Chapter4.tex

\chapter{An investigation of star formation suppression in NGC 628}

\label{Chap4}
\lhead{\emph{Chapter 3: An investigation of star formation suppression in NGC 628}} % Write in your own chapter title to set the page header
\noindent

\section{Introduction}
NGC 628, also known as Messier 74 or Phantom Galaxy, is a nearby (9.84 Mpc) grand-design spiral galaxy seen almost face-on \citep{lang2020phangs, anand2021distances}.
This galaxy was recently brought to wider attention due to its coverage across multiple instruments of the James Webb Space Telescope (JWST; \citealt{lee2023phangs}).
A flurry of new studies has come out inspired by the JWST observations that unveiled intricate details of the galaxy (e.g., \citealt{thilker2023phangs, hoyer2023phangs, watkins2023phangs, mayya2023stellar}).
In addition to the JWST data, archival data on the galaxy exists from multiple instruments covering many wavelength bands.
Such multiwavelength data has been used to address varied problems related to the galaxy (e.g., \citealt{lomaeva2022recent, avdan2023investigation, ujjwal2022understanding, ujjwal2024disentangling}).


NGC 628 has a large central cavity of size $\sim$200 pc $\times$ $\sim$400 pc devoid of gas, dust, and recent star formation \citep{hoyer2023phangs}.
The origin of the central cavity remains an interesting puzzle to be solved.
While central suppression of star formation is commonly observed in barred galaxies, NGC 628 appears unbarred \citep{querejeta2021stellar}.
Another explanation for the central suppression of star formation is AGN feedback, as this has been observed in nearby galaxies with past or present AGN activity \citep{shin2019positive, ngc7252george2018uvit, joseph2022active}.
It is not conclusively known whether an AGN is present in NGC 628.
\cite{koliopanos2017searching} studied the central X-ray source in NGC 628 and reported a low luminosity of 2.49 $\times$ 10$^{38}$ erg s$^{-1}$ in the 0.5-10 keV band.
However, it should be noted that the AGN activity is episodic, and the AGN luminosity can vary rapidly during a typical star-forming episode \citep{hickox2014black}.
The closest example is the SMBH present in our Galaxy (Sagittarius A*) which had significantly higher luminosity in the recent past \citep{ponti2010discovery}.
There is also the case of NGC 7252, where an ionisation echo indicates past AGN activity \citep{schweizer2013iii}.
Therefore, if present, AGN in NGC 628 could have had multiple short and active phases in the recent past.
We might find signs of that activity imprinted in the galaxy through observations.

In this chapter, we investigate the central star suppression in NGC 628 and put forward AGN feedback as a solution to the central cavity origin puzzle with the help of multiwavelength observations. We present indications of AGN activity in NGC 628 using data from JWST \citep{gardner2006james}, Multi-Unit Spectroscopic Explorer (MUSE, \citealt{bacon2014muse}), and Ultra-Violet Imaging Telescope (UVIT, \citealt{tandon2017firstresults}).


\section{Data}

We downloaded publicly available UVIT Level2 data generated using the UVIT Level2 pipeline version 6.3 from the Indian Space Science Data Centre (ISSDC) \textit{AstroSat} archive \citep{ghosh2021performance,
	ghosh2022automated}.
Two observation IDs, G06\_151T01\_9000000836 and A04\_209T01\_9000002378, were available with NGC 628 observations.
We combined all available far ultraviolet (FUV) F154W observations (total exposure time = 4942 seconds).
\revii{The FUV data were combined} using Curvit version 1.7.0 by taking the episode-wise F154W events lists as input, and an image was generated from the combined event list \citep{joseph2021curvit}.
The astrometry was done using the Astrometry.net software \citep{lang2010astrometry}.
The UVIT FUV channel has a point spread function (PSF) of $\sim$1.2 arcseconds full width at half maximum (FWHM).
The image has been smoothed using a Gaussian kernel with a full width at half maximum (FWHM) of 1.2 arcseconds to suppress the noise.

The JWST Mid-Infrared Instrument (MIRI) F770W data of NGC 628
% shown in the right panel of Fig. \ref{fig:uvit_jwst_centre} 
is from Physics at High Angular resolution in Nearby GalaxieS (PHANGS)–JWST Treasury survey \citep{lee2023phangs}.
The reduced JWST data with astrometric calibration is made publicly accessible by the PHANGS team\footnote{\url{https://sites.google.com/view/phangs/home/data?authuser=0}}.
The F770W data has an angular resolution of 0.25 arcseconds FWHM.

We have taken the nebular gas excitation mechanism classification using the Baldwin-Phillips-Terlevich
(BPT) diagram from the PHANGS–MUSE nebular catalogue \citep{baldwin1981classification, kewley2006host, kauffmann2003host, groves2023phangs}.
The catalogue provides BPT classification for 2855 nebulae in the NGC 628 galaxy. The $[\text{O\,\textsc{iii}}]\lambda5007$ emission line flux and $[\text{N\,\textsc{ii}}]\lambda6584$ velocity dispersion maps were obtained from the PHANGS-MUSE Public Release (PMPR; \citealt{emsellem2022phangs}).
We have used PMPR data with homogenised PSF of 0.92 arcseconds FWHM.


\section{Results}

The UVIT imaging in the F154W filter shows a cavity in the NGC 628 central 30$\times$30 arcseconds region FUV profile (see Fig. \ref{fig:uvit_jwst_centre}, left panel).
Since the FUV band probes recent star formation in the last 100 Myr \citep{kennicutt2012star}, the observed cavity suggests that star formation is suppressed in that region.
However, relatively large dust attenuation in the central region can also lead to the observed FUV profile.

The JWST F770W imaging traces the 7.7 $\mu$m mid-infrared (MIR) emission from polycyclic aromatic hydrocarbons (PAHs).
The PAHs produce MIR emission due to excitation by FUV radiation from young massive stars, and the F770W imaging can be used as an extinction-free probe of recent star formation in galaxies \citep{peeters2004polycyclic, xie2019new}.
The JWST F770W image of the NGC 628 central 30$\times$30 arcseconds region is shown in Fig. \ref{fig:uvit_jwst_centre}, right panel.
F154W contours are overlaid on the F770W image, and it can be observed that there is good correspondence between the F154W and F770W intensity maps of the NGC 628 central region.
The F770W imaging confirms that recent star formation is absent in the central region.
While a point-like emission is present in the very centre of NGC 628, it is a nuclear star cluster (NSC) that has not been found to have any recent star formation \citep{hoyer2023phangs}.

\cite{groves2023phangs} identified 2855 nebulae within the NGC 628 galaxy using PHANGS-MUSE survey data and classified them based on gas excitation mechanisms.
The description of nebulae identification, emission line measurements, and classification can be found in \cite{groves2023phangs}.
They estimated the H${\alpha}$,
H${\beta}$,
$[\text{O\,\textsc{iii}}]\lambda5007$,
$[\text{O\,\textsc{i}}]\lambda6300$,
$[\text{N\,\textsc{ii}}]\lambda6584$, and
$[\text{S\,\textsc{ii}}]\lambda\lambda6717, 31$ nebulae emission line fluxes and classified them using
$[\text{O\,\textsc{iii}}]/\text{H}{\beta}$ versus
$[\text{N\,\textsc{ii}}]/\text{H}{\alpha}$ (BPT\_NII),
$[\text{O\,\textsc{iii}}]/\text{H}{\beta}$ versus
$[\text{S\,\textsc{ii}}]/\text{H}{\alpha}$ (BPT\_SII), and
$[\text{O\,\textsc{iii}}]/\text{H}{\beta}$ versus
$[\text{O\,\textsc{i}}]/\text{H}{\alpha}$ (BPT\_OI) line diagnostic diagrams.
The nebulae are classified as star-forming (SF), AGN, composite (SF + AGN), and low-ionisation narrow emission-line region (LINER).
If the line is not detected with a signal-to-noise ratio (S/N) > 5, it is marked as low S/N.

The BPT\_NII, BPT\_SII, and BPT\_OI classifications of nebulae present in the NGC 628 central 15$\times$15 arcseconds region are shown in Fig. \ref{fig:BPT}, with the left panel showing the BPT\_NII classification, BPT\_SII in the middle, and BPT\_OI on the right.
We note seven nebulae with BPT\_NII AGN classification in the cavity region.
However, the nebula associated with the NSC has an SF classification.
The BPT\_NII, BPT\_SII, and BPT\_OI classifications for these eight nebulae in the cavity region are given in Table \ref{tab:nebulae}.

The $[\text{O\,\textsc{iii}}]$ emission line has a relatively high ionisation potential, which is excited by strong ionising sources such as AGN than star formation.
Fig. \ref{fig:OIIIandNII}, left panel, shows the $[\text{O\,\textsc{iii}}]$ flux map with F770W contours overlaid to show the cavity region.
Relatively large levels of $[\text{O\,\textsc{iii}}]$ flux can be observed inside the cavity region, even though no recent star formation exists there.
Also, the $[\text{N\,\textsc{ii}}]$ traced ionised gas velocity dispersion ($\sigma_{\text{gas}}$) is given in Fig. \ref{fig:OIIIandNII}.
Relatively high levels of $\sigma_{\text{gas}}$ can be observed inside the cavity region.
We do not show the $[\text{O\,\textsc{iii}}]$ traced $\sigma_{\text{gas}}$ map due to comparatively poor S/N.

\begin{table}[]
	\caption{The BPT\_NII, BPT\_SII, and BPT\_OI classifications for the eight cavity region nebulae. The region\_ID is the same as provided in the \cite{groves2023phangs} catalogue for NGC 628.}
	\label{tab:nebulae}
	\begin{tabular}{@{}cccccc@{}}
		\toprule
		region\_ID & RA\_J2000  & DEC\_J2000 & BPT\_NII & BPT\_SII & BPT\_OI \\ \midrule
		598        & 24.1739581 & 15.7836692 & SF       & SF       & AGN     \\
		1217       & 24.1731991 & 15.7838129 & AGN      & Low S/N  & Low S/N \\
		1445       & 24.1736601 & 15.7841917 & AGN      & Low S/N  & AGN     \\
		1609       & 24.1751517 & 15.7834045 & AGN      & LINER    & AGN     \\
		2424       & 24.1740632 & 15.7829772 & AGN      & Low S/N  & Low S/N \\
		2474       & 24.1744675 & 15.7829903 & AGN      & Low S/N  & AGN     \\
		2550       & 24.1744765 & 15.7840125 & AGN      & AGN      & AGN     \\
		2855       & 24.1725615 & 15.7839474 & AGN      & LINER    & AGN     \\ \bottomrule
	\end{tabular}
\end{table}

\begin{landscape}

	\begin{figure}
		\centering
		\includegraphics[width=0.495\columnwidth]{NGC_628/NGC628_F154W.pdf}
		\includegraphics[width=0.495\columnwidth]{NGC_628/NGC628_F770W.pdf}
		\caption{The UVIT F154W intensity map is on the left, and the JWST F770W intensity map is on the right. Both images depict the same central 30$\times$30 arcseconds region of NGC 628. The F154W image has been smoothed with a Gaussian kernel of 1.2 arcseconds. F154W contours with contour levels 0.0004 and 0.0006 CPS are overlaid on the F770W image in cyan lines. A square of 15 arcseconds in length is shown in the F770W image in white dashed lines.
			The same 15$\times$15 arcseconds square region is shown in Fig. \ref{fig:BPT} and Fig. \ref{fig:OIIIandNII}. North is up, and east is towards the left.}
		\label{fig:uvit_jwst_centre}
	\end{figure}

	\begin{figure}
		\centering
		\includegraphics[width=0.325\columnwidth]{NGC_628/BPTNII_classification_NGC628.pdf}
		\includegraphics[width=0.325\columnwidth]{NGC_628/BPTSII_classification_NGC628.pdf}
		\includegraphics[width=0.325\columnwidth]{NGC_628/BPTOI_classification_NGC628.pdf}
		\caption{The gas excitation mechanism classification for the nebulae present in the central 15$\times$15 arcseconds region of NGC 628 is shown in all three panels. The classification is as per Table 5 of \cite{groves2023phangs}.
			The left panel shows the BPT\_NII nebulae classification, the middle panel shows the BPT\_SII classification and the BPT\_OI classification is shown on the right panel. The same JWST F770W image is shown as the background image in all three panels.}
		\label{fig:BPT}
	\end{figure}

	\begin{figure}
		\centering
		\includegraphics[width=0.495\columnwidth]{NGC_628/OIII5006_FLUX.pdf}
		\includegraphics[width=0.495\columnwidth]{NGC_628/NII6583_SIGMA.pdf}
		\caption{Emission line $[\text{O\,\textsc{iii}}]$ flux (left panel) and $[\text{N\,\textsc{ii}}]$ traced $\sigma_{\text{gas}}$ (right panel) in the central 15$\times$15 arcseconds region of NGC 628. The F770W contours with contour levels 4 MJy sr$^{-1}$ are overlaid in both panels in black dashed lines.}
		\label{fig:OIIIandNII}
	\end{figure}

\end{landscape}

\section{Analytical considerations}
\label{sec:radmode_analysis}

\revii{To further understand how a past AGN activity may have shaped the central region of NGC 628, we used analytical methods to probe the effect of AGN on gas clouds.
To do this, we compared the energy imparted on a spherical gas cloud by AGN against the cloud’s gravitational potential energy.}

The AGN pressure, $p_{AGN}$, can be estimated using Equation (11) from \cite{silk2013unleashing}:

\begin{equation}
\label{P_agn}
	p_{AGN}=f_p\frac{L_{AGN}}{4\pi R^2 c},
\end{equation}
where $L_{AGN}$ is the AGN luminosity, $R$ is the radius, $c$ is the light speed, and $f_p$ is the mechanical advantage factor.
$f_p$ is described in \cite{wagner2013ultrafast}, and it introduces an additional boost.
$f_p$ can be $>$100 close to the AGN and falls as $R$ increases.
%The AGN pressure on gas clouds can drive turbulence in them and increase $\rho$. 
%However, the work done on the gas cloud by $p_{AGN}$ can also increase $T$.

Assuming a spherical gas cloud with mass $m_{cloud}$ and radius $r_{cloud}$ at a distance $R$ from the AGN,
% of 50 pc size and 100 M$_\odot$ pc$^{-2}$ surface density, 
we can make a rough estimate of the energy gained by the gas cloud when $p_{AGN}$ acts on it, $E_{ext}$, using the equation:
\begin{equation}
	E_{ext} = p_{AGN} \times V_{cloud},
\end{equation}
where $V_{cloud} = 4 \pi r_{cloud}^3 / 3$.
The gravitational potential energy of the gas cloud, $E_{grav}$, can be estimated using:
\begin{equation}
	E_{grav} = \frac{3 G m_{cloud}^2}{5 r_{cloud}}.
\end{equation}
We take the surface density of the cloud as $\Sigma_{cloud}$ and keep $m_{cloud} = \pi r_{cloud}^2 \Sigma_{cloud}$. Then, we can obtain the ratio,
\begin{equation}
\label{E_ext_by_E_grav}
	E_{ext} / E_{grav} = \frac{20 p_{AGN}}{9 \pi G \Sigma_{cloud}^2}.
\end{equation}


The $E_{ext} / E_{grav}$ ratio\footnote{\revii{Please note that $E_{ext} / E_{grav}$ represents the ratio of $p_{AGN}$ to the cloud pressure ($P_{cloud} = \phi_P G \Sigma_{cloud}^2$), where $\phi_P$ is a numerical factor with a value of $\sim$1 for gravitationally bound objects \citep{mckee2003formation, mckee2007theory}, and is derived here as $9 \pi / 20$.}} can be used as a measure of how much the $p_{AGN}$ can affect the gas cloud.
If it is 1 (100\%), the cloud will most likely heat up and not collapse to form stars.
If it is less than 0.01 (1\%), then the $p_{AGN}$ may not have any discernible effect on the cloud.
We can roughly consider that star formation may occur due to $p_{AGN}$ acting on clouds in the 1-10\% zone.
% \textbf{Please note that this is a heuristic argument and a more rigourous treatment will require detailed consideration of shocks acting on gas clouds.}
We fixed the $\Sigma_{cloud}$ to be 100 M$_\odot$ pc$^{-2}$ considering the surface density values of giant molecular clouds (GMC) in the local quiescent galaxies \citep{dessauges2019molecular}.
While $f_p$ could be $\sim$100 for small distances ($<$100 pc) and powerful outflows, it will be $\sim$1 at larger distances.
% We take $f_p$ = 1. 
The $E_{ext} / E_{grav}$ ratio was estimated for a range of $L_{AGN}$ values with $f_p$ = 1, and the results are shown in Fig. \ref{fig:fp_1}.
% , Fig. \ref{fig:fp_10} for $f_p$ = 10, and Fig. \ref{fig:fp_100} for $f_p$ = 100. 

\begin{figure}
	\centering
	\includegraphics[width=\columnwidth]{NGC_628/for_article_conducive_zones_fp_1.png}
	\caption{The $E_{ext} / E_{grav}$ in percentage is plotted against $R$ (oblique coloured lines) for a range of $L_{AGN}$ with $f_p$ = 1.
		The horizontal grey region lies between 1\% and 10\%. The dashed vertical line denotes the 1000 pc radius, and the dotted vertical line denotes the 300 pc radius.}
	\label{fig:fp_1}
\end{figure}

The NGC 3982 galaxy has a central cavity with AGN suppressed star formation, and a star-forming ring appears at $\sim$1 kpc \citep{joseph2022active}.
The NGC 3982 AGN has an X-ray luminosity, L$_{\text{(2-10 keV)}}$, of 10$^{42.83}$ erg s$^{-1}$ \citep{saade2022nustar}.
We estimated the AGN bolometric luminosity, L$_{\text{bol}}$,
to be 10$^{44.03}$ erg s$^{-1}$ following \cite{duras2020universal}.
%\begin{equation}
%	\text{L}_{\text{bol}} = \text{K}_{\text{X}} \times \text{L}_{\text{(2-10 KeV)}}
%\end{equation}
%where $\text{K}_{\text{X}}$ is estimated using Equation (3) 
%and Table 1 from \cite{duras2020universal}:
%\begin{equation}
%	\text{K}_{\text{X}} =  15.33 \left[ 1 + 
%	\left( \frac{\text{log}_{10}(\text{L}_{\text{(2-10 KeV)}} /
%		\text{L}_{\odot})}
%	{11.48} 
%	\right)^{16.2} 
%	\right]
%\end{equation}
%$\text{K}_{\text{X}}$ is found to be 15.79, and $\text{L}_{\text{bol}}$ is obtained as 10$^{44.03}$ erg s$^{-1}$. 
Note that the $L_{AGN} = 10^{44}$ erg s$^{-1}$ line is still at $\sim$100\% level ($E_{ext} / E_{grav}$ $\approx$ 1)  around 1 kpc in Fig. \ref{fig:fp_1}.
At 1 kpc, it is the $L_{AGN} = 10^{43}$ erg s$^{-1}$ line that is in the 1-10\% zone.
One possible explanation for the factor of 10 difference is the interstellar medium (ISM) opacity.
The AGN luminosity may have been reduced by a factor of 10 due to the ISM opacity before it triggered star formation.
% For large distances like 1 kpc, $f_p$ can be considered to be close to $\sim$1. 

NGC 5728 also has a star-forming ring at $\sim$1 kpc with an $\text{L}_{\text{bol}}$ of $1.46\times10^{44}$ erg s$^{-1}$ \citep{shin2019positive}.
Similar to NGC 3982, a factor of 10 reduction in $L_{AGN}$ is possible due to the ISM opacity before it triggered star formation.

The NGC 628 galaxy has a central star formation cavity with a star formation ring appearing at $\sim$300 pc.
The observed X-ray luminosity, $L_{(0.5-10keV)}$, of the NGC 628 central source is $\sim$10$^{38}$ erg s$^{-1}$, which corresponds to a bolometric luminosity of $\sim$$10^{39}$ erg s$^{-1}$  \citep{koliopanos2017searching}.
	Assuming a $f_p$ = 1, the $L_{AGN}$ has to be around $10^{42}$ erg s$^{-1}$ to generate the observed profile in NGC 628 (see Fig. \ref{fig:fp_1}).
	Even if we take $f_p$ = 10, an $L_{AGN}$ of $10^{41}$ erg s$^{-1}$ is required.
	Provided the star formation cavity in NGC 628 is indeed caused by AGN activity, $L_{AGN}$ has now faded by a factor of 10$^2$-10$^3$.
	If we also consider the effect of opacity, there could be a factor of 10$^2$-10$^4$ reduction in the AGN luminosity.


	\section{Discussion}
	The UVIT F154W and JWST F770W imaging show no recent star formation in the central region of NGC 628 (see Fig. \ref{fig:uvit_jwst_centre}).
	The origin of NGC 628 central star formation cavity is an interesting puzzle to solve.
	While star-forming rings with centrally suppressed star formation are commonly observed in barred galaxies \citep{erwin2024frequency}, NGC 628 is not found to have a bar \citep{querejeta2021stellar}.
	Additionally, the NGC 628 cavity appears box-shaped even though we view the galaxy with a very low inclination angle \citep{lang2020phangs}---the star-forming rings in barred galaxies are nearly circular \citep{comeron2014arrakis}.
	AGN activity is known to suppress star formation in the central regions of galaxies and create a star formation cavity \citep{ngc7252george2018uvit, joseph2022active, pak2023origin}.
	We consider the possibility of a recent AGN activity causing the observed star formation cavity.

	The BPT classifications of the eight nebulae present inside the cavity region are all AGN/LINER except for the nebula with region\_ID 598 (see Table \ref{tab:nebulae} and Fig. \ref{fig:BPT}).
	Even if we keep aside the \revii{region\_ID} 598 nebula and consider that some nebula are not BPT\_SII/BPT\_OI classified due to low S/N, the predominant AGN/LINER nebulae classifications across multiple line diagnostic diagrams are a strong indication that there has been a strong ionising source present in the cavity region.
	The \revii{region\_ID} 598 nebula is interesting because it coincides with the NSC---its BPT\_NII and BPT\_SII classifications are SF, while the BPT\_OI classification is AGN.
	Due to the \revii{region\_ID} 598 nebula having SF classification in two different line diagnostic diagrams, we looked at the available UVIT FUV data to check for recent star formation in the NSC.
	We do not detect any point-like FUV emission in the F154W filter image coinciding with the \revii{region\_ID} 598 nebula.
	% We also tried co-adding all available FUV data across multiple filters.  
	\cite{hoyer2023phangs} do not detect any young stellar population of their NSC analysis using spectral energy distribution (SED) modelling.
	However, it may be possible that the NSC is hosting few young stars like the Milky Way NSC (MWNSC, \citealt{neumayer2020nuclear}), and the UV emission is not detected due to large levels of dust attenuation in the NSC region similar to that found in MWNSC \citep{chatzopoulos2015dust}.

	The relatively large levels of $[\text{O\,\textsc{iii}}]$ emission found $\sim$100 pc northeast of the NSC (see Fig. \ref{fig:OIIIandNII}) necessitates the presence of a strong ionising source.
	However, the $[\text{O\,\textsc{iii}}]$ emission is found inside the cavity region where no recent star formation is present.
	The $\sigma_{\text{gas}}$, as traced by the $[\text{N\,\textsc{ii}}]$ emission, shows relatively large levels covering almost all the cavity region.
	Again, there is no evidence of recent star formation to drive  $\sigma_{\text{gas}}$ inside the cavity region.
	AGN activity is known to produce high $\sigma_{\text{gas}}$ in galaxy central regions \citep{ruschel2021agnifs}.

	% Due to the low resolution of the NVSS survey, the observed radio emission cannot be associated conclusively with a relativistic jet from NGC 628 \citep{koliopanos2017searching}. 
	The evidence presented here suggests that there has been a strong ionising source, most likely an AGN, in the central region of NGC 628.
	While there is no current indication of AGN activity---\citealt{koliopanos2017searching} reports low X-ray and radio fluxes for the NGC 628 nucleus---AGN activity is known to be episodic and random in secularly evolving pseudo-bulge disk galaxies like NGC 628 \citep{kormendy2013coevolution}.
	As per our analysis, the AGN luminosity that caused the presently observed star formation cavity should have faded by a factor of 10$^2$-10$^4$.

	\section{Summary}
	We investigated the central star formation cavity found in the NGC 628 galaxy using JWST, MUSE, and UVIT data.
	There is no recent star formation in the central  $\sim$200 pc $\times$ $\sim$400 pc region as confirmed by UVIT F154W and JWST F770W imaging.
	NGC 628 is not known to have a bar, which can generate such central suppression of star formation.
	Therefore, we have looked at the alternate possibility of AGN feedback.
	Interestingly, BPT diagram AGN classification of cavity region nebulae points toward a recent AGN activity.
	The presence of a recent AGN activity is further supported by $[\text{O\,\textsc{iii}}]$ and $\sigma_{\text{gas}}$ maps of the cavity region.
	Our analysis suggests that the AGN luminosity that caused the presently observed star formation cavity should have faded by a factor of 10$^2$-10$^4$.