Intermediate-age Stellar Populations Research Articles

Hunting young stars in the Galactic centre

Context. The Galactic centre stands out as the most prolific star-forming environment of the Galaxy when averaged over volume. In the last 30 million years, it has witnessed the formation of ∼106 M⊙ of stars. However, crowding and high extinction hamper their detection and, up to now, only a small fraction of the expected mass of young stars has been identified. Aims. We aim to detect hidden young stars at the Galactic centre by analysing the stellar population in Sagittarius (Sgr) C. This is a region at the western edge of the nuclear stellar disc whose HII emission makes it a perfect candidate to host young stars. Methods.We built dereddened luminosity functions for Sgr C and a control field in the central region of the nuclear stellar disc, and fitted them with a linear combination of theoretical models to analyse their stellar population. Results. We find that Sgr C hosts several 105 M⊙ of young stars. We compared our results with the recently discovered young stellar population in Sgr B1, which is situated at the opposite edge of the nuclear stellar disc. We estimated that the Sgr C young stars are ∼20 Myr old, and likely show the next evolutionary step of the slightly younger stars in Sgr B1. Our findings contribute to addressing the discrepancy between the expected and the detected number of young stars in the Galactic centre, and shed light on their evolution in this extreme environment. As a secondary result, we find an intermediate-age stellar population in Sgr C (∼50% of its stellar mass with an age of between 2 and 7 Gyr), which is not present in the innermost regions of the nuclear stellar disc (dominated by stars > 7 Gyr). This supports the existence of an age gradient and favours an inside-out formation of the nuclear stellar disc.

AGN feedback and star formation in the peculiar galaxy NGC 232: insights from VLT-MUSE observations

ABSTRACT We use VLT-MUSE integral field unit data to study the ionized gas physical properties and kinematics as well as the stellar populations of the Seyfert 2 galaxy NGC 232 as an opportunity to understand the role of AGN feedback on star formation. The data cover a field of view of 60 × 60 arcsec2 at a spatial resolution of ∼850 pc. The emission-line profiles have been fitted with two Gaussian components, one associated to the emission of the gas in the disc and the other due to a bi-conical outflow. The spectral synthesis suggests a predominantly old stellar population with ages exceeding 2 Gyr, with the largest contributions seen at the nucleus and decreasing outwards. Meanwhile, the young and intermediate age stellar populations exhibit a positive gradient with increasing radius and a circum-nuclear star-forming ring with radius of ∼0.5 kpc traced by stars younger than 20 Myr, is observed. This, along with the fact that AGN and SF dominated regions present similar gaseous oxygen abundances, suggests a shared reservoir feeding both star formation and the AGN. We have estimated a maximum outflow rate in ionized gas of ∼1.26 M⊙ yr−1 observed at a distance of ∼560 pc from the nucleus. The corresponding maximum kinetic power of the outflow is ∼3.4 × 1041 erg s−1. This released energy could be sufficient to suppress star formation within the ionization cone, as evidenced by the lower star formation rates observed in this region.

Are “Changing-look” Active Galactic Nuclei Special in the Coevolution of Supermassive Black Holes and Their Hosts? I.

The nature of the so-called “changing-look” (CL) active galactic nucleus (AGN), which is characterized by spectral-type transitions within ∼10 yr, remains an open question. As the first in our series of studies, we attempt here to understand the CL phenomenon from the view of the coevolution of AGNs and their host galaxies (i.e., if CL-AGNs are at a specific evolutionary stage) by focusing on the SDSS local “partially obscured” AGNs in which the stellar population of the host galaxy can be easily measured in the integrated spectra. A spectroscopic follow-up program using the Xinglong 2.16 m, Lick/Shane 3 m, and Keck 10 m telescopes enables us to identify in total nine CL-AGNs from a sample of 59 candidates selected by their mid-infrared variability. Detailed analysis of these spectra shows that the host galaxies of the CL-AGNs are biased against young stellar populations and tend to be dominated by intermediate-age stellar populations. This motivates us to propose that CL-AGNs are probably particular AGNs at a specific evolutionary stage, such as a transition stage from “feast” to “famine” fueling of the supermassive black hole. In addition, we reinforce the previous claim that CL-AGNs tend to be biased against both a high Eddington ratio and a high bolometric luminosity, suggesting that the disk-wind broad-line-region model is a plausible explanation of the CL phenomenon.

Mapping the stellar population and gas excitation of MaNGA galaxies with megacubes. Results for AGN versus control sample

ABSTRACT We present spaxel-by-spaxel stellar population fits for the ∼10 000 MaNGA data cubes. We provide multiple extension fits files, nominated as megacubes, with maps of several properties as well as emission-line profiles that are provided for each spaxel. All the megacubes are available through a web interface (https://manga.linea.org.br/ or http://www.if.ufrgs.br/~riffel/software/megacubes/). We also defined a final Active Galactic Nuclei (AGN) sample, as well as a control sample matching the AGN host galaxy properties. We have analysed the stellar populations and spatially resolved emission-line diagnostic diagrams of these AGNs and compared them with the control galaxies sample. We find that the relative fractions of young (t ≤56 Myr) and intermediate-age (100 Myr ≤t ≤ 2 Gyr) show predominantly a positive gradient for both AGNs and controls. The relative fraction of intermediate-age stellar population is higher in AGN hosts when compared to the control sample, and this difference becomes larger for higher [O iii] luminosity AGNs. We attribute this to the fact that extra gas is available in these more luminous sources and that it most likely originates from mass-loss from the intermediate-age stars. The spatially resolved diagnostic diagrams reveal that the AGN emission is concentrated in the inner 0.5 Re (effective radius) region of the galaxies, showing that the AGN classification is aperture dependent and that emission-line ratios have to be taken together with the H α equivalent width for proper activity classification. We present a composite ‘BPT+WHAN’ diagram that produces a more comprehensive mapping of the gas excitation.

Search for Young and Intermediate-age Stellar Populations in the Galactic Center with Subaru/MOIRCS and CO Narrowband Filters

Extinction toward the Galactic Center (GC) is extreme and limits observations of its stars to the infrared. In addition, the extinction varies on scales of arcseconds. It is therefore highly challenging to distinguish between different stars photometrically and to identify young, massive, or intermediate-age stars that can trace recent star formation. Here we report the results of near-infrared imaging observations of a region at the GC that encompasses the northwest part of the nuclear star cluster and parts of the nuclear stellar disk. We carried out the observations with the wide-field camera MOIRCS of the Subaru Telescope, using three narrowband (NB) filters. Two of the filters trace the stellar continuum, while we use the third one to measure the strength of the CO absorption features that are prominent in the spectra of cool giants, shallower in the spectra of warmer stars, and absent in young massive stars. Thus, we aim to distinguish between old late-type giants and young and intermediate-age stars. We have found 131 candidates of young and intermediate-age stars. The magnitude limit is ≈11.25 in the extinction-corrected K-band magnitude. The contamination of the candidates by old giants is estimated to be 26%. We show that our method allows us to identify correctly four out of five spectroscopically confirmed massive stars, and 68 of 75 spectroscopically confirmed cool stars (91%). Our results suggest that imaging observations with a CO NB filter can be complementary to other methods to identify massive stars in the GC.

Spectroscopy of the supernova H0pe host galaxy at redshift 1.78

Supernova (SN) H0pe was discovered as a new transient in James Webb Space Telescope (JWST) NIRCam images of the galaxy cluster PLCK G165.7+67.0 taken as part of the Prime Extragalactic Areas for Reionization and Lensing Science (PEARLS) JWST GTO program (ID 1176) on 2023 March 30. The transient is a compact source associated with a background galaxy that is stretched and triply imaged by the strong gravitational lensing of the cluster. This paper reports spectra in the 950–1370 nm observer frame of two of the galaxy images obtained with Large Binocular Telescope (LBT) Utility Camera in the Infrared (LUCI) in long-slit mode two weeks after the JWST observations. The individual and average spectra show the [O II] λλ3727,3730 doublet and the Balmer and 4000 Å breaks at redshift z = 1.783 ± 0.002. The code investigating galaxy emission (CIGALE) best-fit model of the spectral energy distribution indicates that the host galaxy of SN H0pe is massive (Mstar ≃ 6 × 1010 M⊙ after correcting for a magnification factor μ ∼ 7) with a predominantly intermediate-age (∼2 Gyr) stellar population, moderate extinction, and a magnification-corrected star formation rate ≃13 M⊙ yr−1, consistent with being below the main sequence of star formation. These properties suggest that H0pe might be a type Ia SN. Additional observations of SN H0pe and its host recently carried out with JWST (JWST-DD-4446; PI: B. Frye) will be able to both determine the SN classification and confirm its association with the galaxy analyzed in this work.

A Three-dimensional Map of the Milky Way Using 66,000 Mira Variable Stars

We study the three-dimensional structure of the Milky Way using 65,981 Mira variable stars discovered by the Optical Gravitational Lensing Experiment survey. The spatial distribution of the Mira stars is analyzed with a model containing three barred components that include the X-shaped boxy component in the Galactic center (GC) and an axisymmetric disk. We take into account the distance uncertainties by implementing the Bayesian hierarchical inference method. The distance to the GC is R 0 = 7.66 ± 0.01(stat.) ± 0.39(sys.) kpc, while the inclination of the major axis of the bulge to the Sun–GC line of sight is θ = 20.°2 ± 0.°6(stat.) ± 0.°7(sys.). We present, for the first time, a detailed three-dimensional map of the Milky Way composed of young and intermediate-age stellar populations. Our analysis provides independent evidence for both the X-shaped bulge component and the flaring disk (being plausibly warped). We provide the complete data set of properties of Miras that were used for calculations in this work. The table includes mean brightness and amplitudes in nine photometric bands (covering a range of wavelengths from 0.5 to 12 μm), photometric chemical type, estimated extinction, and calculated distance with its uncertainty for each Mira variable. The median distance accuracy to a Mira star is at the level of 6.6%.

Long-Period Variables as distance and age indicators in the era of Gaia and LSST

AbstractLong-period variables are bright, evolved red giant stars showing periodic photometric changes due to stellar pulsation. They follow one or more period-luminosity and period-age relations, which make them highly promising distance indicators and tracers of young and intermediate-age stellar populations. Such a potential is especially interesting in view of the massive amount of data delivered by modern large-scale variability surveys. Crucially, these applications require a clear theoretical understanding of pulsation physics in connection with stellar evolution. Here, I describe an ongoing effort from our collaboration dedicated to the modelling of stellar pulsation in evolved stars, and how this work is impacting our capability of investigating long-period variables and exploiting them for other astrophysical studies. Furthermore, I present our ongoing work aimed at assessing the potential of semi-regular variables, an often neglected sub-type of long-period variables, to be distance indicators complementary to their better-known, more evolved counterparts, the Mira variables.

Accretion and Host-Galaxy Properties of 14 New “Changing-Look” Active Galactic Nuclei Identified from the SDSS-V Survey

Abstract The widely accepted active galactic nucleus (AGN) paradigm has been recently challenged by the discovery of the so-called ``changing-look'' (CL) phenomenon characterized by spectral-type transitions. By comparing the SDSS-V and SDSS DR16 spectroscopic datasets, here we report the identification of 14 new CL-AGNs (redshift $z<0.5$) exhibiting spectral-type changes on a timescale of $\sim 10$\,yr. Follow-up spectroscopy was conducted with the Lick Shane 3\,m and Keck 10\,m telescopes for three of the objects. Detailed analysis of these spectra enables us to arrive at the following two main results. (1) By compiling a sample of 65 CL-AGNs with good measurements, we reinforce the previous claim that CL-AGNs tend to be biased against both a high Eddington ratio ($\lesssim 0.1$) and a high bolometric luminosity ($\lesssim 10^{46}\,\mathrm{erg\,s^{-1}}$). This bias suggests that the disk-wind broad-line-region model is a plausible explanation of the CL phenomenon. (2) The host galaxies of CL-AGNs tend to be dominated by intermediate stellar populations, which motivates us to propose that CL-AGNs are probably particular AGNs at a special evolutionary stage, such as a transition stage from ``feast'' to ``famine'' fueling of the supermassive black hole. In addition, with our spectra, we identify SDSS\,J025951.22+003744.2 as a new repeat CL narrow-line Seyfert 1 galaxy with a rapid ``turn-on'' timescale of $\sim 1$\,yr.

The relic galaxy NGC 1277 rules out intermediate-age stellar populations origin of CO-strong absorptions in massive early-type galaxies

ABSTRACT Massive early-type galaxies (ETGs) show several strong CO absorption features in their H- and K-band spectra that cannot be explained by state-of-the-art stellar population models. For many years, the disagreement has been attributed to the presence of intermediate-age stellar components that are dominated by stars in the asymptotic giant branch phase. However, no robust evidence of this scenario has been provided so far. One way to test this claim is by comparison of CO indices for ETGs and for relic galaxies. Lacking the intermediate-age stellar populations, relic galaxies provide us with a unique opportunity to address the origin of strong CO absorptions in ETGs. Here, we utilize the prototype relic galaxy NGC 1277 and compare the CO absorption features of this galaxy with the ones of a representative sample of massive ETGs. We show that the CO lines in both systems have similar strengths, significantly stronger than the predictions of stellar population synthesis models. We conclude that intermediate-age stellar populations in massive ETGs are not the culprit of the strong CO absorptions.

Stellar populations of a sample of far-infrared AGN and non-AGN green valley galaxies

ABSTRACT We present a study on the stellar populations and stellar ages of a subsample of far-infrared (FIR) active galactic nucleus (AGN) and non-AGN green valley galaxies at ${0.6\, \lt \, z\, \lt \, 1.0}$ using the data from the Cosmological Evolution Survey field. We used long-slit spectroscopy and derived stellar populations and stellar ages using the stellar population synthesis code ‘starlight’ and analysed the available Lick/IDS indices, such as Dn4000 and HδA. We find that both FIR AGN and non-AGN green valley galaxies are dominated by intermediate stellar populations (67 per cent and 53 per cent, respectively). The median stellar ages for AGNs and non-AGNs are ${\log t\, =\, 8.5}$ and ${\, 8.4\, \rm yr}$, respectively. We found that majority of our sources (62 per cent of AGNs and 66 per cent of non-AGNs) could have experienced bursts and continuous star formation (SF). In addition, most of our FIR AGNs (38 per cent) compared to FIR non-AGNs (27 per cent) might have experienced a burst of SF more than 0.1 Gyr ago. We also found that our FIR AGN and non-AGN green valley galaxies have similar quenching time-scales of ∼ 70 Myr. Therefore, the results obtained here are in line with our previous results where we do not find that our sample of FIR AGNs in the green valley shows signs of negative AGN feedback, as has been suggested previously in optical studies.

Gemini NIFS survey of feeding and feedback processes in nearby active galaxies – VI. Stellar populations

ABSTRACT We use Gemini Near-Infrared Integral Field Spectrograph (NIFS) adaptive optics assisted data cubes to map the stellar population of the inner few hundred parsec of a sample of 18 nearby Seyfert galaxies. The near-infrared light is dominated by the contribution of young to intermediate-age stellar populations, with light-weighted mean ages ‹t›L ≲ 1.5 Gyr. Hot dust (HD) emission is centrally peaked (in the unresolved nucleus), but it is also needed to reproduce the continuum beyond the nucleus in nearly half of the sample. We have analysed the stellar population properties of the nuclear region and their relation with more global properties of the galaxies. We find a correlation between the X-ray luminosity and the contributions from the HD, featureless continuum (FC), and reddening AV. We attribute these correlations to the fact that all these properties are linked to the mass accretion rate to the active galactic nuclei (AGNs). We also find a correlation of the bolometric luminosity log($L_{\rm Bol_{\rm obs}}$) with the mass-weighted mean age of the stellar population, interpreted as due a delay between the formation of new stars and the triggering/feeding of the AGN. The gas reaching the supermassive black hole is probably originated from mass loss from the already somewhat evolved intermediate-age stellar population (‹t›L ≲ 1.5 Gyr). In summary, our results show that there is a significant fraction of young to intermediate-age stellar populations in the inner few 100 pc of active galaxies, suggesting that this region is facing a rejuvenation process in which the AGN, once triggered, precludes further star formation, in the sense that it can be associated with the lack of new star formation in the nuclear region.

A Systematic Analysis of Stellar Populations in the Host Galaxies of Changing-look AGNs

Abstract “Changing-look” active galactic nuclei (CL-AGNs) are a newly discovered class of AGNs that show the appearance (or disappearance) of broad emission lines within short timescales (months to years), and are often associated with dramatic changes in their continuum emissions. They provide us with an unprecedented chance to directly investigate the host galaxy properties with minimal contamination from the luminous central engine during the turn-off state, which is difficult for normal luminous AGNs. In this work, for the first time, we systematically characterize the stellar populations and star formation histories of host galaxies for 26 turn-off CL-AGNs using the stellar population synthesis code STARLIGHT. We find that the stellar populations of CL-AGNs are similar to those of normal AGNs, except that the intermediate stellar populations contribute more fractions. We estimate their stellar velocity dispersions (σ ⋆) and black hole masses (M BH,vir), and find that CL-AGNs also follow the overall M BH–σ ⋆ relationship. We also confirm the previous claims that CL-AGNs tend to be biased toward lower Eddington ratios, and that their extreme variabilities are more likely due to the intrinsic changes of the accretion rates. In addition, CL-AGNs with recent star formations tend to have higher Eddington ratios. Compared with previous studies, our analysis suggests that there may be a correlation between CL-AGN host galaxy properties and their CL phenomena.

Dust and gas in the central region of NGC 1316 (Fornax A)

Context. The early-type galaxy NGC 1316, associated with the radio source Fornax A, hosts about 107 M⊙ of dust within a central radius of 5 kpc. These prominent dust structures are believed to have an external origin, which is also a popular interpretation for other dusty early-type galaxies. Moreover, it has been long known that ionised gas is present in NGC 1316, but to date there has been a lack of detailed investigation. Aims. Our aim is to understand the nature of ionised gas and dust in NGC 1316 and to offer an interpretation for the origin of the dust. Methods. We use archival Hubble Space Telescope/Advanced Camera for Surveys data to construct colour maps that delineate the dust pattern in detail, and we compare these data with maps constructed with data from the Multi Unit Spectroscopic Explorer (MUSE) instrument of the Very Large Telescope at the European Southern Observatory. Twelve MUSE pointings in wide field mode form a mosaic of the central 3.3 × 2.4 arcmin2. We use the tool PyParadise to fit the stellar population. We use the residual emission lines and the residual interstellar absorption NaI D-lines, and we measure line strengths, the velocity field, and the velocity dispersion field. Results. The emission lines resemble low-ionisation nuclear emission-line region lines, with [NII] being the strongest line everywhere. Ionising sources are plausibly the post-asymptotic giant branch stars of the old or intermediate-age stellar population. There is a striking match between the dust structures, ionised gas, and atomic gas distributions, the last of which is manifested by interstellar absorption residuals of the stellar NaI D-lines. In the dust-free regions, the interstellar NaI D-lines appear in emission, which is indicative of a galactic wind. The velocity field of the ionised gas (and thus of the dust) is characterised by small-scale turbulent movements that indicate short lifetimes. At the very centre, a bipolar velocity field of the ionised gas is observed, which we interpret as an outflow. The low-velocity part is associated with dust. We identify a strongly inclined gaseous dusty disc along the major axis of NGC 1316. A straight beam of ionised gas with a length of about 4 kpc emanates from the centre. Conclusions. The dust in NGC 1316 has different origins. Our findings are strongly suggestive of a dusty outflow that is curved along the line-of-sight. Nuclear outflows may be important dust-producing machines in galaxies. Another dusty gaseous component forms a disc that we identify as the predecessor of a central dust lane.

Understanding the Discrepancy between IRX and Balmer Decrement in Tracing Galaxy Dust Attenuation

Abstract We compare the infrared excess (IRX) and Balmer decrement ( ) as dust attenuation indicators in relation to other galaxy parameters using a sample of ∼32,000 local star-forming galaxies (SFGs) carefully selected from the Sloan Digital Sky Survey, the Galaxy Evolution Explorer, and the Wide-field Infrared Survey Explore. While at fixed , IRX turns out to be independent on galaxy stellar mass, the Balmer decrement does show a strong mass dependence at fixed IRX. We find the discrepancy, parameterized by the color excess ratio , is not dependent on the gas-phase metallicity and axial ratio, but on the specific star formation rate (SSFR) and galaxy size (R e) following . This finding reveals that the nebular attenuation as probed by the Balmer decrement becomes increasingly larger than the global (stellar) attenuation of SFGs with decreasing SSFR surface density. This can be understood in the context of an enhanced fraction of intermediate-age stellar populations that are less attenuated by dust than the H ii region-traced young population, in conjunction with a decreasing dust opacity of the diffuse interstellar matter when spreading over a larger spatial extent. Once the SSFR surface density of an SFG is known, the conversion between attenuation of nebular and stellar emission can be well estimated using our scaling relation.

Evolutionary Population Synthesis model with binary stars – Yunnan-II model

AbstractBy considering a modified version of the evolutionary population synthesis (EPS) model for stellar populations (SPs) comprising binary stars, the retrieved galaxy and HII-region parameters/properties differ from the case of neglecting binary stars. The retrieved age, stellar metallicity and mass of galaxies increase (e.g. ∼ 0.2 dex when using spectral fitting algorithm), whilst the star formation rate decreases (∼0.2 dex). The radiation fields from intermediate-age SPs with binary stars could be potentially important ionizing sources in HII regions. Under this possibility, the theoretical division between star forming galaxy and AGN on the diagnostic diagrams would move towards the up-right corner and the retrieved gaseous metallicity would decrease.Our prediction for the birth rate of binary neutron stars in SPs ranges from 10−9 to 10−6${\M {^\minus 1_\odot}} $ yr−1 when the kick velocity is from 0 to 190 km s−1.

M 31 circum-nuclear region: A molecular survey with the IRAM interferometer

We analysed molecular observations performed at IRAM interferometer in CO(1-0) of the circum-nuclear region (within 250 pc) of Andromeda with 2.9″ = 11 pc resolution. We detected 12 molecular clumps in this region, corresponding to a total molecular mass of (8.4 ± 0.4)×104 M⊙. These clumps follow Larson’s mass-size relation, but lie well above the velocity-size relation. We discuss the possibility that these clumps are probably not virialised, but are transient agglomerations of smaller entities that might be virialised. Three of these clumps have been detected in CO(2-1) in a previous work, and we find a temperature line ratio below 0.5 in this work. With a radiative transfer analysis, we show that this gas is in non-local thermal equilibrium with a low excitation temperature (Tex = 5 − 9 K). We find a surface beam filling factor of order 5% and a gas density in the range 60 − 650 cm−3, which is well below the critical density. With a gas-to-stellar mass fraction of 4 × 10−4 and dust-to-gas ratio of 0.01, this quiescent region has exhausted its gas budget. Its spectral energy distribution is compatible with passive templates assembled from elliptical galaxies. While weak dust emission is present in the region, we show that no star formation is present and support the previous results that the dust is heated by the old and intermediate stellar population. We study the possibility that this region lies formally in the low-density part of the Kennicutt-Schmidt law in a regime where the star formation rate estimators are not completely reliable. We confirm the quiescence of the inner part of this galaxy known to lie on the green valley.

Stellar populations in hosts of giant radio galaxies and their neighbouring galaxies

Context. Giant radio galaxies (with projected linear size of radio structure larger than 0.7 Mpc) are very rare and unusual objects. Only ∼5% of extended radio sources reach such sizes. Understanding of the processes responsible for their large sizes is crucial to further our knowledge about the radio source’s evolution. Aims. We check the hypothesis that giants become extremely large due to the specific history of their host galaxy formation, as well as in the context of the cluster or group of galaxies where they evolve. Therefore we study the star formation histories in their host galaxies and in galaxies located in their neighbourhood. Methods. We studied 41 giant-size radio galaxies as well as galaxies located within a radius of 5 Mpc around giants to verify whether the external conditions of the intergalactic medium somehow influence the internal evolution of galaxies in the group/cluster. We compared the results with a control sample of smaller-sized Fanaroff–Riley type II radio galaxies and their neighbouring galaxies. We fit stellar continua in all galaxy spectra using the spectral synthesis code STARLIGHT and provide statistical analysis of the results. Results. We find that hosts of giant radio galaxies have a larger amount of intermediate age stellar populations compared with smaller-sized FRII radio sources. The same result is also visible when we compare neighbouring galaxies located up to 1.5 Mpc around giants and FRIIs. This may be evidence that star formation in groups with giants was triggered due to global processes occurring in the ambient intergalactic medium. These processes may also contribute to mechanisms responsible for the extremely large sizes of giants.

An Extended Emission Line Region around Mrk 1172

AbstractWe serendipitously found an intriguing Extended Emission Line Region (EELR) near the quiescent and massive early-type Mrk 1172, with a projected extension of approximately 14 × 14 kpc. Its irregular shape, high gas content, strong emission lines and proximity to an isolated possible faded quasar raise questions about the ionization of this gas and the nature of this object. Analyzing the stellar population in both objects we observe that the EELR has a dominance of young-intermediate and intermediate stellar populations (200 Myr < t < 1 Gyr) with significant star formation activity, while Mrk 1172 is dominated by old stellar population (t > 5 Gyr). BPT diagnostic diagrams indicate that the gas in the EELR is photoionized by hot massive stars rather than by a hard radiation field or by shocks. Further analysis on abundances of the gas and its kinematics shall be performed to better comprehend the nature of this object and how it is interacting with Mrk 1172.

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

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 low-mass 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 intermediate-age (0.1--5 Gyr) stellar populations. Liny RGs show higher dust attenuation and $W3$ luminosities than their lineless counterparts. We also find that the amount of warm gas needed to explain the observed \Ha luminosity in liny RGs is $10^5$--$10^8$\msun, and that their \nii/\oii 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.