Euclid: Galaxy morphology and photometry from bulge-disc decomposition of Early Release Observations

TL;DR

This work jointly models the brightness distributions of background galaxies in Euclid’s Perseus Early Release Observations with both single-Sérsic and bulge–disc decompositions, quantifying how morphology-driven biases affect sizes, colours, and photometry. By leveraging SourceXtractor++ and multi-band fits, the authors show that single-Sérsic radii are intermediate between bulge and disc scales and that bulge–disc decomposition provides more reliable measurements of bulges and discs, including rest-frame colours and colour gradients up to redshifts of $z\lesssim0.54$. They document a robust bulge–disc colour dichotomy and disc colour gradients, interpret these in terms of stellar population and dust effects, and highlight implications for cosmic shear and spin-alignment analyses in Euclid. The study underscores the necessity of multi-component morphological modelling for accurate galaxy characterization in large-area, high-resolution surveys and lays groundwork for forward-modeling approaches in future Euclid data releases.

Abstract

The background galaxies in Euclid Early Release Observations images of the Perseus cluster make up a remarkable sample in its combination of 0.57 deg$^2$ area, 25.3 and 23.2 AB mag depth, as well as 0.1" and 0.3" angular resolutions, in optical and near-IR bands, respectively. Towards characterising the history of the Hubble sequence, we perform a morphological analysis of 2445 and 12,786 galaxies with $I_E < 21$ and $I_E < 23$, respectively. We use single-Sérsic profiles and the sums of a Sérsic bulge and an exponential disc to model these galaxies with SourceXtractor++ and analyse their parameters in order to assess their consistencies and discrepancies. The fitted galaxies to $I_E < 21$ span the various Hubble types with ubiquitous bulge and disc components, and a bulge-to-total light ratio B/T taking all values from 0 to 1. The effective radius of the single-Sérsic profile is an intermediate estimate of galaxy size, between the bulge and disc effective radii, depending on B/T. The axis ratio of the single-Sérsic profile is higher than the disc axis ratio, increasingly so with B/T. The model impacts the photometry with -0.08 to 0.01 mag median systematic $I_E$ offsets between single-Sérsic and bulge+disc total magnitudes, and a 0.05 to 0.15 mag dispersion, from low to high B/T. We measure a median 0.3 mag bulge-disk colour difference in rest-frame $M_g - M_i$ that originates from the disc-dominated galaxies, whereas bulge-dominated galaxies have similar median colours of their components. Remarkably, we also measure redder-inside disc colour gradients, based on 5 to 10% systematic variations of disc effective radii between the optical and near-IR bands. This analysis demonstrates the usefulness and limitations of single-Sérsic profile modelling, and the power of bulge-disc decomposition for reliably characterising the morphology of lenticulars and spirals in Euclid images.

Figures (37)

• Figure 1: Distribution of the angular radii corresponding to the isophotal areas calculated by SourceXtractor++ on all objects identified as galaxies in the ERO-Perseus field by EROPerseusOverview versus their SExtractor's MAG_AUTO magnitude. All $38\,032$ modelled sources with $\IE\le24.5$ are plotted, as described in Sect. \ref{['sct-source-selection']}.
• Figure 2: magnitude distribution of the $212\,975$ sources classified as galaxies in the ERO-Perseus field by EROPerseusOverview and labelled as "Detected", compared to the $38\,082$ galaxies fit with the single-Sérsic profile and the bulge-disc decomposition using SourceXtractor++, labelled as "Modelled". The magnitudes of the Detected objects are from the photometric redshift catalogue based on MAG_AUTO photometry using SExtractor, whereas those for the "Modelled" objects are auto_mag photometry calculated with SourceXtractor++.
• Figure 3: Ratios of the disc-to-bulge effective radii as a function of the ratios of the disc-to-single-Sérsic effective radii, all in the band, for the $2445$ galaxies with $\IE\le21$. In the upper vertical concentration of disc-dominated galaxies (in blue, $B/T\approx0$) and on the right diagonal concentration of bulge-dominated galaxies (in red, $B/T\approx1$), the single-Sérsic effective radius is consistent with that of either a dominating disc or bulge component. The single-Sérsic effective radius of the $74.7\%$ of galaxies in the top-right cone is intermediate between the disc effective radius (the largest) and the bulge effective radius (the smallest), with a smooth gradient in the ratio of the disc to single-Sérsic effective radius, while $B/T$() varies from zero to one. These galaxies are visually indistinguishable from the types in the present-time Hubble sequence, whereas objects in other regions of the diagram are identified as either non-physical bulge-disc modelling, or biased bulge fits due to bars.
• Figure 4: Correlation between the disc-to-single Sérsic ratio of effective radii relative to the disc-to-bulge ratio of radii (in logarithmic scale) and $B/T$ in the band, for the $1826$ galaxies with $\IE\le21$, verifying $R_\mathrm{e,bulge} < R_\mathrm{e,1p} < R_\mathrm{e,disc}$.
• Figure 5: Distances between the centres of the single-Sérsic profile and those of the disc and bulge components normalised by the effective radius of the single-Sérsic component $R_\mathrm{e,1p}$ (left), and distances between the bulge and disc centres, normalised by the bulge and disc effective radii (right). Points are colour-coded by $B/T$() of the bulge-disc decomposition for the $2445$ ERO-Perseus field galaxies with $\IE\le21$. As expected, galaxies with a dominant bulge or disc have this component closer than the other one to the centre of the single-Sérsic profile.