XMM-Newton RGS Spectroscopy of the M31 Bulge. I. Evidence for a Past AGN Half a Million Years Ago

Abstract

Existing analysis based on spectra from the Reflection Grating Spectrometer (RGS) onboard XMM-Newton already shows that the G-ratio of the O vii Heα triplet in the inner bulge of M31 is too high to be consistent with a pure optically thin thermal plasma in collisional ionization equilibrium (CIE). Different processes that may affect properties of diffuse hot plasma were proposed, such as resonance scattering (RS) and charge exchange (CX) with cold gas. To determine which physical process(es) may be responsible for this inconsistency, we present a systematic spectroscopic analysis based on 0.8 Ms XMM-Newton/RGS data, together with complementary Chandra/ACIS-S images. The combination of these data enables us to reveal multiple non-CIE spectroscopic diagnostics, including but not limited to the large G-ratios of Heα triplets (O vii, N vi, and Ne ix) and the high Lyman series line ratios (O viii Lyβ/Lyα and Lyγ/Lyα, and N vii Lyβ/Lyα), which are not expected for a CIE plasma, and the high iron line ratios (Fe xviii 14.2Å/Fe xvii 17Å and Fe xvii 15Å/17Å), which suggest much higher temperatures than other line ratios, as well as their spatial variations. Neither CX nor RS explains all these spectroscopic diagnostics satisfactorily. Alternatively, we find that an active galactic nucleus (AGN) relic scenario provides a plausible explanation for virtually all the signatures. We estimate that an AGN was present at the center of M31 about half a million years ago and that the initial ionization parameter ξ of the relic plasma is in the range of 3-4.

Inflow of low-metallicity cool gas in the halo of the Andromeda galaxy

“…Finally, in addition to the star formation rate, a central AGN could also have an impact on the surrounding CGM. Zhang et al (2019) found, using X-ray spectroscopy, a possible evidence in M31 for a past AGN event that happened half a million years ago. In this very short timescale, even if the clouds were ejected at 2000 km s −1 they would reach a distance of about 1 kpc from the centre, much smaller than the distances that we are probing in this study.…”

HST/COS data from Fox et al. 2020 ApJ Paper "Kinematics of the Magellanic Stream and Implications for its Ionization"

“…This question is given additional relevance by the compelling evidence that now exists for recent (Myr-timescale) activity at the GC, including the giant γ-ray emitting Fermi Bubbles (Su et al 2010;Dobler et al 2010;Ackermann et al 2014), their counterparts in X-rays (Bland-Hawthorn & Cohen 2003;Miller & Bregman 2016), microwaves (Finkbeiner 2004), and polarized radio emission (Carretti et al 2013), the presence of smaller-scale (≈400 pc) radio lobes within the Bubbles (Heywood et al 2019), and the tentative detection of radio jets (Su & Finkbeiner 2012), though see Ackermann et al (2014). Furthermore, AGN activity on ∼Myr timescales has recently been inferred from X-ray studies of M31 (Zhang et al 2019), suggesting that such processes are common in MW-mass spiral galaxies. Our UV kinematic analysis of the Magellanic Stream brings a new method for gauging the GC activity: assessing its impact on the extended gaseous environment.…”

Chemical enrichment in the cool core of the Centaurus cluster of galaxies

“…The RGS spectra provide us spatial information of extended emission over 5 arcmin width along the cross-dispersion direction when the X-ray peak of the target locates near the pointing position (e.g., Chen et al 2018;Zhang et al 2019). Thus, we used only two datasets in 2002 and 2007 which have quite smaller offsets than the others (see Table 1).…”