Is The Lens SW05, A Fossil Group? (Astronomy)

Fossil galaxy groups, fossil groups, or fossil clusters are believed to be the end-result of galaxy merging within a normal galaxy group, leaving behind the X-ray halo of the progenitor group. Galaxies within a group interact and merge. The physical process behind this galaxy-galaxy merger is dynamical friction.

Now, Denzel and colleagues presented models for the system J143454.4+522850 (SW05) of the lensing and stellar mass content, followed by a comparison with simulations, which indicate several differences compared with regular early-type galaxies, suggesting the system may indeed be a fossil group.

“Its mass estimate fits the expectations of a massive elliptical galaxy, with a halo mass in the galaxy group range. Its stellar mass dominates the centre, and in total it makes up to 2.7% of the entire mass budget. These results, along with a lack of nearby bright galaxies suggest that SW05 is a fossil group.”

— told Denzel, first author of the study.

J143454.4+522850 or SW05 was discovered in the SpaceWarps citizen-science project. Out of the 29 promising lens candidates in that work, SW05 probably has the best lens image quality. It is a relatively large gravitational lens with four clearly separated images within a radial distance between 3.5 and 5.25 arcsec from the centre (as shown in Fig. 1).

Figure 1. Stacked observational picture of SW05: Observational data from CFHTLS (stored in the CFHT Science Archive) was taken with the wide-field imager MegaPrime in five optical bands (u, g, r, i, and z). The false-colour image was generated using a stacking procedure, where the i, r, and g bands are transformed into rgb colours. © Denzel et al.

Denzel and colleagues combined gravitational lensing with stellar population-synthesis to separate the total mass of the lens into stars and dark matter. They contrasted enclosed mass profiles with state-of-the-art galaxy formation simulations, to conclude that SW05 is likely a fossil group with a high stellar to dark matter mass fraction (0.027±0.003) with respect to expectations from abundance matching (0.012±0.004), indicative of a more efficient conversion of gas into stars in fossil groups.

Figure 2. Cumulative mass profiles of the SW05 lensing galaxy models and galaxies from MassiveFIRE simulations (A1, A4, C1, D7). The solid curves (lens model in dark-blue) denote the ensemble median of the enclosed total mass. The dashed curves (lens model in dark-blue) give the ensemble median of the enclosed stellar mass. The grey areas show the 99.7 % confidence range of the SW05 lens models. The enclosed lens mass is best constrained at the radial location of the images of the background source (vertical thin dotted lines). © Denzel et al.

“Our lens models indicate that there is no other major group member in the neighbourhood of SW05, supporting our conclusion. This explanation also seems to fit with the discrepancies found in the comparison with numerical simulations.”

— wrote authors of the study in their paper.

Featured image: a false-colour image enhancing the candidate lensed images. © Denzel et al.

Reference: Philipp Denzel, Onur Çatmabacak, Jonathan P. Coles, Claude Cornen, Robert Feldmann, Ignacio Ferreras, Xanthe Gwyn Palmer, Rafael Küng, Dominik Leier, Prasenjit Saha, Aprajita Verma, “The lens SW05 J143454.4+522850: a fossil group at redshift 0.6?”, ArXiv, pp. 1-8, 2021.

Copyright of this article totally belongs to our author S. Aman. One is allowed to reuse it only by giving proper credit either to him or to us

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