Physical drivers of stellar-gas kinematic misalignments in the nearby Universe

Andrei Ristea, Amelia Fraser-McKelvie, Julia Bryant

This is part of Andrei Ristea's PhD thesis

How galaxies get their gas remains one of the most outstanding question in galaxy evolution. A decade ago, with the raise of the concept of cold accretion identified in numerical simulations, the observational community thought cold accretion via filaments was key at z~0 and the hope was to detect atomic hydrogen accretion via 21cm emission observations of nearby galaxies. Unfortunately, it gradually became clear that cold accretion via filaments, while an important mechanisms to get gas into galaxies, is mostly efficient at higher redshifts, and at z~0 is almost negligible (with the potential exception of isolated dwarf galaxies) as filaments no longer reach the galactic disk. Instead, accretion at z~0 most likely happens via galactic fountain and/or via accretion of satellites. This means that HI observations may not be the best (or the only) tool to identify gas accretion. With the advent of large IFS surveys, the identification of a significant population of galaxies showing misalignments between gas and stellar rotation axis is potentially providing a complementary way to investigate gas accretion. Studies based on both MaNGA and SAMI survey (Jin et al. 2016; Bryant et al. 2019) have started to investigate this issue, but so far they have been limited by relatively small number statistics. Moreover, to use misalignments to properly study accretion, we need to differentiate between misalignments due to accretion, due to bars, to environmental effects, etc. Once this is done, the first key question is to understand where misalignments are more likely found. Previous works identified that they are most likely present in ETG, but this is clearly a selection effect. The most fundamental question is in which environments accretion is most likely to happen, and whether this depends only on the galaxy being a central or a satellite, or also on the position of galaxies within the large scale structure.

This project will extend previous works by providing a case-by-case analysis of the physical processes causing stellar-gas kinematic misalignments in the nearby Universe.

The first step will be to extract reliable gas and stellar PAs for all galaxies in both samples using a consistent procedure. This is critical as – at least in case of SAMI – it is clear that automatic estimates of PAs are not always reliable and wrong estimates could significant effect any trends.

Once reliable position angles, misaligned galaxies will be identified and the plan is to a step further by discriminating - for example – objects in which both gas and stars are rotating, from objects where stars are not rotating. Following this, we will distinguish between cases where such features are caused by gas accretion and those where outflows are at work. Already this will significantly expand on previous works.

However, the primary goal of this project is to then investigate the connection between misalignments and morphology/gas content, individually. At the same time, we will determine whether the misaligned gas is feeding star formation or is dominated by other ionisation processes and what is its metallicity, as this will provide key insights into the accretion process responsible for this feature. Lastly, thanks to the availability of HI detections or upper limits for most of both SAMI and MANGA we will connect the evidence of accretion via IFS with information on the cold gas reservoir to determine whether galaxies with misalignments are special also from an HI point of view.

Potential conflicts: This project partially overlap with papers by Julia Bryant, but both parties are collaborating on both projects and have established a strategy so that both papers can go ahead. Also, collaboration with Stefania Barsanti to coordinate alignment analysis is underway.

Publication Date: 
June 2022