1,721,086 research outputs found
Interacting galaxies in the IllustrisTNG simulations - III. (The rarity of) quenching in post-merger galaxies
Full text linkGalaxy 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
The interconnection between galaxy mergers, AGN activity and rapid quenching of star formation in simulated post-merger galaxies
Full text linkWe 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 -- IV: Enhanced Supermassive Black Hole Accretion Rates in Post-Merger Galaxies
Full text linkWe 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 > . 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 ( erg/s) within
500 Myrs after coalescence, and fewer than 10\% achieve a luminosity in excess
of erg/s. Moreover, only ~10\% of the highest luminosity
( 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
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
The Enhancement of Supermassive Black Hole Accretion Rates in Post-Merger Galaxies from Cosmological Simulations
No full textNumerical simulations of galaxy mergers predict that in a merger event, the angular momentum of gas is efficiently drained, leading to gaseous inflows resulting in high central gas densities and bursts of star formation. The merger-starburst connection is strongly corroborated by observational studies of interacting and post-merger galaxies. In addition to the enhancement of central star formation rate, simulations predict that gaseous inflows will increase the accretion onto central supermassive black holes (SMBHs) which may trigger or enhance nuclear activity. However, observational studies continue to debate the connection between galaxy mergers and active galactic nuclei (AGN), with contrasting conclusions on whether mergers are likely to trigger AGN and whether the majority of AGN are triggered by merger events. In this thesis, I present an analysis of the SMBH accretion rates in post-merger galaxies drawn from the IllustrisTNG, Illustris, and EAGLE cosmological simulations. The post-merger samples consist of galaxies that have experienced a merger in the last simulation snapshot in the redshift range 01:10 and post-merger stellar masses > 10^10 M_sun. I compare the post-merger SMBH accretion rates to a control sample of galaxies that are matched to the post-mergers in stellar mass, redshift, environment (and AGN feedback mode in the case of IllustrisTNG) but that have not experienced a merger (of mass ratio >1:10) within at least two Gyrs. I find that all three simulations demonstrate a population averaged enhancement in SMBH accretion rate in the post-merger sample, with accretion rates ~2 times higher than controls for IllustrisTNG and EAGLE and ~4 times higher in Illustris. In addition, I find that post-mergers are 2-4 times more likely to host a highly accreting SMBH compared with matched non-merger galaxies. In all three simulations, the population averaged SMBH accretion rate enhancement persists for ~2-3 Gyrs after coalescence, significantly exceeding the ~500 Myr lifetime of star formation rate enhancements. In the IllustrisTNG simulation, I investigate the presence of simultaneous enhancements in both the star formation and SMBH accretion rates; I find that co-incidence occurs predominantly in the first few hundred Myrs post-coalescence and depends on both the mass ratio of the merger and on the gas mass of the post-merger galaxy. Furthermore, in TNG, I find that despite the overall SMBH accretion rate enhancement, only approximately 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. My 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.Graduat
Probing galaxy evolution through numerical simulations: mergers, gas, and star formation
Full text linkLarge observational surveys have compiled substantial galaxy samples with an array of different properties across cosmic time. While we have a broad understanding of how galaxies grow and build their observable properties, the details of galaxy growth and evolution pose a fundamental challenge to galaxy evolution theories. Nonetheless, galaxy evolution is ultimately regulated by the properties of the gas reservoir. In this thesis I use numerical simulations to answer key questions related to the galactic gas reservoir, and galaxy mergers: a major transformational process.
In Chapter 2 I present an analysis of 28 simulated L* galaxies to understand the physical processes that shape the massive gas reservoir surrounding galaxies (i.e. the circum-galactic medium; CGM). I show that (1) the gas and metal content of the CGM is driven by galaxy growth and the strength/presence of feedback processes, and (2) the ionisation and internal structures of the CGM are shaped by galactic outflows, and active galactic nucleus luminosity. Albeit dependent on internal galactic properties and the physical processes that shape them, the CGM remains greatly diverse, thus posing a challenge for observational surveys.
As a follow-up to my study of normal L* galaxy gas halos, in Chapter 3 I present a theoretical study of the effect of galaxy mergers on the CGM. I demonstrate that galaxy mergers can leave a strong imprint on the CGM's gas and metal content, metallicity, and size. The merger can increase (1) the CGM's metallicity by 0.2-0.3 dex within 0.5 Gyr post-merge, and (2) the metal covering fractions by factors of 2-3. In spite of the increase in the CGM's metal content, the hard ionising field during the merger can drive a decline in the covering fractions of commonly observed ions.
In Chapter 4 I shift focus to star formation, particularly the effects of galaxy mergers on star formation. While the effects of galaxy mergers have been proven observationally, theoretical predictions are limited to small binary merger suites and cosmological zoom-in studies. I present a statistical study of 27,691 post-merger galaxies from IllustrisTNG to quantify the effect of galaxy mergers on galactic star formation. I report a dependence in the merger-induced star formation rate (SFR) on mass ratio, stellar mass, gas fraction, and galaxy SFR. I also track the evolution of the effects of galaxy mergers demonstrating their decay over ~500 Myr.
In Chapter 6, I leverage galactic scaling relations to extend my work on the effects of galaxy mergers to resolved scales. However, before using the simulated resolved scaling relations, I first examine their existence and robustness. In Chapter 5, I demonstrate the emergence of the kpc-scale star forming main sequence (rSFMS) in the FIRE-2 simulations. Nonetheless, the slope of the rSFMS is dependent on the (1) star formation tracer's timescale, and (2) observed resolution, which I propose is caused by the clumpiness of star formation. I develop a toy model that quantitatively captures the effects of clumpy star formation. I then illustrate how the model can be used to characterise the mass of star-forming clumps.
Having demonstrated the existence and robustness of known scaling relations in numerical simulations, I explore the effects of galaxy mergers on resolved scales in Chapter 6. I generate synthetic observations for 1,927 post-mergers in IllustrisTNG and examine the radially-dependent merger-driven SFR enhancement, and metallicity suppression in post-mergers. Galaxy mergers preferentially boost star formation in the centres and suppress metallicities globally. The effects of the merger depends on galaxy properties such as stellar mass, SFR, mass ratio, and gas fraction.Graduat
Metal Strong Damped Lyman Alpha Systems And Their Context With The Local Group
No full textDamped Lyman α systems (DLAs) are useful probes of the chemical enrichment of the universe as they provide accurate abundance measurements of many chemical species. Using a sample of 30 DLAs (with large metal column densities) observed with the High Resolution Echelle Spectrometer on the Keck I telescope, the abundances of several elements (i.e. iron, zinc, chromium, silicon, sulphur, phosphorus, manganese, and boron) are derived and presented. A comparison is drawn between the abundances from these metal-rich DLAs with literature samples encompassing the largest compilation of high resolution observations of other DLAs, and stars from the Milky Way and its satellite galaxies to understand the astrophysical nature of DLAs.
Furthermore, the first ever extragalactic study of boron is presented. Using the sample of 30 metal-rich DLAs, two 3σ detections and one near detection 2.97σ) were found. From the comparison of [B/O] and, for the first time, [B/S], with studies in the Milky Way, there appears to be an excess of boron relative to its parent nucleus (oxygen) in these three DLA systems, suggesting that there may be a higher cosmic ray flux in DLAs than in the Milky Way.Graduate060
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