Chapter5.tex

% Chapter 4
\chapter[Summary and future prospects]{Summary and future prospects}

\label{Chap5}
\lhead{\emph{Chapter 7: Summary and future prospects}}


\section{Summary}

In this work, we conducted a detailed study of three nearby galaxies, each located within 16 Mpc, to evaluate the role of AGN in regulating star formation.
Our primary data source was ultraviolet observations from UVIT and GALEX.
This was supplemented by multiwavelength data, which allowed us to construct a comprehensive picture of star formation activities in these galaxies.
The high-resolution imaging capabilities of UVIT played a critical role in our analysis and findings.

Centaurus A is the nearest active galaxy known, and we studied the mechanical mode of AGN feedback in this galaxy using UVIT observations augmented by multiwavelength data.
An edge-on galaxy with a jet approximately perpendicular to the galaxy disk, Centaurus A has ongoing star formation in and outside the galaxy disk.
We present a census of the star formation properties in the Northern Star-forming Region of Centaurus A.
In this region, we identified 352 ultraviolet sources associated with Centaurus A using UVIT observations at an angular resolution of $<$1.5 arcseconds.
These observations were carried out in one far-ultraviolet ($\lambda_{\text{mean}}$ = 1481 \AA) and three near-ultraviolet ($\lambda_{\text{mean}}$ of 2196 \AA, 2447 \AA, and 2792 \AA, respectively) bands.
The star-forming sources identified in ultraviolet tend to lie in the direction of the jet of Cen A, thereby suggesting jet triggering of star formation.
Separating the Northern Star-forming Region into Outer and Inner regions, we found the stars in the Inner region to have a relatively younger age than the Outer region, thereby suggesting that the two regions may have different star formation histories.
We have also found the likely path of a past jet activity and detected new candidate jet-induced star-forming sources.

NGC 3982 is a nearby galaxy that hosts a
Seyfert-type AGN, and we studied the
radiative mode of AGN feedback in this galaxy.
Observational evidence for AGN impact on star formation can be searched for in galaxies by combining ultraviolet imaging and optical integral field unit data.
The ultraviolet flux directly traces recent star formation, and the integral field unit data can reveal dust attenuation, gas ionisation mechanisms, and gas kinematics from the central regions of the galaxy disk.
Our study on NGC 3982 shows star formation suppression in the central regions of the galaxy, likely due to negative AGN feedback, and enhanced star formation in the outer regions.
The case of NGC 3982 could be observational evidence of AGN feedback operating in a Seyfert galaxy.

We investigated the central star formation suppression in the NGC 628 galaxy to look for signs of AGN activity.
UVIT and JWST imaging data were used to probe the spatial distribution of recent star formation, while MUSE data was used to look for the signatures of AGN activity.
We found that nebulae inside the star formation cavity region are excited by a strong ionising source.
This strong ionising source is most likely a recent AGN activity.

Our study of three nearby galaxies with AGN feedback on star formation has resulted in new results and insights, which are further described below.


\subsection{Comparison of AGN mechanical and radiative mode effects on star formation}

While our sample size is too small to draw definitive conclusions, detailed observations of nearby galaxies suggest potential differences in how AGN mechanical and radiative modes influence star formation.
Notably, the AGN jet of Cen A can shape star formation well beyond the galaxy.
Additional examples of this phenomenon are provided in Table \ref{tab:jetinduced}.
The collimated nature of AGN jets allows them to traverse distances greater than the galaxy itself, reaching and interacting with gas clouds orbiting the AGN host galaxy to trigger star formation.
In contrast, radiative mode AGN feedback appears to be largely confined to distances within the galaxy, as observed in NGC 3982, NGC 628, and NGC 5728.
Although the AGN in Cen A has a bolometric luminosity an order of magnitude lower than that of NGC 3982 \citep{borkar2021multiphase}, the Cen A AGN has influenced star formation up to approximately 15 kpc, compared to around 1 kpc of the NGC 3982 AGN.
However, an important corollary to collimated jets shaping star formation at such large distances is that the gas clouds must fall within the jet's path for interactions to occur.

Upon interacting with the gas cloud, the AGN jet of Cen A has triggered star formation up to approximately 3 kpc away from its path (See Fig. \ref{fig:separation}).
This distance could be even greater when considering projection effects.
The relative efficiency in triggering star formation is likely due to the ability of low-power jets to effectively couple with gas clouds \citep{mukherjee2016relativistic}.


\subsection{The role of AGN radiative mode feedback in galaxy evolution}
Section \ref{sec:radmode_analysis} presents an analysis of AGN radiative mode feedback on star formation.
Fig. \ref{fig:fp_1} from this section serves as a valuable tool for understanding the impact of AGN on star formation via radiative mode feedback, and consequently, on galaxy evolution.
This figure can be used to infer a minimum radius from the AGN at which AGN radiation pressure may trigger star formation for various AGN luminosities under certain assumptions described in Section \ref{sec:radmode_analysis}.
Within this radius, star formation is suppressed due to the heating of molecular clouds by AGN radiation pressure, creating a star formation cavity.
Fig. \ref{fig:fp_1} is constructed using Equation \ref{E_ext_by_E_grav}, and this equation can be used to derive a relationship between star formation cavity radius ($R$) and AGN luminosity ($L_{AGN}$) assuming $E_{ext} / E_{grav}$ = 0.1 and $f_p$ = 1:
\begin{equation}
\label{L_AGN_to_R}
	R\ (\text{pc})= 2.6 \times 10^{-19} \sqrt{L_{AGN} \ (\text{erg s}^{-1})}.
\end{equation}

However, limited comparisons with observations of AGN feedback in NGC 3982 and NGC 5728 suggest that $L_{AGN}$ needs to be an order of magnitude higher, possibly due to the effects of ISM opacity, to create a star formation cavity of $R$ radius.
This requirement can be accommodated in Equation \ref{L_AGN_to_R} as:
\begin{equation}
		\label{L_AGN_to_R_correct}
		R\ (\text{pc})= 8.23 \times 10^{-20} \sqrt{L_{AGN} \ (\text{erg s}^{-1})}.
\end{equation}

The relationship between AGN luminosity and star formation cavity radius, as described by Equation \ref{L_AGN_to_R_correct}, can be improved when additional observations of AGN radiative mode feedback caused star formation cavities are available.
Table \ref{tab:LAGN_R} gives the $L_{AGN}$ and corresponding $R$ values derived using Equation \ref{L_AGN_to_R_correct}.
Such a relationship can be extremely useful in understanding galaxy evolution---we can predict the size of the star formation cavities caused by a particular AGN luminosity and vice versa.
We have used our present understanding of this relationship, as represented in Equation \ref{L_AGN_to_R_correct}, to interpret observations of galaxy properties.


\begin{table}[]
	\centering
	\caption{$L_{AGN}$ and corresponding $R$ values derived using Equation \ref{L_AGN_to_R_correct}.}
	\label{tab:LAGN_R}
\begin{tabular}{@{}cc@{}}
	\toprule
$L_{AGN}$ (erg s$^{-1}$)  & $R$ (pc) \\ \midrule
			10$^{41}$         & 26     \\
			10$^{42}$         & 82     \\
			10$^{43}$         & 260    \\
			10$^{44}$         & 823    \\
			10$^{45}$         & 2603   \\
			10$^{46}$         & 8231   \\
			10$^{47}$         & 26028  \\ \bottomrule
\end{tabular}
\end{table}

Dwarf galaxies typically exhibit AGN luminosities around $10^{41}$ erg s$^{-1}$ \citep{Singha2024llagn}.
According to Equation \ref{L_AGN_to_R_correct}, the corresponding star formation cavity radius for this luminosity is 26 pc.
%\revi{This radius decreases to 26 pc if the effective AGN luminosity reduces by a factor of 10 due to the ISM opacity.}
Given that this radius is small compared to the overall size of dwarf galaxies, AGN radiative mode feedback in dwarf galaxies is not expected to have a substantial impact on their host galaxy's star formation properties.
Observations also confirm that AGN radiative mode feedback does not significantly affect star formation in dwarf galaxies \citep{bichang2024properties}.

NGC 7172 is a Seyfert galaxy with a bolometric luminosity of $\sim$$10^{44}$ erg s$^{-1}$.
\cite{herrero2023agn} discovered a cold molecular gas ring with a radius of 720 pc.
Although the authors attributed the ring's origin to a bar, there is little evidence supporting the presence of one in this galaxy.
Given that AGN radiation pressure can compress gas, leading to molecular cloud formation and subsequent star formation, it is plausible that the AGN activity in NGC 7172 contributed to the creation of this molecular gas ring.
According to Equation \ref{L_AGN_to_R_correct},
a star-forming ring of 823 pc is predicted for $L_{AGN}$ = $10^{44}$ erg s$^{-1}$.
The 14\% discrepancy between the observed and predicted ring radii is likely due to uncertainties in our $L_{AGN}$-$R$ relationship.

In spiral galaxies such as NGC 628, NGC 3982 and NGC 5728, the AGN radiative mode feedback appears to have affected star formation at distances of $\sim$0.3-1 kpc, with estimated AGN luminosities of 10$^{42-44}$ erg s$^{-1}$.
If the luminosity increases to 10$^{46}$ erg s$^{-1}$ or higher, the AGN can suppress star formation at distances exceeding 10 kpc, roughly the size of a typical massive galaxy \citep{kormendy2011revised}.
At such high luminosities, AGN radiative mode feedback has the potential to quench star formation across the entire galaxy.
%This quenching is likely a rapid process due to the light-speed propagation of AGN radiation.
%The rapid cessation of star formation across the galaxy aligns with the proposed swift transition of galaxies from the blue cloud to the red sequence \citep{salim2014green}.


If the AGN luminosity is high but not sufficient to quench star formation throughout the entire galaxy, star formation can be triggered in a ring-like region in the galaxy's outer parts due to AGN pressure.
Interestingly, green valley galaxies, which are transitional between the blue cloud and the red sequence, often exhibit a significant number of rings \citep{kelvin2018galaxy}.
If green valley galaxies with rings are indeed caused by AGN activity, these galaxies may show signs of past AGN activity.
The ring-like star formation caused by AGN activity with the ring radius increasing as the AGN luminosity increases (see Table \ref{tab:LAGN_R}) may also contribute to the inside out growth observed in galaxies \citep{perez2013evolution}.

%\subsection{\revi{Efficacy of high spatial resolution observations in uncovering how AGN shapes star formation}}
%
%\revi{The large field of view and high spatial resolution of UVIT was particularly useful in studying the jet triggered star forming regions in Cen A.}

\subsection{AGN mechanical mode feedback and star formation}

Our study of AGN mechanical mode feedback in Cen A suggests that jet-triggered star formation might be ubiquitous.
In Cen A, the paths of current and past jet activity differ, leading to star-forming regions at distinct locations outside the galaxy.
This implies that AGN jets can potentially trigger star formation in various directions and at multiple distances outside galaxies.
Even if the jet activity is presently not observed or is oriented differently, the distribution of star-forming regions outside a galaxy---especially in areas with well-defined star formation cavities---could be linked to past jet activity.

\subsection{Final remarks}
Our findings contribute to the growing evidence that AGN can both suppress and trigger star formation.
The results presented in this work sheds light on how AGN shapes star formation in and outside their host galaxies.
Studying galaxies in close proximity has allowed us to gain a detailed understanding of AGN feedback mechanisms.
Our analysis reveals how AGN feedback plays a crucial role in galaxy evolution.
The cases of NGC 3982 and NGC 628 add to the limited observations of ring-like star formation induced by AGN feedback.

\section{Future prospects}
The work carried out in this thesis raises significant questions about the impact of AGN on star formation.
To further investigate these effects and improve our derived $L_{AGN}$-$R$ relationship, more spatially resolved studies are necessary.
Our future research will mainly focus on identifying additional face-on, non-barred galaxies with centrally suppressed star formation to determine the potential role of AGN.
Additionally, we will examine whether AGN activity may enhance star formation in certain galaxies.

An important avenue for future research involves investigating galaxies that host both bar structures and AGN activity.
Specifically, we will focus on galaxies that exhibit AGN activity, bar formations, and ring-like star formation patterns.
Since both AGN and bars can independently induce ring-like star formation, our objective will be to develop methods to determine whether the AGN or the bar is the primary driver behind these formations.
Such a study can enhance our understanding of the respective roles and interactions of AGN and bar dynamics in shaping galactic star formation.


We aim to investigate whether there are additional cases similar to Centaurus A, where the AGN jet has significantly influenced the environment beyond the galaxy itself.
Detailed studies of known instances like Minkowski's Object can provide new insights into the mechanisms and extent of such AGN-induced environmental shaping.

Future studies focusing on the central regions of galaxies, akin to our research on NGC 3982 and NGC 628, can greatly benefit from ultraviolet imaging instruments offering even higher spatial resolution than UVIT.
Upcoming UV missions with enhanced capabilities will be crucial in advancing this line of research, allowing for more precise observations and a deeper understanding of the intricate relationship between AGN activity and star formation suppression or enhancement.