Why do many early-type galaxies lack emission lines? I. Fossil clues

Abstract

Early-type retired galaxies (RGs, i.e. galaxies which no longer form stars) can be divided into two classes: those with no emission lines, here dubbed lineless RGs, and those with emission lines, dubbed liny RGs. Both types of galaxies contain hot lowmass evolved stars (HOLMES) which emit ionizing photons. The difference must thus lie in the presence or absence of a reservoir of ionizable gas. From a volume-limited sample of 38 038 elliptical galaxies, we explore differences in physical properties between liny and lineless using data from the SDSS, WISE and GALEX catalogues. To avoid biases in the comparison, we pair-match liny and lineless in stellar-mass, redshift and half-light Petrosian radius. We detect marginal differences in their optical stellar ages and NUV luminosities, indicating that liny RGs have an excess of intermediateage (0.1-5 Gyr) stellar populations. Liny RGs show higher dust attenuation and W 3 luminosities than their lineless counterparts. We also find that the amount of warm gas needed to explain the observed Hα luminosity in liny RGs is 10 5 -10 8 M , and that their [N ii]/[O ii] emission-line ratios are typical of those of the most massive star-forming galaxies. Taken together, these results rules out the following sources for the warm gas in liny RGs: mass-loss from intermediate-mass stars, mergers with metal-poor galaxies and intergalactic streams. They imply instead an inflow of enriched gas previously expelled from the galaxy or a merger with a metal-rich galaxy. The ionization source and the origin of the gas producing the emission lines are thus disconnected.

Clues on the history of early-type galaxies from SDSS spectra and GALEX photometry

“…7 are comparable to the ones of In the top panel, we plot only galaxies with S/N > 3 in Hα, while in the bottom panel we exclude 3 galaxies for which there is no WISE data. Herpich et al (2018), who find that, at all stellar masses, ETGs with emission lines have larger A V and redder W2−W3 when compared to ETGs without emission lines. We have shown that ETGs with young components have more ionised gas than other ETGs; however, the source of this ionisation is not clear.…”

“…The molecular gas fractions are found to be related to both star-formation histories (Young et al 2014) and kinematics (Cappellari et al 2013), in that galaxies with rotating stellar disks have larger gas fractions. Ionised gas can also be detected in the ISM of ETGs of all masses (Herpich et al 2018). In this section, we will investigate the connection between the presence or absence of young components and the ISM properties of ETGs.…”

“…Welch et al 2010, Agius et al 2013, Janowiecki et al 2020, see also Knapp 1999), and the presence of molecular gas have been associated with younger ages (Young et al 2014). Herpich et al (2018) have investigated differences between ETGs with and without emission lines. The authors have shown that the galaxies where emission lines are detected also have more 12 µm emission (WISE W3 band), attributed to the emission from the policyclic aromatic hydrocarbon (PAH) powered by a hard ionizing field produced by hot low-mass evolved stars (HOLMES; Stasińska et al 2008;Flores-Fajardo et al 2011).…”

UV bright red-sequence galaxies: how do UV upturn systems evolve in redshift and stellar mass?

“…with its classes, according to the description by Cid Fernandes et al (2011), which in brief segragates galaxies into 5 groups: star-forming (SF), strong and weak AGN (sAGN and wAGN), retired galaxies (RG), and passive (lineless) galaxies, as follows: Differently from the BPT chart, WHAN enables us to spot the retired and some of the passive population, which is suitable for this investigation. Additionally, a number of objects cannot be detected in neither of them, mostly due to the simple fact that our sample is focused on RSGs, hence including a sizeable amount of lineless galaxies (Herpich et al 2018); nevertheless, this does not guarantee that all of these objects are, in fact, quiescent, for they can be unclassified due to other unknown issues (e.g. low S/N) -these systems are tagged as 'unclassified'.…”

“…The presence of AGN could also contribute to the UV flux (e.g. Cid Fernandes et al 2000;Heinis et al 2016;Padovani et al 2017) in red-sequence galaxies (RSGs); those are often classified as low-ionisation nuclear emission-line region (LINERs): their ionisation mechanisms are historically attributed to AGN activity -which has been strongly questioned, for many of such galaxies seem to be ionised by hot low-mass evolved (post-main sequence) stars (AKA HOLMES, Cid Fernandes et al 2011;Singh et al 2013;Belfiore et al 2016;Padovani et al 2017;Herpich et al 2018). On the other hand, it seems that AGN activity contributes only to a tiny part of the far-UV emission in systems with very bright nuclei (e.g.…”