Studio degli indici di Lick come traccianti evolutivi in galassie passive

Le galassie passive sono sistemi dominati da popolazioni stellari vecchie, non mostrano tracce di formazione stellare (spettri compatibili con assenza di righe in emissione e caratterizzati da righe in assorbimento), e ci sono evidenze osservative che indicano che le galassie passive abbiano iniziato ad assemblare la loro massa stellare a z. Gli spettri delle galassie passive conservano traccia dei meccanismi fisici ed evolutivi della loro popolazione stellare. Laddove si hanno a disposizione spettri di buona qualità, ovvero che abbiano un rapporto segnale-rumore elevato, l’informazione contenuta in tali spettri può essere dedotta dalla misura dell’intensità di alcune righe in assorbimento. Burstein et al. (1984) hanno costruito un insieme di indici spettroscopici, chiamato sistema di indici di Lick, i quali misurano l’intensità delle principali righe in assorbimento (nella regione di lunghezze d’onda ottiche tra 4000-6000 Å), in termini di larghezza equivalente. in questa tesi è stato adottato il metodo degli indici di Lick per stimare i parametri evolutivi di un campione di galassie passive. Gli obiettivi principali di questa tesi sono due: 1.) studiare l’evoluzione col redshift dei parametri di età, metallicità totale e abbondanze relative di elementi α rispetto al ferro di un campione di galassie estratto spettroscopicamente dalla SDDS (Moresco et al., 2011). L’obiettivo finale è quello di dedurre informazioni sulla storia di formazione stellare delle galassie del campione. 2.) realizzare una simulazione per valutare la possibilità di misurare gli indici di Lick negli spettri di galassie passive che verranno osservate con la missione futura Euclid. Da questo studio è emerso un chiaro andamento evolutivo del campione in linea con quello previsto dallo scenario evolutivo del mass-downsizing, per il quale la SFH di una popolazione stellare è fortemente vincolata dalla massa della popolazione stessa, nel senso che al crescere della massa la formazione delle galassie passive si colloca in epoche progressivamente più remote, e l’assemblaggio della loro massa stellare avviene in tempi scala via via inferiori. Dalla simulazione è emerso un risultato molto importante che deriva dalla robustezza delle misure del D4000 e riguarda la possibilità di determinare il redshift di galassie a z ≥ 1.5 con Euclid

Catching galaxies in the act of quenching star formation

Detecting galaxies when their star-formation is being quenched is crucial to understand the mechanisms driving their evolution. We identify for the first time a sample of quenching galaxies selected just after the interruption of their star formation by exploiting the [O III]5007/Halpha ratio and searching for galaxies with undetected [O III]. Using a sample of ~174000 star-forming galaxies extracted from the SDSS-DR8 at 0.04 < z < 0.21,we identify the ~300 quenching galaxy best candidates with low [O III]/Halpha, out of ~26000 galaxies without [O III] emission. They have masses between 10^9.7 and 10^10.8 Mo, consistently with the corresponding growth of the quiescent population at these redshifts. Their main properties (i.e. star-formation rate, colours and metallicities) are comparable to those of the star-forming population, coherently with the hypothesis of recent quenching, but preferably reside in higher-density environments.Most candidates have morphologies similar to star-forming galaxies, suggesting that no morphological transformation has occurred yet. From a survival analysis we find a low fraction of candidates (~0.58% of the star-forming population), leading to a short quenching timescale of tQ~50Myr and an e-folding time for the quenching history of tauQ~90Myr, and their upper limits of tQ<0.76 Gyr and tauQ<1.5Gyr, assuming as quenching galaxies 50% of objects without [O III] (~7.5%).Our results are compatible with a 'rapid' quenching scenario of satellites galaxies due to the final phase of strangulation or ram-pressure stripping. This approach represents a robust alternative to methods used so far to select quenched galaxies (e.g. colours, specific star-formation rate, or post-starburst spectra).Comment: 22 pages, 23 figures, accepted for publication in MNRA

A methodology to select galaxies just after the quenching of star formation

We propose a new methodology aimed at finding star-forming galaxies in the phase which immediately follows the star-formation (SF) quenching, based on the use of high- to low-ionization emission line ratios. These ratios rapidly disappear after the SF halt, due to the softening of the UV ionizing radiation. We focus on [O III] $\lambda$5007/H$\alpha$ and [Ne III] $\lambda$3869/[O II] $\lambda$3727, studying them with simulations obtained with the CLOUDY photoionization code. If a sharp quenching is assumed, we find that the two ratios are very sensitive tracers as they drop by a factor $\sim$ 10 within $\sim$ 10 Myr from the interruption of the SF; instead, if a smoother and slower SF decline is assumed (i.e. an exponentially declining star-formation history with $e$-folding time $\tau=$ 200 Myr), they decrease by a factor $\sim$ 2 within $\sim$ 80 Myr. We mitigate the ionization -- metallicity degeneracy affecting our methodology using pairs of emission line ratios separately related to metallicity and ionization, adopting the [N II] $\lambda$6584/[O II] $\lambda$3727 ratio as metallicity diagnostic. Using a Sloan Digital Sky Survey galaxy sample, we identify 10 examples among the most extreme quenching candidates within the [O III] $\lambda$5007/H$\alpha$ vs. [N II] $\lambda$6584/[O II] $\lambda$3727 plane, characterized by low [O III] $\lambda$5007/H$\alpha$, faint [Ne III] $\lambda$3869, and by blue dust-corrected spectra and $(u-r)$ colours, as expected if the SF quenching has occurred in the very recent past. Our results also suggest that the observed fractions of quenching candidates can be used to constrain the quenching mechanism at work and its time-scales.Comment: Accepted for publication in MNRAS; 19 pages, 21 figures, 1 tabl

Interacting galaxies in the IllustrisTNG simulations -- IV: Enhanced Supermassive Black Hole Accretion Rates in Post-Merger Galaxies

We present an analysis of the instantaneous supermassive black hole (SMBH) accretion rates in a collection of 1563 post-merger galaxies drawn from the IllustrisTNG simulation. Our sample consists of galaxies that have experienced a merger in the last simulation snapshot (within ~160 Myrs of coalescence) in the redshift range 01:10 and post-merger stellar masses > $10^{10} M_{\odot}$. We find that, on average, the accretion rates of the post-mergers are ~1.7 times higher than in a control sample and that post-mergers are 3-4 times more likely to experience a luminous active galactic nuclei (AGN) phase than isolated galaxies. SMBH accretion rate enhancements persist for ~2 Gyrs after coalescence, significantly exceeding the ~500 Myr lifetime of star formation rate enhancements. We find that the presence of simultaneous enhancements in both the star formation and SMBH accretion rates depends on both the mass ratio of the merger and on the gas mass of the post-merger galaxy. Despite these accretion rate enhancements, only ~35% of post-mergers experience a luminous AGN ($L_{bol}>10^{44}$ erg/s) within 500 Myrs after coalescence, and fewer than 10\% achieve a luminosity in excess of $L_{bol}>10^{45}$ erg/s. Moreover, only ~10\% of the highest luminosity ($L_{bol}>10^{45}$ erg/s) AGN in the IllustrisTNG galaxy sample are recent mergers. Our results are therefore consistent with a picture in which mergers can (but don't always) trigger AGN activity, but where the majority of galaxies hosting high luminosity AGN are not recent mergers.Comment: This article has been accepted for publication in Monthly Notices of the Royal Astronomical Societ

Toward a Better Understanding of Cosmic Chronometers: Stellar Population Properties of Passive Galaxies at Intermediate Redshift

We take advantage of the publicly available LEGA-C spectroscopic survey to measure the stellar population properties of 140 individual massive and passive galaxies at z similar to 0.7. We develop and publicly release PyLick, a flexible Python code to measure UV to near-IR spectral indices. With PyLick we study the H/K ratio as a new diagnostic based on the pseudo-Lick Ca ii H and K indices and find that a cut in H/K &lt; 1.1 can be used jointly with other criteria to select (or verify the purity of) samples of passive galaxies. By combining photometric and spectroscopic criteria, we select a reliable sample of passively evolving galaxies. We constrain single-burst stellar ages, metallicities [Z/H], and [alpha/Fe] with an optimized set of Lick indices, exploring in detail the robustness of our measurement against different combinations. Even without imposing cosmological priors, the derived ages follow a clear trend compatible with the expected cosmological aging of the universe. We observe no significant redshift evolution for the metal abundance with respect to the values derived at z = 0, with median [Z/H] = 0.08 +/- 0.18 and [alpha/Fe] = 0.13 +/- 0.11. Finally, we analyze the relations between log age, [Z/H], [alpha/Fe], and the stellar velocity dispersion, finding slopes of (0.5 +/- 0.1), (0.3 +/- 0.2), and (0.2 +/- 0.1), respectively; the small scatter of &lt;0.2 dex points to rather homogeneous and short star formation histories. Overall, these results confirm and extend low-redshift findings of a mass-downsizing evolution. This work further strengthens the possibility of selecting pure samples of passive galaxies to be exploited reliably as cosmic chronometers to place independent cosmological constraints

The interconnection between galaxy mergers, AGN activity and rapid quenching of star formation in simulated post-merger galaxies

We investigate the role of galaxy mergers on supermassive black hole (SMBH) accretion and star formation quenching in three state-of-the-art cosmological simulations with contrasting physics models: EAGLE, Illustris and IllustrisTNG. We find that recently coalesced 'post-mergers' in all three simulations have elevated SMBH accretion rates by factors of ~2-5. However, rapid (within 500 Myr of coalescence) quenching of star formation is rare, with incidence rates of 0.4% in Illustris, 4.5% in EAGLE and 10% in IllustrisTNG. The rarity of quenching in post-mergers results from substantial gas reservoirs that remain intact after the merger. The post-mergers that do successfully quench tend to be those that had both low pre-merger gas fractions as well as those that experience the largest gas losses. Although rare, the recently quenched fraction of post-mergers is still elevated compared to a control sample of non-mergers by factors of two in IllustrisTNG and 11 in EAGLE. Conversely, quenching is rarer in Illustris post-mergers than in their control. Recent observational results by Ellison et al. have found rapid quenching to be at least 30 times more common in post-mergers, a significantly higher excess than found in any of the simulations. Our results, therefore, indicate that whilst merger-induced SMBH accretion is a widespread prediction of the simulations, its link to quenching depends sensitively on the physics models, and that none of the subgrid models of the simulations studied here can fully capture the connection between mergers and rapid quenching seen in observations.Comment: This article has been accepted for publication in Monthly Notices of the Royal Astronomical Societ

Interacting galaxies in the IllustrisTNG simulations - III. (The rarity of) quenching in post-merger galaxies

Galaxy mergers are traditionally one of the favoured mechanisms for the transformation of spiral galaxies to spheroids and for quenching star formation. To test this paradigm in the context of modern cosmological simulations, we use the IllustrisTNG simulation to investigate the impact of individual merger events on quenching star formation [i.e. star formation rate (SFR) at least 3σ below the star-forming main sequence] within 500 Myr after the coalescence phase. The rate of quenching amongst recently merged galaxies is compared with a control sample that is matched in redshift, stellar mass, SFR, black hole mass, and environment. We find quenching to be uncommon among the descendants of post-merger galaxies, with only ∼5 per cent of galaxies quenching within 500 Myr after the merger. Despite this low absolute rate, we find that quenching occurs in post-mergers at twice the rate of the control galaxies. The fraction of quenched post-merger descendants 1.5 Gyr after the merger become statistically indistinguishable from that of non-post-mergers, suggesting that mergers could speed up the quenching process in those post-mergers whose progenitors had physical conditions able to sustain effective active galactic nuclei (AGN) kinetic feedback, thus capable of removing gas from galaxies. Our results indicate that although quenching does not commonly occur promptly after coalescence, mergers none the less do promote the cessation of star formation in some post-mergers. We find that, in IllustrisTNG, it is the implementation of the AGN kinetic feedback that is responsible for quenching post-mergers, as well as non-post-merger controls. As a result of the released kinetic energy, galaxies experience gas loss and eventually they will quench. Galaxies with an initially low gas fraction show a preferable pre-disposition towards quenching. The primary distinguishing factor between quenched and star-forming galaxies is gas fraction, with a sharp boundary at fgas ~ 0.1 in TNG

Galaxy mergers can rapidly shut down star formation

Galaxy mergers trigger both star formation and accretion onto the central supermassive black hole. As a result of subsequent energetic feedback processes, it has long been proposed that star formation may be promptly extinguished in galaxy merger remnants. However, this prediction of widespread, rapid quenching in late stage mergers has been recently called into question with modern simulations and has never been tested observationally. Here we perform the first empirical assessment of the long-predicted end phase in the merger sequence. Based on a sample of ~500 post-mergers identified from the Ultraviolet Near Infrared Optical Northern Survey (UNIONS), we show that the frequency of post-merger galaxies that have rapidly shutdown their star formation following a previous starburst is 30-60 times higher than expected from a control sample of non-merging galaxies. No such excess is found in a sample of close galaxy pairs, demonstrating that mergers can indeed lead to a rapid halt to star formation, but that this process only manifests after coalescence.PostprintPeer reviewe

Studying the final stages of galaxy evolution across cosmic time

One of the key open questions of galaxy evolution is to understand when, how and where the star formation ceases (the so called star formation quenching). It is well known that galaxies have pronounced bimodal distributions of their main prop- erties, and that they are segregated into the two populations of blue star-forming (spiral and irregular) and red passive (elliptical and lenticular) galaxies. There is a general consensus on a scenario in which blue/star-forming galaxies quench their star formation transforming into red passive systems. However, the processes which drive this change in galaxy properties and structure are still unclear. The main goal of this Thesis work is twofold. On the one hand, we aim at defining new methods to select galaxies that are in the critical phase of quenching, or that have recently (e.g. within 0.5 Gyr) terminated their star formation. On the other hand, the final goal is to study the physical properties of the selected galaxies in order to investigate the origin of the quenching, place constraints on how and where (within the galaxies) star formation terminates and understand the possible evolutionary links with the population of E/S0 galaxies