Information for Astronomers

1. Environmental-Quenching of Low Mass Galaxies
2. Hierarchical Assembly of Baryons
3. Cluster Dynamics and Masses

Environment-Quenching of Low Mass Galaxies

Despite great efforts and good theoretical foundations for the gravitational growth of dark matter structure, all galaxy formation models have consistently been unable to explain:

♦ The rate of decline in global Star Formation Rate (SFR)
♦ The mass dependence of this decline
♦ The star formation histories of satellite galaxies

These problems may all be related, as all of these properties are sensitive to assumptions about gas accretion, ejection and heating processes that depend on epoch, environment and halo mass. Failing to correctly reproduce these properties of satellites means our understanding of these details is incomplete.

The conventional view of the interaction between galaxies and their surroundings proposes that galaxies enter dense environments with a reservoir of gas, and that the SFR declines as this reservoir is removed. New indirect observations have made this model obsolete. Simulations explain this instead by proposing that galaxies grow as a result of continuous supply from surrounding filaments. Although reservoirs may be of substantial relevance at low redshifts, the effect of a permanent gas infall completely dominates over this at high redshifts. Thus, following from this, the new model (McGee et al. 2014) predicts that the SFR shuts down even more rapidly at z > 1.

Quenching (the suppression of SFR) is strongly seen in:

♦ The evolution of the quiescent galaxy stellar mass function
♦ The stellar mass dependence of the quiescent fraction of all galaxies

Generally, the quiescent galaxy stellar mass function evolves rapidly, with star formation shutting down in the most massive galaxies first, and taking longer to do so in dwarfs. At this redshift, most models predict an elevated number of low-mass, quiescent galaxies. It is unknown if this problem persists at higher redshift. In this context, the strongest test for current models is the abundance of faint red cluster members, a measurement which can now be made reliably at z > 1 with GOGREEN, and at the depth required to reach the low stellar-mass galaxies that are predicted to be most sensitive to environmental quenching.

One of the analytic goals of this survey is to classify galaxies and measure the quiescent fraction. From the spectra, the [OII] emission line and the D4000 break are characteristic feature of star–forming galaxies and thus can be used to identify and categorize members. The “UVJ” colour–colour diagram compliments further in providing another way to classify galaxies, as it does an excellent job of separating dusty star-forming galaxies from truly passive galaxies (e.g. Balogh et al. 2011; van der Burg et al. 2013).

 Hierarchical Assembly of Baryons

Λ-CDM (LambdaCold Dark Matter) theory predicts that massive clusters are built from haloes of lower mass, i.e. groups and isolated galaxies. To preferentially remove stars from dark matter dominated systems like these is very difficult and unrealistic. Thus, when these smaller systems merge together the fraction of total mass in stars can only increase (via star formation) or remain constant. Therefore, measurements of the stellar fraction, gas fraction, and SFR in haloes of a given mass provide one of the closest possible links between galaxies and this basic prediction of the underlying theoretical framework. To understand this, we need a precise, system-by-system determination of the stellar mass content of groups and clusters at z > 1. Such a precise determination can now be achieved via deep spectroscopy, including the faint red objects that will be targeted using data in GOGREEN.

Cluster Dynamics and Masses

At low redshift, the total mass content and distribution of galaxy clusters can be estimated by either:

♦ Gravitational lensing
♦ The assumption of hydrostatic equilibrium of the intracluster plasma
♦ The distribution and kinematics of cluster galaxies

The latter method is especially important for clusters at high redshifts, which are much more difficult to detect due to their weak lensing effect and X-ray emissions.  Due to this limitation, knowledge of the mass profiles of z >1.0 galaxy clusters is therefore restricted to only a few individual clusters. GOGREEN will provide breakthrough measurements of the dynamics of individual haloes at 1 < z < 1.5.

Dynamical analyses of nearby clusters have shown their M(r)mass as a function of cluster-centric distance – have well-behaved profiles; passive galaxy orbits are isotropic while star-forming galaxy orbits are radially elongated. Current models (e.g. NFW and Einasto) also appear to agree with observations of galaxy clusters in the redshift range of z ~ 0.6. However it is observed that M(r) profiles evolve with redshift, and at z ~ 0.6 passive galaxy orbits begin to resemble more those of star-forming galaxies.  With GOGREEN, it will be possible to test whether the NFW and Einasto models remain valid representations of the cluster M(r) at new, unprecedented redshifts. For analysis of dynamics and structure, we will have more than 50 confirmed members in all Virgo- and Coma-progenitor clusters, sufficient to obtain estimates of total masses from their velocity dispersion. More detailed and accurate analysis can be acquired from the previously discussed stacked samples. To exemplify, members from our “Virgo-progenitor” sample are enough to constrain both the average total mass radial profile M(r), and also the velocity anisotropy profile β(r).