Abstract
ABSTRACT We present the identification and analysis of an unbiased sample of active galactic nuclei (AGN) that lie within the local galaxy population. Using the MPA-JHU catalogue (based on SDSS DR8) and 3XMM DR7 we define a parent sample of 25 949 local galaxies (z ≤ 0.33). After confirming that there was strictly no AGN light contaminating stellar mass and star-formation rate calculations, we identified 917 galaxies with central, excess X-ray emission likely originating from an AGN. We analysed their optical emission lines using the BPT diagnostic and confirmed that such techniques are more effective at reliably identifying sources as AGN in higher mass galaxies: rising from 30 per cent agreement in the lowest mass bin to 93 per cent in the highest. We then calculated the growth rates of the black holes powering these AGN in terms of their specific accretion rates (∝LX/M*). Our sample exhibits a wide range of accretion rates, with the majority accreting at rates $\le 0.5\ \mathrm{ per \, cent}$ of their Eddington luminosity. Finally, we used our sample to calculate the incidence of AGN as a function of stellar mass and redshift. After correcting for the varying sensitivity of 3XMM, we split the galaxy sample by stellar mass and redshift and investigated the AGN fraction as a function of X-ray luminosity and specific black hole accretion rate. From this we found the fraction of galaxies hosting AGN above a fixed specific accretion rate limit of 10−3.5 is constant (at $\approx 1\ \mathrm{ per \, cent}$) over stellar masses of 8 < log M*/M⊙ < 12 and increases (from $\approx 1\ \mathrm{ per \, cent}$ to 10 per cent) with redshift.
Highlights
The degree to which a supermassive black hole (SMBH) and its host galaxy interact and affect each other’s evolution is an important and contested question
We found a weak increase in active galactic nuclei (AGN) fraction with stellar mass, it was consistent with being flat for the range probed
One black hole growing at a higher accretion rate in a lower mass galaxy could produce a similar X-ray luminosity to another black hole with a lower accretion rate in a high mass galaxy. To break this degeneracy and understand more about how the black hole activity is distributed across our observed AGN sample, we looked at the specific black hole accretion rate, λsBHAR
Summary
The degree to which a supermassive black hole (SMBH) and its host galaxy interact and affect each other’s evolution is an important and contested question. By combining the large galaxy sample available in the SDSS and the X-ray data from XMM-Newton Serendipitous Sky Survey, Birchall et al (2020) found power law distributions describing the AGN incidence in dwarf galaxies as a function of X-ray luminosity and sBHAR. From this we identified a redshift-independent fraction out to z ∼ 0.7. Any SDSS DR8 spectra classified by the pipeline as a galaxy was considered for further processing by the MPA-JHU value-added catalogue 1 It provides estimates of galaxy properties such as stellar mass, star formation rate (SFR) and emission line fluxes, for 1,472,583 objects in this release. We continue to use this catalogue, instead of 4XMM, because our analysis requires the use of comprehensive upper limits data accessible through Flix (Carrera et al 2007). 3XMM DR7 is the most recent version of the serendipitous sky survey available with this infrastructure
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