MAGPI Magic IV: Forming stars in galaxies from the inside out

By Marcie Mun
[email protected]

As some of the most complex structures in the universe, galaxies can be subject to multiple physical processes that make their evolutionary pathways unique. These processes include feedback from active galactic nuclei (AGN) and supernovae, on top of interactions with the environment leading to continuous gas accretion and/or stripping. All such mechanisms have a profound impact on shaping the star formation (SF) history of galaxies. One way to infer the possible combinations of such events throughout a galaxy’s evolutionary history is to measure the SF activity on global and spatially resolved scales. Doing so usually requires a powerful instrument that can resolve the light distribution of galaxies, such that we can map the star formation rates from the centre to the outskirts of galaxies. Otherwise, such analyses have typically been limited to nearby galaxies, where previous studies have used both the global and resolved star-forming main sequence (SFMS) as a reference point to quantify the level of SF. 

Using the adaptive optics (AO) capabilities of the Multi-Unit Spectroscopic Explorer (MUSE) instrument on the Very Large Telescope (VLT), the Middle Ages Galaxy Properties with Integral Field Spectroscopy (MAGPI) survey allows us to explore the “middle ages” (i.e., ~4 billion years ago or z ~ 0.3) of the universe at an unprecedented spatial resolution at this epoch. Using a combination of two star formation rate (SFR) tracers – the Hα emission line and the 4000 Å break (D4000), we measure population-averaged radial profiles of the local SF activity as a function of the offset from the global SFMS. We show the distribution of 302 galaxies in SFR-stellar mass (M_*) space in Figure 1 below, where galaxies with Hα- and D4000-measured SFRs are colour-coded in blue and red, respectively. We then categorize our sample into 4 global star-forming states as a function of the offset from the SFMS, otherwise labelled as ΔSFR, as shown by the histogram in Figure 2 below. Positive offsets denote star-forming galaxies above the SFMS, whereas negative offsets denote galaxies below the SFMS, undergoing SF quenching.

The population-averaged profiles per global star-forming state are shown in Figure 3 below, along with those of a sample of local (i.e., z ~ 0) galaxies from the MaNGA survey, categorized into the same states. We use a parameter analogous to ΔSFR to measure the local variation in SF – ΔΣ_SFR, the offset with respect to the resolved SFMS. Key highlights of our findings, which are currently under review for publication in MNRAS, include:

• a striking difference in the galaxies above the SFMS (pink profiles) at the two redshift regimes. This suggests an evolution in radial trends from uniformly enhanced SF at z ~ 0.3, to centrally concentrated SF at z ~ 0. Centrally concentrated SF typically points toward processes such as galaxy-galaxy interactions that preferentially funnel star-forming fuel toward the centre. The fact that we observe a flat profile for MAGPI suggests that these galaxies at z ~ 0.3 are undergoing a mixture of physical processes at play along with a sufficient gas reservoir fueling SF extending towards the galaxy outskirts.
• higher ΔΣ_SFR values observed in the two populations of MAGPI galaxies below the SFMS (green and red profiles), which is likely a combination of physical and empirical differences. The local universe hosts a larger population of quenched galaxies, shifting the population-averaged profile to lower levels of SF. On the other hand, the different treatments of the limitations of D4000 probing the lowest levels of SF may also play a role in the different ranges of ΔΣ_SFR probed by MaNGA and MAGPI. Overall, the galaxies below the SFMS show positive gradients in their SF, suggestive of inside-out quenching, showing a preference for internal mechanisms (e.g., AGN feedback) dominating over external mechanisms (e.g., gas stripping due to the environment).