Giant molecular clouds and their type classification in M 74: Toward understanding star formation and cloud evolution

Abstract

We investigated the giant molecular clouds (GMCs) in M 74 (NGC 628), using data obtained from the PHANGS (Physics at High Angular resolution in Nearby GalaxieS) project. We applied GMC types according to the activity of star formation: Type I without star formation, Type II with H$\alpha$ luminosity ($L_{\mathrm{H\alpha }}$) less than $10^{37.5}\ \rm{erg\ s ^{-1}}$, and Type III with $L_{\mathrm{H\alpha }}$ greater than $10^{37.5}\ \rm{erg\ s^{-1}}$. A total of 432 GMCs were identified, with 59, 201, and 172 GMCs, for Types I, II, and III, respectively. The size and mass of the GMCs range from 23 to 238 pc and $10^{4.9}$ to $10^{7.1}\, M_{\odot }$, indicating that the mass and radius increase from Types I to III. Clusters younger than 4 Myr and H ii regions are concentrated within 150 pc of a GMC, indicating a tight association between these young objects and GMCs. The virial ratio decreases from Type I to Type III, indicating that Type III GMCs are the most gravitationally relaxed among the three. We interpret that the GMCs evolve from Type I to Type III, as previously observed in the Large Magellanic Cloud. Based on a steady-state assumption, the estimated evolutionary timescales of Types I, II, and III are 1, 5, and 4 Myr, respectively. We assume that the timescale of Type III is equal to the age of the associated clusters, indicating a GMC lifetime of 10 Myr or longer. Although Chevance et al. (2020, MNRAS, 493, 2872) investigated GMCs using the same PHANGS dataset of M 74, they did not define a GMC, reaching an evolutionary picture with a 20 Myr duration of the non-star-forming phase, which is five times longer than 4 Myr. We compare the present results with those of Chevance et al. (2020, MNRAS, 493, 2872) and argue that defining individual GMCs is essential for understanding GMC evolution.