Medición y análisis del desplazamiento entre el fotocentro y el agujero negro supermasivo de M87

Abstract

It is generally assumed that supermassive black holes (SBHs) reside at the centers of their host galaxies. However, there are several mechanisms that seem capable of displacing the SBH from its equilibrium position. Most recently, interest has focused on gravitational recoil resulting from the coalescence of a SBH–SBH binary. Other possibilities include a sustained acceleration due to intrinsic asymmetries in jet power, orbital motion of SBH binaries and interactions 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 the galaxy. Nevertheless, in Lena et al. (2014) [2] the results from some images differ significantly from the others, and the origin of these differences is unclear. The active giant elliptical M87 is a reference to the study of jets in AGNs, and an explanation of these results is essential to understand the evolution of this galaxy. Under these circumstances, we aim to investigate whether M87 presents a displaced SBH or not, and also to evaluate the possible explanations to the discrepancies. Our method consists of measuring the relative positions of the AGN point source and the photocenter of the galaxy, assuming that the former marks the SBH position and the latter marks the minimum of the galactic potential. Similarly to previous studies, we perform a standard photometric analysis by means of isophotal fitting to locate the photocenter. In order to do that, we have analyzed several images with the task ellipse within the astronomical software IRAF. Besides, we adjusted gaussians along the x and y axis to compute the position of the point nuclear source. The data selected for this study consisted of seven images with high angular resolution and red filters (from 0.8 to 2.2 µm). Five of them are from the Hubble Space Telescope, taken with the Advanced Camera for Surveys, the Wide Field Planetary Camera 2 and the Wide Field Camera 3. The remaining images were taken with NaCo in the Very Large Telescope. These images were taken on different dates, which allows 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 irregularities associated with the knots of the jet. The results from the photometric analysis show that the displacement varies in amplitude and direction depending on the image. There are several images in which the position of the photocenter is consistent with the AGN and others that show a relative displacement ≥ 50 mas. The maximum displacement of the photocenter relative to the AGN is ≈100 mas, and is in agreement with the results obtained by the previous studies [1] and [2]. It is remarkable that these discrepancies are similar to those from Lena et al [2]., but obtained from different images. On the other hand, we observe that the photocenter position depends on the semimajor axis length (SMA) of the galaxy used in the isophotal fitting. The displacements obtained within a radius between 100 and 300 from the centre (range used by Lena et al. [2] and Batcheldor et al. [1]) vary in amplitude but are roughly aligned in the same direction as the jet, implying that the SBH is displaced in the counter-jet direction. In a wider range of SMA, the photocenter displacement of each image decreases in amplitude and its position angle separates from the jet direction. In order to compare the positions of both the AGN and the photocenter from all of the images (obtained with different instruments and filters), we define a common reference frame by means of several globular clusters that appear in every image analyzed . This comparison indicates that the differences in the displacements obtained for each image are due largely to differences in the positions of the photocenters. This result implies that the mechanisms that were thought as the origin of the displaced SBH are no longer valid in this case, as the AGN 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 shows a displacement of the photocenter was taken in the same period of time, and that it coincides with a huge and well studied outburst that occurred between 2003 and 2007. This outburst caused an increase of the flux density in the nucleus and in the HST-1 knot (at ≈ 0.8500 from the nucleus) along the entire wavelength range. We believe that this event is capable of affecting the isophotal fitting, and it could explain the discrepancies in the results of the different images and between the different ranges of SMA. However, we analyzed an image from 1998 that also presents an offset of about 100 mas that is not well explained with a flux variability in M87 nucleus that year. There is also evidence of instabilities in photocenter positions in other galaxies in the bibliography. Anton et al. (2012) [3] observed a set of selected AGNs and reported that photocenter jitters at mas level, accompanied by flux variation. According to them, those jitters translate from few parsecs (in most of the objects), to tens of parsec. They discussed possible 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 new blob 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 to the photocenter by up to several mas. The similarity of these conclusions with our results supports the theory that the instability of the photocenter is due to an additional contribution of light that comes from the jet. In conclusion, we determine that the most likely origin of the displacement between the photocenter and the SBH -found in this study and also in previous references- is a consequence of an instability of the photocenter caused by the intrinsic variability of M87.