Euclid: A blue galaxy population and a brightest cluster galaxy in the making in a $z\sim1.74$ MaDCoWS2 galaxy cluster candidate

Abstract

We present an example cluster follow-up study with Euclid. Our target, a $z\sim 1.74$ candidate cluster nicknamed the `Puddle', was initially discovered by the Massive and Distant Clusters of WISE Survey 2 (MaDCoWS2) as a $z_{phot}\sim 1.65$ candidate cluster. It was also detected independently as a $z_{phot}\sim 1.5$ candidate with both cluster-finding algorithms in Euclid Quick Release 1 (Q1). A Keck MOSFIRE spectrum shows the brightest nucleus is at $z=1.74$ and is AGN-dominated. We focus our analysis on the galaxy population and the Brightest Cluster Galaxy (BCG), using a combination of Euclid and ancillary photometry. Compared to similar fields, we measure an overdensity of $110\pm 14$ galaxies with $H_\mathrm{E}\leq 22.25$ in a 2' radius around the BCG. We estimate that $18\pm 4$% of the completeness-corrected galaxy population is red, which is consistent with some clusters at $z>1.5$ but lower than others. \textit{Euclid} imaging reveals that six or seven galaxies appear to be assembling to form the future BCG. Spectral energy distribution (SED) fitting suggests that the merging BCG has a stellar mass of $5.7\pm 0.3\times 10^{11}\,M_\odot$ and experienced a short burst of star formation about $300\,$Myr ago. Its morphology, stellar mass, and star-formation history suggest that the proto-BCG is a more evolved version of the merging core of SPT2349$-$56. These systems indicate that multiobject mergers might be a common BCG formation process. Assuming a similar density of mergers in the Euclid Wide Survey, we expect that Euclid will discover approximately 400 assembling BCGs by the end of its mission.

Figures (7)

• Figure 1: Left: A $\ang{;2} \times \ang{;2}$ tricolor image (, , and ) of the Puddle cluster. The position of the MaDCoWS2 detection is indicated by a light green 'X'. The coordinates of the Puddle cluster in Q1-SP050 are indicated by crosses: light green for the PZWav coordinates and pink for the AMICO coordinates. MaDCoWS2 most likely photometric members (galaxies for which the integrated probability density function between $z_\mathrm{phot}-\sigma_z$ and $z_\mathrm{phot}+\sigma_z$ is equal or greater than 0.3) are circled in mauve. The white box indicates the $\ang{;0.3} \times \ang{;0.3}$ field of view of the zoom-in in the right panel. Right: View of the BCG complex in the Puddle cluster. The white contours indicate the edges of the aperture used to measure the photometry. The white dashed box shows the approximate position of the spectroscopic slit. The centre of the MIPS $24\,\mathrm{\micron}$ emission is indicated by a black cross.
• Figure 2: The $z-\HE$ CMD diagram for a $\ang{;2}$ region centred on the BCG. Only the galaxies with $S\slash N>3\,\sigma$ in $z$ and bands are shown. The expected location of the red sequence at $z = 1.74$ is shown in pink, while the red sequences at $z = 1.4$, $z = 1.7$, and $z = 2.0$ are in grey. The photometry of the BCG complex is indicated by a cyan symbol. The solid purple line indicates the 50% combined $z$ and completeness limit.
• Figure 3: Left: Computation of the completeness correction for the $z$ band. The galaxy counts in the field are in green in the region where it is considered complete and black elsewhere. The purple line indicates the fit used to compute the completeness correction. Right: Same for the band.
• Figure 4: Bottom left: Field density-subtracted, completeness-corrected CMD. Each colour encodes a galaxy overdensity (purple) or underdensity (orange), expressed in count per arcminute square. We restrict the magnitude to $\HE \leq 22.25$. As in Fig. \ref{['fig_cmd']}, the locations of the red sequence at different redshifts are indicated by grey lines, and the location at $z = 1.74$ is indicated in pink. On the $z=1.74$ red sequence, $\HE= 22.25$ corresponds to $z=24.69$ and to a completeness level of 76%. The cyan line on the very top right on the panel indicates the 50% $z$ completeness limits and the dashed green line shows the location of the red sequence/blue cloud boundary. The proto-BCG photometry is marked by a cyan dot. Top Left: Count of member galaxies as a function of -band magnitude, after subtraction of the background level. The solid green and black step lines indicate the background-subtracted counts before and after the application of the completeness correction, while the shaded regions indicate count uncertainties. Bottom Right: Background-subtracted members count as a function of colour with and without the completeness correction.
• Figure 5: Top: Portion of the proto-BCG 2D spectrum from MOSFIRE, showing emission lines. The spectrum shows the entire slit height but only a part of the spectral range, centered on the emission lines. Middle: Same portion of the 1D spectrum. The blue and pink lines show possible decompositions into one and two H$\,\alpha$ emission lines respectively. Bottom: As in the middle panel, but presenting a third possible decomposition (in purple): H$\,\alpha$ with the [Nii] doublet on each side. The dotted lines trace the individual components of each fit. The shaded zones on the 1D spectra correspond to the regions with the 10% lowest weights in the fits, i.e. regions with subtracted sky lines. The fits are performed on the 1D spectrum.