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Register a new userMapping stellar content to dark matter halos. II. Halo mass is the main driver of galaxy quenching
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We develop a simple yet comprehensive method to distinguish the underlying drivers of galaxy quenching, using the clustering and galaxy-galaxy lensing of red and blue galaxies in SDSS. Building on the iHOD framework developed by Zu & Mandelbaum (2015a), we consider two quenching scenarios: 1) a halo quenching model in which halo mass is the sole driver for turning off star formation in both centrals and satellites; and 2) a hybrid quenching model in which the quenched fraction of galaxies depends on their stellar mass while the satellite quenching has an extra dependence on halo mass. The two best-fit models describe the red galaxy clustering and lensing equally well, but halo quenching provides significantly better fits to the blue galaxies above $10^{11} M_odot/h^2$. The halo quenching model also correctly predicts the average halo mass of the red and blue centrals, showing excellent agreement with the direct weak lensing measurements of locally brightest galaxies. Models in which quenching is not tied to halo mass, including an age-matching model in which galaxy colour depends on halo age at fixed $M_*$, fail to reproduce the observed halo mass for massive blue centrals. We find similar critical halo masses responsible for the quenching of centrals and satellites (~$1.5times10^{12} Modot/h^2$), hinting at a uniform quenching mechanism for both, e.g., the virial shock-heating of infalling gas. The success of the iHOD halo quenching model provides strong evidence that the physical mechanism that quenches star formation in galaxies is tied principally to the masses of their dark matter halos rather than the properties of their stellar components.
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Recent studies suggest that the quenching properties of galaxies are correlated over several mega-parsecs. The large-scale galactic conformity phenomenon around central galaxies has been regarded as a potential signature of galaxy assembly bias or pre-heating, both of which interpret conformity as a result of direct environmental effects acting on galaxy formation. Building on the iHOD halo quenching framework developed in Zu & Mandelbaum (2015, 2016), we discover that our fiducial halo mass quenching model, without any galaxy assembly bias, can successfully explain the overall environmental dependence and the conformity of galaxy colours in SDSS, as measured by the mark correlation functions of galaxy colours and the red galaxy fractions around isolated primaries, respectively. Our fiducial iHOD halo quenching mock also correctly predicts the differences in the spatial clustering and galaxy-galaxy lensing signals between the more vs. less red galaxy subsamples, split by the red-sequence ridge-line at fixed stellar mass. Meanwhile, models that tie galaxy colours fully or partially to halo assembly bias have difficulties in matching all these observables simultaneously. Therefore, we demonstrate that the observed environmental dependence of galaxy colours can be naturally explained by the combination of 1) halo quenching and 2) the variation of halo mass function with environment --- an indirect environmental effect mediated by two separate physical processes.
We study the dependence of the galaxy content of dark matter halos on large-scale environment and halo formation time using semi-analytic galaxy models applied to the Millennium simulation. We analyze subsamples of halos at the extremes of these distributions and measure the occupation functions for the galaxies they host. We find distinct differences in these occupation functions. The main effect with environment is that central galaxies (and in one model also the satellites) in denser regions start populating lower-mass halos. A similar, but significantly stronger, trend exists with halo age, where early-forming halos are more likely to host central galaxies at lower halo mass. We discuss the origin of these trends and the connection to the stellar mass -- halo mass relation. We find that, at fixed halo mass, older halos and to some extent also halos in dense environments tend to host more massive galaxies. Additionally, we see a reverse trend for the satellite galaxies occupation where early-forming halos have fewer satellites, likely due to having more time for them to merge with the central galaxy. We describe these occupancy variations also in terms of the changes in the occupation function parameters, which can aid in constructing realistic mock galaxy catalogs. Finally, we study the corresponding galaxy auto- and cross-correlation functions of the different samples and elucidate the impact of assembly bias on galaxy clustering. Our results can inform theoretical models of assembly bias and attempts to detect it in the real universe.
Measurements of the total amount of stars locked up in galaxies as a function of host halo mass contain key clues about the efficiency of processes that regulate star formation. We derive the total stellar mass fraction f_star as a function of halo mass M500c from z=0.2 to z=1 using two complementary methods. First, we derive f_star using a statistical Halo Occupation Distribution model jointly constrained by data from lensing, clustering, and the stellar mass function. This method enables us to probe f_star over a much wider halo mass range than with group or cluster catalogs. Second, we derive f_star at group scales using a COSMOS X-ray group catalog and we show that the two methods agree to within 30%. We quantify the systematic uncertainty on f_star using abundance matching methods and we show that the statistical uncertainty on f_star (~10%) is dwarfed by systematic uncertainties associated with stellar mass measurements (~45% excluding IMF uncertainties). Assuming a Chabrier IMF, we find 0.012<f_star<0.025 at M500c=10^13 Msun and 0.0057<f_star<0.015 at M500c=10^14 Msun. These values are significantly lower than previously published estimates. We investigate the cause of this difference and find that previous work has overestimated f_star due to a combination of inaccurate stellar mass estimators and/or because they have assumed that all galaxies in groups are early type galaxies with a constant mass-to-light ratio. Contrary to previous claims, our results suggest that the mean value of f_star is always significantly lower than f_gas for halos above 10^13 Msun. Combining our results with recently published gas mas fractions, we find a shortfall in f_star+f_gas at R500c compared to the cosmic mean. This shortfall varies with halo mass and becomes larger towards lower halos masses.
The similarity between the distributions of spins for galaxies ($lambda_{rm g}$) and for dark-matter haloes ($lambda_{rm h}$), indicated both by simulations and observations, is naively interpreted as a one-to-one correlation between the spins of a galaxy and its host halo. This is used to predict galaxy sizes in semi-analytic models via $R_{rm e}simeqlambda_{rm h} R_{rm v}$, with $R_{rm e}$ the half-mass radius of the galaxy and $R_{rm v}$ the halo radius. Utilizing two different suites of zoom-in cosmological simulations, we find that $lambda_{rm g}$ and $lambda_{rm h}$ are in fact only barely correlated, especially at $zgeq 1$. A general smearing of this correlation is expected based on the different spin histories, where the more recently accreted baryons through streams gain and then lose significant angular momentum compared to the gradually accumulated dark matter. Expecting the spins of baryons and dark matter to be correlated at accretion into $R_{rm v}$, the null correlation at the end reflects an anti-correlation between $lambda_{rm g}/lambda_{rm h}$ and $lambda_{rm h}$, which can partly arise from mergers and a compact star-forming phase that many galaxies undergo. On the other hand, the halo and galaxy spin vectors tend to be aligned, with a median $costheta=0.6$-0.7 between galaxy and halo, consistent with instreaming within a preferred plane. The galaxy spin is better correlated with the spin of the inner halo, but this largely reflects the effect of the baryons on the halo. Following the null spin correlation, $lambda_{rm h}$ is not a useful proxy for $R_{rm e}$. While our simulations reproduce a general relation of the sort $R_{rm e}=AR_{rm vir}$, in agreement with observational estimates, the relation becomes tighter with $A=0.02(c/10)^{-0.7}$, where $c$ is the halo concentration, which in turn introduces a dependence on mass and redshift.
Narrow stellar streams in the Milky Way halo are uniquely sensitive to dark-matter subhalos, but many of these subhalos may be tidally disrupted. I calculate the interaction between stellar and dark-matter streams using analytical and $N$-body calculations, showing that disrupting objects can be detected as low-concentration subhalos. Through this effect, we can constrain the lumpiness of the halo as well as the orbit and present position of individual dark-matter streams. This will have profound implications for the formation of halos and for direct and indirect-detection dark-matter searches.
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