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Compact groups of dwarf galaxies (CGDs) have been observed at low redshifts (z < 0.1) and are direct evidence of hierarchical assembly at low masses. To understand the formation of CGDs and the galaxy assembly in the low-mass regime, we search for analogues of compact (radius ≤100 kpc) groups of dwarfs (7 ≤ log [ M */M⊙] ≤ 9.5) in the IllustrisTNG highest resolution simulation. Our analysis shows that TNG50-1 can successfully produce CGDs at z  = 0 with realistic total and stellar masses. We also find that the CGD number density decreases towards the present, especially at z ≲ 0.26, reaching |$n \approx 10^{-3.5} \ \rm cMpc^{-3}$| at z  = 0. This prediction can be tested observationally with upcoming surveys targeting the faint end of the galaxy population and is essential to constrain galaxy evolution models in the dwarf regime. The majority of simulated groups at z ∼ 0 formed recently (⁠|$\lesssim 1.5 \ \rm Gyr$|⁠), and CGDs identified at z ≤ 0.5 commonly take more than 1 Gyr to merge completely, giving origin to low- to intermediate-mass (8 ≤ log [ M */M⊙] ≤ 10) normally star-forming galaxies at z  = 0. We find that haloes hosting CGDs at z  = 0 formed later when compared to haloes of similar mass, having lower stellar masses and higher total gas fractions. The simulations suggest that CGDs observed at z ∼ 0 arise from a late hierarchical assembly in the last ∼3 Gyr, producing rapid growth in total mass relative to stellar mass and creating dwarf groups with median halo masses of |$\sim 10^{11.3} \ \rm M_\odot$| and B -band mass-to-light ratios mostly in the range 10 ≲ M / L ≲ 100, in agreement with previous theoretical and observational studies. [ABSTRACT FROM AUTHOR]

Most of the observed galaxies cannot be resolved into individual stars and are studied through their integrated spectrum using simple stellar populations (SSPs) models, with stellar libraries being a key ingredient in building them. Spectroscopic observations are increasingly being directed towards the near-infrared (NIR), where much is yet to be explored. SSPs in the NIR are still limited, and there are inconsistencies between different sets of models. One of the ways to minimize this problem is to have reliable NIR stellar libraries. The main goal of this work is to present smarty (mileS Moderate resolution neAr-infRared sTellar librarY) a ∼0.9−2.4 |$\mu$| m stellar spectral library composed of 31 stars observed with the Gemini Near-IR Spectrograph (GNIRS) at the 8.1 m Gemini North telescope and make it available to the community. The stars were chosen from the miles library, for which the atmospheric parameters are reliable (and well-tested), to populate different regions of the Hertzsprung–Russell (HR) diagram. Furthermore, five of these stars have NIR spectra available that we use to assess the quality of smarty. The remaining 26 stars are presented for the first time in the NIR. We compared the observed smarty spectra with synthetic and interpolated spectra, finding a mean difference of |$\sim 20~{{\ \rm per\ cent}}$| in the equivalent widths and ∼1 per cent in the overall continuum shape in both sets of comparisons. We computed the spectrophotometric broad-band magnitudes and colours and compared them with the 2MASS ones, resulting in mean differences up to 0.07 and 0.10 mag in magnitudes and colours, respectively. In general, a small difference was noted between the smarty spectra corrected using the continuum from the interpolated and the theoretical stars. [ABSTRACT FROM AUTHOR]

We investigate the formation and evolution of z = 0 massive compact galaxies (MCGs) in the IllustrisTNG cosmological simulation. We found that, as in observations, MCGs are mainly old (median age ∼10.8 Gyr), have supersolar metallicities (median log  Z /Z⊙ ∼ 0.35), and are α-enhanced (median [α/Fe] ∼ 0.25). The age distribution extends to younger ages, however, and a few MCGs are as young as ∼7 Gyr. In general, MCGs assemble their mass early and accrete low angular momentum gas, significantly increasing their mass while growing their size much slower. A small fraction of MCGs follow another evolutionary path, going through a compaction event, with their sizes shrinking by 40 per cent or more. The accretion of low angular momentum gas leads to enhanced supermassive black hole (SMBH) growth, and MCGs reach the threshold SMBH mass of |$\log M_\mathrm{BH}\sim 10^{8.5}\ \mathrm{ M}_\odot$| – when kinetic active galactic nucleus feedback kicks in and quenches the galaxy – earlier than non-compact galaxies. Comparing MCGs to a sample of median-sized quiescent galaxies matched in effective velocity dispersion, we find that their accretion histories are very different. 71 per cent of MCGs do not merge after quenching compared to 37 per cent of median-sized quiescent galaxies. Moreover, tracing these populations back in time, we find that at least a third of median-sized quiescent galaxies do not have a compact progenitor, underscoring that both dry mergers and progenitor bias effects are responsible for the differences in the kinematics and stellar population properties of MCGs and median-sized quiescent galaxies. [ABSTRACT FROM AUTHOR]

Near-infrared long-slit spectroscopy has been used to study the stellar population (SP) of the low luminosity active galactic nuclei (AGNs) and matched analogues (LLAMA) sample. To perform the SP fits we have employed the X-shooter simple stellar population models together with the starlight code. Our main conclusions are: The star formation history of the AGNs is very complex, presenting many episodes of star formation during their lifetimes. In general, AGN hosts have higher fractions of intermediate-age SP (light-weighted mean ages, < t > L ≲ 4.5 Gyr) when compared with their analogues (< t > L ≲ 8.0 Gyr). AGNs are more affected by reddening and require significant fractions of featureless continuum and hot dust components. The ratio between the AGN radiated energy and the gravitational potential energy of the molecular gas (E Rad/ E PG) for the AGN is compared with the < t > L and a possible anticorrelation is observed. This suggests that the AGN is affecting the star formation in these galaxies, in the sense that more energetic AGN [log(E Rad/ E PG) ≳ 3] tend to host nuclear younger SP (< t > L ≲4 Gyr). We found that the recent (t <2 Gyr) returned (recycled) stellar mass is higher in AGN than in the controls. We also provide evidence that the mass-loss of stars would be enough to feed the AGN, thus providing observational constraints for models that predict that AGN feeding is partially due to the recycled gas from dying stars. [ABSTRACT FROM AUTHOR]

We use Gemini integral field unit observations to map the stellar population properties in the inner region (∼680 × 470 pc2) of the galaxy NGC 6868. In order to understand the physical and chemical properties of the stellar content of this galaxy, we performed stellar population synthesis using the starlight code with the MILES simple stellar population models. We measured the absorption line indices Fe4383, Mg2, Mgb, Fe5270, and Fe5335 for the whole FoV, and used them to derive Fe3 and [MgFe]'. These indices were used to derive [α/Fe]. This galaxy is dominated by old metal-rich populations (12.6 Gyr; 1.0 and 1.6 Z⊙) with a negative metallicity gradient. We also found a recent (∼63 Myr) metal-rich (1.6 Z⊙) residual star formation in the centre of the galaxy. A dust lane with a peak extinction in the V band of 0.65 mag is seen. No signs of ordered stellar motion are found and the stellar kinematics is dispersion dominated. All indices show a spatial profile varying significantly along the FoV. Mg2 shows a shallow gradient, compatible with the occurrence of mergers in the past. Mgb and Fe3 profiles suggest different enrichment processes for these elements. We observe three distinct regions: for R < 100 pc and R > 220 pc, Mg2, Mgb anticorrelate with respect to Fe3 and [MgFe]', and for 100 pc < R < 220 pc, they correlate, hinting at different enrichment histories. The [α/Fe] profile is really complex and has a central value of ∼0.2 dex. We interpret this as the result of a past merger with another galaxy with a different [α/Fe] history, thus explaining the [α/Fe] maps. [ABSTRACT FROM AUTHOR]