Measures of galaxy environment – III. Difficulties in identifying protoclusters at z ∼ 2
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Galaxy environment is frequently discussed, but inconsistently defined. It is especially difficult to measure at high redshift where only photometric redshifts are available. With a focus on early forming protoclusters, we use a semi-analytical model of galaxy formation to show how the environment measurement around high-redshift galaxies is sensitive to both scale and metric, as well as to cluster viewing angle, evolutionary state and the availability of either spectroscopic or photometric data. We use two types of environment metrics (nearest-neighbour and fixed aperture) at a range of scales on simulated high-z clusters to see how ‘observed’ overdensities compare to ‘real’ overdensities. We also ‘observationally’ identify z = 2 protocluster candidates in our model and track the growth histories of their parent haloes through time, considering in particular their final state at z = 0. Although the measured environment of early forming clusters is critically dependent on all of the above effects (and in particular the viewing angle), we show that such clusters are very likely ( ≳ 90 per cent) to remain overdense at z = 0, although many will no longer be among the most massive. Object-to-object comparisons using different methodologies and different data, however, require much more caution.Abstract Galaxy cluster outskirts mark the transition region from the mildly non-linear cosmic web to the highly non-linear, virialised, cluster interior. It is in this transition region that the intra-cluster medium (ICM) begins to influence the properties of accreting galaxies and groups, as ram pressure impacts a galaxy’s cold gas content and subsequent star formation rate. Conversely, the thermodynamical properties of the ICM in this transition region should also feel the influence of accreting substructure (i.e. galaxies and groups), whose passage can drive shocks. In this paper, we use a suite of cosmological hydrodynamical zoom simulations of a single galaxy cluster, drawn from the nIFTy comparison project, to study how the dynamics of substructure accreted from the cosmic web influences the thermodynamical properties of the ICM in the cluster’s outskirts. We demonstrate how features evident in radial profiles of the ICM (e.g. gas density and temperature) can be linked to strong shocks, transient and short-lived in nature, driven by the passage of substructure. The range of astrophysical codes and galaxy formation models in our comparison are broadly consistent in their predictions (e.g. agreeing when and where shocks occur, but differing in how strong shocks will be); this is as we would expect of a process driven by large-scale gravitational dynamics and strong, inefficently radiating, shocks. This suggests that mapping such shock structures in the ICM in a cluster’s outskirts (via e.g. radio synchrotron emission) could provide a complementary measure of its recent merger and accretion history.
Substructure
Ram pressure
Structure formation
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We describe the construction of the QDOT survey, which is publicly available from an anonymous FTP account. The catalogue consists of infrared properties and redshifts of an all-sky sample of 2387 IRAS galaxies brighter than the IRAS PSC 60-μm completeness limit (S60>0.6 Jy), sparsely sampled at a rate of one-in-six. At |b|>10°, after removing a small number of Galactic sources, the redshift completeness is better than 98 per cent (2086/2127). New redshifts for 1401 IRAS sources were obtained to complete the catalogue; the measurement and reduction of these are described, and the new redshifts tabulated here. We also tabulate all sources at |b|>10° with no redshift so far, and sources with conflicting alternative redshifts either from our own work, or from published velocities. A list of 95 ultraluminous galaxies (i.e. with L60 μm>1012 L⊙) is also provided. Of these, ∼20 per cent are AGN of some kind; the broad-line objects typically show strong Fe ii emission. Since the publication of the first QDOT papers, there have been several hundred velocity changes: some velocities are new, some QDOT velocities have been replaced by more accurate values, and some errors have been corrected.
Completeness (order theory)
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