Abstract
Galaxy mergers play a fundamental role in the hierarchical model of galaxy evolution. They are also thought to be an important process in fueling the growth of supermassive black holes (SMBH) and thus powering active galactic nuclei (AGN) activity. NGC6240 is a local merger and luminous infrared galaxy (LIRG). It has two AGNs in its center, and it shows massive molecular outflows extending by > 10 kpc. The aim of this work is to study the cold dust emission in NGC6240 both in the nuclear region (affected by vigorous starburst and AGN activity) and in an extended region (affected by outflows and tidal tails). This is done by exploiting high sensitivity archival ALMA datasets at observed frequencies ranging from 84 GHz (λ ∼ 3 mm) to 680 GHz (λ ∼ 0.4 mm).
The first result of my work is the detection of a significant amount of emission beyond the central 1.5 kpc region, which was the portion of the galaxy on which most previous interferometric studies have focused. This extended component represents 32% of the total emission measured within the frequency range 336.6-352.2 GHz. Since these single-pointing interferometric observations have a limited field of view (FoV 63” in band 3, but only 22” in band 9) and therefore are not sensitive to larger scales, the extended emission contribution could be considered a lower limit. The shape of the extended emission I detect is spatially correlated with the blueshifted and redshifted components of the molecular outflow studied in [CI] and CO by previous works.
I have also identified three new point sources. Two of them are found in between the two AGNs which i have labelled C-4 and C-3. C-4 is at the midpoint between the two AGNs, while C-3 is closer to the southern nucleus. The position of C-4 coincides with the peak of the redshifted [CI] outflow, and the position of C-3 is close to the peak of the blueshifted [CI] outflow. This suggests that the outflow is closely related to the source that heats the dust in this region. The third source, labelled C-11, is found in the north-east ∼ 4.3 kpc from the northern AGN at Ra = 16:52:59.256 and Dec =
02.24.11.458.
I have compared my (sub)mm continuum flux measurements from the nuclear and extended region and from the five point sources (the two AGNs and three new sources detected in this work) with a model for the spectral energy distribution (SED) and previous observations of this galaxy. Unfortunately, the new data are insifficient to perform a detailed SED fitting, but I have robustly detected some differences in the low-frequency part of the SED (at 1-3 mm). In particular, for the box probing the extended continuum emission I find a flux increase from the datapoints at 130 GHz to 97 GHz. This long wavelength part of the SED is often associated with AGN activity, iv
but it has been suggested that this emission may also be attributed to synchrotron emission from outflows. The rising trend and the cospatiality between the molecular outflow and these extended continuum features support this hypothesis. From the SED of C-3 and C-4, I find that both point sources have a SED that resembles that of the two known AGNs. C-3 also has a counterpart in the radio (VLBI observations at 5 GHz), which suggests that this might be an additional obscured AGN. However, with the current data, I cannot rule out that C-3 and C-4 are starburst regions. The source C-11 does not follow the same rising slope at low frequencies in its SED, and does not have a counterpart in the X-ray, which suggests that it is unlikely an AGN. It may rather be a supernova or another nucleus. Since tidal dwarf galaxies (TDGs) have previously been observed in the tidal tails of gas rich mergers, and this work suggests that there are additional continuum sources outside the nuclear region, it is also possible that this source is a TDG.
The detection of extended emission spatially correlated with outflows in addition to three new point sources in NGC6240 gives greater insight into the origin of the (sub)mm continuum emission, and may explain the extreme ISM of this galaxy. The possible detection of multiple nuclei in a local merger is of great importance both observationally, as the potential for observing obscured AGNs with (sub)-mm continuum emission is very promising, and also from a cosmological point of view, as this helps understanding the hierarchical model of galaxy evolution.
Future studies targeting line-free frequency ranges and with a higher sensitivity to larger scales will help us understand the origin of the sub-mm continuum emission in NGC6240. As there are few observations contributing to the long wavelength part of the SED of NGC6240, studies in the far-IR to radio would also contribute to constraining the SED further and give insight into the components that dominate at these wavelengths.
Veileder: Førsteamanuensis Claudia Cicone, Institutt for teoretisk astrofysikk, UiO
Intern sensor: Professor Øystein Elgarøy, Institutt for teoretisk astrofysikk, UiO
Ekstern sensor: Doctor (PhD) Paola Severgnini, INAF - Osservatorio astronomico di Brera