The Nuclear Star Cluster of M 74: a fossil record of the very early stages of a star-forming galaxy

TL;DR

Using PHANGS-MUSE integral field spectroscopy, we perform a spectro-photometric decomposition to isolate the NSC in M 74 from its central host and map their stellar populations. The NSC is ancient (∼11 Gyr), metal-poor ([M/H] ∼ −0.55) and only mildly α-enhanced ([Mg/Fe] ∼ 0.07), contrasting with a central region of the host that is younger (~4–5 Gyr) and more metal-rich. Spatially resolved maps reveal a metal-rich central ~500 pc surrounding the NSC, with star formation largely confined to the surrounding disk and spiral arms. The SFHs indicate the NSC formed early and remained quiescent since, while the host center experienced more recent star formation, suggesting decoupled evolution between the NSC and its host.

Abstract

Nuclear star clusters (NSC) are dense and compact stellar systems, of sizes of few parsecs, located at galactic centers. Their properties and formation mechanisms seem to be tightly linked to the evolution of the host galaxy, with potentially different formation channels for late- and early-type galaxies (respectively, LTGs and ETGs). While most observations target ETGs, here we focus on the NSC in M 74 (NGC 628), a relatively massive, gas-rich and star-forming spiral galaxy, part of the PHANGS survey. We analyzed the central arcmin of the PHANGS-MUSE mosaic, in which the NSC is not spatially resolved. We performed a two-dimensional spectro-photometric decomposition of the MUSE cube, employing a modified version of the C2D code, to disentangle the NSC from the host galaxy. Here we used three components: a bulge, a disk and a NSC approximated to the point spread function (PSF), obtaining three data cubes, one for each component. This allowed us to extract separately the age, metallicity and [Mg/Fe] abundance for the NSC and the host galaxy. Our results show a very old and metal-poor NSC, in contrast to the surrounding regions. While similar properties were found in NSCs hosted by galaxies of different masses and/or morphological types from M 74, they are somewhat unexpected for a relatively massive star-forming spiral galaxy. The spatially resolved stellar populations of the host galaxy display much younger (light-weighted) ages and higher metallicities, especially in the central region (${\sim}500$ pc) surrounding the NSC. This suggests that this NSC formed a long time ago, and evolved passively until today, without any further growth. Our results show that the NSC was not involved in the active recent star-formation history of its host galaxy.

Figures (8)

• Figure 1: White image obtained from the cropped MUSE data cube of the central arcmin, combining the light in the full MUSE wavelength range.
• Figure 2: From left to right: full kinematic-corrected data cube (given to C2D as an input), NSC, bulge and disk, the three of them resulting from the C2D spectrophotometric decomposition. Masked regions are plotted in black.
• Figure 3: Light ratio of each component over the total light of the galaxy, delivered by C2D. From left to right: NSC fitted as a PSF, bulge and disk. The NSC dominates the light in the central spaxels within one PSF FWHM. Masked regions are plotted in white.
• Figure 4: Spectra obtained by integrating, within an aperture of $1\sigma$ of the PSF, the prepared ( original) data cube before the spectrophotometric decomposition ( blue solid), the NSC data cube ( red dashed), and the host-galaxy data cube ( green dashed).
• Figure 5: Stellar population maps of the host-galaxy component of M 74, after subtracting the NSC. From top to bottom: light-weighted mean age, total metallicity [M/H] and [Mg/Fe] abundance. Masked regions are depicted in white and isophotal contours in black. The physical scale is given as a reference in the top $X$ axis.