MAGPI Magic II: Unwinding spiral galaxies

By Qianhui Chen
[email protected]

Hosting the majority of star formation in the local Universe, spiral galaxies are unique and complex astronomical objects. The interplay between spiral features and star formation activities through the distribution of gas in the interstellar medium (ISM) is diverse but remains mysterious. We have limited knowledge of the formation of nearby spiral galaxies and even fewer resolved studies on spiral galaxies at higher redshifts.

The density wave theory predicts an offset between the young stellar population and the old stellar population, while the dynamic spiral theory predicts comparable ISM properties and stellar ages on both sides of the spiral arms. Taking advantage of the adaptive optics of MUSE, the Middle Ages Galaxy Properties with Integral Field Spectroscopy (MAGPI) survey enables resolved studies on galaxies at a lookback time of 3-4 Gyr, providing new pathways to discern the density wave theory and dynamic spiral theory at z~0.3.

MAGPI field 1202
Fig. 1: The field of view of MAGPI field 1202. The three-armed spiral galaxy in the centre is one of our pilot objects (hereafter SG1202).

Our pilot study on two three-armed spiral galaxies at z~0.3 was recently published in the Monthly Notices of the Royal Astronomical Society. This work examines the 2D distribution of star formation rate (SFR), gas-phase metallicity, and D4000, as a stellar age proxy. We separate the disc region of spiral galaxies into three sections (see Fig. 2 below): spiral arms (grey), leading edge (orange), and trailing edge (purple). One of our spiral galaxies (SG1202, shown in Fig. 1 above) shows a higher SFR in the leading edge than the trailing edge, consistent with the density wave theory rather than the dynamic spiral theory. We don’t observe a significant offset in the metallicity, while the Kolmogorov–Smirnov test rejects the hypothesis that the metallicity on each side of the spiral is drawn from the same distribution. Our work, along with kinematic models from other MAGPI team members, suggests that D4000 is not a suitable indicator to trace the spiral arms of SG1202. This is because the sensitive timescale of D4000 is 1-2 Gyr, which is longer than the estimated crossing time between the spiral arms.

Fig. 2: The Δφ map of SG1202, indicating the azimuthal distance to the nearest spiral arms. The spiral arm regions are marked as grey, while the spaxels on the leading edge are assigned with negative Δφ shown as orange. The purple spaxels with positive Δφ are on the trailing edge.


Michael Murphy is the Australian representative on the ESO Science Technical Committee. Contact: [email protected]

Sarah Sweet is the Australian representative on the ESO Users Committee. Contact: [email protected]

Stuart Ryder is a Program Manager with AAL. Contact: [email protected]

Guest posts are also welcome – please submit these to [email protected]