Supermassive black holes, star clusters, and stellar discs in galactic nuclei

In this thesis we investigated the structures of the central regions of galaxies. This was done by characterising some aspects of the central massive objects (CMOs) found to live in the galactic nuclei, such as studying the possible formation scenarios, stellar properties and scaling relations using a variety of methods. In Chapter 1 we highlighted the importance of understanding the physical properties of CMOs in galaxies by exploring their possible connection with the host galaxies. We reviewed the previous works on the properties, formations scenarios, and scaling relations of supermassive black holes (SBHs), nuclear star clusters (NSCs), and nuclear stellar discs (NSDs) which reside in galactic nuclei. SBHs are believed to be the cause of the past or present activity of the galaxies. The masses of the SBHs range between 10^6 and 10^10 M_⊙ and can be measured with several methods. SBHs are found to correlate with several properties of their host galaxy and their formation is still unclear. NSCs are commonly found in the centres of both elliptical and disc galaxies. They are very massive (M_NSC∼ 10^5 − 10^8 M_⊙), very compact (r_e ∼ 5 pc), and very bright (−14 < M_I < −10). They can have multiple stellar populations, possessing both an old spheroidal component and a younger elongated disc or ring component. The mass of NSCs tightly correlates with the total mass of the galaxy, but several other correlations have been proposed to link the properties of the NSCs with those of the host galaxy. A combined scenario where star formation occurs in the centre of galaxies after dissipation processes and mass accretes via the mergers of globular clusters seems the more plausible way to form NSCs. NSDs are small (h ∼ 10 − 50 pc) and bright (μ_0,V ∼ 16 − 19 mag arcsec^(−2^) discs. They never dominate the light distribution of the galactic nuclei, and locally contribute at most half the galaxy surface brightness. They are fragile systems and do not survive a major merger. Their stellar population has been studied in details in only a few objects, showing a variety of phenomena. NSDs follow the same relation between the central face-on surface-brightness and the scalelength as the main discs of lenticular and spiral galaxies and embedded discs of early-type galaxies. No other relation was found with the properties of the host galaxy. The external capture or the secular infall of gas into the centre where it accumulates, dissipates and forms stars are the most studied scenarios to form NSDs. SBHs, NSCs, and NSDs have been observed to coexist in some galaxies, rising the question whether they are incarnations of the same object and share a common formation scenario. Then in Chapter 2 we investigated the formation and properties of NSDs by looking for their presence in a set of N−body simulations studying the dissipationless merging of multiple star clusters in galactic nuclei. A few tens of star clusters with sizes and masses comparable to those of globular clusters observed in the Milky Way are accreted onto a pre-existing nuclear stellar component: either a massive super star cluster or a rapidly rotating, compact disc with a scalelength of a few parsecs, mimicking the variety of observed nuclear structures. Images and kinematic maps of the simulation time-steps were then built and analysed as if they were real and at the distance of the Virgo cluster. We used the Scorza-Bender method to search for the presence of disc structures via photometric decomposition. In one case the merger remnant had all the observed photometric and kinematic properties of NSDs observed in real galaxies. This shows that current observations are consistent with most of the NSD mass being assembled from the migration and accretion of star clusters into the galactic centre. In the other simulation instead, we detected an elongated structure from the unsharp masked image, that does not develop the photometric or kinematic signature of an NSD. Thus, in the context of searches for a disc structure, the Scorza-Bender method is a robust and necessary tool. In Chapter 3 we investigated the structure and properties of the stellar population of the nuclear regions of the interacting SB0 galaxy NGC 1023 through a detailed analysis of archival Hubble Space Telescope (HST) imaging and ground-based integral-field spectroscopy. The stars of the nuclear disc are remarkably younger and more metal-rich with respect to the host bulge. These findings support a scenario in which the nuclear disc is the end result of star formation in gas piled up in the galaxy centre. The gas can be of either internal or external origin, i.e. from either the main disc of NGC 1023 or the nearby interacting satellite NGC 1023A. The dissipationless formation from already formed stars through the migration and accretion of star clusters into the galactic centre is rejected. In Chapter 4 we presented a dynamical analysis aimed at constraining the mass of the CMOs in the lenticular galaxy NGC 383 at a distance of 63.4 Mpc. The central stellar velocity dispersion is consistent with a putative SBHwith a mass of 5.8 x 10^8 M_⊙. We presented archival HST imaging and spectroscopic observations obtained with the Wide Field and Planetary Camera 2 mounting the F814W filter and the Space Telescope Imaging Spectrograph using the G570M grism, respectively. The data provide detailed information on the structure and mass profile of the stellar component, the dust optical depth, and the spatial distribution and kinematics of the ionised gas within the innermost region of the galaxy. Dynamical models, which account for the observed stellar mass profile and include the contribution of a NSC and a central SBH, were constructed to reproduce the kinematics derived from the [N II]λ6583 emission line along three slit positions crossing the nucleus and parallel to the galaxy major axis. A secure SBH detection with a mass of 8.5 (+1.8 -1.3) x 10^8 M_⊙ was obtained when a single CMO is considered. If we account for the presence of the NSC, then the masses of the SBH and NSC were 6.0 (+1.8 -1.2) x 10^8 M_⊙ and M_NSC = 8.9 (+5.0 - 3.9) x10^7 M_⊙, respectively. Both are consistent with the scaling relations linking the mass of CMOs with the properties of their host galaxy. These measurements prove that SBHs can coexist with NSCs and represent an important step forward in the characterisation of CMOs. The main conclusions of this thesis can be summarised as follows: 1) NSDs can form via accretion events, but a certain amount of gas is necessary; 2) the young stellar population of the NSD of NGC 1023 suggests a formation via gas dissipation; 3) a SBH and a NSD coexist in NGC 383 and follow different scaling relations with the host galaxy. For the first time we were able to disentangle simultaneously the mass of both the CMOs using dynamical modelling

Simulations of ELT-GMCAO performance for deep field observations

The Global-Multi Conjugated Adaptive Optics (GMCAO) approach offers an alternative way to correct an adequate scientific Field of View (FoV) using only natural guide stars (NGSs) to extremely large ground-based telescopes. Thus, even in the absence of laser guide stars, a GMCAO-equipped ELT-like telescope can achieve optimal performance in terms of Strehl Ratio (SR), retrieving impressive results in studying star-poor fields, as in the cases of the deep field observations. The benefits and usability of GMCAO have been demonstrated by studying 6000 mock high redshift galaxies in the Chandra Deep Field South region. However, a systematic study simulating observations in several portions of the sky is mandatory to have a robust statistic of the GMCAO performance. Technical, tomographic and astrophysical parameters, discussed here, are given as inputs to GIUSTO, an IDL-based code that estimates the SR over the considered field, and the results are analyzed with statistical considerations. The best performance is obtained using stars that are relatively close to the Scientific FoV; therefore, the SR correlates with the mean off-axis position of NGSs, as expected, while their magnitude plays a secondary role. This study concludes that the SRs correlate linearly with the galactic latitude, as also expected. Because of the lack of natural guide stars needed for low-order aberration sensing, the GMCAO confirms as a promising technique to observe regions that can not be studied without the use of laser beacons. It represents a robust alternative way or a risk mitigation strategy for laser approaches on the ELTs.Comment: 18 pages, 10 figures, accepted for publication on PAS

Ingot Laser Guide Stars Wavefront Sensing

We revisit one class of z-invariant WaveFront sensor where the LGS is fired aside of the telescope aperture. In this way there is a spatial dependence on the focal plane with respect to the height where the resonant scattering occurs. We revise the basic parameters involving the geometry and we propose various merit functions to define how much improvement can be attained by a z-invariant approach. We show that refractive approaches are not viable and we discuss several solutions involving reflective ones in what has been nicknamed "ingot wavefront sensor" discussing the degrees of freedom required to keep tracking and the basic recipe for the optical design.Comment: 6 pages, 4 figures, AO4ELT5 Conference Proceeding, 201

Multiple Spatial Frequencies Pyramid WaveFront Sensing

A modification of the pyramid wavefront sensor is described. In this conceptually new class of devices, the perturbations are split at the level of the focal plane depending upon their spatial frequencies, and then measured separately. The aim of this approach is to increase the accuracy in the determination of some range of spatial frequency perturbations, or a certain classes of modes, disentangling them from the noise associated to the Poissonian fluctuations of the light coming from the perturbations outside of the range of interest or from the background in the pupil planes; the latter case specifically when the pyramid wavefront sensor is used with a large modulation. While the limits and the effectiveness of this approach should be further investigated, a number of variations on the concept are shown, including a generalization of the spatial filtering in the point-diffraction wavefront sensor. The simplest application, a generalization to the pyramid of the well-known spatially filtering in wavefront sensing, is showing promise as a significant limiting magnitude advance. Applications are further speculated in the area of extreme adaptive optics and when serving spectroscopic instrumentation where “light in the bucket” rather than Strehl performance is required

A Holographic Diffuser Generalised Optical Differentiation Wavefront Sensor

The wavefront sensors used today at the biggest World's telescopes have either a high dynamic range or a high sensitivity, and they are subject to a linear trade off between these two parameters. A new class of wavefront sensors, the Generalised Optical Differentiation Wavefront Sensors, has been devised, in a way not to undergo this linear trade off and to decouple the dynamic range from the sensitivity. This new class of WFSs is based on the light filtering in the focal plane from a dedicated amplitude filter, which is a hybrid between a linear filter, whose physical dimension is related to the dynamic range, and a step in the amplitude, whose size is related to the sensitivity. We propose here a possible technical implementation of this kind of WFS, making use of a simple holographic diffuser to diffract part of the light in a ring shape around the pin of a pyramid wavefront sensor. In this way, the undiffracted light reaches the pin of the pyramid, contributing to the high sensitivity regime of the WFS, while the diffused light is giving a sort of static modulation of the pyramid, allowing to have some signal even in high turbulence conditions. The holographic diffuser zeroth order efficiency is strictly related to the sensitivity of the WFS, while the diffusing angle of the diffracted light gives the amount of modulation and thus the dynamic range. By properly choosing these two parameters it is possible to build a WFS with high sensitivity and high dynamic range in a static fashion. Introducing dynamic parts in the setup allows to have a set of different diffuser that can be alternated in front of the pyramid, if the change in the seeing conditions requires it.Comment: 11 pages, 5 figure

MICADO Italian Contribution

Citato dalla notizia media inaf https://www.media.inaf.it/2020/01/21/micado-elt/The Webpages of the Istituto Nazionale di Astrofisica (INAF) contribution to the MICADO project for the European Southern Observatory (ESO) Extremely Large Telescope (ELT). The Webpages serve as white pages for easy access to information relative to the MICADO and in particular to the Point Spread Function (PSF) reconstruction working group

Multiple spatial frequencies wavefront sensing

We describe the concept of splitting spatial frequency perturbations into some kind of pupil planes wavefront sensors. Further to the existing approach of dropping higher spatial frequency to suppress aliasing effects (the so-called spatial filtered Shack-Hartmann), we point out that spatial frequencies splitting and mixing of these in a proper manner, could be handled in order to exhibit some practical or fundamental advantages. In this framework we describe the idea behind such class of concepts and we derive the relationship useful to determine if, by which extent, and under what kind of merit function, these devices can overperform existing conventional sensors.Comment: 6 pages, 3 figures, in AO4ELT5 Proceeding

BRUTE, PSF Reconstruction for the SOUL pyramid-based Single Conjugate Adaptive Optics facility of the LBT

The astronomical applications greatly benefit from the knowledge of the instrument PSF. We describe the PSF Reconstruction algorithm developed for the LBT LUCI instrument assisted by the SOUL SCAO module. The reconstruction procedure considers only synchronous wavefront sensor telemetry data and a few asynchronous calibrations. We do not compute the Optical Transfer Function and corresponding filters. We compute instead a temporal series of wavefront maps and for each of these the corresponding instantaneous PSF. We tested the algorithm both in laboratory arrangement and in the nighttime for different SOUL configurations, adapting it to the guide star magnitudes and seeing conditions. We nick-named it "BRUTE", Blind Reconstruction Using TElemetry, also recalling the one-to-one approach, one slope-to one instantaneous PSF the algorithm applies.Comment: 11 pages, 7 figures, Proceeding of the SPIE Conference 12185, Adaptive Optics Systems VIII, 1218540 (29 August 2022

Prospects of Deep Field Surveys with Global-MCAO on an ELT

Several astronomical surveys aimed at the investigation of the extragalactic components were carried out in order to map systematically the universe and its constituents. An excellent level of detail is needed, and it is possible only using space telescopes or with the application of adaptive optics (AO) techniques for ground-based observatories. By simulating K-band observations of 6000 high-redshift galaxies in the Chandra Deep Field South region, we have already shown how an extremely large telescope can carry out photometric surveys successfully using the Global-MCAO, a natural guide stars based technique that allows the development of extragalactic research, otherwise impracticable without using laser guide stars. As the outcome of the analysis represents an impact science case for the new instruments on upcoming ground-based telescopes, here we show how the investigation of other observed deep fields could profit from such a technique. Further to an overview of the surveys suitable for the proposed approach, we show preliminary estimations both on geometrical (FoV and height) and purely AO perspectives (richness and homogeneity of guide stars in the area) for planned giant telescope.Comment: 8 pages, 6 figures, AO4ELT5 conferenc