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Register a new userDissipation and Extra Light in Galactic Nuclei: I. Gas-Rich Merger Remnants
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Philip F. Hopkins
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We study the origin and properties of extra or excess central light in the surface brightness profiles of gas-rich merger remnants. Combining a large set of hydrodynamical simulations with data on observed mergers (spanning a broad range of profiles at various masses and degrees of relaxation), we show how to robustly separate the physically meaningful extra light -- stellar populations formed in a compact central starburst during a gas-rich merger -- from the outer profile established by violent relaxation acting on stars already present in the progenitors prior to the final merger. This separation is sensitive to the profile treatment, and we demonstrate that certain fitting procedures can yield physically misleading results. We show that our method reliably recovers the younger starburst population, and examine how the properties of this component scale with mass, gas content, and other aspects of the progenitors. We consider the time evolution of profiles in different bands, and estimate biases introduced by observational studies at different times and wavelengths. We show that extra light is ubiquitous in observed and simulated gas-rich merger remnants, with sufficient mass (~3-30% of the stellar mass) to explain the discrepancy in the maximum phase-space densities of ellipticals and their progenitor spirals. The nature of this central component provides powerful new constraints on the formation histories of observed systems.
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We study the origin and properties of extra or excess central light in the surface brightness profiles of cusp or power-law ellipticals. Dissipational mergers give rise to two-component profiles: an outer profile established by violent relaxation acting on stars present in the progenitors prior to the final merger, and an inner stellar population comprising the extra light, formed in a compact starburst. Combining a large set of hydrodynamical simulations with data that span a broad range of profiles and masses, we show that this picture is borne out -- cusp ellipticals are indeed extra light ellipticals -- and examine how the properties of this component scale with global galaxy properties. We show how to robustly separate the extra light, and demonstrate that observed cusps are reliable tracers of the degree of dissipation in the spheroid-forming merger. We show that the typical degree of dissipation is a strong function of stellar mass, tracing observed disk gas fractions at each mass. We demonstrate a correlation between extra light content and effective radius at fixed mass: systems with more dissipation are more compact. The outer shape of the light profile does not depend on mass, with a mean outer Sersic index ~2.5. We explore how this relates to shapes, kinematics, and stellar population gradients. Simulations with the gas content needed to match observed profiles also reproduce observed age, metallicity, and color gradients, and we show how these can be used as tracers of the degree of dissipation in spheroid formation.
We investigate how extra central light in the surface brightness profiles of cusp ellipticals relates to the profiles of ellipticals with cores. Cusp elliptical envelopes are formed by violent relaxation in mergers acting on stars in progenitor disks, while their centers are structured by dissipational starbursts. Core ellipticals are formed by subsequent merging of (now gas-poor) cusp ellipticals, with the fossil starburst components combining to preserve a compact component in the remnant (although the transition is smoothed). Comparing hydrodynamical simulations and observed profiles, we show how to observationally isolate the relic starburst components in core ellipticals. We demonstrate that these survive re-mergers and reliably trace the dissipation in the initial gas-rich merger(s). The typical degree of dissipation is a strong function of stellar mass, tracing observed disk gas fractions. We find a correlation between dissipation and effective radius: systems with more dissipation are more compact. The survival of this component and scattering of stars into the envelope naturally explain high-Sersic index profiles characteristic of massive core ellipticals. This is also closely related to the kinematics and isophotal shapes: only systems with matched starburst components from their profile fits also reproduce the observed kinematics of boxy/core ellipticals. We show that it is critical to adopt physically motivated profiles when attempting to quantify how much mass has been scoured or scattered out of the inner regions by binary black holes. Estimates of scoured mass ignoring multi-component structure can be strongly biased, potentially explaining observed systems with large inferred core masses in apparent conflict with core-scouring models.
We develop a model for the origins and redshift evolution of spheroid scaling relations. We consider spheroid sizes, velocity dispersions, masses, profile shapes (Sersic indices), and black hole (BH) masses, and their related scalings. Our approach combines advantages of observational constraints in halo occupation models and hydrodynamic merger simulations. This allows us to separate the relative roles of dissipation, dry mergers, formation time, and progenitor evolution, and identify their effects on scalings at each redshift. Dissipation is the most important factor determining spheroid sizes and fundamental plane (FP) scalings, and can account for the FP tilt and differences between disk and spheroid scalings. Because disks at high-z have higher gas fractions, mergers are more gas-rich, yielding more compact spheroids. This predicts mass-dependent evolution in spheroid sizes, in agreement with observations. This relates to subtle evolution in the FP, important to studies that assume a fixed intrinsic FP. This also predicts mild evolution in BH-host correlations, towards larger BHs at higher z. Dry mergers are significant, but only for massive systems which form early: they form compact, but undergo dry mergers (consistent with observations) such that their sizes at later times are similar to spheroids of similar mass formed more recently. We model descendants of observed compact high-z spheroids: most will become cores of BCGs, with sizes, velocity dispersions, and BH masses consistent with observations, but we identify a fraction that might survive to z=0 intact.
We present < 1 kpc resolution CO imaging study of 37 optically-selected local merger remnants using new and archival interferometric maps obtained with ALMA, CARMA, SMA and PdBI. We supplement a sub-sample with single-dish measurements obtained at the NRO 45 m telescope for estimating the molecular gas mass (10^7 - 10^11 M_sun), and evaluating the missing flux of the interferometric measurements. Among the sources with robust CO detections, we find that 80 % (24/30) of the sample show kinematical signatures of rotating molecular gas disks (including nuclear rings) in their velocity fields, and the sizes of these disks vary significantly from 1.1 kpc to 9.3 kpc. The size of the molecular gas disks in 54 % of the sources is more compact than the K-band effective radius. These small gas disks may have formed from a past gas inflow that was triggered by a dynamical instability during a potential merging event. On the other hand, the rest (46 %) of the sources have gas disks which are extended relative to the stellar component, possibly forming a late-type galaxy with a central stellar bulge. Our new compilation of observational data suggests that nuclear and extended molecular gas disks are common in the final stages of mergers. This finding is consistent with recent major-merger simulations of gas rich progenitor disks. Finally, we suggest that some of the rotation-supported turbulent disks observed at high redshifts may result from galaxies that have experienced a recent major merger.
The study of how stars distribute themselves around a massive black hole (MBH) in the center of a galaxy is an important prerequisite for the understanding of many galactic-center processes. These include the observed overabundance of point X-ray sources at the Galactic center, the prediction of rates and characteristics of tidal disruptions of extended stars by the MBH and of inspirals of compact stars into the MBH, the latter being events of high importance for the future space borne gravitational wave interferometer LISA. In relatively small galactic nuclei, hosting MBHs with masses in the range 10^5-10^7 Msun, the single most important dynamical process is 2-body relaxation. It induces the formation of a steep density cusp around the MBH and strong mass segregation, as more massive stars lose energy to lighter ones and drift to the central regions. Using a spherical stellar dynamical Monte-Carlo code, we simulate the long-term relaxational evolution of galactic nucleus models with a spectrum of stellar masses. Our focus is the concentration of stellar black holes to the immediate vicinity of the MBH. We quantify this mass segregation for a variety of galactic nucleus models and discuss its astrophysical implications. Special attention is given to models developed to match the conditions in the Milky Way nucleus; we examine the presence of compact objects in connection to recent high-resolution X-ray observations.
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