Context. The morphological, spectroscopic, and kinematical properties of the warm interstellar medium (wim) in early-type galaxies (ETGs) hold key observational constraints to nuclear activity and the buildup history of these massive, quiescent systems. High-quality integral field spectroscopy (IFS) data with a wide spectral and spatial coverage, such as those from the CALIFA survey, offer an unprecedented opportunity for advancing our understanding of the wim in ETGs. Aims. This article centers on a 2D investigation of the wim component in 32 nearby ( < ∼ 150 Mpc) ETGs from CALIFA, complementing a previous 1D analysis of the same sample. Methods. The analysis presented here includes Hα intensity and equivalent width (EW) maps and radial profiles, diagnostic emission-line ratios, and ionized-gas and stellar kinematics. It is supplemented by τ-ratio maps, which are a more efficient means to quantify the role of photoionization by the post-AGB stellar component than alternative mechanisms (e.g., AGN, low-level star formation). Results. Confirming and strengthening our previous conclusions, we find that ETGs span a broad continuous sequence in the properties of their wim, exemplified by two characteristic classes. The first (type i) comprises systems with a nearly constant EW(Hα) in their extranuclear component, which quantitatively agrees with (but is no proof of) the hypothesis that photoionization by the post-AGB stellar component is the main driver of extended wim emission. The second class (type ii) stands for virtually wim-evacuated ETGs with a very low (≤0.5 Å), outwardly increasing EW(Hα). These two classes appear indistinguishable from one another by their LINER-specific emission-line ratios in their extranuclear component. Here we extend the tentative classification we proposed previously by the type i+, which is assigned to a subset of type i ETGs exhibiting ongoing low-level star-forming activity in their periphery. This finding along with faint traces of localized star formation in the extranuclear component of several of our sample galaxies points to a non-negligible contribution by OB stars to the global ionizing photon budget in ETGs. Additionally, our data again highlight the diversity of ETGs in their gaseous and stellar kinematics. While in one half of our sample, gas and stars show similar (yet not necessarily identical) velocity patterns that are both dominated by rotation along the major galaxy axis, our analysis also documents several cases of kinematical decoupling between gas and stars, or rotation along the minor galaxy axis. We point out that the generally very low ( < ∼ 1 Å) EW(Hα) of ETGs requires a careful quantitative assessment of potential observational and analysis biases in studies of their wim. With standard emission-line fitting tools, Balmer emission lines become progressively difficult to detect below an EW(Hα) ∼ 3 Å, therefore our current understanding of the presence and 2D emission patterns and kinematics of the diffuse wim ETGs may be severely incomplete. We demons...

Based on a combined analysis of SDSS imaging and CALIFA integral field spectroscopy data, we report on the detection of faint (24 < µ r mag/ < 26) star-forming spiral-arm-like features in the periphery of three nearby early-type galaxies (ETGs). These features are of considerable interest because they document the still ongoing inside-out growth of some local ETGs and may add valuable observational insight into the origin and evolution of spiral structure in triaxial stellar systems. A characteristic property of the nebular component in the studied ETGs, classified i+, is a two-radial-zone structure, with the inner zone that displays faint (EW(Hα) 1 Å) low-ionization nuclear emission-line region (LINER) properties, and the outer one (3 Å < EW(Hα) < ∼ 20 Å) H-region characteristics. This spatial segregation of nebular emission in two physically distinct concentric zones calls for an examination of aperture effects in studies of type i+ ETGs with single-fiber spectroscopic data.

Context. A key subject in extragalactic astronomy concerns the chronology and driving mechanisms of bulge formation in late-type galaxies (LTGs). The standard scenario distinguishes between classical bulges and pseudo-bulges (CBs and PBs, respectively), the first thought to form monolithically prior to disks and the second gradually out of disks. These two bulge formation routes obviously yield antipodal predictions on the bulge age and bulge-to-disk age contrast, both expected to be high (low) in CBs (PBs). Aims. Our main goal is to explore whether bulges in present-day LTGs segregate into two evolutionary distinct classes, as expected from the standard scenario. Other questions motivating this study center on evolutionary relations between LTG bulges and their hosting disks, and the occurrence of accretion-powered nuclear activity as a function of bulge stellar mass M and stellar surface density Σ . Methods. In this study we have combined three techniques -surface photometry, spectral modeling of integral field spectroscopy data and suppression of stellar populations younger than an adjustable age cutoff with the code REMOVEYOUNG (RY) -toward a systematic analysis of the physical and evolutionary properties (e.g., M , Σ and mass-weighted stellar age t M and metallicity Z M , respectively) of a representative sample of 135 nearby (≤130 Mpc) LTGs from the CALIFA survey that cover a range between 10 8.9 M and 10 11.5 M in total stellar mass M ,T . In particular, the analysis here revolves around <δµ 9G >, a new distance-and formally extinction-independent measure of the contribution by stellar populations of age ≥9 Gyr to the mean r-band surface brightness of the bulge. We argue that <δµ 9G > offers a handy semi-empirical tracer of the physical and evolutionary properties of LTG bulges and a promising means for their characterization. Results. The essential insight from this study is that LTG bulges form over three dex in M and more than one dex in Σ a tight continuous sequence of increasing <δµ 9G > with increasing M , Σ , t M and Z M . Along this continuum of physical and evolutionary properties, our sample spans a range of ∼4 mag in <δµ 9G >: high-<δµ 9G > bulges are the oldest, densest and most massive ones ( t M ∼11.7 Gyr, Σ > 10 9 M kpc −2 , M ≥ 10 10 M ), whereas the opposite is the case for low-<δµ 9G > bulges ( t M ∼7 Gyr) that generally reside in low-mass LTGs. Furthermore, we find that the bulge-to-disk age and metallicity contrast, as well as the bulge-todisk mass ratio, show a positive trend with M ,T , raising from, respectively, ∼0 Gyr, ∼0 dex and 0.25 to ∼3 Gyr, ∼0.3 dex and 0.67 across the mass range covered by our sample. Whereas gas excitation in lower-mass ( 10 9.7 M ) bulges is invariably dominated by star formation (SF), LINER-and Seyfert-specific emission-line ratios were exclusively documented in high-mass ( 10 10 M ), high-Σ ( 10 9 M kpc −2 ) bulges. This is in agreement with previous work and consistent with the notion that the Eddington ratio or the black hole-to-bulge mass rati...

Radial age gradients hold the cumulative record of the multitude of physical processes driving the build-up of stellar populations and the ensuing star formation (SF) quenching process in galaxy bulges, therefore potentially sensitive discriminators between competing theoretical concepts on bulge formation and evolution. Based on spectral modeling of integral field spectroscopy (IFS) data from the CALIFA survey, we derive mass-and light-weighted stellar age gradients (∇(t ,B ) L,M ) within the photometrically determined bulge radius (R B ) of a representative sample of local face-on late-type galaxies that span 2.6 dex in stellar mass (8.9 ≤ logM ,T ≤ 11.5). Our analysis documents a trend for decreasing ∇(t ,B ) L,M with increasing M ,T , with high-mass bulges predominantly showing negative age gradients and vice versa. The inversion from positive to negative ∇(t ,B ) L,M occurs at logM ,T 10, which roughly coincides with the transition from lower-mass bulges whose gas excitation is powered by SF to bulges classified as Composite, LINER or Seyfert. We discuss two simple limiting cases for the origin of radial age gradients in massive LTG bulges. The first one assumes that the stellar age in the bulge is initially spatially uniform (∇(t ,B ) L,M ≈ 0), thus the observed age gradients (∼ -3 Gyr/R B ) arise from an inside-out SF quenching (ioSFQ) front that is radially expanding with a mean velocity v q . In this case, the age gradients for massive bulges translate into a slow (v q ∼1-2 km s −1 ) ioSFQ that lasts until z ∼ 2, suggesting mild negative feedback by SF or an AGN. If, on the other hand, negative age gradients in massive bulges are not due to ioSFQ but primarily due to their inside-out formation process, then the standard hypothesis of quasi-monolithic bulge formation has to be discarded in favor of a scenario that involves gradual buildup of stellar mass over 2-3 Gyr through, e.g., inside-out SF and inward migration of SF clumps from the disk. In this case, rapid ( 1 Gyr) AGN-driven ioSFQ cannot be ruled out. While the M ,T vs. ∇(t ,B ) L,M relation suggests that the assembly history of bulges is primarily regulated by galaxy mass, its large scatter (∼1.7 Gyr/R B ) reflects a considerable diversity that calls for an in-depth examination of the role of various processes (e.g., negative and positive AGN feedback, bar-driven gas inflows) with higher-quality IFS data in conjunction with advanced spectral modeling codes.

Context. The field of galaxy evolution will make a great leap forward in the next decade as a consequence of the huge effort by the scientific community in multi-object spectroscopic facilities. Various future surveys will enormously increase the number of available galaxy spectra, providing new insights into unexplored areas of research. To maximise the impact of such incoming data, the analysis methods must also step up, extracting reliable information from the available spectra. It is therefore urgent to refine and test reliable analysis tools that are able to infer the properties of a galaxy from medium- or high-resolution spectra. Aims. In this paper we aim to investigate the limits and the reliability of different spectral synthesis methods in the estimation of the mean stellar age and metallicity. These two quantities are fundamental to determine the assembly history of a galaxy by providing key insights into its star formation history. The main question this work aims to address is which signal-to-noise ratios (S/N) are needed to reliably determine the mean stellar age and metallicity from a galaxy spectrum and how this depends on the tool used to model the spectra. Methods. To address this question we built a set of realistic simulated spectra containing stellar and nebular emission, reproducing the evolution of a galaxy in two limiting cases: a constant star formation rate and an exponentially declining star formation with a single initial burst. We degraded the synthetic spectra built from these two star formation histories (SFHs) to different S/N and analysed with three widely used spectral synthesis codes, namely FADO, STECKMAP, and STARLIGHT, assuming similar fitting set-ups and the same base of spectral templates. Results. For S/N ≤ 5 all the three tools show a large diversity in the results. The FADO and STARLIGHT tools find median differences in the light-weighted mean stellar age of ∼0.1 dex, while STECKMAP shows a higher value of ∼0.2 dex. For S/N >  50 the median differences in FADO are ∼0.03 dex (∼7%), a factor 3 and 4 lower than the 0.08 dex (∼20%) and 0.11 dex (∼30%) obtained from STARLIGHT and STECKMAP, respectively. Detailed investigations of the best-fit spectrum for galaxies with overestimated mass-weighted quantities point towards the inability of purely stellar models to fit the observed spectral energy distribution around the Balmer jump. Conclusions. Our results imply that when a galaxy enters a phase of high specific star formation rate (sSFR) the neglect of the nebular continuum emission in the fitting process has a strong impact on the estimation of its SFH when purely stellar fitting codes are used, even in presence of high S/N spectra. The median value of these differences are of the order of 7% (FADO), 20% (STARLIGHT), and 30% (STECKMAP) for light-weighted quantities, and 20% (FADO), 60% (STARLIGHT), and 20% (STECKMAP) for mass-weighted quantities. More specifically, for a continuous SFH both STECKMAP and STARLIGHT overestimate the stellar age by > 2 dex within the first ∼100 Myr even for high S/N spectra. This bias, which stems from the neglect of nebular continuum emission, obviously implies a severe overestimation of the mass-to-light ratio and stellar mass. But even in the presence of a mild contribution from nebular continuum, there is still the possibility to misinterpret the data as a consequence of the poor quality of the observations. Our work underlines once more the importance of a self-consistent treatment of nebular emission, as implemented in FADO, which, according to our analysis, is the only viable route towards a reliable determination of the assembly of any high-sSFR galaxy at high and low redshift.

Context. A fundamental subject in extragalactic astronomy concerns the formation and evolution of late-type galaxies (LTGs). The standard scenario envisages a two-phase build-up for these systems, comprising the early assembly of the bulge followed by disk accretion. However, recent observational evidence points to a joint formation and perpetual coevolution of these structural components. Our current knowledge on the properties of the bulge and the disk is, to a large degree, founded on photometric decomposition studies, which sensitively depend on the adopted methodology and enclosed assumptions on the structure of LTGs. A critical assumption whose validity had never been questioned before is that galactic disks conserve their exponential nature up to the galactic center. This, although seemingly plausible, implies that bulge and disk coexist without significant dynamical interaction and mass exchange over nearly the entire Hubble time. Aims. Our goal is to examine the validity of the standard assumption that galactic disks preserve their exponential intensity profile inside the bulge radius (RB) all the way to the galactic center, as is generally assumed in photometric decomposition studies. Methods. We developed a spectrophotometric bulge-disk decomposition technique that provides an estimation for the net (i.e., disk-subtracted) spectrum of the bulge. Starting from an integral field spectroscopy (IFS) data cube, this tool computes the integrated spectrum of the bulge and the disk, scales the latter considering the light fraction estimated from photometric decomposition techniques, and subtract it from the former, thereby allowing for the extraction of the net-bulge spectrum. Considering that the latter depends on the underlying assumption for the disk luminosity profile, checking its physical plausibility (for instance, positiveness and spectral slope) places indirect constraints on the validity of the disk’s assumed profile inside the radius R⋆ < RB. In this pilot study, we tested the following three different disk configurations: the standard exponential disk profile as well as a centrally flattened or down-bending exponential disk profile. Results. A systematic application of our spectrophotometric bulge-disk decomposition tool to a representative sample of 135 local LTGs from the CALIFA survey yields a significant fraction (up to ∼30 (20)%) of unphysical net-bulge spectra when a purely exponential (centrally flattened) intensity profile is assumed for the disk. This never occurs for disks’ profiles involving a centrally decreasing intensity. Conclusions. The obtained results suggest that, for a significant fraction of LTGs, the disk component shows a down-bending beneath the bulge. If proven to be true, this result will call for a substantial revision of structural decomposition studies for LTGs and it will have far-reaching implications in our understanding of the photometric properties of their bulges. Given its major relevance, it appears worthwhile to explore the central stellar surface density of galactic disks further, through an improved version of the spectrophotometric decomposition tool presented here and its application combining deep surface photometry, spatially resolved spectral synthesis, and kinematical analyses.

Extreme emission line galaxies (EELGs) are a notable galaxy genus, ultimately being regarded as local prototypes of early galaxies at the cosmic noon. Robust characterisation of their stellar content, however, is hindered by the exceptionally high nebular emission present in their optical spectroscopic data. This study is dedicated into recovering the stellar properties of a sample of 414 EELGs as observed by the SDSS Survey. Such is achieved by means of the spectral synthesis code FADO, which self-consistently considers the stellar and nebular emission in an optical spectrum. Additionally, a comparative analysis was carried on, by further processing the EELGs sample with the purely stellar spectral synthesis code STARLIGHT, and by extending the analysis to a sample of 697 normal star-forming galaxies, expected to be less affected by nebular contribution. We find that, for both galaxy samples, stellar mass and mean age estimates by STARLIGHT are systematically biased towards higher values, and that an adequate determination of the physical and evolutionary properties of EELGs via spectral synthesis is only possible when nebular continuum emission is taken into account. Moreover, the differences between the two population synthesis codes can be ascribed to the degree of star-formation activity through the specific star-formation rate and the sum of the flux of the most prominent emission lines. As expected, on the basis of the theoretical framework, our results emphasise the importance of considering the nebular emission while performing spectral synthesis, even for galaxies hosting typical levels of star-formation activity.

Integral field spectroscopy (IFS) studies based on CALIFA survey data have recently revealed ongoing low-level star formation (SF) in the periphery of a small fraction (∼10%) of local early-type galaxies (ETGs), witnessing a still ongoing inside-out galaxy growth process. A distinctive property of the nebular component in these ETGs, classified i+, is a structure with two radial zones, the inner of which displays LINER emission with a Hα equivalent width EW(Hα) 1 Å, the outer (3 Å < EW(Hα) < ∼ 20 Å) H-region characteristics. Using CALIFA IFS data, we empirically demonstrate that the confinement of nebular emission to the galaxy periphery leads to a strong aperture (or, correspondingly, redshift) bias in spectroscopic single-fiber studies of type i+ ETGs: at low redshift (z < ∼ 0.45), SDSS spectroscopy is restricted to the inner (SF-devoid LINER) zone, which causes the galaxies to be erroneously classified as "retired", that is, systems entirely lacking SF, and whose faint nebular emission is solely powered by the post-AGB stellar component. The SDSS aperture progressively encompasses the outer SF zone only at higher z, at which the galaxies are unambiguously classified as "composite SF/LINER". We also empirically demonstrate that the principal effect of a decreasing spectroscopic aperture on the classification of i+ ETGs through standard [N]/Hα vs.[O]/Hβ emission-line (BPT) ratios consists of a monotonic shift upward and to the right precisely along the upper right wing of the "seagull" distribution on the BPT plane, that is, along the pathway connecting composite SF/H galaxies with AGN/LINERs. Motivated by these observational insights, we also investigate theoretically observational biases in aperture-limited studies of inside-out growing galaxies as a function of z. To this end, we devise a simple 1D model that involves an outward-propagating exponentially decreasing SF process since z ∼ 10 and reproduces the radial extent and two-zone EW(Hα) distribution of local i+ ETGs. By simulating the 3 spectroscopic SDSS aperture in this model, we find that SDSS studies at z < ∼ 1 are progressively restricted to the inner (SF-devoid LINER) zone and miss an increasingly larger portion of the Hα-emitting periphery. This leads to the incorrect spectroscopic classification of these inside-out assembling galaxies as retired ETG/LINERs and also to a severe underestimation of their total star formation rate (SFR) in a manner inversely related to z. More specifically, the SFR inferred from the Hα luminosity registered within the SDSS fiber is reduced by 50% at z ∼ 0.86, reaching only 0.1% of its integral value at z = 0.1. We argue that the aperture-driven biases described above pertain to any morphological analog of i+ ETGs (e.g., SF-quiescent bulges within star-forming disks), regardless of whether it is viewed from the perspective of inside-out growth or inside-out SF quenching, and might be of considerable relevance to galaxy taxonomy and studies of the cosmic SFR density as a function of z.