Signatures of Stellar Accretion in MaNGA Early-type Galaxies

Abstract

The late assembly of massive galaxies is thought to be dominated by stellar accretion in their outskirts (beyond 2 effective radii R e ) due to dry, minor galaxy mergers. We use observations of 1010 passive early-type galaxies (ETGs) within z < 0.15 from SDSS IV MaNGA to search for evidence of this accretion. The outputs from the stellar population fitting codes FIREFLY, pPXF, and Prospector are compared to control for systematic errors in stellar metallicity (Z) estimation. We find that the average radial logZ/Z profiles of ETGs in various stellar mass (M * ) bins are not linear. As a result, these profiles are poorly characterized by a single gradient value, explaining why weak trends reported in previous work can be difficult to interpret. Instead, we examine the full radial extent of stellar metallicity profiles and find them to flatten in the outskirts of M * 10 11 M ETGs. This is a signature of stellar accretion. Based on a toy model for stellar metallicity profiles, we infer the ex-situ stellar mass fraction in ETGs as a function of M * and galactocentric radius. We find that ex-situ stars at R∼2R e make up 20% of the projected stellar mass of M * 10 10.5 M ETGs, rising up to 80% for M * 10 11.5 M ETGs. 5 pPXF

Old and new major mergers in the SOSIMPLE galaxy, NGC 7135

“…The figure shows a clear preference for ex-situ material to be located at the outer edges of the galaxy, with no detectable ex-situ material in the centre of the galaxy. This is akin to simulated results showing the same preference for ex-situ fraction increase with galactocentric radius (Schaye et al 2014;Crain et al 2015;Rodriguez-Gomez et al 2016;Davison et al 2020), as well as observational studies showing the same principles (Forbes et al 2011;Pillepich, Madau & Mayer 2015;Oyarzún et al 2019). The mean ex-situ fraction measured for NGC 7135 at approximately 0.6 effective radii (the greatest extent captured by the MUSE image) is 7 per cent.…”

SDSS-IV MaNGA: Global and local stellar population properties of elliptical galaxies

“…Regarding the dependence of the metallicity gradient on mass, previous observational results are in disagreement (see for a review Oyarzún et al 2019). We find that the gradient slopes tend to become flattened as the CLEs are less massive, which agrees with the proposal that SN-driven winds lower the SF efficiency in galaxies with weaker gravitational potentials.…”

Mapping accreted stars in early-type galaxies across the mass–size plane

“…In comparison to Oyarzún et al (2019) who use stellar metallicity profiles to estimate the ex-situ fractions of MaNGA galaxies, results are reasonably well matched. The results in the Oyarzún et al (2019) similarly show an increase in ex-situ fraction with both mass and galactocentric radius, however the scatter in these values at >2r e essentially cover the entire ex-situ axis. Similar to population analyses in Edwards et al (2020) we find population gradients in essentially all the galaxies studied, occurring strongly with both age and metallicity.…”

“…This is in line with a two-phase formation scenario in which central galaxy regions are formed by relatively stable in-situ processes, and the outskirts are formed through far more stochastic accretion and so show a greater scatter in the surface mass density. This is also found to be the case in Oyarzún et al (2019) in which the authors find a flattening in the metallicity profile of z<0.15 early type galaxies beyond a radius of 1.5r e , and conclude the most reasonable explanation of this is stellar accretion to the galaxy outskirts. This result is also seen for samples of brightest cluster galaxies (Edwards et al 2020) who likewise find signatures of the two-phase scenario in profiles of kinematics and metallicity.…”

“…Further population based evidence comes from the MaNGA survey (Goddard et al 2017;Zheng et al 2017;Li et al 2018). In Oyarzún et al (2019) the authors find variable metallicity gradients and flattening in the outskirts of MaNGA galaxies, suggesting that this is a sign of stellar accretion, and these features are also found in simulated environments (see e.g. Taylor & Kobayashi 2017).…”