Daryl Joe D. Santos
The black-hole galaxy coevolution scenario remains as one of the greatest mysteries of the Universe. Studying active galactic nuclei (AGNs) which are manifestations of accretion of supermassive black holes (SMBHs) which reside in the center of galaxies can give us clues in further understanding this scenario. Galaxy environment is seen as the last key to understanding this scenario. It refers to the immediate vicinity of a galaxy, whether it belongs to a cluster/denser environment, or field/less dense environment. Galaxies living in different environments are shown to possess different characteristics. However, the current understanding of the relationship between AGN activity and galaxy environment remains unclear, most especially for galaxies detected in the infrared (IR) region. To address this issue, we aim to unveil this relationship with galaxies detected in the mid-infrared (MIR) by the AKARI IR satellite, which possesses a unique 9-band filter coverage compared to other IR telescopes (WISE and Spitzer), allowing us to have a better constraint of the galaxies’ photometry and properties. A spectral energy distribution (SED) fitting code named CIGALE was utilized to derive the galaxy properties of our sample. Galaxy environments were also derived based on their local galaxy density (which is related to the 10th nearest neighbor distance of the galaxy). Our current results show that more IR luminous galaxies tend to prefer less dense environments, supporting the merger system scenario for very luminous IR galaxies. The AGN contribution fraction (ratio between the AGN luminosity and total IR luminosity) and the AGN number fraction (ratio of the number of AGNs over the total number of galaxies) increase with density only for ultraluminous IR galaxies (ULIRGs). Looking at other galaxy environment definitions such as cluster-field classification and clustercentric distance will help us determine the cause of this trend in the near future.
The last key to uncovering the mystery of SMBHs is the role of the galaxy environment. Galaxy environment refers to the immediate vicinity of galaxies, whether they are in a galaxy cluster or denser environment, or in the field or less dense environment, and it is widely known that galaxy environment affects certain galaxy properties. However, there is no clear consensus yet about the environmental effects on AGNs.
The main aim of our research is to unveil this mystery by covering dust-obscured AGNs using the AKARI Infrared Telescope. These AGNs are missed by commonly used radio, optical, and X-ray surveys. Since dust emits radiation in the mid-infrared (MIR) region, it makes sense to use MIR surveys to observe these dust-obscured AGNs. Other MIR telescopes like WISE and Spitzer have gaps in their filter responses (transmission vs. wavelength), which may be disadvantageous if the spectral features of AGNs/galaxies lie on these gaps. But with AKARI’s 9-band continuous filter coverage, we can overcome this problem.
For a summary of our sample selection, we selected MIR galaxies in the AKARI North Ecliptic Pole (NEP) Wide Field which is observed by numerous telescopes in different wavelength ranges, providing multiwavelength observations for our sources. We constrained the galaxies’ properties via spectral energy distribution (SED; wavelength vs. energy) fitting using a code called CIGALE. For the galaxy environment, galaxy number density was calculated using optically selected galaxies by the HSC telescope in the AKARI NEP Wide Field. It is inversely proportional to the square of the target galaxy’s distance to the 10th nearest neighbor galaxy.
We classified our galaxies based on their total IR luminosity (LIR): infrared galaxies (IRGs), luminous IRGs (LIRGs), and ultraluminous IRGs (ULIRGs), with the former as the faintest group, and the latter as the brightest group. We also defined two probes of AGN activity: AGN number fraction which refers to the ratio of the number of AGNs and the total number of galaxies in a density bin, and AGN contribution fraction which refers to the ratio of the AGN luminosity and total IR luminosity of the source.
For our results, we found out that more luminous galaxies live in less dense environments as shown by our histogram plot of the galaxies’ densities. This supports previous works which suggest that very luminous IRGs (e.g. ULIRGs) are merger systems since less dense environments provide better frequency for dynamical merging (e.g. Goto, 2005). We also found out that only the ULIRG AGNs show a trend with density: ULIRG AGNs live and are more powerful in denser environments. AGN number and contribution fraction increases with density as shown in our plots.
As we covered previously-missed dust-obscured AGNs, we found out that only ULIRG AGNs are the only ones affected by environmental effects. The exact reason why is yet unclear. Looking into other environment definitions such as cluster-field classification (whether a galaxy belongs to a cluster or not) and clustercentric distance (distance of galaxy to the nearest galaxy cluster center) will definitely help us pinpoint this reason in the near future.