One of the main question of astrophysics concerns star formation in galaxies, and how it governs galaxy evolution. Star formation is the results of a complex balance between gravity and opposite processes (heating, cooling, chocs, radiation field, turbulence?, magnetic field?) in molecular clouds. While the reservoir of molecular gas is a key parameter, the physical properties, large-scale gaz dynamics and micro-physics also play a role in star formation. All these processes have effect at various scales, from a fraction of parsec to several kiloparsecs. It is thus essential to adopt a multi-scale approach.
The core of my research is to understand the processes at play in star formation, and the effect of environment of the gas/star formation relation. In particular, I study the evolution of interstellar gas in its different phases, from the large-scale (several tens of parsecs, several kiloparsecs) diffuse medium to the sites of star formation. To do so, I adopt an observational approach, mainly based on radio astronomy. I mostly observe the CO emission (to trace the cold molecular gas) and the HI emission (to trace the atomic phases). The wealth of exiting (e.g. ALMA, NOEMA, IRAM 30m, APEX, GBT, VLA) or upcoming (e.g. SKA) observatories, as well as the near launch of JWST, bring more precised constrains of the different phases of the interstellar medium.
1. AGN feedback and radio jet-gas interaction
AGN feedback is commonly invoked to explain regulation of star formation in massive galaxies. Accretion of gas onto the central supermassive black hole produces energy (radiative or kinetic) which may quench star formation by heating or removing the gas of the host galaxy. However, it is sometimes claimed that AGN can locally enhance star formation by compressing the gas (positive feedback). In particular, the jets of plasma produced by radio galaxies may trigger star formation around few radio galaxies (jet-induced star formation).
First, I focused on 3C 285/09.6 and Minkowski’s Object, two star-forming regions that lie at several kpc from nearby radio galaxies, along the jet direction. This study was based on IRAM 30m data I obtained in March 2014. These two unresolved regions show a possible enhanced star formation along shocked regions inside radio jets, favouring a jet-induced star formation scenario (Salomé et al. 2015). Then, I focused on the northern filaments of Centaurus A that lie in projection along the radio jet. Being at a distance of 3.8 Mpc, this source is the perfect target to study the jet-gas interaction at intermediate scales (few hundreds parsecs). Archival CO data from SEST and new CO(2-1) observations from APEX reveal the existence of a large molecular gas reservoir. However, star formation is very inefficient in the filaments (Salomé et al. 2016a,b).
ΣSFR vs. ΣH2 for 3C285/09.6, Minkowski’s Object and the filaments of Centaurus A. The diagonal
dashed lines show lines of constant molecular gas depletion times. For the filaments of Centaurus A,
I plotted each position separately (red and blue corsses), and the average value (black cross).
The contours are from Leroy et al. (2013) for nearby spiral galaxies.
2. Centaurus A
Centaurus A (hosted by the galaxy NGC 5128) is the most nearby powerful AGN, widely studied at all wavelengths. The galaxy is surrounded by faint arc-like stellar and gaseous HI shells (Schiminovich et al. 1994), at a radius of several kpc around the galaxy. Bright star-forming filaments lie along the radio jet, including at the interaction with a HI shell (the so-called outer filament). I mapped the northern filaments with APEX and discovered a huge amount of molecular gas outside the HI shell, where star formation seems to be very inefficient, compared with spiral star-forming galaxies (Salomé et al. 2016b).
I also mapped the CO emission in the filaments with ALMA (project 2015.1.01019.S; Salomé et al. 2017). The spatial resolution (1.3″~20 pc) enables to distinguish molecular clouds, without resolving them. ALMA reveals the presence of multiple clouds, distributed in two main structures: (1) the « Horseshoe complex » outside the HI shell, and (2) the « Vertical filament » at the edge of the HI cloud.
Using a clustering method, we identified 140 GMCs, with similar properties (size, velocity dispersion, mass) that observed in the inner Milky Way. Gravity does not seem to be dominant, however the clouds located in the HI shell have virial parameters slightly lower than in the « Horseshoe complex ». Moreover, molecular gas at the edge of the HI shell is associated with recent star formation, while the « Horseshoe complex » shows Hα emission mostly excited by large scales shocks. This highlights the existence of two star formation regimes in the filaments of Centaurus A, likely due to different energy injection rate and cooling efficiency.
Left: Optical image of Centaurus A with the HI emission (orange color) from Schiminovich et al. (1994).
Middle: CO intensity from APEX (27.5″ resolution). The black contours are the HI emission.
Right: CO flux from ALMA (1.3″ resolution). The dashed contours show the region mapped with ALMA.
3. Phases transitions
Star formation consumes the molecular gas reservoir. At the current rate efficiency in most of spiral galaxies, star formation should already be stopped without gas inflow. For the Milky Way, it is well known that warm atomic gas is continuously falling onto the disc. In addition to the molecular gas, it is important to study the atomic gas which, via phase transitions, will cool and form molecular gas, the key ingredient for star formation.
I thus also study the warm-to-cold neutral medium and atomic-to-molecular phase transitions in molecular clouds at Galactic latitudes.
TBC