RT info:eu-repo/semantics/masterThesis
T1 Medición y análisis del desplazamiento entre el fotocentro y el agujero negro supermasivo de M87
A1 López Navas, Elena
K1 Astrofísica
AB It is generally assumed that supermassive black holes (SBHs) reside at the centers of theirhost galaxies. However, there are several mechanisms that seem capable of displacing theSBH from its equilibrium position. Most recently, interest has focused on gravitational recoilresulting from the coalescence of a SBH–SBH binary. Other possibilities include a sustainedacceleration due to intrinsic asymmetries in jet power, orbital motion of SBH binaries andinteractions with massive perturbers such as globular clusters or massive molecular clouds.Over the past years, there have been many researches to seek displaced SBH candidates.It was reported in Batcheldor et al. (2010) [1] that the photocenter of NGC4486 (M87)and the AGN are displaced significantly. The displacement was confirmed by Lena et al.(2014) [2] and this fact suggests that the SBH is not located at the center of mass of thegalaxy. Nevertheless, in Lena et al. (2014) [2] the results from some images differ significantlyfrom the others, and the origin of these differences is unclear. The active giant elliptical M87is a reference to the study of jets in AGNs, and an explanation of these results is essential tounderstand the evolution of this galaxy. Under these circumstances, we aim to investigatewhether M87 presents a displaced SBH or not, and also to evaluate the possible explanationsto the discrepancies.Our method consists of measuring the relative positions of the AGN point source and thephotocenter of the galaxy, assuming that the former marks the SBH position and the lattermarks the minimum of the galactic potential. Similarly to previous studies, we perform astandard photometric analysis by means of isophotal fitting to locate the photocenter. Inorder to do that, we have analyzed several images with the task ellipse within the astronomicalsoftware IRAF. Besides, we adjusted gaussians along the x and y axis to computethe position of the point nuclear source. The data selected for this study consisted of sevenimages with high angular resolution and red filters (from 0.8 to 2.2 µm). Five of them arefrom the Hubble Space Telescope, taken with the Advanced Camera for Surveys, the WideField Planetary Camera 2 and the Wide Field Camera 3. The remaining images were takenwith NaCo in the Very Large Telescope. These images were taken on different dates, whichallows a temporal examination of the results. Since the jet of M87 is visible in all the images,a mask was created before the analysis in order to minimize the photometric irregularitiesassociated with the knots of the jet.The results from the photometric analysis show that the displacement varies in amplitudeand direction depending on the image. There are several images in which the position of thephotocenter is consistent with the AGN and others that show a relative displacement ≥ 50mas. The maximum displacement of the photocenter relative to the AGN is ≈100 mas, andis in agreement with the results obtained by the previous studies [1] and [2]. It is remarkablethat these discrepancies are similar to those from Lena et al [2]., but obtained from differentimages.On the other hand, we observe that the photocenter position depends on the semimajoraxis length (SMA) of the galaxy used in the isophotal fitting. The displacements obtainedwithin a radius between 100 and 300 from the centre (range used by Lena et al. [2] andBatcheldor et al. [1]) vary in amplitude but are roughly aligned in the same direction asthe jet, implying that the SBH is displaced in the counter-jet direction. In a wider range ofSMA, the photocenter displacement of each image decreases in amplitude and its positionangle separates from the jet direction.In order to compare the positions of both the AGN and the photocenter from all of theimages (obtained with different instruments and filters), we define a common reference frameby means of several globular clusters that appear in every image analyzed . This comparisonindicates that the differences in the displacements obtained for each image are due largelyto differences in the positions of the photocenters. This result implies that the mechanismsthat were thought as the origin of the displaced SBH are no longer valid in this case, as theAGN remains fairly stable within a precision of 13 x 10 mas2.In an attempt to explain these discrepancies we notice that almost every image that showsa displacement of the photocenter was taken in the same period of time, and that it coincideswith a huge and well studied outburst that occurred between 2003 and 2007. This outburstcaused an increase of the flux density in the nucleus and in the HST-1 knot (at ≈ 0.8500from the nucleus) along the entire wavelength range. We believe that this event is capableof affecting the isophotal fitting, and it could explain the discrepancies in the results of thedifferent images and between the different ranges of SMA. However, we analyzed an imagefrom 1998 that also presents an offset of about 100 mas that is not well explained with a fluxvariability in M87 nucleus that year.There is also evidence of instabilities in photocenter positions in other galaxies in thebibliography. Anton et al. (2012) [3] observed a set of selected AGNs and reported thatphotocenter jitters at mas level, accompanied by flux variation. According to them, thosejitters translate from few parsecs (in most of the objects), to tens of parsec. They discussedpossible origins for such photocenter displacements and concluded that in the case of “jetted”objects, enhancements in flux due to shocks along the jet, or the appearance of a newblob of plasma are natural candidates. Furthermore, the study of Popović et al.(2012) [4]revealed that perturbations in the quasar inner structure can cause a significant offset tothe photocenter by up to several mas. The similarity of these conclusions with our resultssupports the theory that the instability of the photocenter is due to an additional contributionof light that comes from the jet.In conclusion, we determine that the most likely origin of the displacement between thephotocenter and the SBH -found in this study and also in previous references- is a consequenceof an instability of the photocenter caused by the intrinsic variability of M87.
YR 2018
FD 2018
LK http://riull.ull.es/xmlui/handle/915/7406
UL http://riull.ull.es/xmlui/handle/915/7406
LA es
DS Repositorio institucional de la Universidad de La Laguna
RD 22-dic-2024