The outer halos of elliptical galaxies

The outer halos of galaxies are known to store vital information about the formation history and merger-induced evolution of their central galaxies, since the relaxation timescales are much larger than in the innermost parts and thus the memory of the events is conserved over a long period. This information provides fundamental insights into the processes of mass growth and morphological changes, broadening our understanding of the different mechanisms of structure formation. Additionally, the radius regime where the stellar component starts to dominate over the dark matter component is the perfect place to study the interplay between dark matter and stars. This interaction between the collisionless components of a galaxy, although much slower than the gas-induced processes, significantly alters the appearance of a galaxy in the long term. A better understanding of those processes can help to shed light on the dark sides of the galaxies. In this work, we use the combined strength of idealized high-resolution simulations of individual galaxies and large cosmological simulations to unveil some of the information encoded in the outer halos of galaxies. The high resolution simulations allow us to disentangle the impact of selected physics on the formation and evolution of galaxies in particular, while the large cosmological simulations provide a statistically meaningful sample of galaxies covering a large range in masses and environments. The first part of this thesis focuses on the interplay between dark matter and stars, revealing that both parts actually do interact through their common potential by re-ordering into a stable state where the total halo is isothermal and its density distribution follows a $\rho \propto r^{-2}$ profile. The gas, which dissipates energy and sinks towards the center on much shorter timescales, disturbs this process, forcing the total halo into a more compact state with approximately $\rho \propto r^{-3}$. Therefore, as long as gas is present, the collisionless attractor state can not be reached, but every dry merger evolves the system towards it. This is also apparent by the fact that more evolved halos have higher central dark matter fractions and smaller amounts of stars formed in situ, and that the slopes are generally steeper at high redshifts. We conclude that the equilibrium attractor state of dry merging systems provides a new test case for $\Lambda$CDM and prove that, if $\Lambda$CDM is correct, the dark matter and the stars do communicate through their common gravitational potential. The second part of this thesis deals with the information provided by the stellar halo. We show that the radial density profiles of all stellar halos have a universal shape which can be described by a curved exponential, independent of the morphology of their central galaxy. The strength of the curvature appears to be an indication for the amount of merging a galaxy suffered, since the stellar halo mostly grows through merging as the cold gas density in the halo region is much too low to cause a significant amount of star formation at all redshifts. With such a universal shape at hand, it is possible to study the deviations from this shape to learn about the details of the stellar accretion history of a galaxy, since different types of events leave distinct signatures. We suggest that more emphasis on the understanding of those different signatures is needed in the future to fully exploit the rich information contained in the outer halos, to learn more about the accretion driven but also the secular evolution of galaxies.Es ist bekannt, dass die äußeren Halos der Galaxien wichtige Informationen über die Entstehungsgeschichte und die durch Verschmelzungen von Strukturen verursachten Entwicklungen der zentralen Galaxien speichern, da die Relaxationszeiten in den Außenbereichen wesentlich länger sind als im Zentralbereich. Daher bleibt die Erinnerung an diese Ereignisse dort wesentlich länger erhalten. Die Verschmelzungsgeschichte gibt Einsicht in die fundamentalen Prozesse, die zum Wachstum der Galaxien und zur Veränderung ihrer Morphologien beitragen. Die Entschlüsselung dieser Information wird massiv dazu beitragen, unser Verständnis der verschiedenen Mechanismen der Strukturentwicklung zu erweitern. Des Weiteren gewinnt in diesem Bereich der Galaxien die Dunkle Materie im Vergleich zu den Sternen an Dominanz, wodurch diese Region perfekt geeignet ist, um das Zusammenspiel der Sterne mit der Dunklen Materie zu untersuchen. Diese dynamische Interaktion der kollisionsfreien Komponenten der Galaxien kann das Erscheinungsbild einer Galaxie signifikant prägen, wenn auch über deutlich längere Zeiträume hinweg als gasbedingte Prozesse. Ein besseres Verständnis dieser dynamischen Prozesse kann maßgeblich dazu beitragen, Licht in die dunklen Bereiche der Galaxien und ihrer Entstehung zu bringen. In dieser Dissertation nutzen wir die kombinierte Stärke idealisierter, hochaufgelöster Simulationen individueller Galaxien und großer kosmologischer Simulationen, um einige der Informationen zu entschlüsseln, die in den äußeren Halos der Galaxien verborgen sind. Mittels der hochaufgelösten Simulationen kann der Einfluss einzelner physikalischer Prozesse auf die dynamischen Strukturen der Galaxien im Detail untersucht werden, während die großen kosmologischen Simulationen ein statisch relevantes Sample an Galaxien verschiedenster Massen in unterschiedlichen Umgebungen bereitstellen. Der Fokus des ersten Teils der Dissertation liegt auf der Untersuchung des Zusammenspiels von Dunkler Materie und stellarer Komponente im Falle sphärischer Galaxien wie beispielsweise Ellipsen. Wir zeigen, dass diese Interaktion mittels des gemeinsamen Potentials stattfindet und die Komponenten sich so anordnen, dass ihr Gesamtprofil isotherm ist und die Gesamtdichteverteilung einem Potenzprofil der Form $\rho \propto r^{-2}$ entspricht. Dieser Zustand, wenn er erreicht ist, erweist sich als ausgesprochen stabil. Das Gas, das auf deutlich kürzeren Zeitskalen interagiert und seine Energien mittels Dissipation umverteilen kann, stört diesen Prozess und verursacht eine deutliche Komprimierung der Gesamtdichteverteilung, die dadurch eher einem Potenzprofil der Form $\rho \propto r^{-3}$ entspricht. Solange das Gas innerhalb der Galaxie Sterne bilden kann, wird der dynamische Gleichgewichtszustand zwischen den stoßfreien Komponenten nicht erreicht, jedoch sorgt jedes gasarme Akkretionsereignis für einen Schub in diese Richtung. Dies wird auch durch die Tatsache verdeutlicht, dass dynamisch weiter entwickelte Systeme einen größeren Anteil an Dunkler Materie im Zentrum besitzen und der Anteil der Sterne, die innerhalb der Galaxie selbst geboren wurden, kleiner ist. Generell sehen wir, dass die Gesamtdichteverteilung der sphärischen Galaxien bei höheren Rotverschiebungen komprimierter ist. Zusammenfassend schlußfolgern wir, dass der Gleichgewichtszustand, den kollisionsfreie Systeme anstreben, einen hervorragenden Testfall bietet, um einen Eckpfeiler der modernen Kosmologie -- die Existenz Dunkler Materie -- zu überprüfen, da nur in einem solchen Falle die Dunkle Materie und die Sterne auf eine Art miteinander interagieren, dass sich ein Dichteprofil obengenannter Form ausbildet. Im zweiten Teil dieser Dissertation analysieren wir Informationen, die in den stellaren Halos der Galaxien verschlüsselt sind. Wir zeigen, dass das Dichteprofil aller stellarer Halos von ähnlicher Form ist und sich durch ein gekrümmtes Exponentialgesetz beschreiben lässt. Diese universelle Form ist dabei unabhängig von der Morphologie der Galaxie im Zentrum. Der Krümmungsgrad kann als Indiz dafür gewertet werden, wie viele (kleinere) Strukturen die Galaxie bereits verschlungen hat, da der stellare Halo im Wesentlichen durch Akkretion kleinerer Strukturen wächst. Dies liegt darin begründet, dass die Gasdichte in den äußeren Bereichen der Galaxien dauerhaft zu niedrig ist, um einen signifikanten Anteil an Sternen zu erzeugen. Mittels eines derartigen universalen Dichteprofils ist es nunmehr möglich, die Abweichungen von diesem Profil zu bestimmen und daraus Details der Akkretionsgeschichte individueller Galaxien zu rekonstruieren, da die unterschiedlichen Akkretionsprozesse (wie zum Beispiel der Einfall kleiner oder großer (Zwerg-)Galaxien oder das Akkretieren einzelner Sterne von vorbeifliegenden Strukturen) unterschiedliche Signaturen im Halo hinterlassen. Daher schlagen wir vor, dem detailierten Verständnis der Signaturen der einzelnen Akkretionsprozesse in der Zukunft eine größere Bedeutung zukommen zu lassen, um die ergiebigen Informationen, die in den stellaren Halos enthalten sind, auswerten zu können, wodurch ein deutlicher Fortschritt im Verständnis sowohl der akkretionsgetriebenen als auch der sekularen Entwicklung von Galaxien erreicht würde

Accreted or Not Accreted? The Fraction of Accreted Mass in Galaxies from the Magneticum Simulations and Observations

In the two-phase scenario of galaxy formation, a galaxy's stellar mass growth is first dominated by in-situ star formation, and subsequently by accretion. We analyze the radial distribution of the accreted stellar mass in similar to 500 galaxies from the (48 Mpc/h)(3) box volume of the hydrodynamical cosmological simulation Magneticum, in a stellar-mass range of 10(10) to 10(12) M (circle dot). We find that higher-mass galaxies have larger accreted fractions, as found in previous works, but predict generally higher accretion fractions for low-mass galaxies. Based on the 3D radial distribution of the accreted and in-situ components, we define six galaxy classes, from completely accretion to completely in-situ dominated, and measure the transition radii between in-situ and accretion-dominated regions for galaxies that reveal a transition. About 70% of our galaxies have one transition radius. However, about 10% of the galaxies are accretion dominated everywhere, and about 13% have two transition radii, with the center and the outskirts both being accretion dominated. We show that these classes are strongly correlated with the galaxy merger histories, especially with the cold gas fraction at the time of merging. We find high total in-situ (low accretion) fractions to be associated with smaller, lower-mass galaxies, lower central dark-matter fractions, and larger transition radii. Finally, we show that the dips in observed surface brightness profiles seen in many early-type galaxies do not correspond to the transition from in-situ to accretion-dominated regions, and that any inferred mass fractions are not indicative of the true accreted mass but contain information about the galaxies' dry-merger history

Relight the Candle: What happens to High Redshift Massive Quenched Galaxies

A puzzling population of extremely massive quiescent galaxies at redshifts beyond z=3 has recently been revealed by JWST and ALMA, some of them with stellar ages that show their quenching times to be as high as z=6, while their stellar masses are already above 5e10Msun. These extremely massive yet quenched galaxies challenge our understanding of galaxy formation at the earliest stages. Using the hydrodynamical cosmological simulation suite Magneticum Pathfinder, we show that such massive quenched galaxies at high redshifts can be successfully reproduced with similar number densities as observed. The stellar masses, sizes, formation redshifts, and star formation histories of the simulated quenched galaxies match those determined with JWST. Following these quenched galaxies at z=3.4 forward in time, we find 20% to be accreted onto a more massive structure by z=2, and from the remaining 80% about 30% rejuvenate up to z=2, another 30% stay quenched, and the remaining 40% rejuvenated on a very low level of star formation. Stars formed through rejuvenation are mostly formed on the outer regions of the galaxies, not in the centres. Furthermore, we demonstrate that the massive quenched galaxies do not reside in the most massive nodes of the cosmic web, but rather live in side-nodes of approximately Milky-Way halo mass. Even at z=0, only about 10% end up in small-mass galaxy clusters, while most of the quenched galaxies at z=3.4 end up in group-mass halos, with about 20% actually not even reaching 1e13Msun in halo mass.Comment: 18 pages, 14 figures. Submitted to ApJ, Comments welcom

The Morphology-Density-Relation: Impact on the Satellite Fraction

In the past years several authors studied the abundance of satellites around galaxies in order to better estimate the halo masses of host galaxies. To investigate this connection, we analyze galaxies with $M_\mathrm{star}\geq\,10^{10}\,M_{\odot}$ from the hydrodynamical cosmological simulation Magneticum. We find that the satellite fraction of centrals is independent of their morphology. With the exception of very massive galaxies at low redshift, our results do not support the assumption that the dark matter (DM) haloes of spheroidal galaxies are significantly more massive than those of disc galaxies at fixed $M_\mathrm{star}$. We show that the density-morphology-relation starts to build up at $z\sim2$ and is independent of the star-formation properties of central galaxies. We conclude that environmental quenching is more important for satellites than for centrals. Our simulations indicate that conformity is already in place at $z=2$, where formation redshift and current star-formation rate (SFR) of central and satellite galaxies correlate. Centrals with low SFRs have formed earlier (at fixed $M_\mathrm{star}$) while centrals with high SFR formed later, with typical formation redshifts well in agreement with observations. However, we confirm the recent observations that the apparent number of satellites of spheroidal galaxies is significantly larger than for disc galaxies. This difference completely originates from the inclusion of companion galaxies, i.e. galaxies that do not sit in the potential minimum of a DM halo. Thus, due to the density-morphological-relation the number of satellites is not a good tracer for the halo mass, unless samples are restricted to the central galaxies of DM haloes.Comment: 17 pages, submitted to MNRAS, www.magneticum.or

The Young and the Wild: What happens to Protoclusters forming at z = 4?

Using one of the largest volumes of the hydrodynamical cosmological simulation suit Magneticum, we study the evolution of protoclusters identified at redshift = 4, with properties similar to SPT2349-56. We identify 42 protoclusters in the simulation, as massive and equally rich in substructures as observed, confirming that these structures are already virialized. The dynamics of the internally fast rotating member galaxies within these protoclusters resembles observations, merging rapidly to form the cores of the BCGs of the assembling clusters. Half of the gas reservoir of these structures is in a hot phase, with the metal-enrichment at a very early stage. These systems show a good agreement with the observed amount of cold star-forming gas, largely enriched to solar values. We predict that some of the member galaxies are already quenched at z = 4, rendering them undetectable through measurements of their gas reservoir. Tracing the evolution of protoclusters reveals that none of the typical mass indicators at high redshift are good tracers to predict the present-day mass of the system. We find that none of the simulated protoclusters with properties as SPT2349-56 at z = 4.3, are among the top ten most massive clusters at redshift z = 0, with some barely reaching masses of M = 2 x 10^14Msun. Although the average star-formation and mass-growth rates in the simulated galaxies match observations at high redshift reasonably well, the simulation fails to reproduce the extremely high total star-formation rates within observed protoclusters, indicating that the sub-grid models are lacking the ability to reproduce higher star-formation efficiency (or lower depletion timescales).Comment: 20 pages, 15 figures. Submitted to Ap

Gone after one orbit: How cluster environments quench galaxies

The effect of galactic orbits on a galaxy's internal evolution within a galaxy cluster environment has been the focus of heated debate in recent years. To understand this connection, we use both the $(0.5 \,$Gpc)$^3$ and the Gpc$^3$ boxes from the cosmological hydrodynamical simulation set Magneticum Pathfinder. We investigate the velocity-anisotropy, phase space, and the orbital evolution of up to $\sim 5 \cdot 10^{5}$ resolved satellite galaxies within our sample of 6776 clusters with $M_{\mathrm{vir}} > 10^{14} \, \mathrm{M_{\odot}}$ at low redshift, which we also trace back in time. In agreement with observations, we find that star-forming satellite galaxies inside galaxy clusters are characterised by more radially dominated orbits, independent of cluster mass. Furthermore, the vast majority of star-forming satellite galaxies stop forming stars during their first passage. We find a strong dichotomy both in line-of-sight and radial phase space between star-forming and quiescent galaxies, in line with observations. The tracking of individual orbits shows that the star-formation of almost all satellite galaxies drops to zero within $1 \, \mathrm{Gyr}$ after in-fall. Satellite galaxies that are able to remain star-forming longer are characterised by tangential orbits and high stellar mass. All this indicates that in galaxy clusters the dominant quenching mechanism is ram-pressure stripping.Comment: 22 pages, 16 figures, accepted by MNRA

A refined sub-grid model for black hole accretion and AGN feedback in large cosmological simulations

In large scale cosmological hydrodynamic simulations simplified sub-grid models for gas accretion onto black holes and AGN feedback are commonly used. Such models typically depend on various free parameters, which are not well constrained. We present a new advanced model containing a more detailed description of AGN feedback, where those parameters reflect the results of recent observations. The model takes the dependency of these parameters on the black hole properties into account and describes a continuous transition between the feedback processes acting in the so-called radio-mode and quasar-mode. In addition, we implement a more detailed description of the accretion of gas onto black holes by distinguishing between hot and cold gas accretion. Our new implementations prevent black holes from gaining too much mass, particularly at low redshifts so that our simulations are now very successful in reproducing the observed present-day black hole mass function. Our new model also suppresses star formation in massive galaxies slightly more efficiently than many state-of-the-art models. Therefore, the simulations that include our new implementations produce a more realistic population of quiescent and star-forming galaxies compared to recent observations, even if some discrepancies remain. In addition, the baryon conversion efficiencies in our simulation are - except for the high mass end - consistent with observations presented in literature over the mass range resolved by our simulations. Finally, we discuss the significant impact of the feedback model on the low-luminous end of the AGN luminosity function.Comment: 25 pages, 19 figures. MNRAS accepted. Magneticum website: http://www.magneticum.or

Rise and fall of post-starburst galaxies in Magneticum Pathfinder

Post-starburst galaxies (PSBs) belong to a short-lived transition population between star-forming (SF) and quiescent galaxies. Deciphering their heavily discussed evolutionary pathways is paramount to understanding galaxy evolution. We aim to determine the dominant mechanisms governing PSB evolution in both the field and in galaxy clusters. Using the cosmological hydrodynamical simulation suite Magneticum Pathfinder, we identify 647 PSBs with $z \sim 0$ stellar mass $M_* \geq 5 \cdot 10^{10} \, \mathrm{M_{\odot}}$. We track their galactic evolution, merger history, and black hole activity over a time-span of 3.6Gyr. Additionally, we study cluster PSBs identified at different redshifts and cluster masses. Independent of environment and redshift, we find that PSBs, like SF galaxies, have frequent mergers. At z=0, 89% of PSBs have experienced mergers and 65% had at least one major merger within the last 2.5Gyr, leading to strong star formation episodes. In fact, 23% of z=0 PSBs were rejuvenated during their starburst. Following the mergers, field PSBs are generally shutdown via a strong increase in AGN feedback (power output $P_{AGN,PSB} \geq 10^{56}\,$erg/Myr). We find agreement with observations for both stellar mass functions and z = 0.9 line-of-sight phase space distributions of PSBs in galaxy clusters. Finally, we find that $z \lesssim 0.5$ cluster PSBs are predominantly infalling, especially in high mass clusters and show no signs of enhanced AGN activity. Thus, we conclude that the majority of cluster PSBs are shutdown via an environmental quenching mechanism such as ram-pressure stripping, while field PSBs are mainly quenched by AGN feedback.Comment: 28 pages, 15 figures, accepted by MNRA