Planetary nebulae populations in the haloes of nearby massive early-type galaxies

TL;DR

This work addresses how massive early-type galaxies assemble their haloes and intra-group light (IGL) by using planetary nebulae (PNe) as discrete tracers in low-surface-brightness regions. It combines the ePN.S kinematic survey with deep narrow-band PN imaging to measure the $\alpha$-parameter and the PNLF slope $c_2$ across halos and IGL, enabling a LOSVD-based decomposition into distinct stellar components. Key results include a direct link in M105 between a high $\alpha$-parameter and a metal-poor IGL, and in M49 the discovery of multiple PN populations with kinematics indicating an outer IGL component, together with constraints on blue IGL formation channels. These findings support a two-phase halo formation scenario and place quantitative constraints on the early accretion of metal-poor systems, while outlining a path for future PN-based studies with next-generation instrumentation and simulations. $\alpha$-parameter and PNLF slope variations are central to interpreting halo and IGL assembly, and the work highlights the role of PNe as tracers across a broad range of galactic environments.

Abstract

Planetary nebulae (PNe) are excellent tracers of the metal-poor haloes of nearby early-type galaxies. They are commonly used to trace spatial distribution and kinematics of the halo and intracluster light at distances of up to 100 Mpcs. The results on the early-type galaxy M105 in the Leo I group represent a benchmark for the quantitative analysis of halo and intragroup light. Since the Leo I group lies at just a 10 Mpc distance, it is at the ideal location to compare results from resolved stellar populations with the homogeneous constraints over a much larger field of view from the PN populations. In M105, we have -- for the first time -- established a direct link between the presence of a metal-poor halo as traced by resolved red-giant branch stars and a PN population with a high specific frequency ($α$-parameter). This confirms our inferences that the high $α$-parameter PN population in the outer halo of M49 in the Virgo Cluster traces the metal-poor halo and intra-group light.

Figures (5)

• Figure 1: Overview of methods and tools used to study extragalactic PN populations. In this proceeding, we focused on photometry, i.e. deep [OIII] narrow-band surveys and spectroscopy, with the goal of obtaining the PN velocities.
• Figure 2: Variation of the M49 $\alpha$-parameter with galaxy colour 2013apj...764l..20m in comparison with data from other galaxies collated in 2006mnras.368..877b. The colour-$\alpha$-parameter profile for M49 is based on the two-component model discussed in Sect. \ref{['sec:m49-phot']} and extrapolated beyond the radial range of the colours measured by 2013apj...764l..20m assuming a linear colour gradient in the inner halo and a constant colour for the IGL. Adapted from 2017aa...603a.104h.
• Figure 3: Line-of-sight velocity dispersion profile as a function of major-axis radius for all PNe (black), and the bright (blue) and faint (red) subsamples. Stellar kinematics derived from 2018mnras.473.5446v are denoted by the dashed line and those of dwarf galaxies in the Virgo Subcluster B 2014apjs..215...22k by grey error bars. Adapted from 2018AA...616A.123H.
• Figure 4: Observed tracer profiles and models in the halo of M105. Green dots with errorbars denote the PN number density scaled to observed $B$-band surface brightness via the $\alpha$-parameter measured in the inner halo. The best fit two-component model (including a high $\alpha$-parameter halo component, see text) and its uncertainty are indicated by the solid green line. Stars show the integrated surface brightness profile from 2014apj...791...38w, red squares [blue triangles] the number density of metal-rich [metal-poor] RGB stars from 2016apj...822...70l. The RGB number density profile (solid grey line) is decomposed into an exponential profile (dashed blue line) for the metal-poor ([M/H] $<-1$), and two Sérsic profiles for the intermediate-metallicity ($\-1\leq$[M/H]$<-0.5$, dotted blue line) and metal-rich ([M/H]$\geq-0.5$, solid red line) populations. Adapted from 2020AA...642A..46H.
• Figure 5: Line-of-sight velocity dispersion profiles in the halo of M105. Black squares denote absorption-line kinematics from 2009mnras.398..561w, black circles the PN velocity dispersion based on the central PN.S pointing 2009mnras.395...76d, grey circles the more extended data from 2022AA...663A..12H, and purple and yellow symbols the decomposition into envelope and halo kinematics. Blue symbols on the right panel denote the dwarf galaxy kinematics derived from data compiled by 2018AA...615A.105M. The vertical lines denote the kinematic transition from rotating core to inner halo already identified in 2018AA...618A..94P, and from the inner halo to the exponential envelope (grey dotted). The latter transition radius agrees with the inner photometric transition radius identified independently by 2022FrASS...952810R. We also show the outer photometric transition radius that 2022FrASS...952810R interpret as the photometric transition to the IGL (dashed grey line).