A fresh look at the starburst-AGN connection

There is ever mounting evidence for a connection between the evolution of AGN and starburst-activity, as, for instance, suggested by a relation between the mass of the central black hole and the velocity dispersion of the bulge stars. However, the nature of this connection remains unclear, raising a number of questions, e. g.: Is the AGN triggered by the starburst or is the starburst activity caused by AGN? Which physical processes link starbursts and AGN? Here we will give a short review of crucial observations and theoretical work and describe our plans in that regard

Starbursts und aktive Kerne in Galaxien

Starbursts and active galactic nuclei (AGN) are predominantly driven by galaxy mergers, i.e., the collision of two galaxies, but isolated galaxies, e.g., our Milky Way, can also be subject to these phenomena. Therefore this thesis is divided into two parts: in partwe investigate starbursts and AGN due to galaxy mergers and in part II we examine the Galactic Center (GC), i.e., the center of our Milky Way. In part I we will focus in particular on the timing of starbursts and AGN activity during a merger event. Recent observations indicate a time lag between these two phenomena in the sense that first the merger produces a starburst, followed by, after a time span of 200-300 million years, an episode of AGN activity. To explain this time lag we develop a scenario where the galaxies’ gas, after having formed stars, first forms an accretion disk around the central black hole due to its angular momentum. The gas then loses its angular momentum due to viscous forces and is finally accreted by the black hole, causing the galactic nucleus to become active. Thus the observed time lag is related to the viscous timescale the gas needs, after having formed stars, to move through the accretion disk before reaching the black hole. We develop a new subgrid model to account for the gas forming an accretion disk and for the subsequent evolution of this disk. Our numerical simulations of galaxy mergers reproduce the observed time lag between starburst and AGN activity and fulfill further observational constraints, like the well known correlation between black hole mass and the stellar velocity dispersion of the host galaxy. In part II we will investigate the center of the Milky Way. The GC hosts a black hole that is surrounded by a gaseous disk, the so-called circumnuclear disk (CND). This disk has an inner cavity with a radius of 1 parsec which is populated by a stellar cluster that launches a strong spherical wind. The GC is currently in a quiescent state where no significant star formation or black hole accretion is taking place, but there is evidence that the GC experienced a starburst and AGN activity several million years ago. We will perform magnetohydrodynamical simulations of the CND to investigate the interaction of the stellar cluster’s wind with the CND. It has previously been argued that this wind is responsible for the quiescent state of the GC by pushing the disk’s gas outwards and thus preventing accretion of gas onto the black hole. Our results disprove this scenario, invoking only the wind actually leads to a collapse of the inner cavity within a short period of time. However, including the effects of magnetic fields, which are observed in the CND, stabilizes the inner cavity against collapse. Thus our results explain why the GC is currently in a quiescent state. Furthermore we discuss whether the fading of the stellar winds will subsequently lead to the accretion of gas by the black hole, which in turn could trigger an episode of star formation and AGN activity.Starbursts und aktive galaktische Kerne (AGK) werden vorwiegend durch die Verschmelzung zweier Galaxien ausgelöst, aber auch in isolierten Galaxien, wie z. B. in unserer Milchstraße, können derartige Prozesse stattfinden. Daher gliedern wir diese Arbeit in zwei Teile: im ersten Teil untersuchen wir Starbursts und AGK, welche durch Galaxienkollisionen entstehen, im zweiten Teil widmen wir uns dem Galaktischen Zentrum (GZ), d. h. dem Zentrum unserer Milchstraße. Im ersten Teil untersuchen wir insbesondere, in welcher zeitlichen Beziehung Starbursts und AGK zueinander stehen. Kürzlich durchgeführte Beobachtungen zeigen nämlich eine Zeitdifferenz zwischen diesen beiden Phänomenen, und zwar generiert die Galaxienkollision zunächst einen Starburst, gefolgt von, nach einer Zeitdauer von etwa 200-300 Myr, einer Episode erhöhter AGK Aktivität. Um diese Zeitdifferenz zu erklären entwickeln wir ein Szenario, in dem das Gas der Galaxien, nachdem es den Starburst erzeugt hat, infolge seines Drehimpulses zunächst eine Akkretionsscheibe um das zentrale Schwarze Loch bildet. Durch viskose Prozesse verliert das Gas dann diesen Drehimpuls und wird schließlich vom Schwarzen Loch verschluckt, dieser Prozess ist gekennzeichnet durch eine hohe Aktivität des galaktischen Kerns. Die beobachtete Zeitdifferenz ist also eine viskose Zeitskala, welche das Gas, nachdem es Sterne gebildet hat, zusätzlich benötigt um zum Schwarzen Loch zu gelangen. Wir entwickeln ein neues Subgrid-Modell, welches die Entstehung und die darauf folgende Entwicklung der Akkretionscheibe berücksichtigt. Die Ergebnisse unserer numerischen Simulationen von Galaxienkollisonen reproduzieren die beobachtete Zeitdifferenz zwischen Starburst und AGK Aktivität und erfüllen weitere Beobachtungsbefunde, wie die allgemein bekannte Korrelation zwischen der Masse des zentralen Schwarzen Loches und der stellaren Geschwindigkeitsdispersion der Galaxie. Im zweiten Teil untersuchen wir das Zentrum unserer Milchstraße. Das GZ beherbergt ein Schwarzes Loch, welches von einer gasförmigen Scheibe umgeben ist. Diese Scheibe hat ein zentrales Loch mit einem Radius von etwa einem Parsec, darin befindet sich ein Sternhaufen, welcher einen starken, sphärischen Wind ausstößt. Das GZ befindet sich zur Zeit in einem Stadium geringer Aktivität, es gibt aber Hinweise auf Starbursts und AGK Aktivität, welche vor einigen Millionen Jahren stattfanden. Wir werden magnetohydrodynamische Simulationen durchführen, um die Interaktion des Innenrandes der Scheibe mit dem Wind des zentralen Sternhaufens zu untersuchen. Bisher wurde angenommen, dass dieser Wind für die derzeitige geringe Aktivität des GZs verantwortlich ist, indem dieser einen Druck auf das Gas der Scheibe ausübt und so die Akkretion des Gases auf das Schwarze Loch verhindert. Unsere Ergebnisse widerlegen dieses Szenario, berücksichtigt man nur die Interaktion der Scheibe mit dem Wind führt dies sogar zum Kollaps des inneren Loches. Nur die zusätzliche Berücksichtigung von Magnetfeldern stabilisiert das innere Loch gegen den Kollaps. Unsere Ergebnisse erklären somit, warum sich das GZ zur Zeit in einem Stadium geringer Aktivität befindet. Wir werden außerdem diskutieren, ob das Nachlassen des Windes vom zentralen Sternhaufen eine Phase erhöhter Akkretion und somit Starbursts und AGK Aktivität auslösen könnte

Clues to the nature of dark matter from first galaxies

We use thirty-eight high-resolution simulations of galaxy formation between redshift 10 and 5 to study the impact of a 3 keV warm dark matter (WDM) candidate on the high-redshift Universe. We focus our attention on the stellar mass function and the global star formation rate and consider the consequences for reionization, namely the neutral hydrogen fraction evolution and the electron scattering optical depth. We find that three different effects contribute to differentiate warm and cold dark matter (CDM) predictions: WDM suppresses the number of haloes with mass less than few $10^9$ M$_{\odot}$; at a fixed halo mass, WDM produces fewer stars than CDM; and finally at halo masses below $10^9$ M$_{\odot}$, WDM has a larger fraction of dark haloes than CDM post-reionization. These three effects combine to produce a lower stellar mass function in WDM for galaxies with stellar masses at and below $\sim 10^7$ M$_{\odot}$. For $z > 7$, the global star formation density is lower by a factor of two in the WDM scenario, and for a fixed escape fraction, the fraction of neutral hydrogen is higher by 0.3 at $z \sim 6$. This latter quantity can be partially reconciled with CDM and observations only by increasing the escape fraction from 23 per cent to 34 per cent. Overall, our study shows that galaxy formation simulations at high redshift are a key tool to differentiate between dark matter candidates given a model for baryonic physics.Comment: 11 pages, 8 figures, submitted to MNRA

NIHAO XX: The impact of the star formation threshold on the cusp-core transformation of cold dark matter haloes

We use cosmological hydrodynamical galaxy formation simulations from the NIHAO project to investigate the impact of the threshold for star formation on the response of the dark matter (DM) halo to baryonic processes. The fiducial NIHAO threshold, $n=10\, {\rm cm}^{-3}$, results in strong expansion of the DM halo in galaxies with stellar masses in the range $10^{7.5} < M_{star} < 10^{9.5} M_{\odot}$. We find that lower thresholds such as $n=0.1$ (as employed by the EAGLE/APOSTLE and Illustris/AURIGA projects) do not result in significant halo expansion at any mass scale. Halo expansion driven by supernova feedback requires significant fluctuations in the local gas fraction on sub-dynamical times (i.e., < 50 Myr at galaxy half-light radii), which are themselves caused by variability in the star formation rate. At one per cent of the virial radius, simulations with $n=10$ have gas fractions of $\simeq 0.2$ and variations of $\simeq 0.1$, while $n=0.1$ simulations have order of magnitude lower gas fractions and hence do not expand the halo. The observed DM circular velocities of nearby dwarf galaxies are inconsistent with CDM simulations with $n=0.1$ and $n=1$, but in reasonable agreement with $n=10$. Star formation rates are more variable for higher $n$, lower galaxy masses, and when star formation is measured on shorter time scales. For example, simulations with $n=10$ have up to 0.4 dex higher scatter in specific star formation rates than simulations with $n=0.1$. Thus observationally constraining the sub-grid model for star formation, and hence the nature of DM, should be possible in the near future.Comment: 18 pages, 13 figures, accepted to MNRA

Co-Evolution vs. Co-existence: The Effect of Accretion Modelling on the Evolution of Black Holes and Host Galaxies

We append two additional black hole (BH) accretion models, namely viscous disc and gravitational torque-driven accretion, into the Numerical Investigation of a Hundred Astrophysical Objects (NIHAO) project of galaxy simulations. We show that these accretion models, characterized by a weaker dependence on the BH mass compared to the commonly used Bondi-Hoyle accretion, naturally create a common evolutionary track (co-existence) between the mass of the BH and the stellar mass of the galaxy, even without any direct coupling via feedback (FB). While FB is indeed required to control the final BH and stellar mass of the galaxies, our results suggest that FB might not be the leading driver of the cosmic co-evolution between these two quantities; in these models, co-evolution is simply determined by the shared central gas supply. Conversely, simulations using Bondi-Hoyle accretion show a two-step evolution, with an early growth of stellar mass followed by exponential growth of the central supermassive black hole (SMBH). Our results show that the modelling of BH accretion (sometimes overlooked) is an extremely important part of BH evolution and can improve our understanding of how scaling relations emerge and evolve, and whether SMBH and stellar mass co-exist or co-evolve through cosmic time.Comment: 13 pages, 10 figure

The edge of galaxy formation III: The effects of warm dark matter on Milky Way satellites and field dwarfs

In this third paper of the series, we investigate the effects of warm dark matter with a particle mass of $m_\mathrm{WDM}=3\,\mathrm{keV}$ on the smallest galaxies in our Universe. We present a sample of 21 hydrodynamical cosmological simulations of dwarf galaxies and 20 simulations of satellite-host galaxy interaction that we performed both in a Cold Dark Matter (CDM) and Warm Dark Matter (WDM) scenario. In the WDM simulations, we observe a higher critical mass for the onset of star formation. Structure growth is delayed in WDM, as a result WDM haloes have a stellar population on average two Gyrs younger than their CDM counterparts. Nevertheless, despite this delayed star formation, CDM and WDM galaxies are both able to reproduce the observed scaling relations for velocity dispersion, stellar mass, size, and metallicity at $z=0$. WDM satellite haloes in a Milky Way mass host are more susceptible to tidal stripping due to their lower concentrations, but their galaxies can even survive longer than the CDM counterparts if they live in a dark matter halo with a steeper central slope. In agreement with our previous CDM satellite study we observe a steepening of the WDM satellites' central dark matter density slope due to stripping. The difference in the average stellar age for satellite galaxies, between CDM and WDM, could be used in the future for disentangling these two models.Comment: 10 pages, 11 figures, accepted for publication on MNRA