Equal-mass Galaxy Mergers : Initial Conditions and Subresolution Astrophysics

Interactions and mergers play important roles in the evolution of galaxies. In this Master's thesis I have studied some basic properties of merging galaxies using numerical simulations. I have created initial conditions for equal-mass disk galaxy mergers and run the simulations using the numerical simulation codes GADGET-3 and its recently updated version SPHGal. In this Thesis I have also reviewed the basic physics modeled by GADGET-3 and the differences between the astrophysical subresolution models in the standard GADGET-3 and SPHGal versions of the code. The aim of the Thesis was to see how different initial conditions affect the mergers of disk galaxies, and how the updated astrophysics affect the properties of the mergers. The simulations were run on the supercomputer Sisu and the supercluster Taito at CSC, the Finnish IT Center for Science. From the results of the simulations I have studied the effect of having a dark matter halo with either an NFW density profile or a Hernquist density profile. Four observed mergers were succesfully reproduced with NFW profiles using position and velocity maps, and then compared to their Hernquist profile analogues. By comparing the results of the mergers between galaxies with the two types of haloes, I found that the steeper gravitational potentials of the NFW haloes produced more prominent tidal tails than the Hernquist haloes. The more compact morphology of the disks within the Hernquist haloes before the final coalescence enabled also faster coalescence times in the Hernquist halo mergers. As expected, the NFW halo mergers experienced slower orbital decay, were more violent, experienced higher star formation rates and produced thus 2-17% more new stars than the Hernquist halo mergers. In the Thesis I have also studied the effect of the updated subresolution models on the star formation process in the merging galaxies. The star formation in the standard GADGET-3 is self-regulated, whereas the star formation in SPHGal is regulated by detailed feedback processes. The star formation rates vary both spatially and on much shorter time scales in the SPHGal compared to the analogous simulations with the standard GADGET-3. I analyzed also the metal yields in the merger remnants of the Antennae galaxies, and found a metallicity gradient within the inner 2 kpc from the center of mass of the remnant. A gradient was also present in the oxygen abundance of the remnants, following the fact that the metallicity was dominated by the abundance of oxygen. Gradients in the inner regions of the merger remnants are to be expected, since star formation is most active in the central regions of the remnant

The fate of the Antennae galaxies

27 pages, 18 figures, submitted to MNRASWe present a high-resolution smoothed particle hydrodynamics simulation of the Antennae galaxies (NGC 4038/4039) and follow the evolution $3$ Gyrs beyond the final coalescence. The simulation includes metallicity dependent cooling, star formation, and both stellar feedback and chemical enrichment. The simulated best-match Antennae reproduces well both the observed morphology and the off-nuclear starburst. We also produce for the first time a simulated two-dimensional metallicity map of the Antennae and find good agreement with the observed metallicity of off-nuclear stellar clusters, however the nuclear metallicities are overproduced by $\sim 0.5$ dex. Using the radiative transfer code SKIRT we produce multi-wavelength observations of both the Antennae and the merger remnant. The $1$ Gyr old remnant is well fitted with a S\'ersic profile of $n=4.05$, and with an $r$-band effective radius of $r_{\mathrm{e}}= 1.8$ kpc and velocity dispersion of $\sigma_{\mathrm{e}}=180$ km$/$s the remnant is located on the fundamental plane of early-type galaxies (ETGs). The initially blue Antennae remnant evolves onto the red sequence after $\sim 2.5$ Gyr of secular evolution. The remnant would be classified as a fast rotator, as the specific angular momentum evolves from $\lambda_R\approx0.11$ to $\lambda_R\approx0.14$ during its evolution. The remnant shows ordered rotation and a double peaked maximum in the mean 2D line-of-sight velocity. These kinematical features are relatively common among local ETGs and we specifically identify three local ETGs (NGC 3226, NGC 3379 and NGC 4494) in the ATLAS$^\mathrm{3D}$ sample, whose photometric and kinematic properties most resemble the Antennae remnant.We present a high-resolution smoothed particle hydrodynamic simulation of the Antennae galaxies (NGC 4038/4039) and follow the evolution 3 Gyr beyond the final coalescence. The simulation includes metallicity-dependent cooling, star formation, and both stellar feed-back and chemical enrichment. The simulated best-match Antennae reproduce well both the observed morphology and the off-nuclear starburst. We also produce for the first time a simulated two-dimensional (2D) metallicity map of the Antennae and find good agreement with the observed metallicity of off-nuclear stellar clusters; however, the nuclear metallicities are overproduced by similar to 0.5 dex. Using the radiative transfer code SKIRT, we produce multiwavelength observations of both the Antennae and the merger remnant. The 1-Gyr-old remnant is well fitted with a Sersic profile of n = 7.07, and with an r-band effective radius of r(e) = 1.6 kpc and velocity dispersion of sigma(e) = 180 km s(-1) the remnant is located on the Fundamental Plane of early-type galaxies (ETGs). The initially blue Antennae remnant evolves on to the red sequence after similar to 2.5 Gyr of secular evolution. The remnant would be classified as a fast rotator, as the specific angular momentum evolves from lambda(Re) approximate to 0.11 to 0.14 during its evolution. The remnant shows ordered rotation and a double peaked maximum in the mean 2D line-of-sight velocity. These kinematical features are relatively common amongst local ETGs and we specifically identify three local ETGs (NGC 3226, NGC 3379, and NGC 4494) in the ATLAS(3D) sample, whose photometric and kinematic properties most resemble the Antennae remnant.Peer reviewe

The challenge of simulating the star cluster population of dwarf galaxies with resolved interstellar medium

We present results on the star cluster properties from a series of high resolution smoothed particles hydrodynamics (SPH) simulations of isolated dwarf galaxies as part of the GRIFFIN project. The simulations at sub-parsec spatial resolution and a minimum particle mass of 4 M-circle dot incorporate non-equilibrium heating, cooling, and chemistry processes, and realize individual massive stars. The simulations follow feedback channels of massive stars that include the interstellar-radiation field variable in space and time, the radiation input by photo-ionization and supernova explosions. Varying the star formation efficiency per free-fall time in the range epsilon(ff) = 0.2-50 per cent neither changes the star formation rates nor the outflow rates. While the environmental densities at star formation change significantly with epsilon(ff), the ambient densities of supernovae are independent of epsilon(ff) indicating a decoupling of the two processes. At low epsilon(ff), gas is allowed to collapse more before star formation, resulting in more massive, and increasingly more bound star clusters are formed, which are typically not destroyed. With increasing epsilon(ff), there is a trend for shallower cluster mass functions and the cluster formation efficiency Gamma for young bound clusters decreases from 50 per cent to similar to 1 per cent showing evidence for cluster disruption. However, none of our simulations form low mass (Peer reviewe

Formation of star clusters and enrichment by massive stars in simulations of low-metallicity galaxies with a fully sampled initial stellar mass function

We present new GRIFFIN project hydrodynamical simulations that model the formation of galactic star cluster populations in low-metallicity ($Z=0.00021$) dwarf galaxies, including radiation, supernova and stellar wind feedback of individual massive stars. In the simulations, stars are sampled from the stellar initial mass function (IMF) down to the hydrogen burning limit of $0.08$ M$_\odot$. Mass conservation is enforced within a radius of $1$ pc for the formation of massive stars. We find that massive stars are preferentially found in star clusters and follow a correlation set at birth between the highest initial stellar mass and the star cluster mass that differs from pure stochastic IMF sampling. With a fully sampled IMF, star clusters lose mass in the galactic tidal field according to mass-loss rates observed in nearby galaxies. Of the released stellar feedback, $60\%$ of the supernova material and up to $35\%$ of the wind material reside either in the hot interstellar medium (ISM) or in gaseous, metal enriched outflows. While stellar winds (instantaneously) and supernovae (delayed) start enriching the ISM right after the first massive stars form, the formation of supernova-enriched stars and star clusters is significantly delayed (by $>50$ Myr) compared to the formation of stars and star clusters enriched by stellar winds. Overall, supernova ejecta dominate the enrichment by mass, while the number of enriched stars is determined by continuous stellar winds. These results present a concept for the formation of chemically distinct populations of stars in bound star clusters, reminiscent of multiple populations in globular clusters.Comment: 26 pages, 23 figures. Accepted for publication in MNRA

Where do stars explode in the ISM? -- The distribution of dense gas around massive stars and supernova remnants in M33

Star formation in galaxies is regulated by turbulence, outflows, gas heating and cloud dispersal -- processes which depend sensitively on the properties of the interstellar medium (ISM) into which supernovae (SNe) explode. Unfortunately, direct measurements of ISM environments around SNe remain scarce, as SNe are rare and often distant. Here we demonstrate a new approach: mapping the ISM around the massive stars that are soon to explode. This provides a much larger census of explosion sites than possible with only SNe, and allows comparison with sensitive, high-resolution maps of the atomic and molecular gas from the Jansky VLA and ALMA. In the well-resolved Local Group spiral M33, we specifically observe the environments of red supergiants (RSGs, progenitors of Type II SNe), Wolf-Rayet stars (WRs, tracing stars $>$30 M$_{\odot}$, and possibly future stripped-envelope SNe), and supernova remnants (SNRs, locations where SNe have exploded). We find that massive stars evolve not only in dense, molecular-dominated gas (with younger stars in denser gas), but also a substantial fraction ($\sim$45\% of WRs; higher for RSGs) evolve in lower-density, atomic-gas-dominated, inter-cloud media. We show that these measurements are consistent with expectations from different stellar-age tracer maps, and can be useful for validating SN feedback models in numerical simulations of galaxies. Along with the discovery of a 20-pc diameter molecular gas cavity around a WR, these findings re-emphasize the importance of pre-SN/correlated-SN feedback evacuating the dense gas around massive stars before explosion, and the need for high-resolution (down to pc-scale) surveys of the multi-phase ISM in nearby galaxies.Comment: 34 pages, 14 figures. Submitted to ApJ. Comments welcome! The density distributions will be made publicly available after journal acceptance of manuscript. Please feel free to contact us in the meantime if you would like to use the

Galaksitörmäysten hydrodynaamiset simulaatiot ja synteettiset havainnot

Interactions and mergers between galaxies are among the most spectacular astrophysical phenomena that drive morphological transformations of galaxies as they evolve throughout cosmic times. Specifically, galactic encounters induce star formation due to the compression of the interstellar medium through tidal torques, ram pressure and shocks. The in-situ star formation process is in turn self-regulated by various stellar feedback processes, such as ultraviolet radiation from young massive stars and energetic supernova explosions. The thermodynamical processes in the interstellar gas with temperatures ranging from a few degrees to millions of Kelvins, coupled with the stellar lifecycle, are therefore the subjects of a wide range of ongoing observational and numerical studies. Significant technological advances in recent decades have resulted in a general framework for the formation and evolution of galaxies, but the complete astrophysical picture still remains incomplete. Here we study the evolution of galaxies undergoing mergers by running high-resolution hydrodynamical simulations. We use state-of-the-art numerical methods, post-processing methods, and observational data analysis tools. The simulations presented here span a wide range of initial conditions from gas-rich dwarf galaxies, through Milky Way-like disk galaxies, to massive early-type galaxies which include central supermassive black holes. The employed simulation methods include some of the most sophisticated astrophysical models available for galactic-scale simulations. The cooling of the star-forming gas is modelled in detail using a chemical network, and the newly formed stars sample a mass resolution down to the masses of individual massive stars. We also follow the spatially and the temporally evolving interstellar radiation field emanating from the individually modelled stars into the surrounding interstellar medium, while simultaneously accounting for dust attenuation and gas self-shielding. In this thesis we investigate how the extreme star formation environment produced by a gas-rich, low-metallicity dwarf galaxy merger can be used as a proxy for the turbulent star formation conditions present in the high-redshift Universe. Specifically, we follow the formation of a population of young star clusters during the interactions of dwarf galaxies. We show that the star cluster formation proceeds most efficiently during the starburst phase. Young star clusters are, however, already present with an observationally consistent power-law mass function after the first pericentric passage. We take special interest in the formation and early evolution of the three most massive star clusters, which form hierarchically during the most intense starburst. These objects are shown to evolve in terms of their sizes and surface mass densities to resemble the present-day globular clusters observed in the Local Group. Another simulation, specifically set up to reproduce the observed properties of the Antennae galaxy merger (NGC 4038/4039), is in turn used to study the spatially extended star formation during a disk galaxy merger. The simulation output is post-processed using radiative transfer and the results are reduced with observational data analysis methods. We compare the spatial star formation properties and the metallicity distribution to the observed present-day Antennae. We further follow the enrichment of the interstellar medium through stellar winds and supernovae, and show how the merger remnant evolves into a red and dead elliptical galaxy. We continue simulating the Antennae merger for a prolonged period of time after the coalescence of the galactic disks, and use the surface brightness and kinematic properties of the simulated remnant to search for an observational counterpart to the possible future fate of the present-day Antennae galaxies. The outputs of our numerical simulations are used as well to discern how long a period a galaxy merger can be identified in optical images of observed mergers, and the results are used in building a comprehensive picture of the origin of post-starburst galaxies. Finally, we show how the supermassive black holes, found in the centres of all massive early-type galaxies, end in binaries at the centres of merger remnants of elliptical galaxies. The binaries scour the galactic centres while producing cored surface brightness profiles often observed in ellipticals, and coalesce as a result of gravitational wave driven binary evolution.Galaksienväliset vuorovaikutukset ja yhteentörmäykset ovat vaikuttavimpia astrofysikaalisia ilmiöitä, jotka aiheuttavat galakseissa morfologisia muodonmuutoksia niiden kehittyessä. Galaktiset vuorovaikutukset edistävät tähtien syntyä, kun tähtienvälinen kaasu puristuu vuorovesivoimien, paine-erojen ja shokkirintamien johdosta. Tähtien syntyä puolestaan rajoittavat monet galaksien sisäiset prosessit kuten kuumien, massiivisten nuorten tähtien ultraviolettisäteily ja energeettiset supernovat. Tähtien elämänkaareen kytketyn tähtienvälisen kaasun termodynaamiset prosessit ovatkin monen käynnissä olevan havaitsevan ja numeerisen tähtitieteen tutkimushankkeen keskiössä. Merkittävät teknologiset edistysaskeleet ovat viime vuosikymmeninä auttaneet luomaan yleiskuvan galaksien synnyn ja kehityksen eri vaiheista, mutta prosessin astrofysikaalinen ymmärrys on vielä paikoittain keskeneräinen. Tutkimme galaksien evoluutiota hydrodynaamisten simulaatioiden avulla. Käytämme viimeisimpiä simulaatio- ja datankäsittelymetodeja, sekä havaitsevan tähtitieteen data-analyysityökaluja. Tutkimamme simulaatiot koostuvat laajasta valikoimasta alkuehtoja kaasurikkaista kääpiögalakseista Linnunradan kaltaisten kiekkogalaksien kautta suurimpiin, supermassiivisia mustia aukkoja sisältäviin ellipsigalakseihin. Käyttämämme simulaatiometodologia sisältää hienostuneimpia astrofysikaalisia malleja joita galaksiskaalan simulaatioihin tänä päivänä hyödynnetään. Tähtiä synnyttävän kaasun jäähtymistä mallinnetaan yksityiskohtaisesti kemiallisten reaktioiden verkoston avulla, ja vastasyntyneiden tähtien massaresoluutio yltää jopa yksittäisten massiivisten tähtien tasolle. Mallinnamme myös yksittäisten tähtien muodostamaa paikka- ja aikariippuvaista tähtienvälistä säteilykenttää ottaen samalla huomioon pölyn ja kaasun säteilyn kulkuun vaikuttavat ominaisuudet. Tässä väitöskirjassa tutkimme kuinka runsaskaasuisten, matalametallipitoisten kääpiögalaksien törmäyksessä syntyvää äärimmäistä tähtiensynty-ympäristöä voidaan käyttää nuoressa maailmankaikkeudessa vallinneiden turbulenttien tähtiensyntyolosuhteiden jäljittelyyn. Tarkemmin sanoen, tutkimme nuorten tähtijoukkopopulaatioiden syntyä kääpiögalaksien yhteentörmäyksissä. Osoitamme kuinka nuorten tähtijoukkojen synty tapahtuu tehokkaimmin tähtiryöppyvaiheen aikana. Nuoria tähtijoukkoja syntyy kuitenkin jo ensimmäisen lähiohituksen aikana, ja tähtijoukkojen massajakauma vastaa havaittujen tähtijoukkojen jakaumaa koko vuorovaikutusprosessin ajan. Keskitymme erityisesti kolmen massiivisimman, hierarkisesti kasautuvan tähtijoukon syntyyn ja kehitykseen. Näytämme kuinka nämä tähtijoukot kehittyvät kokojensa ja pintatiheyksiensä perusteella nykypäivänä paikallisessa galaksijoukossa havaittujen pallomaisten tähtijoukkojen kaltaisiksi kohteiksi. Toista keskeisimmistä simulaatioistamme, jonka tarkoitus on ollut alusta alkaen tuottaa mahdollisimman tarkka mallinnus havaitusta Antennae-galaksitörmäyksestä (Tuntosarvigalaksi, NGC 4038/4039), käytetään galaksitörmäyksen aikaisen laaja-alaisen tähtiensynnyn tutkimiseen. Simulaatiotuotokset käsitellään säteilynkuljetustekniikalla ja tulokset käsitellään havaitsevan tähtitieteen data-analyysimetodeilla. Vertailemme tähtiensynty- ja metallisuusominausuuksia havaitun Antennaen vastaaviin ominaisuuksiin. Seuraamme tähtienvälisen kaasun tähtituulien ja supernovien aiheuttamaa kemiallista rikastumista, ja näytämme kuinka galaksitörmäyksen jäännös kehittyy kuolleeksi punaiseksi ellipsigalaksiksi. Simuloimme Antennae-galakseja miljardeja vuosia galaksikiekkojen yhteensulautumisen jälkeen, ja käytämme lopputuloksen pintakirkkaus- ja nopeusprofiileja etsiäksemme nykypäivän ellipsigalakseista kandidaattia Antennae-galaksien tulevalle kohtalolle. Simulaatiolopputuloksia käytetään myös galaksitörmäysten havaittavissa olevien ominaisuuksien himmenemisen arviointiin, ja tähtiryöppygalaksien jatkokehityksen kokonaiskuvan rakentamiseen. Lopuksi näytämme myös kuinka supermassiiviset mustat aukot, joita löytyy kaikkien massiivisten ellipsigalaksien keskustoista, päätyvät kaksoisaukoiksi törmäyksissä muodostuneiden ellipsigalaksien keskustoihin. Kaksoisaukot tyhjentävät törmäysjäännösten keskustoja tähdistä tuottaen havaittujen galaksien odottamattoman himmeät keskusvaloprofiilit, ja sulautuvat yhteen säteillen voimakkaasti gravitaatioaaltoja