MNRAS 539, 1471–1479 (2025) https://doi.org/10.1093/mnras/staf514 Advance Access publication 2025 March 31 INSPIRE: INvestigating Stellar Population In RElics VIII. Emission lines and UV colours in ultracompact massi v e galaxies Chiara Spiniello , 1 ‹ Mario Radovich , 2 Anna Ferr ´e-Mateu , 3 , 4 Roberto De Propris , 5 , 6 Magda Arnaboldi, 7 Francesco La Barbera, 8 Johanna Hartke , 5 , 9 Giuseppe D’Ago , 10 Crescenzo Tortora , 8 Davide Be v acqua , 11 , 12 Michalina Maksymo wicz-Maciata, 13 John Mills, 1 Nicola R. Napolitano , 8 , 14 Claudia Pulsoni , 15 Paolo Saracco 11 and Diana Scognamiglio 16 1 Sub-Department of Astrophysics, Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK 2 INAF – Osservatorio astronomico di Padova, Vicolo Osservatorio 5, I-35122 Padova, Italy 3 Instituto de Astrof ´ısica de Canarias, V ´ıa L ´actea s/n, E-38205 La Laguna, Tenerife, Spain 4 Departamento de Astrof ´ısica, Universidad de La Laguna, E-38200 La Laguna, Tenerife, Spain 5 Finnish Centre for Astronomy with ESO (FINCA), University of Turku, FI-20014 Turku, Finland 6 Department of Physics and Astronomy, Botswana International University of Science and Technology, Private Bag 16, Palapye, Botswana 7 European Southern Observatory, Karl-Sc hwarzsc hild-Str aße 2, D-85748 Garching, Germany 8 INAF – Osservatorio Astronomico di Capodimonte, Via Moiariello 16, I-80131 Naples, Italy 9 T uorla Observatory , Department of Physics and Astronomy , University of T urku, FI-20014 T urku, Finland 10 Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK 11 INAF – Osservatorio Astronomico di Brera, via Brera 28, I-20121 Milano, Italy 12 DiSAT, Universit ´a degli Studi dell’Insubria, via Valleggio 11, I-22100 Como, Italy 13 Astr ophysics Gr oup, H. H. W ills Physics Laboratory , University of Bristol, Tyndall A venue, Bristol BS1 8TL, UK 14 Department of Physics E. Pancini, University Federico II, Via Cinthia 21, I-80126 Naples, Italy 15 Max-Planck-Institut f ¨ur extr aterrestrisc he Physik, Giessenbac hstr asse, D-85748 Garching, Germany 16 Jet Propulsion Laboratory, California Institute of Technology, 4800, Oak Grove Drive - Pasadena, CA 91109, USA Accepted 2025 March 26. Received 2025 March 26; in original form 2024 October 9 A B S T R A C T We report the disco v ery of emission lines in the optical spectra of ultracompact massive galaxies (UCMGs) from IN- SPIRE including relics, which are the oldest galaxies in the Universe. Emission-lines diagnostic diagrams suggest that all these UCMGs, independently of their star formation histories, are ‘retired galaxies’. They are inconsistent with being star- forming but lie in the same region of shock-driven emissions or photoionization models, incorporating the contribution from post-asymptotic giant branch (pAGB) stars. Furthermore, all but one INSPIRE objects have a high [O II ]/H α ratio, resembling what has been reported for normal-size red and dead galaxies. The remaining object (J1142 + 0012) is the only one to show clear evidence for strong active galactic nucleus activity from its spectrum. We also provide near -UV (far -UV) fluxes for 20 (5) INSPIRE objects that match in GALEX . Their NUV − r colours are consistent with those of galaxies lying in the UV green valley, but also with the presence of recently ( ≤ 0 . 5 Gyr) formed stars at the sub-per cent fraction level. This central recent star formation could have been ignited by gas that was originally ejected during the pAGB phases and then re-compressed and brought to the core by the ram-pressure stripping of Planetary Nebula envelopes. Once in the centre, it can be shocked and re-emit spectral lines. Key words: galaxies: elliptical and lenticular, cD – galaxies: evolution – galaxies: formation – galaxies: star formation – galaxies: stellar content. 1 R a c 2  2 f S l 2 t © P C p D ow nloaded from https://academ ic.oup.com /m nras/article/539/2/1471/8101535 by Turun Yliopiston Kirjasto user on 09 June 2025 I N T RO D U C T I O N elic galaxies (Trujillo et al. 2009 , 2014 ; Ferr ´e-Mateu et al. 2017 ) re the local descendants of high-redshift red nuggets that have ompletely missed the size-growth evolutionary phase (Daddi et al. 005 ; Trujillo et al. 2007 ; Buitrago et al. 2008 ; van Dokkum et al. E-mail: chiara.spiniello@physics.ox.ac.uk t i e 2025 The Author(s). ublished by Oxford University Press on behalf of Royal Astronomical Society. Th ommons Attribution License ( https:// creativecommons.org/ licenses/ by/ 4.0/ ), whic rovided the original work is properly cited. 008 ; Naab et al. 2014 ) and have e volved passi vely and undisturbed rom their first intense and fast high- z star formation (SF) burst. ince they are made almost e xclusiv ely of ‘in situ’ very old stars, ike the innermost regions of massive galaxies (e.g. La Barbera et al. 019 ; Barbosa et al. 2021 ) they provide a unique opportunity to rack the evolution of this stellar component, which is mixed with he accreted one in normal early-type galaxies (ETGs). They are the deal systems to investigate and understand the mass assembly in the arly Universe with the amount of detail currently available only foris is an Open Access article distributed under the terms of the Creative h permits unrestricted reuse, distribution, and reproduction in any medium, 1472 C. Spiniello M g o a r d o g T d a t R l m s n p t m s l i e T A f t b f t o t r S t w l w e 2 I R d 5 S i ( a S l F d 2 t i r g S i ( ( [ M c s I f s a w t r e t D e 3 I l f U b g e e ( c s w ( l a H e fi t W [ t o a p u v w 1 Publicly available through the ESO Phase 3 Science Archive 2 https:// lmfit.github.io/ lmfit-py/ D ow nloaded from https://academ ic.oup.com /m nras/article/539/2/1471/8101535 by Turun Yliopiston Kirjasto user on 09 June 2025alaxies in the local Univ erse. Moreo v er, since the number density f relics and its redshift evolution depends strongly on the processes cting during the size growth and how they are modelled, counting elics at low-z is an incredibly valuable way to disentangle between ifferent galaxy evolution models. Recent claims have reported the presence of a sub-per cent fraction f young stellar populations in the innermost region of very massive alaxies and bright cluster galaxies (Salvador-Rusi ˜ nol et al. 2021 ). his is the region where the pristine (i.e. in situ), oldest stars should ominate the light budget (Barbosa et al. 2021 ). Moreo v er, the same mount of younger stars ( ∼ 1 per cent ) have also been found in he most extreme relic in the local Universe, NGC 1277 (Salvador- usi ˜ nol et al. 2022 ), by fitting near -ultra violet (NUV) and optical ine-strength indices. Since NGC 1277 has not experienced any ergers or interactions with other galaxies, the presence of younger tars in its centre indicates that the gas for the formation of these ew stars must be associated with the galaxy itself, i.e. with intrinsic rocesses rather than external ones. Dopita et al. ( 2000 ) highlight hat the presence of gas and dust at the centres of passive galaxies ay originate from the mass-loss of evolved stars. According to their imulation and analysis, in high-pressure interstellar environments ike the centres of massive ETGs or UCMGs and relics, the material n the shocked shell of planetary nebulae (PNe) will cool and its xpansion rev ersed, causing a collapse into the denser central re gion. his will, in turn, recompress the dusty material ejected during the GB phase of stellar evolution and allow the cold dusty clouds to all intact (without being destroyed by the hot interstellar medium) owards the nucleus of the galaxy. During this process, the gas could e shocked, creating shock-driven emission lines and, in some cases, orm a small percentage of new stars, as in NGC 1277. The paper is organized as follows. In Section 2 we briefly describe he data used in this paper, highlighting the definition and selection f ultracompact massive galaxies (UCMGs) and the confirmation of heir relic nature. In Section 3 we analyse the emission lines, obtain atios, and plot diagnostic diagrams to investigate their origins. In ection 4 we focus on the UV, analysing the NUV − r colours. We hen discuss the findings and conclude in Section 5 investigating hich could be the possible source of both colour and emission ines. Throughout the paper, we assume a standard  CDM cosmology ith H 0 = 69.6 km s −1 Mpc −1 ,  = 0.714 and M = 0 . 286 (Bennett t al. 2014 ). T H E SAMPLE n this paper, we leverage the INvestigating Stellar Population In Elics ( INSPIRE ; Spiniello et al. 2021b , 2024 ; D’Ago et al. 2023 ; ) ata set. INSPIRE is based on an ESO Large Programme that targets 2 spectroscopically confirmed UCMGs at 0 . 1 < z < 0 . 4 with the X- hooter spectrograph (XSH; Vernet et al. 2011 ). These objects were nitially found in Tortora et al. ( 2016 ) from the Kilo Degree Survey KiDS; Kuijken 2011 ) third data release (DR3; de Jong et al. 2017 ), nd then spectroscopically confirmed in Tortora et al. ( 2018 ) and cognamiglio et al. ( 2020 ) (hereafter T18 and S20, respectively) with ow signal-to-noise (SNR) and medium-resolution optical spectra. or each galaxy, structural parameters and stellar masses have been erived from ugri KiDS photometry (Roy et al. 2018 ; Tortora et al. 018 ; Scognamiglio et al. 2020 ). All objects are UCMGs in the sense hat they are clear outliers in the stellar mass-size plane (see fig. 2 n Spiniello et al. 2021a ), having very small sizes (with ef fecti ve adii R e < 2 kpc) with respect to the o v erall population of passive alaxies with relatively large stellar masses ( M  > 6 × 10 10 M  ).NRAS 539, 1471–1479 (2025) tellar masses and sizes for the entire INSPIRE sample are provided n table 1 of Spiniello et al. ( 2024 , hereafter S24). From UVB-to-NIR high-resolution, medium signal-to-noise SNR) spectra, 1 we have measured the integrated stellar kinematics D’Ago et al. 2023 ), as well as stellar population age, metallicity, Mg/Fe] abundances, and the IMF slope (Mart ´ın-Navarro et al. 2023 ; aksymowicz-Maciata et al. 2024 ). Of these UCMGs, 38 have been lassified as relics, since they formed more than 75 per cent of their tellar masses already by z > 2 (Spiniello et al. 2021b , 2024 ). Hence, NSPIRE has enlarged by at least a factor of 5 the number of nearby ully confirmed relics, providing the first statistically significant ample of these objects. Here, we focus on optical emission lines nd UV colours, aiming at investigating on their sources and on hether a correlation exists between UV-light, emission lines, and he degree of relicness (DoR). The DoR is a dimensionless number anging from 0 (non-relic) to 1 (extreme relic), that quantifies how xtreme and peaked the SFH of these UCMGs is. We refer the reader o Spiniello et al. ( 2024 , hereafter S24) for more details on how the oR is computed. We use here the 52 INSPIRE UVB + VIS XSH spectra to measure mission lines, as described in the next section. EMISSION LI NES n roughly half of the spectra released in the third INSPIRE data re- ease (DR3, S24), we observe convincing evidence for emission lines rom [O II ] ( λ ∼ 3727 Å) and [N II ] ( λ ∼ 6583 Å) in the combined VB + VIS spectra. This is broadly consistent with the statistics, ased on a much larger sample of normal-sized galaxies ( ∼ 300 000 alaxies from SDSS DR4; Adelman-McCarth y et al. 2006 ), of Yan t al. ( 2006 ): 38 per cent of all red galaxies have detectable [O II ] mission. Weak emissions are also detected in [O III ] ( λ ∼ 5007 Å) and [S II ] λ ∼ 6720 Å). Finally, H β and H α also hav e v ery weak emission omponents in some cases, although they are contaminated by tellar absorption lines. No other emission lines are detected in the avelength range [2700–9500] Å. Unfortunately, the SNRs of most emission lines are generally low ≤ 10), with the exception of J1142 + 0012 (which will be discussed ater). Moreo v er, the continuum underlying [O II ] has many narrower bsorption features that could bias the measurement of the flux. ence, to compute the equi v alent width (EW) of the line and flux in mission, we first subtract the stellar component and then perform a t to the emission lines. To compute EWs and fluxes, we use a Python code, based on he LMFIT library, 2 that fits emission lines with Gaussian profiles. e also note that for [O II ]( λ3736,29), H α+ [N II ]( λ6548,83) and S II ]( λ6716,31), the fit is performed using several Gaussians simul- aneously. Ho we ver, in order to ensure that the results are based only n reliable measurements, we consider only lines with an SNR > 2 nd a FWHM > 1 Å. The fluxes of all emission lines that pass the above thresholds are rovided in the central block of columns in Table 1 , along with their ncertainties, in units of 10 −17 erg s −1 cm −2 Å−1 . We note that the elocity dispersion of the emission lines is similar to that of the stars, hich disfa v our an origin due to AGN or extreme shock wa ves. INSPIRE VIII. Emissions & UV colours in UCMGs 1473 MNRAS 539, 1471–1479 (2025) Ta bl e 1. Em iss io n lin es flu xe s fo r t he I N SP IR E ga la xi es . F or ea ch o bje ct, w e gi v e ID an d D oR (co mp ute d i n S2 4) in th e fir st bl oc k o f c o lu m ns , t he flu x w ith u n ce rt ai nt ie s o f a ll th e lin es w e u se in th is le tte r in th e se co n d bl oc k, an d G AL EX o bs er ve d m ag ni tu de s a n d th e N U V − r co lo ur in th e th ird bl oc k. M iss in g m ea su re m en ts ar e n o t p as sin g th e th re sh ol ds de sc rib ed in Se ct io n 3 . U ni ts fo r s pe ct ra ar e in 10 −1 7 er g s − 1 c m −2 Å− 1 . G A LA X Y D oR [O II ] H β [O II I ] H α [N II ] [S II ] FU V N U V N U V- r ID λ 37 27 + 37 29 λ 48 61 λ 50 07 λ 65 63 λ 65 83 λ 67 17 + 67 31 (m ag ) (m ag ) J0 21 1- 31 55 0. 72 0 . 72 ± 0 . 19 0 . 57 ± 0 . 21 0 . 50 ± 0 . 18 – – – – 23 .4 2 3. 64 J0 22 4- 31 43 0. 56 – – – – – – – – – J0 22 6- 31 58 0. 12 – – 1 . 21 ± 0 . 35 – – – – 23 .2 7 4. 02 J0 24 0- 31 41 0. 43 0 . 87 ± 0 . 21 0 . 72 ± 0 . 42 2 . 23 ± 0 . 73 – – – – – – J0 31 4- 32 15 0. 42 2 . 03 ± 0 . 21 0 . 59 ± 0 . 28 1 . 47 ± 0 . 46 1 . 09 ± 0 . 27 1 . 90 ± 0 . 31 1 . 34 ± 0 . 24 – – – J0 31 6- 29 53 0. 40 1 . 36 ± 0 . 19 – – – 1 . 55 ± 0 . 30 – – 22 .8 7 3. 21 J0 31 7- 29 57 0. 51 – – – – – – – – – J0 32 1- 32 13 0. 37 6 . 10 ± 0 . 34 1 . 03 ± 0 . 32 1 . 02 ± 0 . 34 3 . 40 ± 0 . 30 4 . 27 ± 0 . 34 – – – – J0 32 6- 33 03 0. 25 – 0 . 84 ± 0 . 28 – – – – – – – J0 83 8 + 00 52 0. 54 1 . 39 ± 0 . 18 – – – 1 . 18 ± 0 . 30 – – – – J0 84 2 + 00 59 0. 73 2 . 72 ± 0 . 22 2 . 10 ± 0 . 53 0 . 77 ± 0 . 34 1 . 24 ± 0 . 28 2 . 00 ± 0 . 32 – – – – J0 84 4 + 01 48 0. 45 1 . 32 ± 0 . 21 – 2 . 42 ± 0 . 68 1 . 26 ± 0 . 30 2 . 09 ± 0 . 36 – – 22 .2 6 2. 48 J0 84 7 + 01 12 0. 83 5 . 09 ± 0 . 25 0 . 94 ± 0 . 69 3 . 30 ± 0 . 97 3 . 18 ± 0 . 46 3 . 15 ± 0 . 48 4 . 72 ± 0 . 71 – 23 .4 0 4. 99 J0 85 7 − 01 08 0. 39 – – 0 . 83 ± 0 . 21 – – – – 24 .0 7 4. 86 J0 90 4 − 00 18 0. 32 – – – – – – 23 .1 7 22 .2 0 3. 09 J0 90 9 + 01 47 0. 79 4 . 74 ± 0 . 64 2 . 55 ± 0 . 76 – – – – – – – J0 91 7 − 01 23 0. 44 2 . 79 ± 0 . 23 0 . 69 ± 0 . 32 1 . 59 ± 0 . 29 3 . 17 ± 0 . 35 5 . 36 ± 0 . 41 4 . 43 ± 0 . 54 – – – J0 91 8 + 01 22 0. 43 – 0 . 83 ± 0 . 49 – – 1 . 86 ± 0 . 46 – – 24 .1 2 4. 99 J0 92 0 + 01 26 0. 25 – – 0 . 85 ± 0 . 29 – 0 . 72 ± 0 . 28 – – – – J0 92 0 + 02 12 0. 64 – – 3 . 03 ± 0 . 97 – – – 22 .7 7 22 .2 7 3. 40 J1 02 6 + 00 33 0. 29 4 . 17 ± 0 . 50 – 4 . 65 ± 1 . 77 – 4 . 11 ± 10 00 . 0 0 – 22 .9 2 21 .5 8 4. 19 J1 04 0 + 00 56 0. 77 6 . 05 ± 0 . 18 – 4 . 44 ± 0 . 38 2 . 62 ± 0 . 40 2 . 76 ± 0 . 42 3 . 89 ± 0 . 34 – – – J1 11 4 + 00 39 0. 40 – – – – – – – – – J1 12 8 − 01 53 0. 34 1 . 47 ± 0 . 32 – 2 . 28 ± 0 . 44 – – – – 23 .2 9 4. 73 J1 14 2 + 00 12 0. 18 46 . 0 5 ± 2 . 23 21 . 1 6 ± 0 . 76 17 3 . 99 ± 4 . 94 93 . 0 4 ± 4 . 27 16 2 . 70 ± 4 . 90 88 . 3 5 ± 3 . 59 21 .9 7 21 .3 4 4. 32 J1 15 4 − 00 16 0. 11 – – – – – – – 23 .7 1 4. 19 J1 15 6 − 00 23 0. 30 2 . 50 ± 0 . 24 1 . 18 ± 0 . 58 2 . 63 ± 0 . 79 – 2 . 10 ± 0 . 57 – – – – J1 20 2 + 02 51 0. 36 – – – – – – – – – J1 21 8 + 02 32 0. 45 3 . 47 ± 0 . 24 – 2 . 07 ± 1 . 94 1 . 66 ± 0 . 33 2 . 68 ± 0 . 38 – – 23 .2 3 4. 00 J1 22 8 − 01 53 0. 39 2 . 07 ± 0 . 29 0 . 91 ± 0 . 35 0 . 78 ± 0 . 36 – 1 . 36 ± 0 . 34 – – – – J1 40 2 + 01 17 0. 31 1 . 08 ± 0 . 14 1 . 10 ± 0 . 41 1 . 16 ± 0 . 42 1 . 14 ± 0 . 22 1 . 33 ± 0 . 24 – – – – J1 41 1 + 02 33 0. 41 1 . 54 ± 0 . 22 – – – – – – – – J1 41 2 − 00 20 0. 61 – – – – – – – 22 .5 4 3. 35 J1 41 4 + 00 04 0. 36 3 . 09 ± 0 . 34 1 . 20 ± 0 . 91 1 . 41 ± 0 . 32 1 . 71 ± 0 . 32 3 . 85 ± 0 . 38 – – – – J1 41 7 + 01 06 0. 33 – – 0 . 65 ± 0 . 38 – – – – – – J1 42 0 − 00 35 0. 41 – – 1 . 07 ± 0 . 59 – – 2 . 39 ± 0 . 55 – – – J1 43 6 + 00 07 0. 33 4 . 15 ± 0 . 35 – – 2 . 75 ± 0 . 33 4 . 21 ± 0 . 36 2 . 43 ± 0 . 46 – – – J1 43 8 − 01 27 0. 78 0 . 83 ± 0 . 15 0 . 65 ± 0 . 28 1 . 64 ± 0 . 35 – 1 . 22 ± 0 . 28 – – – – J1 44 7 − 01 49 0. 38 4 . 94 ± 0 . 34 1 . 99 ± 0 . 65 3 . 05 ± 0 . 66 3 . 21 ± 0 . 35 4 . 75 ± 0 . 39 3 . 45 ± 0 . 48 – 23 .3 2 4. 71 J1 44 9 − 01 38 0. 60 – 0 . 65 ± 0 . 33 0 . 34 ± 0 . 14 0 . 52 ± 0 . 16 – – – – – D ow nloaded from https://academ ic.oup.com /m nras/article/539/2/1471/8101535 by Turun Yliopiston Kirjasto user on 09 June 2025 1474 C. Spiniello MNRAS 539, 1471–1479 (2025) Ta bl e 1 – co n tin ue d G A LA X Y D oR [O II ] H β [O II I ] H α [N II ] [S II ] FU V N U V N U V- r ID λ 37 27 + 37 29 λ 48 61 λ 50 07 λ 65 63 λ 65 83 λ 67 17 + 67 31 (m ag ) (m ag ) J1 45 6 + 00 20 0. 17 4 . 06 ± 0 . 20 1 . 06 ± 0 . 60 0 . 88 ± 0 . 37 2 . 10 ± 0 . 29 4 . 26 ± 0 . 35 – 23 .0 2 23 .1 8 3. 72 J1 45 7 − 01 40 0. 47 – – – – – – – 23 .5 3 4. 10 J1 52 7 − 00 12 0. 38 – – – – – – – – – J1 52 7 − 00 23 0. 37 2 . 26 ± 0 . 21 – – – 2 . 51 ± 0 . 62 – – 23 .9 9 4. 35 J2 20 2 − 31 01 0. 48 2 . 66 ± 0 . 19 2 . 30 ± 0 . 79 2 . 28 ± 0 . 50 2 . 62 ± 1 . 00 3 . 23 ± 1 . 11 – – – – J2 20 4 − 31 12 0. 78 – 1 . 05 ± 0 . 37 0 . 44 ± 0 . 16 – – – – – – J2 25 7 − 33 06 0. 27 – – 0 . 59 ± 0 . 24 – – – – – – J2 30 5 − 34 36 0. 80 – – 0 . 45 ± 0 . 26 – – – – – – J2 31 2 − 34 38 0. 36 – – 0 . 53 ± 0 . 18 – – – – – – J2 32 7 − 33 12 0. 06 3 . 96 ± 0 . 14 – 2 . 57 ± 0 . 25 2 . 34 ± 0 . 28 4 . 65 ± 0 . 34 3 . 00 ± 0 . 73 – – – J2 35 6 − 33 32 0. 44 2 . 41 ± 0 . 15 1 . 27 ± 0 . 40 1 . 94 ± 0 . 39 1 . 03 ± 0 . 32 1 . 07 ± 0 . 32 – – 23 .2 7 3. 46 J2 35 9 − 33 20 0. 71 – – 2 . 79 ± 1 . 00 – – – – – – Figure 1. The distribution of INSPIRE galaxies in log H α – log [O II ] EWs, and colour-coded by DoR. Objects on the left side of the black dashed lines are classified as ‘High [O II ]/H α’. The only ‘Low [O II ]/H α’ galaxy from the INSPIRE sample is J1142 + 0122, which shows clear sign of AGN activity in its spectrum. Squares denote objects with a match in GALEX . l U 3 Y m f O t l l d a s t I t r f a s c t m l o 2 ( 3 D ow nloaded from https://academ ic.oup.com /m nras/article/539/2/1471/8101535 by Turun Yliopiston Kirjasto user on 09 June 2025In the following, we will make use of line-line plots and emission ine diagnostic to investigate the possible origin of emission in CMGs and relics. .1 [O II ]/H α bimodality an et al. ( 2006 , hereafter Y06) reported the disco v ery of a bi- odality in [O ii]/ H α ratio among ∼300 000 galaxies from the ourth SDSS Data Release (DR4; Adelman-McCarthy et al. 2006 ). ne mode is largely associated with star-forming galaxies, while he other mode contains galaxies with line ratios compatible with ow ionization nuclear emission-line regions (LINERs). Narrow- ine Seyferts and transition objects mostly fall in between the two ominant populations, consistent with the picture that both SF nd AGNs (or some other sources, e.g. pAGBs) might contribute ubstantially to the emission in these objects. In Fig. 1 , we reproduce he log ([O II ] EW ) – log (H αEW ) diagnostic plot used by Y06 for the NSPIRE galaxies 3 to check whether a bimodality exists also in his sample (i.e. with objects with higher DoR falling in the LINERs egion while non-relic UCMGs being more compatible with star- orming galaxies). All but one object are classified as’High [O II ]/H α’ galaxies, ccording to the threshold set by equation (6) in Y06. Indeed, Y06 howed that the bimodality in [O II ]/H α ratio echoes the galaxies’ olour bimodality and the great majority of red galaxies reside in he ‘High [O II ]/H α’ cate gory. Moreo v er, ev en though, in theory, SF odels can explain a high [O II ]/H α ratio, they would predict much ower [N II ]/[O II ] ratios (for solar or supersolar metallicities) than the ne observed for the INSPIRE sample (Gutkin, Charlot & Bruzual 016 ). The only system classified as ‘Low [O II ]/H α’ is J1142 + 0012 DoR = 0 . 18, i.e. non-relic). This is the only INSPIRE object that Note that, in Fig. 1 , we plot the EWs of the lines, rather than their fluxes. INSPIRE VIII. Emissions & UV colours in UCMGs 1475 Figure 2. The BPT diagrams with the classifying scheme (dashed lines, from left to right): SF demarcations from K e wley et al. ( 2001 ); Kauf fmann et al. ( 2003 ); SF (K e wley et al. 2001 ) and AGN/LINER (K e wley et al. 2006 ) demarcations; maximum SF value (Kewley, Geller & Jansen 2004 ). Top: The line ratios of INSPIRE points, colour-coded by the DoR, on top of predictions from SF models (grey dots). Squares are galaxies with a match in GALEX . Middle: Emission line ratios from the shock models by Alarie & Morisset ( 2019 ) with the parameters as described in the legend and in the text. Bottom: Emission line ratios from the MP23 photoionization models with parameters as shown in the legend. h s a s h u c 3 W P K g a a a v P t S ‘ c p L c 2 F e D D ow nloaded from https://academ ic.oup.com /m nras/article/539/2/1471/8101535 by Turun Yliopiston Kirjasto user on 09 June 2025as a spectrum consistent with a Seyfert 1 AGN, with broad and trong H α emission. This system will be further investigated in n upcoming publication. The three extreme relics for which a ecure measurement of the [O II ] and H α lines is possible seem to ave a higher log ([O II ] EW ) than objects with lower DoR. Ho we ver, nfortunately, we do not have a large enough sample to draw firm onclusions. .2 BPT and WHAN diagnostic diagrams e use the classical emission-line diagnostics diagram (Baldwin, hillips & Terlevich 1981 ; Rola, Terlevich & Terlevich 1997 ; auffmann et al. 2003 ) to further investigate on the possible source enerating emission lines in red UCMGs, including in those classified s relics (0 . 34 < DoR < 0 . 7) and extreme relics (DoR ≥ 0 . 7). Thispproach ef fecti vely dif ferentiates between a star-forming (SF) and n active galactic nucleus (AGN) origin by examining the ratios of arious emission lines. We start by producing the classical ‘Baldwin, hillips & Terlevich’ (BPT) diagrams (Baldwin et al. 1981 ). In hese diagrams, two main tracks of galaxies are visible and separate. tar-forming objects will dominate the left-bottom region, whereas AGN-like’ objects will lie on the right side. Traditionally, AGNs an then be further divided into subcategories occupying different ositions in the diagrams: Seyfert galaxies in the upper part and INERs towards the bottom region. Both theoretical and empirical lassifications have been reported in the literature (e.g. K e wley et al. 001 , 2004 , 2006 ; Kauffmann et al. 2003 ). In the upper panel of ig. 2 we show these diagrams for the INSPIRE objects where mission lines are securely detected, again colour-coded by their oR. The large uncertainty on the H β emission, mainly due to theMNRAS 539, 1471–1479 (2025) 1476 C. Spiniello M l t s w d S S l a f w c t a ( D u a w p w c p I s s c o c M m r d C e s ( a m t F s i o p a 4 5 r Figure 3. The WHAN diagram (Cid Fernandes et al. 2011 ) for the INSPIRE galaxies, colour-coded by their DoR. Squares are objects with a match in GALEX . Dashed lines separate SF from LINERs and AGNs. The only system consistent with being a LINER is J1142 + 0012. m s c  i r b e s h A s a ( g 2 i a r r o a e p D ow nloaded from https://academ ic.oup.com /m nras/article/539/2/1471/8101535 by Turun Yliopiston Kirjasto user on 09 June 2025ow SNR in the blue, prevents us from obtaining an estimate of he extinction from the Balmer decrement. For comparison, in the ame panel, we also show the line ratios produced by SF models ith an age ≤ 3 Myr, corresponding to ionization by OB stars (grey ots). Clearly, the INSPIRE objects are not well reproduced by F models, having a too high [N II ]/H α and [S II ]/H α ratios. The F models could still reproduce the observed [O II ]/H α ratio (up to og [OII] / H α = 0 . 32, the maximum value allowed by the SF models, s shown by the vertical dotted line in Fig. 2 ), but only if no correction or internal dust extinction is considered. For instance, H α/H β = 4 ould correspond to a correction of + 0.27. The UCMG line ratios an instead be reproduced by shock and/or photoionization models hat include the contribution of pAGB stars, as shown in the middle nd low panels of Fig. 2 . First, we consider the shock models from Alarie & Morisset 2019 ), available in the Mexican Million Database (3MdB: Morisset, elgado-Inglada & Flores-Fajardo 2015 ). 4 The models were derived sing the MAPPINGS V code (Sutherland & Dopita 2017 ), which llows us to retrieve emission lines produced in a shocked gas, both ith and without the presence of the so-called precursor. 5 The middle anel of Fig. 2 displays the line ratios produced by the shock models ith parameters as described in Allen et al. ( 2008 ). In particular, we onsider: (i) only pure shock models, since adding the precursor would roduce [O III ]/H β ratios much higher than the value observed in NSPIRE UCMGs; (ii) models with solar ( Z = 0 . 0183, blue and light blue) and twice olar ( Z = 0 . 0358, red) metallicities; (iii) models with shock velocities v sh with values between 100 km −1 (small points) and 500 km s −1 (big circle); (iv) pre-shock densities of n 0 = 1 cm −3 (light blue) and n 0 = 1 m −3 (blue) for the solar metallicity models and n 0 = 1 cm −3 (the nly available) for the supersolar models; (v) models with transverse magnetic fields of 10 −4 , 1, 5, 10 μG m 3 / 2 . Secondly, we use the models by Mart ´ınez-Paredes et al. ( 2023 , P23 hereafter) who recently presented a large set of photoionization odels, which are publicly available in the CB 19 table of 3MdB. We efer to MP23 for details of how the models were computed and the escription of their parameters. In short, the photoionization code LOUDY (Ferland et al. 2017 , v. 17.03) has been used to compute mission line ratios adopting as a ionizing source the population ynthesis models described in Plat et al. ( 2019 ) and S ´anchez et al. 2022 ) that include the contribution from pAGB stars (also defined s HOLMES: hot low-mass evolved stars). We select from the MP23 odels the ones that give line ratios close to those measured in he INSPIRE galaxies. They are displayed in the bottom panel of ig. 2 , where the models are chosen to have the following: (i) a single stellar population (SSP) with a Kroupa IMF up to 100 olar masses (RB–SSP–Kroup–MU100) and an age of 1 Gyr as the onizing source. Ho we ver we note that similar line ratios would be btained changing the age of the models up to 10 Gyr; (ii) metallicity of Z = 0 . 03 ([M/H] = 0.22), as from the stellar opulation analysis performed in S24 we conclude that UCMGs re consistent with supersolar metallicities. We note that selectingNRAS 539, 1471–1479 (2025) https:// sites.google.com/ site/ mexicanmillionmodels/ The precursor is where the gas entering the shock is photoionized by the UV adiation emitted by the shocked gas. l 6 iodels with solar metallicities would produce BPT ratios only lightly shifted to the SF region; (iii) an ionization parameter log U < −3; (iv) a density n H = [10 , 100] cm −3 ; (v) values of the H β fraction defining the thickness of the ionized loud between 0 and 1; (vi) CNO gas abundances: C/O = [0.1, 0.44], N/O = [ −0 . 25 , 0 , 0 . 25], 6 12 + log OH = 9.14. With the exception of J1142 + 0012, models with ionization dom- nated by pAGB stars allow to reproduce well the emission line atios observed in the INSPIRE UCMGs. Nevertheless, as shown y Stasi ´nska et al. ( 2008 ) and further discussed by Cid Fernandes t al. ( 2011 ), retired galaxies (i.e. emission-line galaxies that have topped forming stars and are ionized by hot low-mass evolved stars) ave the same location in the BPT diagram as galaxies hosting weak GNs. According to Lee et al. ( 2024 ), photoionization by pAGB tars and interstellar shocks can only be distinguished with in-depth nalysis, for instance using temperature predictions. To try to more precisely separate pure SF galaxies, AGN hosts strong and weak) and passive galaxies (i.e. retired, red, and dead alaxies) we use the so-called WHAN diagram (Cid Fernandes et al. 011 ) showing the [N II ]/H α ratio against the H α EW. This is plotted n Fig. 3 for the INSPIRE galaxies for which we could measure H α nd [N II ] emissions with high confidence. Here, all galaxies with eliable emission lines but J1142 + 0012 lie in the ‘retired galaxies’ egion, hence completely ruling out SF and disfa v ouring an AGN rigin as the most probable source of emission lines in UCMGs nd relics (with no DoR dependence). We therefore argue that the mission lines in INSPIRE UCMGs can be explained by either the resence of pAGB stars or shocks of the gas produced by the mass- oss of evolved stars, which collects at the centres of the UCMGs. Defined in MP23 as the deviation from the (N/O) to (O/H) ratio as defined n Gutkin et al. ( 2016 ) INSPIRE VIII. Emissions & UV colours in UCMGs 1477 4 T o I s I I ( w o s H r P f ( w S s a o ( p D a t g b ( a T l w ( I f r fi s e w M 2 s p a t n p t t ( 7 e 8 c Figure 4. Top: NUV − r colour for the INSPIRE galaxies with a detection in GALEX plotted versus their DoR. On the top, black crosses indicate objects without a match to show that both detection and non-detection co v er a wide range of DoR. The red and yellow lines (upper and lower and shaded regions) indicate the mean colour (and standard deviation) for quiescent and AGNs, respectively (Ardila et al. 2018 ). Bottom: NUV − r prediction from E-MILES SSP (Vazdekis et al. 2016 ) old models with a small percentage of younger stars, as reported in the legend. The green shaded region shows the NUV − r range co v ered by INSPIRE galaxies. ( m J fl a l u w t D ow nloaded from https://academ ic.oup.com /m nras/article/539/2/1471/8101535 by Turun Yliopiston Kirjasto user on 09 June 2025 U V D ETECTION o acquire an independent line of evidence on the possible origin f emission lines, in this section we look at the UV fluxes for NSPIRE objects. Unfortunately, the SNR of X-Shooter spectra of ingle galaxies is too low in the UV. Hence, to investigate whether the NSPIRE UCMGs have detectable UV fluxes, we cross-match the NSPIRE catalogue with data from the Galaxy Evolution Explorer Morrissey et al. 2007 ), matching sources to GALEX photometry ithin 10 arcsec. We caution the reader that the spatial resolution f GALEX is suboptimal when matching ultracompact galaxies, with izes smaller than the nominal surv e y resolution (FWHM ≈ 6 arcsec). o we ver, we checked that no other source was present within this adius in the u -band images from SDSS or the g-band images from anStarrs. Thus, we are confident that the detected emission comes rom the UCMGs themselves. Among the 52 INSPIRE objects, 20 have a match, with near-UV NUV ) magnitudes ranging between 21.3 and 24.1, and spanning a ide range of DoR, from non-relics (DoR < 0 . 34) with an extended FH, to extreme relics (DoR ≥ 0 . 7) that have assembled all their tellar mass within the first 2 Gyr after the Big Bang (S24). In ddition, 5 UCMGs have also been detected in the far-UV ( FUV ) but nly one of them is a relic, according to the INSPIRE classification J0920 + 0212, DoR = 0 . 64). The final block of columns of Table 1 rovides FUV and NUV magnitudes, as well as NUV − r colours. The top panel of Fig. 4 shows the NUV − r colour 7 versus the oR for the 20 detected objects. We also plot the objects without match in GALEX as black crosses at the top of the panel to show hat, perhaps surprisingly, no correlation with the DoR is found. The alaxies’ colours lie perfectly in between the mean colour computed y Ardila et al. ( 2018 , table 1) for red and dead (quiescent) galaxies 4 . 54 ± 1 . 00, red-shaded region) and the mean colour estimated for ctive galactic nuclei (AGNs, 2 . 48 ± 1 . 24, yellow-shaded region). hey are consistent with the UV green valley (Salim 2014 ) and ie at the redder end of those measured in post-starbust galaxies here the UV flux is mainly caused by intermediate-age pAGB stars Melnick & De Propris 2013 ). The NUV − r colour of the matched NSPIRE objects are instead generally redder than these computed or star-forming galaxies (2 . 01 ± 0 . 68; Ardila et al. 2018 ). Ho we ver, another equally valid scenario can reproduce the NUV − colour of the INSPIRE objects. Indeed, they can be perfectly tted with an o v erall old stellar population plus the addition of a mall percentage of young stars, as suggested by Salvador-Rusi ˜ nol t al. ( 2021 ). This is shown in the bottom panel of Fig. 4 where e plot predictions on the NUV − r colour obtained from the E- ILES single stellar population models (SSPs; Vazdekis et al. 016 ). In particular, we computed the expected colour for an old tellar population (10 Gyr) with solar metallicity to which a small ercentage ( < 5 per cent) of young stars ( ≤ 0 . 5 Gyr) is added, lso with solar metallicity. 8 We explore the colour variation due o changes in age and metallicity of the main old population, finding egligible effects on the main conclusions. Importantly, we point out that the presence of a sub-per cental opulation of young stars is not in disagreement with our analysis of he main ionization mechanism of emission lines (Section 3 ). In fact, he latter constrains the presence of SF on a much shorter time-scale of the order of few Myr) compared to UV colours. Optical r-band magnitudes have been retrieved from KiDS DR4 (Kuijken t al. 2019 ). We stress that this assumption does not change the results as the effect of hanging the metallicity on the NUV − r colour is negligible. ∼ i ( 9To further address this issue, we estimate the star formation rate SFR) for the only six systems with a match in GALEX and a easured H α emission (J0844 + 0148, J0847 + 0112, J1218 + 0232, 1447-0149, J1456 + 0020, J2356-3332). 9 First, we convert the H α ux into a total luminosity, considering it unresolved, as the spectra re fully seeing-dominated (see S24). Secondly, we translate H α uminosities into SFRs follo wing K ennicutt ( 1998 ). In particular, we se equation (12) and table 1 of Kennicutt & Evans ( 2012 ): log ˙M  [M yr −1 ] = log (L H α) − log C x , (1) here log C x = 41 . 27 (Murphy et al. 2011 ). The resulting SFRs for he six systems range between 0.01 and 0.02 M yr −1 . At this point, we compute the stellar mass that would be formed in 0.5 Gyr (the oldest age of the young stellar population component n Fig. 4 ) with a constant rate of star formation of 0.02 M yr −1 as inferred from H α), finding that it would be of the order ofMNRAS 539, 1471–1479 (2025) Excluding J1142 + 0012 which is an AGN. 1478 C. Spiniello M M ( 2 4 c r G 6 M H e c m e a t c i i i l I u 5 I s t t a H G t s e 2 e c w 1 B A c 1 D l ‘ i o a L 1 t u ∼ e c r A t s b I w s p t e p o u l g m T m ( a t t o ( ( f U o a e m a o t d v c t 1 L i t t T t d b t l l t D ow nloaded from https://academ ic.oup.com /m nras/article/539/2/1471/8101535 by Turun Yliopiston Kirjasto user on 09 June 2025 , H α ∼ 10 7 M . This is also in agreement with empirical relations e.g. Elbaz et al. 2007 ) and simulations (e.g. Ciesla, Elbaz & Fensch 017 ) predicting that SFRs < 0 . 1 correspond to M  < 10 8 M . In Fig. , the fraction of young stars required to explain the NUV − r olour depends significantly on the age of the young component, anging from ∼ 0 . 001 ( t young = 0 . 07 Gyr) to ∼ 0 . 04 ( t young = 0 . 5 yr). Hence, since the typical mass of INSPIRE objects is M  ≥ × 10 10 M , this implies a stellar mass in the young component , NUV −r ≥ 6 × 10 7 M . In conclusion, since M , NUV −r > M , H α , even assuming that all the α emission comes from newly formed stars (10 7 M ), this is not nough to explain the NUV − r colour. 10 Ho we ver, we must stress a ouple of important points. First, the assumption of a constant SFR ight not be correct. Indeed, the SFR could decrease with time or ven more likely occur in a bursty fashion (e.g. due to residual AGN ctivity), which would make extremely hard to detect it on very short ime-scales as those probed by H α (  10 Myr). Secondly, in the ase of a non-universal IMF, within the integrated galactic stellar nitial mass function (IGIMF) theory (Kroupa & Weidner 2003 ), .e. assuming that the IMF slope steepens with decreasing SFR, the onizing photons that produce H α emission would be significantly o wer, gi ven the smaller number of massive stars, than for a standard MF, meaning that the abo v e value of M , H α might be significantly nderestimated. DISCUSSION A N D C O N C L U S I O N S n this paper we have reported the detection of emission lines and ignificant NUV emission in the UVB + VIS spectra of about half of he INSPIRE UCMGs, including some of the most extreme relics. The [O II ] emission line, which is the strongest that we detect in he INSPIRE galaxies, has been widely used in the literature as n empirical SF rate indicator, calibrated through comparison with ydrogen lines (e.g. Gallagher, Bushouse & Hunter 1989 ; Rosa- onz ´alez, T erlevich & T erlevich 2002 ; Kewley et al. 2004 ). However, he situation is contro v ersial as the same emission line, and others uch as [N II ] and H α, have been detected in spectra of red and dead lliptical galaxies at different redshifts (Caldwell 1984 ; Yan et al. 006 ; La Barbera et al. 2019 ; Maseda et al. 2021 ), where SF is not xpected. Another possible origin for emission lines in red ETGs omes from the photoionization by exposed cores of evolved stars ith ages greater than 100 Myr, such as pAGBs (Greggio & Renzini 990 ; Binette et al. 1994 ; Yan & Blanton 2012 ; Papaderos et al. 2013 ; elfiore et al. 2016 ) and PNe (Taniguchi, Shioya & Murayama 2000 ). lternatively, AGNs and especially LINERs, fast shock waves, and ooling flows might also produce [O II ] emission (Ferland & Netzer 983 ; Halpern & Steiner 1983 ; Dopita & Sutherland 1995 ; Gro v es, opita & Sutherland 2004 ). Hence, we have analysed the emission ine ratios to understand what is most likely causing them in the building blocks’ of massive ETGds, the oldest and densest galaxies n the nearby Universe. We find emission lines for galaxies at all DoR, including some f the most extreme relics. All but one object (J1142 + 0012) have high [O II ]/H α ratio, typical of red and dead galaxies (Y06). a Barbera et al. ( 2019 ) also found strong [O II ] emission in theNRAS 539, 1471–1479 (2025) 0 We note that we did not apply any correction for extinction. Ho we ver, for he 5 systems for which H β emission is also measured, the SFRs can increase p to a factor of 3, given that the Balmer decrement values range between 1 and ∼ 5 (although with very large uncertainties on H β). This is still not nough to explain the NUV − r colours through the H α fluxes. u o A i ( p more of massive, giant ETGs with very old stellar populations, the egion where the ‘pristine’ stellar population dominates the light. high [O II ]/H α is characteristic of ‘retired galaxies’ (Y06) and his conclusion is further corroborated by the lack of clear and trong H α emission, which is still present in a number of systems ut very weak. In BPT and WHAN diagrams, the line ratio of NSPIRE objects cannot be reproduced by SF models. Hence, e fully exclude that the main ionization mechanism is on-going tar formation. Line ratios are, instead, fully consistent with those redicted by shock models (Alarie & Morisset 2019 ) and by models hat include the contribution from HOLMES stars (Mart ´ınez-Paredes t al. 2023 ). Both of these scenarios advocate for internal and passive rocesses, rather than external or environmental ones (e.g. mergers r interactions). Importantly, this opens up new insights into the nderstanding of the mass assembly and cosmic evolution of the ocal massive ETGs. We also looked at the NUV − r colours for the 20 INSPIRE alaxies with a match in GALEX . They are very similar to those easured for post-starbust galaxies (Melnick & De Propris 2013 ). his may hint at the UV emission being powered by the same echanisms (evolved stars) plus, possibly, some AGN contribution Ardila et al. 2018 ), which is, ho we ver, disfa v oured by the line ratio nalysis. It should also be pointed out that the time-scales for the SF raced by emission lines and UV colours are different. In addition, he INSPIRE NUV − r colour can be reproduced with an o v erall ld population ( ∼ 10 Gyr) plus a sub-per cent young population ∼ 0 . 1–0.5 Gyr). Given also the very recent results on NGC 1277 Salvador-Rusi ˜ nol et al. 2022 ) where this is indeed demonstrated rom UV and VIS line-index analysis, we cannot exclude that the V colours are due, at least partially, to a sub-per cent contribution f star formation. In this case, since the relics did not interact with ny other galaxy after the very first assembly phase at high- z, the missions (and the possible residual SF causing blue UV colours) ust have been caused by intrinsic processes. One possible scenario ble to explain all the lines of evidence presented here is the one utlined in Dopita et al. ( 2000 ). The authors attribute the origin of he gas in the centres of red and dead massive ETGs to the mass-loss ue to evolved stars. In particular , pA GBs and PNe, which share the elocity dispersion of the native stellar population, give the largest ontribution to the gas mass. A simple calculation, which assumes he PN birth rate of ( ˙ ≈ 4 × 10 −12 yr −1 L −1  ; Mendez & Soffner 997 ), the typical luminosity and size of UCMGS (L ≈ 6 × 10 10 , R e ≤ 1 . 7 kpc), and the amount of mass-loss by each PN to the nterstellar medium (ISM), ∼ 0 . 4M , leads to a gas loss rate by all he PNe of about 0.24 M  yr −1 . This means that in less than 10 9 yr, hey can feed more than 10 8 M  of gas and dust into the UCMGs. his gas, given the high relative motion of the PN envelopes through he ISM of such very dense stellar systems with high stellar velocity ispersion ( > 200 km s −1 ; D’Ago et al. 2023 ), does not e v aporate ut collapses in dense clouds. These then infall radially towards he centre, where they can be shocked, possibly causing emission ines, and heated, therefore forming new stars at a sub-percentage evel. The analysis we presented here allowed us to completely exclude hat the emission lines are caused by star formation, but it is nfortunately unable to disentangle between shock-driven emission r pAGBs stars. Furthermore, a small contribution from the central GN is still possible, although disfa v oured. The only way forward n this sense is to obtain high spatial resolution spectroscopy and far and near) UV-deep imaging to be able to resolve the stellar opulations and the internal structure of the most compact and dense assive galaxies in the nearby Universe. INSPIRE VIII. Emissions & UV colours in UCMGs 1479 A W ( f J p f c I e G a I D T t I i R A A A A B B B B B B C C C D D v D D E F F F G G G G H d K K K K K K K K K L L M M M M M M M M M N P P R R R S S S S S S S S S S T T T T T T V V Y Y T © P ( D ow nloaded from https://academ ic.oup.com /m nras/article/539/2/1471/8101535 by Turun Yliopiston Kirjasto user on 09 June 2025C K N OW L E D G E M E N T S e acknowledge the usage of the Mexican Million Database Morisset et al. 2015 ). CS, CT, FLB, DB, and PS acknowledge unding from the IN AF PRIN-IN AF 2020 programme 1.05.01.85.11. H and CS acknowledge the financial support from the mobility rogramme of the Finnish Centre for Astronomy with ESO (FINCA), unded by the Academy of Finland grant nr 306531. AFM has re- eived support from RYC2021-031099-I and PID2021-123313NA- 00 of MICIN/AEI/10.13039/501100011033/FEDER,UE, NextGen- rationEU/PRT. CT acknowledges the INAF grant 2022 LEMON. D acknowledges support by UKRI-STFC grants: ST/T003081/1 nd ST/X001857/1. MR acknowledges financial support from the NAF mini-grant 2022 ‘GALCLOCK’. ATA AVA ILA BILITY he INSPIRE spectra used in this paper are publicly available hrough the ESO Phase 3 Archive Science Portal under the collection NSPIRE ( https://ar chive.eso.or g/sciencepor tal/home?data collect on = INSPIRE , ht tps:ht tps:// doi.eso.org/ 10.18727/archive/36 ). EFER ENCES delman-McCarthy J. K. et al., 2006, ApJS , 162, 38 larie A. , Morisset C., 2019, Rev. Mex. Astron. Astrofis. , 55, 377 llen M. G. , Gro v es B. A., Dopita M. A., Sutherland R. S., K e wley L. J., 2008, ApJS , 178, 20 rdila F. et al., 2018, ApJ , 863, 28 aldwin J. A. , Phillips M. M., Terlevich R., 1981, PASP , 93, 5 arbosa C. E. , Spiniello C., Arnaboldi M., Coccato L., Hilker M., Richtler T., 2021, A&A , 649, A93 elfiore F. et al., 2016, MNRAS , 461, 3111 ennett C. L. , Larson D., Weiland J. L., Hinshaw G., 2014, ApJ , 794, 135 inette L. , Magris C. G., Stasi ´nska G., Bruzual A. G., 1994, A&A, 292, 13 uitrago F. , Trujillo I., Conselice C. J., Bouwens R. J., Dickinson M., Yan H., 2008, ApJ , 687, L61 aldwell N. , 1984, PASP , 96, 287 id Fernandes R. , Stasi ´nska G., Mateus A., Vale Asari N., 2011, MNRAS , 413, 1687 iesla L. , Elbaz D., Fensch J., 2017, A&A , 608, A41 ’Ago G. et al., 2023, A&A , 672, A17, INSPIRE DR2 addi E. et al., 2005, ApJ , 626, 680 an Dokkum P. G. et al., 2008, ApJ , 677, L5 opita M. A. , Sutherland R. S., 1995, ApJ , 455, 468 opita M. A. , Massaglia S., Bodo G., Arnaboldi M., Merluzzi P., 2000, in Kastner J. H., Soker N., Rappaport S., eds, ASP Conf. Ser. Vol. 199, Asymmetrical Planetary Nebulae II: From Origins to Microstructures. Astron. Soc. Pac., San Francisco, p. 423 lbaz D. et al., 2007, A&A , 468, 33 erland G. J. , Netzer H., 1983, ApJ , 264, 105 erland G. J. et al., 2017, Rev. Mex. Astron. Astrofis. , 53, 385 err ´e-Mateu A. , Trujillo I., Mart ´ın-Navarro I., Vazdekis A., Mezcua M., Balcells M., Dom ´ınguez L., 2017, MNRAS , 467, 1929 allagher J. S. , Bushouse H., Hunter D. A., 1989, AJ , 97, 700 reggio L. , Renzini A., 1990, ApJ , 364, 35 ro v es B. A. , Dopita M. A., Sutherland R. S., 2004, ApJS , 153, 75 utkin J. , Charlot S., Bruzual G., 2016, MNRAS , 462, 1757 alpern J. P. , Steiner J. E., 1983, ApJ , 269, L37 e Jong J. T. A. et al., 2017, A&A , 604, A134 2025 The Author(s). ublished by Oxford University Press on behalf of Royal Astronomical Society. This is an Open https://cr eativecommons.or g/licenses/by/4.0/), which permits unrestricted reuse, distribution, and repauffmann G. et al., 2003, MNRAS , 341, 54 ennicutt Jr R. C. , 1998, ApJ , 498, 541 ennicutt R. C. , Evans N. J., 2012, ARA&A , 50, 531 e wley L. J. , Dopita M. A., Sutherland R. S., Heisler C. A., Trevena J., 2001, ApJ , 556, 121 e wley L. J. , Geller M. J., Jansen R. A., 2004, AJ , 127, 2002 e wley L. J. , Groves B., Kauffmann G., Heckman T., 2006, MNRAS , 372, 961 roupa P. , Weidner C., 2003, ApJ , 598, 1076 uijken K. , 2011, The Messenger, 146, 8 uijken K. et al., 2019, A&A , 625, A2 a Barbera F. et al., 2019, MNRAS , 489, 4090 ee M.-Y. L. , Yan R., Ji X., Algodon G., Westfall K., Lin Z., Belfiore F., Bundy K., 2024, A&A , 690, A83 aksymowicz-Maciata M. et al., 2024, MNRAS , 531, 2864 art ´ın-Navarro I. et al., 2023, MNRAS , 521, 1408 art ´ınez-Paredes M. , Bruzual G., Morisset C., Kim M., Mel ´endez M., Binette L., 2023, MNRAS , 525, 2916 aseda M. V. et al., 2021, ApJ , 923, 18 elnick J. , De Propris R., 2013, MNRAS , 431, 2034 endez R. H. , Soffner T., 1997, A&A , 321, 898 orisset C. , Delgado-Inglada G., Flores-Fajardo N., 2015, Rev. Mex. Astron. Astrofis. , 51, 103 orrissey P. et al., 2007, ApJS , 173, 682 urphy E. J. et al., 2011, ApJ , 737, 67 aab T. et al., 2014, MNRAS , 444, 3357 apaderos P. et al., 2013, A&A , 555, L1 lat A. , Charlot S., Bruzual G., Feltre A., Vidal-Garc ´ıa A., Morisset C., Che v allard J., Todt H., 2019, MNRAS , 490, 978 ola C. S. , Terlevich E., Terlevich R. J., 1997, MNRAS , 289, 419 osa-Gonz ´alez D. , Terlevich E., Terlevich R., 2002, MNRAS , 332, 283 oy N. et al., 2018, MNRAS , 480, 1057 alim S. , 2014, Serb. Astron. J. , 189, 1 alvador-Rusi ˜ nol N. , Beasley M. A., Vazdekis A., Barbera F. L., 2021, MNRAS , 500, 3368 alvador-Rusi ˜ nol N. , Ferr ´e-Mateu A., Vazdekis A., Beasley M. A., 2022, MNRAS , 515, 4514 ´anchez S. F. et al., 2022, ApJS , 262, 36 cognamiglio D. et al., 2020, ApJ , 893, 4 piniello C. et al., 2021a, A&A , 646, A28, INSPIRE Pilot piniello C. et al., 2021b, A&A , 654, A136, INSPIRE DR1 piniello C. et al., 2024, MNRAS , 527, 8793 tasi ´nska G. et al., 2008, MNRAS , 391, L29 utherland R. S. , Dopita M. A., 2017, ApJS , 229, 34 aniguchi Y. , Shioya Y., Murayama T., 2000, AJ , 120, 1265 ortora C. et al., 2016, MNRAS , 457, 2845 ortora C. et al., 2018, MNRAS , 481, 4728 rujillo I. , Conselice C. J., Bundy K., Cooper M. C., Eisenhardt P., Ellis R. S., 2007, MNRAS , 382, 109 rujillo I. , Cenarro A. J., de Lorenzo-C ´aceres A., Vazdekis A., de la Rosa I. G., Cava A., 2009, ApJ , 692, L118 rujillo I. , Ferr ´e-Mateu A., Balcells M., Vazdekis A., S ´anchez-Bl ´azquez P., 2014, ApJ , 780, L20 azdekis A. , Kole v a M., Ricciardelli E., R ¨ock B., Falc ´on-Barroso J., 2016, MNRAS , 463, 3409 ernet J. et al., 2011, A&A , 536, A105 an R. , Blanton M. R., 2012, ApJ , 747, 61 an R. , Newman J. A., Faber S. M., Konidaris N., Koo D., Davis M., 2006, ApJ , 648, 281 his paper has been typeset from a T E X/L A T E X file prepared by the author. 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