Assessing the connection between galactic conformity and assembly-type bias

TL;DR

The paper investigates whether galactic conformity and assembly-type bias are manifestations of the same large-scale environment. Using the mdpl2-sag catalog at z=0, it compares conformity and assembly-type bias for central galaxies in a low-mass halo range, with samples that include and exclude centrals near massive halos. It shows a strong correlation between the amplitudes of conformity and assembly-type bias across different subsamples, and finds that removing centrals around massive halos substantially reduces both signals. The study further demonstrates that stellar age is a more effective proxy for halo formation time than sSFR in driving assembly-type bias. Overall, the results support a linked physical origin in the large-scale environment for both phenomena and suggest that observed conformity could indicate underlying assembly-type bias in the real universe.

Abstract

Context. Galaxies in the Universe show a conformity in the fraction of quenched galaxies out to large distances, being much larger around quenched central galaxies than for star-forming ones. On the other hand, simulations have shown that the clustering of halos and the galaxies within them depends on secondary properties other than halo mass, a phenomenon termed assembly bias. Aims. Our aim is to study whether samples that show galactic conformity also show assembly bias and to see if the amplitude of these two effects is correlated. Methods. We use synthetic galaxies at $z = 0$ from the semi-analytical model SAG run on the MultiDark Planck 2 (MDPL2) cosmological simulation and measure both conformity and galaxy assembly bias for different samples of central galaxies at fixed host halo mass. We focus on central galaxies hosted by low-mass halos of 10$^{11.6}$ $\leq$ $M_{\rm h}$/$h^{-1}$ M$_{\odot}$ $<$ 10$^{11.8}$ because it is a mass range where the assembly bias has been reported to be strong. The samples of central galaxies are separated according to their specific star formation rate and stellar age. Results. We find that the level of conformity shown by our different samples is correlated with the level of assembly bias measured for them. We also find that removing central galaxies around massive halos diminishes the conformity signal and lowers the amount of assembly bias. Conclusions. The high correlation in the amplitude of conformity and assembly bias for different samples with and without removing galaxies near massive halos clearly indicates the strong relationship between both phenomena.

Figures (6)

• Figure 1: Distribution of sSFR as a function of stellar age for the low-mass central galaxies, represented by the number density of galaxies as indicated in the color bar (main panel). The horizontal dotted line shows the condition used to separate star-forming and quenched galaxies. The top and right panels show the normalized density distributions of stellar age and sSFR, respectively, for the parent samples and sub-samples as indicated in the legends and described in Tables \ref{['tab:parent_samples']} and \ref{['tab:sub_samples']}. The integral of each histogram sums to unity. The lines correspond to the median value for the PrimAll (solid) and PrimB (dashed) samples.
• Figure 2: Mean quenched fractions of neighboring galaxies ($f_{\rm Q}$) as functions of the real-space distance from primary galaxies hosted by low-mass halos of 10$^{11.6}$$\leq$$M_{\rm h}$/$h^{-1}~\rm M_{\sun}$$<$ 10$^{11.8}$ (main panel). Primary samples are separated by their sSFR. The $f_{\rm Q}$ is shown around quenched and star-forming "PrimAll" galaxies as a dark red solid line and navy blue solid circles, respectively. The red dashed line and blue open circles correspond, respectively, to the mean fractions after removing the quenched and star-forming primary galaxies in the vicinity of halos more massive than 10$^{13}$$h^{-1}~\rm M_{\sun}$, case "PrimB". The lower sub-panel shows the difference in the mean quenched fractions of neighboring galaxies around quenched and star-forming primary galaxies at fixed halo mass. The solid line shows the case "PrimAll", whereas the dashed line is the result obtained for "PrimB". The dotted line denotes the case of zero difference, i.e., no conformity.
• Figure 3: The two-point correlation functions of primary galaxies hosted by dark matter halos with masses of 10$^{11.6}$$\leq$$M_{\rm h}$/$h^{-1}~\rm M_{\sun}$$<$ 10$^{11.8}$. The autocorrelation function for the case with all the central galaxies in the primary sample, "PrimAll", is shown as a solid black line, whereas the dark red solid line and navy blue circles correspond to the cross-correlation functions of quenched and star-forming primary galaxies, respectively. On the other hand, the dashed gray line corresponds to the autocorrelation function after removing the central galaxies in the vicinity of halos more massive than 10$^{13}$$h^{-1}~\rm M_{\sun}$ from the primary sample, case "PrimB". The red dashed line and open blue circles correspond to the cross-correlation functions of quenched and star-forming primary galaxies in the case "PrimB", respectively. The dashed lines and open circles are then the results after removing the galaxies that mostly contribute to the two-halo galactic conformity. The sub-panel shows the ratio between the correlation function of each sub-sample and the respective parent sample. The solid and dashed lines are for the cases Q "PrimAll" and Q "PrimB", respectively. The solid and open circles are for the cases SF "PrimAll" and SF "PrimB", respectively. There is an overlap in the circles because both ratios are close to unity.
• Figure 4: Top left (right): Same as Fig. \ref{['fig_AB_ssfr_fixedMhalo']}, but the cross-correlations are between the most quenched (oldest) or the 10% of the most star-forming (youngest) central galaxies and the respective parent sample of primary galaxies. Bottom: Same as Fig. \ref{['fig_GC_ssfr_fixedMhalo']}, but the primary samples are separated between the 10 percent of the most quenched and the 10 percent of the most star-forming central galaxies (left) and between the 10 percent of the oldest and 10 percent of the youngest central galaxies (right).
• Figure 5: Cross-correlation function between the "PrimB" sample and the groups and clusters with masses above $10^{13}~h^{-1}~\rm M_{\sun}$ (gray dashed line). The green dot-dashed line corresponds to the cross-correlation that uses random positions of massive halos with a vector of 5 $h^{-1}~\rm Mpc$ length from the original positions. The sub-panel shows the ratio between the correlation functions using the original and random positions of massive halos. They are the same at scales larger than 10 $h^{-1}~\rm Mpc$.