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We investigate which physical properties are most predictive of the position of local star forming galaxies on the BPT diagrams, by means of different Machine Learning (ML) algorithms. Exploiting the large statistics from the Sloan Digital Sky Survey (SDSS), we define a framework in which the deviation of star-forming galaxies from their median sequence can be described in terms of the relative variations in a variety of observational parameters. We train artificial neural networks (ANN) and random forest (RF) trees to predict whether galaxies are offset above or below the sequence (via classification), and to estimate the exact magnitude of the offset itself (via regression). We find, with high significance, that parameters primarily associated to variations in the nitrogen-over-oxygen abundance ratio (N/O) are the most predictive for the [N II]-BPT diagram, whereas properties related to star formation (like variations in SFR or EW[H$\alpha$]) perform better in the [S II]-BPT diagram. We interpret the former as a reflection of the N/O-O/H relationship for local galaxies, while the latter as primarily tracing the variation in the effective size of the S$^{+}$ emitting region, which directly impacts the [S II]emission lines. This analysis paves the way to assess to what extent the physics shaping local BPT diagrams is also responsible for the offsets seen in high redshift galaxies or, instead, whether a different framework or even different mechanisms need to be invoked., Comment: Accepted for publication on MNRAS (31 pages, 20 figures)

Nebular He ii λ4686 Å line emission is useful to unveil active galactic nuclei (AGN) residing in actively star-forming (SF) galaxies, typically missed by the standard BPT classification. Here, we adopt the He ii diagnostic to identify hidden AGN in the Local Universe using for the first time spatially resolved data from the Data Release 15 of the Mapping Nearby Galaxies at APO survey (MaNGA DR15). By combining results from He ii and BPT diagnostics, we overall select 459 AGN host candidates (∼10 per cent in MaNGA DR15), out of which 27 are identified as AGN by the He ii diagram only. The He ii-only AGN population is hosted by massive (M* ≳ 1010 M⊙) SF Main Sequence galaxies and on average less luminous than the BPT-selected AGN. Given the He ii line faintness, we revisit our census accounting for incompleteness effects due to the He ii sensitivity limit of MaNGA. We thus obtain an overall increased fraction (11 per cent) of AGN in MaNGA compared to the BPT-only census (9 per cent), which further increases to 14 per cent for galaxies more massive than 1010 M⊙; interestingly, on the SF Main Sequence the increase is by about a factor of 2. A substantial number of AGN in SF galaxies points to significant, coeval star formation and black hole accretion, consistently with results from hydrodynamical simulations and with important implications on quenching scenarios. In view of exploring unprecedented high redshifts with JWST and new ground-based facilities, revisiting the standard BPT classification through novel emission-line diagnostics is fundamental to discover AGN in highly SF environments.

We present a comparison of the nitrogen-to-oxygen ratio (N/O) in 37 high-redshift galaxies at $z\sim$2 taken from the KMOS Lensed Emission Lines and VElocity Review (KLEVER) Survey with a comparison sample of local galaxies, taken from the Sloan Digital Sky Survey (SDSS). The KLEVER sample shows only a mild enrichment in N/O of $+$0.1 dex when compared to local galaxies at a given gas-phase metallicity (O/H), but shows a depletion in N/O of $-$0.36 dex when compared at a fixed stellar mass (M$_*$). We find a strong anti-correlation in local galaxies between N/O and SFR in the M$_*$-N/O plane, similar to the anti-correlation between O/H and SFR found in the mass-metallicity relation (MZR). We use this anti-correlation to construct a fundamental nitrogen relation (FNR), analogous to the fundamental metallicity relation (FMR). We find that KLEVER galaxies are consistent with both the FMR and the FNR. This suggests that the depletion of N/O in high-$z$ galaxies when considered at a fixed M$_*$ is driven by the redshift-evolution of the mass-metallicity relation in combination with a near redshift-invariant N/O-O/H relation. Furthermore, the existence of an fundamental nitrogen relation suggests that the mechanisms governing the fundamental metallicity relation must be probed by not only O/H, but also N/O, suggesting pure-pristine gas inflows are not the primary driver of the FMR, and other properties such as variations in galaxy age and star formation efficiency must be important.

We investigate the presence of ionised gas outflows in a sample of 141 main-sequence star-forming galaxies at $1.2 10.8$, AGN-dominated galaxies, suggesting that AGNs may be the primary drivers of these gas flows. Surprisingly, at $\log(M_\star/M_{\odot})\leq 9.6$, the observed line profiles are fully consistent with a rotating disc model, indicating that ionised gas outflows in dwarf galaxies might play a negligible role even during the peak of cosmic star-formation activity. Finally, we find that the observed mass loading factor scales with stellar mass as expected from the TNG50 cosmological simulation, but the ionised gas mass accounts for only 2$\%$ of the predicted value. This suggests that either the bulk of the outflowing mass is in other gaseous phases or the current feedback models implemented in cosmological simulations need to be revised., Comment: Accepted for publication in MNRAS, 28 pages, 20 figures

Black hole feedback has been widely implemented as the key recipe to quench star formation in massive galaxies in modern semi-analytic models and hydrodynamical simulations. As the theoretical details surrounding the accretion and feedback of black holes continue to be refined, various feedback models have been implemented across simulations, with notable differences in their outcomes. Yet, most of these simulations have successfully reproduced some observations, such as stellar mass function and star formation rate density in the local Universe. We use the recent observation on the change of neutral hydrogen gas mass (including both ${\rm H_2}$ and ${\rm HI}$) with star formation rate of massive central disc galaxies as a critical constraint of black hole feedback models across several simulations. We find that the predictions of IllustrisTNG agree with the observations much better than the other models tested in this work. This favors IllustrisTNG's treatment of active galactic nuclei - where kinetic winds are driven by black holes at low accretion rates - as more plausible amongst those we test. In turn, this also indirectly supports the idea that the massive central disc galaxy population in the local Universe was likely quenched by AGN feedback., Comment: 15 pages, 3+4 figures, accepted for publication in ApJ

The phenomenological study of evolving galaxy populations has shown that star forming galaxies can be quenched by two distinct processes: mass quenching and environment quenching (Peng et al. 2010). To explore the mass quenching process in local galaxies, we study the massive central disk galaxies with stellar mass above the Schechter characteristic mass. In Zhang et al. (2019), we showed that during the quenching of the massive central disk galaxies as their star formation rate (SFR) decreases, their molecular gas mass and star formation efficiency drop rapidly, but their HI gas mass remains surprisingly constant. To identify the underlying physical mechanisms, in this work we analyze the change during quenching of various structure parameters, bar frequency, and active galactic nucleus (AGN) activity. We find three closely related facts. On average, as SFR decreases in these galaxies: (1) they become progressively more compact, indicated by their significantly increasing concentration index, bulge-to-total mass ratio, and central velocity dispersion, which are mainly driven by the growth and compaction of their bulge component; (2) the frequency of barred galaxies increases dramatically, and at a given concentration index the barred galaxies have a significantly higher quiescent fraction than unbarred galaxies, implying that the galactic bar may play an important role in mass quenching; and (3) the "AGN" frequency increases dramatically from 10% on the main sequence to almost 100% for the most quiescent galaxies, which is mainly driven by the sharp increase of LINERs. These observational results lead to a self-consistent picture of how mass quenching operates., Comment: 20 pages, 11 figures. Published in ApJ

We report the detection of [O I]145.5um in the BR 1202-0725 system, a compact group at z=4.7 consisting of a quasar (QSO), a submillimeter-bright galaxy (SMG), and three faint Lya emitters. By taking into account the previous detections and upper limits, the [O I]/[C II] line ratios of the now five known high-z galaxies are higher than or on the high-end of the observed values in local galaxies ([O I]/[C II]$\gtrsim$0.13). The high [O I]/[C II] ratios and the joint analysis with the previous detection of [N II] lines for both the QSO and the SMG suggest the presence of warm and dense neutral gas in these highly star-forming galaxies. This is further supported by new CO (12-11) line detections and a comparison with cosmological simulations. There is a possible positive correlation between the [NII]122/205 line ratio and the [O I]/[C II] ratio when all local and high-z sources are taken into account, indicating that the denser the ionized gas, the denser and warmer the neutral gas (or vice versa). The detection of the [O I] line in the BR1202-0725 system with a relatively short amount of integration with ALMA demonstrates the great potential of this line as a dense gas tracer for high-z galaxies., STFC ERC

Star formation and quenching are two of the most important processes in galaxy formation and evolution. We explore in the local Universe the interrelationships among key integrated galaxy properties, including stellar mass $M_*$, star formation rate (SFR), specific SFR (sSFR), molecular gas mass $M_{\rm H_2}$, star formation efficiency (SFE) of the molecular gas and molecular gas to stellar mass ratio $\mu$. We aim to identify the most fundamental scaling relations among these key galaxy properties and their interrelationships. We show the integrated $M_{\rm H_2}$-SFR, SFR-$M_*$ and $M_{\rm H_2}$-$M_*$ relation can be simply transformed from the $\mu$-sSFR, SFE-$\mu$ and SFE-sSFR relation, respectively. The transformation, in principle, can increase or decrease the scatter of each relation. Interestingly, we find the latter three relations all have significantly smaller scatter than the former three corresponding relations. We show the probability to achieve the observed small scatter by accident is extremely close to zero. This suggests that the smaller scatters of the latter three relations are driven by a more fundamental physical connection among these quantities. We then show the large scatters in the former relations are due to their systematic dependence on other galaxy properties, and on star formation and quenching process. We propose the sSFR-$\mu$-SFE relation as the Fundamental Formation Relation (FFR), which governs the star formation and quenching process, and provides a simple framework to study galaxy evolution. Other scaling relations, including integrated Kennicutt-Schmidt law, star-forming main sequence and molecular gas main sequence, can all be derived from the FFR., Comment: This is the revised version accepted for publication in ApJ

Feedback from AGN is thought to be key in shaping the life cycle of their host galaxies by regulating star-formation activity. Therefore, to understand the impact of AGN on star formation, it is essential to trace the molecular gas out of which stars form. In this paper we present the first systematic study of the CO properties of AGN hosts at z~2 for a sample of 27 X-ray selected AGN spanning two orders of magnitude in AGN bolometric luminosity (Lbol= 10^44.7-10^46.9 erg/s) by using ALMA Band 3 observations of the CO(3-2) transition (~1" angular resolution). To search for evidence of AGN feedback on the CO properties of the host galaxies, we compared our AGN with a sample of inactive (i.e., non-AGN) galaxies from the PHIBSS survey with similar redshift, stellar masses, and SFRs. We used the same CO transition as a consistent proxy for the gas mass for the two samples in order to avoid systematics involved when assuming conversion factors. By adopting a Bayesian approach to take upper limits into account, we analyzed CO luminosities as a function of stellar masses and SFRs, as well as the ratio LCO(3-2)/M* (proxy for the gas fraction). The two samples show statistically consistent trends in the LCO(3-2)-Lfir and LCO(3-2)-M* planes. However, there are indications that AGN feature lower CO(3-2) luminosities (0.4-0.7 dex) than inactive galaxies at the 2-3sigma level when we focus on the subset of parameters where the results are better constrained and on the distribution of the mean LCO(3-2)/M*. Therefore, even by conservatively assuming the same excitation factor r31, we would find lower molecular gas masses in AGN, and assuming higher r31 would exacerbate this difference. We interpret our result as a hint of the potential effect of AGN activity (e.g., radiation and outflows), which may be able to heat, excite, dissociate, and/or deplete the gas reservoir of the host galaxies. (abridged), Comment: Key figures: 2, 3, 4. 22 pages, 9 figures (5 in appendices), 3 tables. Accepted for publication in A&A

The SINFONI survey for Unveiling the Physics and Effect of Radiative feedback (SUPER) aims at tracing and characterizing ionized gas outflows and their impact on star formation in a statistical sample of X-ray selected Active Galactic Nuclei (AGN) at z$\sim$2. We present the first SINFONI results for a sample of 21 Type-1 AGN spanning a wide range in bolometric luminosity (log $\mathrm{L_{bol}}$ = 45.4-47.9 erg/s). The main aims of this paper are determining the extension of the ionized gas, characterizing the occurrence of AGN-driven outflows, and linking the properties of such outflows with those of the AGN. We use Adaptive Optics-assisted SINFONI observations to trace ionized gas in the extended narrow line region using the [OIII]5007 line. We classify a target as hosting an outflow if its non-parametric velocity of the [OIII] line, $\mathrm{w_{80}}$, is larger than 600 km/s. We study the presence of extended emission using dedicated point-spread function (PSF) observations, after modelling the PSF from the Balmer lines originating from the Broad Line Region. We detect outflows in all the Type-1 AGN sample based on the $\mathrm{w_{80}}$ value from the integrated spectrum, which is in the range 650-2700 km/s. There is a clear positive correlation between $\mathrm{w_{80}}$ and the AGN bolometric luminosity (99% correlation probability), but a weaker correlation with the black hole mass (80% correlation probability). A comparison of the PSF and the [OIII] radial profile shows that the [OIII] emission is spatially resolved for $\sim$35% of the Type-1 sample and the outflows show an extension up to $\sim$6 kpc. The relation between maximum velocity and the bolometric luminosity is consistent with model predictions for shocks from an AGN driven outflow. The escape fraction of the outflowing gas increase with the AGN luminosity, although for most galaxies, this fraction is less than 10%., Accepted for publication in A&A, 34 pages, 14 figures and 5 tables

In Dou et al. (2021), we introduced the Fundamental Formation Relation (FFR), a tight relation between specific SFR (sSFR), H$_2$ star formation efficiency (SFE$_{\rm H_2}$), and the ratio of H$_2$ to stellar mass. Here we show that atomic gas HI does not follow a similar FFR as H$_2$. The relation between SFE$_{\rm HI}$ and sSFR shows significant scatter and strong systematic dependence on all of the key galaxy properties that we have explored. The dramatic difference between HI and H$_2$ indicates that different processes (e.g., quenching by different mechanisms) may have very different effects on the HI in different galaxies and hence produce different SFE$_{\rm HI}$-sSFR relations, while the SFE$_{\rm H_2}$-sSFR relation remains unaffected. The facts that SFE$_{\rm H_2}$-sSFR relation is independent of other key galaxy properties, and that sSFR is directly related to the cosmic time and acts as the cosmic clock, make it natural and very simple to study how different galaxy populations (with different properties and undergoing different processes) evolve on the same SFE$_{\rm H_2}$-sSFR $\sim t$ relation. In the gas regulator model (GRM), the evolution of a galaxy on the SFE$_{\rm H_2}$-sSFR($t$) relation is uniquely set by a single mass-loading parameter $\lambda_{\rm net,H_2}$. This simplicity allows us to accurately derive the H$_2$ supply and removal rates of the local galaxy populations with different stellar masses, from star-forming galaxies to the galaxies in the process of being quenched. This combination of FFR and GRM, together with the stellar metallicity requirement, provide a new powerful tool to study galaxy formation and evolution., Comment: 16 pages, 7 figures, accepted for publication in ApJ

Outflows driven by active galactic nuclei (AGN) are expected to have a significant impact on the host galaxy evolution, but it is still debated how they are accelerated and propagate on galaxy-wide scales. This work addresses these questions by studying the link between X-ray, nuclear ultra-fast outflows (UFOs) and extended ionised outflows, for the first time in two quasars close to the peak of AGN activity ($z\sim2$), where AGN feedback is expected to be more effective. As targets, we selected two multiple-lensed quasars at $z\sim1.5$, HS 0810+2554 and SDSS J1353+1138, known to host UFOs and observed with the near-IR integral field spectrometer SINFONI at the VLT. We performed a kinematical analysis of the [O III]$\lambda$5007 optical emission line, in order to trace the presence of ionised outflows. We detected spatially resolved ionised outflows in both galaxies, extended more than 8 kpc and moving up to $v>2000$ km/s. We derived mass outflow rates of $\sim$12 M$_{sun}$/yr and $\sim$2 M$_{sun}$/yr for HS 0810+2554 and SDSS J1353+1138. Comparing with the co-hosted UFO energetics, the ionised outflow energetics in HS 0810+2554 is broadly consistent with a momentum-driven regime of wind propagation, while in SDSS J1353+1138 it differs by a factor of $\sim$100 from theoretical predictions, requiring either a massive molecular outflow or a high variability of the AGN activity to account for such a discrepancy. By additionally considering our results with those from the small sample of well-studied objects (all local but one), with both UFO and extended (ionised/atomic/molecular) outflow detections, we found that in 10 out of 12 galaxies the large-scale outflow energetics is consistent with the theoretical predictions of either a momentum- or an energy-driven scenario. This suggests that such models explain relatively well the acceleration mechanism of AGN-driven winds on large scales., Comment: 19 pages, 9 figures. Accepted for publication in A&A

The evolution of the content of heavy elements in galaxies, the relative chemical abundances, their spatial distribution, and how these scale with various galactic properties, provide unique information on the galactic evolutionary processes across the cosmic epochs. In recent years major progress has been made in constraining the chemical evolution of galaxies and inferring key information relevant to our understanding of the main mechanisms involved in galaxy evolution. In this review we provide an overview of these various areas. After an overview of the methods used to constrain the chemical enrichment in galaxies and their environment, we discuss the observed scaling relations between metallicity and galaxy properties, the observed relative chemical abundances, how the chemical elements are distributed within galaxies, and how these properties evolve across the cosmic epochs. We discuss how the various observational findings compare with the predictions from theoretical models and numerical cosmological simulations. Finally, we briefly discuss the open problems the prospects for progress in this field in the nearby future., Comment: Invited review fo Astronomy & Astrophysics Review, 139 pages, 60 figures

The massive galaxy population above the characteristic Schechter mass M*~10^10.6 Msun contributes to about half of the total stellar mass in the local Universe. These massive galaxies usually reside in hot dark matter haloes above the critical shock-heating mass ~10^12 Msun, where the external cold gas supply to these galaxies is expected to be suppressed. When galaxies run out of their cold gas reservoir, they become dead and quiescent. Therefore, massive quiescent galaxies living in hot haloes are commonly believed to be gas-poor. Based on the data from SDSS, ALFALFA, GASS and COLD GASS surveys, here we show that the vast majority of the massive, quiescent, central disk galaxies in the nearby Universe have a remarkably large amount of cold atomic hydrogen gas, surprisingly similar to star-forming galaxies. Both star-forming and quiescent disk galaxies show identical symmetric double-horn HI spectra, indicating similar regularly rotating HI disks. Relative to their star-forming counterparts, massive quiescent central disk galaxies are quenched because of their significantly reduced molecular gas content, lower dust content, and lower star formation efficiency. Our findings reveal a new picture, which clearly demonstrates the detailed star-formation quenching process in massive galaxies and provides a stringent constraint on the physical mechanism of quenching., 10 pages, 4 figures (+2 figures in the appendix); accepted for publication in The Astrophysical Journal Letters