Beecroft Institute for Particle Astrophysics and Cosmology

Abstract:

The Euclid Early Release Observations (ERO) programme targeted the Perseus cluster of galaxies, gathering deep data in the central region of the cluster over 0.7 deg², including the cluster core up to 0.25 r200. The dataset reaches a point-source depth of IE = 28.0 (YE, JE, HE = 25.3), AB magnitudes at 5 σ with a 0^′′ . 16 (0^′′ . 48) full width at half maximum (FWHM), and a surface brightness limit of 30.1 (29.2) mag arcsec⁻² for radially integrated galaxy profiles. The exceptional depth and spatial resolution of this wide-field multi-band data enable simultaneous detection and characterisation of both bright galaxies and low surface brightness ones, along with their globular cluster systems, from the optical to the near-infrared (NIR). Cluster membership was determined using several methods in order to maximise the completeness and minimise the contamination of foreground and background sources. We adopted a catalogue of 1100 dwarf galaxies, detailed in the corresponding ERO paper, that includes their photometric and structural properties. We identified all other sources in the Euclid images and obtained accurate photometric measurements using AutoProf or AstroPhot for 137 bright cluster galaxies and SourceExtractor for half a million compact sources. This study advances beyond previous analyses of the cluster and enables a range of scientific investigations, which are summarised here. We derived the luminosity and stellar mass functions (LF and SMF) of the Perseus cluster in the Euclid IE band thanks to supplementary u, g, r, i, z, and Hα data from the Canada-France-Hawai’i Telescope (CFHT). Our LF and SMF are the deepest recorded for the Perseus cluster, highlighting the groundbreaking capabilities of the Euclid telescope. We fit the LF and SMF with a Schechter plus Gaussian model. The LF features a dip at M(IE) ≃ −19 and a faint-end slope of αS ≃ −1.2 to −1.3. The SMF displays a low-mass-end slope of αS ≃ −1.2 to −1.35. These observed slopes are flatter than those predicted for dark matter halos in cosmological simulations, offering significant insights for models of galaxy formation and evolution.

Abstract:

We make use of the unprecedented depth, spatial resolution, and field of view of the Euclid Early Release Observations (EROs) of the Perseus galaxy cluster to detect and characterise the dwarf galaxy population in this massive system. Using a dedicated annotation tool, the Euclid high-resolution VIS and combined VIS+Near Infrared Spectrometer and Photometer (NISP) colour images were visually inspected and dwarf galaxy candidates were identified. Their morphologies, the presence of nuclei, and their globular cluster (GC) richness were visually assessed richness were visually assessed, complementing an automatic detection of the GC candidates. Structural and photometric parameters, including Euclid filter colours, were extracted from two-dimensional fitting. Based on this analysis, a total of 1100 dwarf candidates were found across the image; 606 of these appear to be new identifications. The majority (96%) are classified as dwarf ellipticals, 53% are nucleated, 26% are GC-rich, and 6% show disturbed morphologies. A relatively high fraction of galaxies, 8%, are categorised as ultra diffuse galaxies. The majority of the dwarfs follow the expected scaling relations of galaxies. Globally, the GC specific frequency, SN, of the Perseus dwarf candidates is intermediate between those measured in the Virgo and Coma clusters. While the dwarf candidates with the largest GC counts are found throughout the Euclid field of view, the dwarfs located around the east–west strip, where most of the brightest cluster members are found, exhibit higher SN values on average. The spatial distribution of the dwarfs, GCs, and intracluster light show a main iso-density and isophotal centre displaced to the west of the bright galaxy light distribution. The ERO imaging of the Perseus cluster demonstrates the unique capability of Euclid to concurrently detect and characterise large samples of dwarf galaxies, their nuclei, and their GC systems, allowing us to construct a detailed picture of the formation and evolution of galaxies over a wide range of mass scales and environments.