. These observed morphology-density relation may intrinsically just be the $M_{*}$-density relation and the morphology-$M_{*}$ relation. We show that once the stellar mass is fixed, the morphology-density relation disappears (the top panel of Figure 5).

Similarly, the color-density relation (or the SFR-density relation) originates from the $M_{*}$-density relation and the $M_{halo}$-density relation. We show that when the $M_{*}$ is fixed at high stellar masses and when the $M_{halo}$ is fixed at the low mass end, the color-density relation/SFR-density relation disappears (the top panel and the middle panel of Figure 6).

Bamford et al. (2009) studied the Galaxy Zoo sample and found no significant relation between the morphology evolution and the environments with the color fixed, while at fixed morphologies, the environmental dependence of color is strong, particularly for lower stellar-mass galaxies. They used the local density as the indicator of the environments, which has different implications from our large-scale environment parameters. Similar findings are also mentioned in Skibba et al. (2009), even though they probed environmental effect through the two-point clustering statistics. We have similar results, in particular, we also find that at lower stellar mass, the quenched fractions (red color) among fixed morphology strongly correlate with the large-scale environment (Figure 6 upper left). However, these variations disappear when the halo masses are fixed (Figure 6 middle left). More detailed comparison of our conclusion and Bamford et al. (2009) and Skibba et al. (2009) can be complicated, as the color and the SFR are not strictly correlated (Woo et al., 2013), and the local density is sometimes difficult to interpret (e.g. Woo et al., 2013; Wang et al., 2018b)