A numerous population of weak line galaxies (WLGs) is often left out of statistical studies on emission-line galaxies (ELGs) due to the absence of an adequate classification scheme, since classical ...diagnostic diagrams, such as O iii/Hβ versus N ii/Hα (the BPT diagram), require the measurement of at least four emission lines. This paper aims to remedy this situation by transposing the usual divisory lines between star-forming (SF) galaxies and active galactic nuclei (AGN) hosts and between Seyferts and LINERs to diagrams that are more economical in terms of line quality requirements. By doing this, we rescue from the classification limbo a substantial number of sources and modify the global census of ELGs. More specifically, (1) we use the Sloan Digital Sky Survey Data Release 7 to constitute a suitable sample of 280 000 ELGs, one-third of which are WLGs. (2) Galaxies with strong emission lines are classified using the widely applied criteria of Kewley et al., Kauffmann et al. and Stasińska et al. to distinguish SF galaxies and AGN hosts and Kewley et al. to distinguish Seyferts from LINERs. (3) We transpose these classification schemes to alternative diagrams keeping N ii/Hα as a horizontal axis, but replacing Hβ by a stronger line (Hα or O ii), or substituting the ionization-level sensitive O iii/Hβ ratio with the equivalent width of Hα(WHα). Optimized equations for the transposed divisory lines are provided. (4) We show that nothing significant is lost in the translation, but that the new diagrams allow one to classify up to 50 per cent more ELGs. (5) Introducing WLGs in the census of galaxies in the local Universe increases the proportion of metal-rich SF galaxies and especially LINERs. In the course of this analysis, we were led to make the following points. (i) The Kewley et al. BPT line for galaxy classification is generally ill-used. (ii) Replacing O iii/Hβ by WHα in the classification introduces a change in the philosophy of the distinction between LINERs and Seyferts, but not in its results. Because the WHα versus N ii/Hα diagram can be applied to the largest sample of ELGs without loss of discriminating power between Seyferts and LINERs, we recommend its use in further studies. (iii) The dichotomy between Seyferts and LINERs is washed out by WLGs in the BPT plane, but it subsists in other diagnostic diagrams. This suggests that the right wing in the BPT diagram is indeed populated by at least two classes, tentatively identified with bona fide AGN and ‘retired’ galaxies that have stopped forming stars and are ionized by their old stellar populations.
We investigate the cosmic evolution of the absolute and specific star formation rate (SFR, sSFR) of galaxies as derived from a spatially resolved study of the stellar populations in a set of 366 ...nearby galaxies from the Calar Alto Legacy Integral Field Area (CALIFA) survey. The sample spans stellar masses from M⋆ ~ 109 to 1012M⊙ and a wide range of Hubble types. The analysis combines images obtained with the Galaxy Evolution Explorer (GALEX; far-ultraviolet and near-ultraviolet) and Sloan Digital Sky Survey (SDSS; u, g, r, i, z) with the 4000 Å break, Hβ, and MgFe′ indices measured from the CALIFA data cubes to constrain parametric models for the star formation history (SFH), which are then used to study the cosmic evolution of the SFR density (ρSFR), the sSFR, the main sequence of star formation (MSSF), and the stellar mass density (ρ⋆). Several SFH laws are used to fit the observational constrains. A delayed-τ model, SFR ∝ (t0 − t)exp(−(t0 − t)∕τ), provides the best results, in good agreement with those obtained from cosmological surveys. Our main results from this model are that (a) the mass currently in the inner (≤0.5 half-light radius, HLR) regions formed at earlier epochs than the mass in the outer (1–2 HLR) regions of galaxies. The time since the onset of the star formation is longer in the inner regions (t0 ~ 13−10 Gyr) than in the outer ones (t0 ~ 11−9 Gyr) for all the morphologies, while the e-folding timescale τ in the inner region is similar to or shorter than in the outer regions. These results confirm that galaxies of any Hubble type grow inside-out. (b) The sSFR declines rapidly as the Universe evolves, and faster for early- than for late-type galaxies, and for the inner than for the outer regions of galaxies. (c) The evolution of ρSFR and ρ⋆ agrees well with results from cosmological surveys, particularly with the recent results from the Galaxy And Mass Assembly (GAMA), the G10-Cosmological Evolution Survey (COSMOS), and the 3D Hubble Space Telescope (HST) survey. At low redshift, z ≤ 0.5, most star formation takes place in the outer regions of late spiral galaxies, while at z > 2, the inner regions of the progenitors of the current E and S0 are the main contributors to ρSFR. (d) Similarly, the inner regions of galaxies are the main contributor to ρ⋆ at z > 0.5, growing their mass faster than the outer regions, with a lookback time at 50% ρ⋆ of t50 ~ 9 and 6 Gyr for the inner and outer regions. (e) The MSSF follows a power law at high redshift, with the slope evolving with time but always remaining sub-linear, in good agreement with the Illustris simulation. (f) In agreement with galaxy surveys at different redshifts, the average SFH of CALIFA galaxies indicates that galaxies grow their mass mainly in a mode that is well represented by a delayed-τ model, with the peak at z ~ 2 and an e-folding time of ~3.9 Gyr.
This paper, the sixth in the Semi-Empirical Analysis of Galaxies series, studies the evolution of 82 302 star-forming (SF) galaxies from the Sloan Digital Sky Survey. Star formation histories (SFHs) ...are derived from detailed spectral fits obtained with our publicly available spectral synthesis code starlight. Our main goals are to explore new ways to derive SFHs from the synthesis results and apply them to investigate how SFHs vary as a function of nebular metallicity (Zneb). A number of refinements over our previous work are introduced, including (1) an improved selection criterion; (2) a careful examination of systematic residuals around Hβ; (3) self-consistent determination of nebular extinctions and metallicities; (4) tests with several Zneb estimators; (5) a study of the effects of the reddening law adopted and of the relation between nebular and stellar extinctions and the interstellar component of the Na i D doublet. Our main achievements may be summarized as follows. (1) A conventional correlation analysis is performed to study how global properties relate to Zneb, leading to the confirmation of previously known relations, such as those between Zneb and galaxy luminosity, mass, dust content, mean stellar metallicity and mean stellar age. (2) A simple formalism which compresses the results of the synthesis while at the same time yielding time-dependent star formation rates (SFR) and mass assembly histories is presented. (3) A comparison of the current SFR derived from the population synthesis with that obtained from Hα shows that these independent estimators agree very well, with a scatter of a factor of 2. An important corollary of this finding is that we now have a way to estimate SFR in galaxies hosting active galactic nuclei, where the Hα method cannot be applied. (4) Fully time-dependent SFHs were derived for all galaxies, and then averaged over six Zneb bins spanning the entire SF wing in the diagram. (5) We find that SFHs vary systematically along the SF sequence. Though all SF galaxies formed the bulk of their stellar mass over 1 Gyr ago, low-Zneb systems evolve at a slower pace and are currently forming stars at a much higher relative rate. Galaxies at the tip of the SF wing have current specific SFRs about two orders of magnitude larger than the metal-rich galaxies at its bottom. (6) At any given time, the distribution of specific SFRs for galaxies within a Zneb bin is broad and approximately lognormal. (7) The whole study was repeated grouping galaxies within bins of stellar mass and surface mass density, both of which are more fundamental drivers of SFH. Given the existence of strong Zneb−M★−Σ★ relations, the overall picture described above remains valid. Thus, low-M★ (low-Σ★) systems are the ones which evolve slower, with current specific SFRs much larger than more massive (dense) galaxies. (8) This overall pattern of SFHs as a function of Zneb, M★ or Σ★ is robust against changes in selection criteria, choice of evolutionary synthesis models for the spectral fits, and differential extinction effects.
We present the first systematic study of the stellar populations of ultra-diffuse galaxies (UDGs) in the field, integrating the large area search and characterization of UDGs by the SMUDGes survey ...with the twelve-band optical photometry of the S-PLUS survey. Based on Bayesian modeling of the optical colors of UDGs, we determine the ages, metallicities, and stellar masses of 100 UDGs distributed in an area of ∼330 deg2 in the Stripe 82 region. We find that the stellar masses and metallicities of field UDGs are similar to those observed in clusters and follow the trends previously defined in studies of dwarf and giant galaxies. However, field UDGs have younger luminosity-weighted ages than do UDGs in clusters. We interpret this result to mean that field UDGs have more extended star formation histories, including some that continue to form stars at low levels to the present time. Finally, we examine stellar population scaling relations that show that UDGs are, as a population, similar to other low surface brightness galaxies.
The classification of galaxies as star forming or active is generally done in the (O iii/Hβ, N ii/Hα) plane. The Sloan Digital Sky Survey (SDSS) has revealed that, in this plane, the distribution of ...galaxies looks like the two wings of a seagull. Galaxies in the right wing are referred to as Seyfert/LINERs, leading to the idea that non-stellar activity in galaxies is a very common phenomenon. Here, we argue that a large fraction of the systems in the right wing could actually be galaxies which stopped forming stars. The ionization in these ‘retired’ galaxies would be produced by hot post-asymptotic giant branch stars and white dwarfs. Our argumentation is based on a stellar population analysis of the galaxies via our starlight code and on photoionization models using the Lyman continuum radiation predicted for this population. The proportion of LINER galaxies that can be explained in such a way is, however, uncertain. We further show how observational selection effects account for the shape of the right wing. Our study suggests that nuclear activity may not be as common as thought. If retired galaxies do explain a large part of the seagull's right wing, some of the work concerning nuclear activity in galaxies, as inferred from SDSS data, will have to be revised.
We present the results of prompt optical follow-up of the electromagnetic counterpart of the gravitational-wave event GW170817 by the Transient Optical Robotic Observatory of the South Collaboration. ...We detected highly significant dimming in the light curves of the counterpart ( mag, mag, mag) over the course of only 80 minutes of observations obtained ∼35 hr after the trigger with the T80-South telescope. A second epoch of observations, obtained ∼59 hr after the event with the EABA 1.5 m telescope, confirms the fast fading nature of the transient. The observed colors of the counterpart suggest that this event was a "blue kilonova" relatively free of lanthanides.
ABSTRACT
We present the S-PLUS Transient Extension Program (STEP): a supernova and fast transient survey conducted in the southern hemisphere using data from the Southern Photometric Local Universe ...Survey (S-PLUS) Main Survey and the T80-South telescope. Transient astrophysical phenomena have a range of interest that goes through different fields of astrophysics and cosmology. With the detection of an electromagnetic counterpart to the gravitational wave (GW) event GW170817 from a binary neutron stars merger, new techniques and resources to study fast astrophysical transients in the multimessenger context have increased. In this paper, we present the STEP overview, the SN follow-up data obtained, data reduction, analysis of new transients and deep learning algorithms to optimize transient candidate selection. Additionally, we present prospects and optimized strategy for the search of gravitational wave counterparts in the current LIGO/Virgo/Kagra observational run (O4) in the context of T80-South telescope.
ABSTRACT
In this paper, we use photometric data from the Southern Photometric Local Universe Survey Data Release 4 survey to identify isolated galaxy pairs and analyse their characteristics and ...properties. Our results align with previous spectroscopic studies, particularly in luminosity function parameters, suggesting a consistent trait among galaxy systems. Our findings reveal a high fraction of red galaxies across all samples, irrespective of projected distance, velocity difference, or luminosity ratio. We found that the proximity of a neighbour to its central galaxy influences its colour due to environmental effects. We also found that central and neighbour have different behaviours: central galaxies maintain a stable red colour regardless of luminosity, while neighbour colours vary based on luminosity ratios. When the central is significantly brighter, the neighbour tends to be less red. According to our division in red, blue, and mixed pairs, we found evidence of galactic conformity. The red pair fractions increase in closer pairs and in pairs of similar luminosity, indicating shared environments promoting red galaxy formation. Analysing local density, the expected colour–density relation is of course recovered, but it is strongly determined by the stellar mass of the pair. In denser environments, the red pair fractions increase, blue pairs decrease, and for the mixed pairs it depends on their stellar mass: more massive mixed pairs decrease their fraction, whereas the less massive ones increase it. These results shed light on the intricate relationship between galaxy pairs, their characteristics, and environmental influences on colour, providing insights into their evolutionary histories.
ABSTRACT
In general, the atypical high velocity of some stars in the Galaxy can only be explained by invoking acceleration mechanisms related to extreme astrophysical events in the Milky Way. Using ...astrometric data from Gaia and the photometric information in 12 filters of the S-PLUS, we performed a kinematic, dynamical, and chemical analysis of 64 stars with Galactocentric velocities higher than 400 $\mathrm{km\, s}^{-1}$. All the stars are gravitationally bound to the Galaxy and exhibit halo kinematics. Some of the stars could be remnants of structures such as the Sequoia and the Gaia-Sausage/Enceladus. Supported by orbital and chemical analysis, we identified Gaia DR3 5401875170994688896 as a star likely to be originated at the centre of the Galaxy. Application of a machine learning technique to the S-PLUS photometric data allows us to obtain very good estimates of magnesium abundances for this sample of high-velocity stars.
ABSTRACT
We present tests of a new method to simultaneously estimate stellar population and emission-line (EL) properties of galaxies out of the Southern Photometric Local Universe Survey (S-PLUS) ...photometry. The technique uses the AlStar code, updated with an empirical prior that greatly improves its ability to estimate ELs using only the survey’s 12 bands. The tests compare the output of (noise-perturbed) synthetic photometry of the Sloan Digital Sky Survey (SDSS) galaxies to properties derived from previous full spectral fitting and detailed EL analysis. For realistic signal-to-noise ratios, stellar population properties are recovered to better than 0.2 dex in masses, mean ages, metallicities, and ±0.2 mag for the extinction. More importantly, ELs are recovered remarkably well for a photometric survey. We obtain input − output dispersions of 0.05–0.2 dex for the equivalent widths of O ii, O iii, Hβ, Hα, N ii, and S ii, and even better for lines stronger than ∼5 Å. These excellent results are achieved by combining two empirical facts into a prior that restricts the EL space available for the fits. (1) Because, for the redshifts explored here, Hα and N ii fall in a single narrow band (J0660), their combined equivalent width is always well recovered, even when N ii/Hα is not. (2) We know from SDSS that $W_{\mathrm{H}\alpha +\mathrm{N}\,\small {\rm II} }$ correlates with N ii/Hα, which can be used to tell if a galaxy belongs to the left or right wings in the classical Baldwin–Phillips–Telervich (BPT) diagnostic diagram. Example applications to integrated light and spatially resolved data are also presented, including a comparison with independent results obtained with the Multi-Unit Spectroscopic Explorer (MUSE)-based integral field spectroscopy.
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