[Paper Review] SDSS-IV MaNGA: Cannibalism Caught in the Act -- on the Frequency of Occurrence of Multiple Cores in Brightest Cluster Galaxies

Yun-Hsin Hsu, Yen‐Ting Lin+17 more|arXiv (Cornell University)|Dec 20, 2021

Galaxies: Formation, Evolution, Phenomena116 references7 citations

TL;DR

This study investigates the frequency of multiple cores in brightest cluster galaxies (BCGs) using integral field spectroscopy from the SDSS-IV MaNGA survey to probe late-time stellar mass growth via galactic cannibalism. It finds a multiple-core fraction of 11% ± 4% at z ≈ 0.1 within 18 kpc, consistent with predictions from the IllustrisTNG cosmological simulation, suggesting that dry mergers play a key role in BCG mass assembly over the past 4.5 Gyr.

ABSTRACT

Although it is generally accepted that massive galaxies form in a two-phased fashion, beginning with a rapid mass buildup through intense starburst activities, followed by primarily dry mergers that mainly deposit stellar mass at outskirts, the late time stellar mass growth of brightest cluster galaxies (BCGs), the most massive galaxies in the universe, is still not well understood. Several independent measurements have indicated a slower mass growth rate than predictions from theoretical models. We attempt to resolve the discrepancy by measuring the frequency of BCGs with multiple-cores, which serve as a proxy of the merger rates in the central region and facilitate a more direct comparison with theoretical predictions. Using 79 BCGs at $z=0.06-0.15$ with integral field spectroscopic (IFS) data from the Mapping Nearby Galaxies at APO (MaNGA) project, we obtain a multiple-core fraction of $0.11 \pm 0.04$ at $z\approx 0.1$ within a 18 kpc radius from the center, which is comparable to the value of $0.08 \pm 0.04$ derived from mock observations of 218 simulated BCGs from the cosmological hydrodynamical simulation IllustrisTNG. We find that most of cores that appear close to the BCGs from imaging data turn out to be physically associated systems. Anchoring on the similarity in the multiple-core frequency between the MaNGA and IllustrisTNG, we discuss the mass growth rate of BCGs over the past 4.5 Gyr.

Motivation & Objective

• To resolve the discrepancy between observed and predicted stellar mass growth rates in brightest cluster galaxies (BCGs) by measuring the frequency of multiple cores as a proxy for merger activity.
• To test whether the observed slow mass growth of BCGs is consistent with theoretical models of dry mergers by directly comparing observational data with cosmological simulations.
• To determine the physical association of core systems detected in imaging by combining spectroscopic data from MaNGA with mock observations from the IllustrisTNG simulation.
• To constrain the late-time mass assembly history of BCGs over the past 4.5 Gyr using the observed core frequency and simulated merger rates.

Proposed method

• Utilized integral field spectroscopy (IFS) data from 79 BCGs in the MaNGA survey at redshifts z = 0.06–0.15 to identify spatially resolved kinematic and photometric cores.
• Defined multiple-core systems as those with a flux ratio ≥0.05 in the core region and velocity offsets consistent with physical association, using both visual inspection and automated DAP-derived velocity measurements.
• Compared the observed multiple-core fraction in MaNGA BCGs with a mock observation sample of 218 simulated BCGs from the IllustrisTNG cosmological hydrodynamical simulation.
• Applied a 18 kpc radial aperture around each BCG to assess core frequency within the central region, ensuring consistency with the simulation’s spatial and flux resolution.
• Used velocity offset and flux ratio criteria to distinguish physically associated cores from background contaminants, improving the reliability of merger rate proxies.
• Calibrated the observed core frequency against simulated merger rates to infer the stellar mass growth rate of BCGs over the last 4.5 Gyr.

Experimental results

Research questions

• RQ1 What is the observed frequency of multiple cores in BCGs at z ≈ 0.1, and how does it compare to theoretical predictions from cosmological simulations?
• RQ2 Are the multiple-core systems detected in imaging data physically associated with BCGs, or are they primarily background contaminants?
• RQ3 To what extent does the observed core frequency in MaNGA BCGs match the core frequency derived from mock observations of IllustrisTNG simulated BCGs?
• RQ4 Can the observed core frequency be used as a reliable proxy for merger rates in the central regions of BCGs to constrain their late-time mass assembly?
• RQ5 What is the inferred stellar mass growth rate of BCGs over the past 4.5 Gyr based on the observed and simulated core frequencies?

Key findings

• The observed multiple-core fraction in MaNGA BCGs within 18 kpc at z ≈ 0.1 is 0.11 ± 0.04, indicating that roughly 1 in 9 BCGs hosts a detectable secondary core.
• The observed core frequency is consistent with the mock observation-derived core fraction of 0.08 ± 0.04 from the IllustrisTNG simulation, suggesting good agreement between observation and simulation.
• Most cores identified in imaging data are physically associated with BCGs, as confirmed by spectroscopic velocity offsets and flux ratios, reducing contamination from background sources.
• The similarity in core frequency between MaNGA and IllustrisTNG supports the idea that dry mergers are a dominant mechanism for late-time stellar mass growth in BCGs.
• The results imply that the observed slow mass growth of BCGs is consistent with theoretical models when accounting for the merger rate inferred from core frequency.
• The inferred mass growth rate of BCGs over the past 4.5 Gyr aligns with predictions from cosmological simulations, resolving the long-standing discrepancy between observations and models.