Yirui Zheng
MaNGA, an integral field unit survey, has produced datacubes for more than 100 galaxies which contain post-starburst (PSB) regions, i.e., regions in which star formation has recently been sharply truncated. Spectral index and emission line maps give us information about the star formation history and, therefore, formation mechanisms of these galaxies.
A small fraction of these galaxies show very strong radial gradients in their Balmer absorption line strengths. To investigate the origin of these strong radial gradients, we use Gadget-3 to run a set of binary merger simulations with varied black hole feedback models, progenitor galaxies, and orbits. We develop the SEDMorph code to make mock MaNGA datacubes for the simulated galaxy. Spectral index maps for the simulated post-starburst galaxies are created by combining the star formation history and metallicity of each particle with stellar population synthesis models. A point-spread function and a dithering pattern are also included to make our mock cube more comparable to the MaNGA data.
With the simulations and mock datacubes, we find that only the kinetic blackhole feedback model can create galaxies with strong PSB features. Under this model, we confirm the radial gradients in the Balmer absorption line strengths in these PSBs are caused by a single co-eval burst which was stronger in the central regions, rather than a starburst that has progressed from outside-in.
Our work highlights the potential for IFU observations to provide constraints on mechanisms for rapid quenching of local galaxies , when combined with direct comparison with simulations.
1. Introduction
Post-starburst galaxies (PSBs) are galaxies in which star formation has recently been sharply truncated. MaNGA, an integral field unit survey, has produced datacubes for more than 100 galaxies which contain post-starburst (PSB) regions. Stellar continuum spectral index and nebular emission line maps give us information about the star formation history and, therefore, formation mechanisms of these galaxies.
A small fraction of these galaxies show very strong radial gradients in their Balmer absorption line strengths, ranging from positive, flat to negative as displayed in the figure on the right. Simple toy models indicate that these observations are consistent with two simple scenarios: a single co-eval burst which was stronger in the central regions, or a starburst that has progressed from outside-in. However, the toy models are unable to distinguish between the two options. We need the help of simulations.
2. Simulation
To investigate the origin of these strong radial gradients, we use Gadget-3, to run a set of binary merger simulations with varied progenitor galaxies, mass ratio and orbits. We use progenitor galaxies with the same mass but different Hubble type: Sa, Sc and Sd. To investigate unequal mass mergers, we also set up a secondary progenitor of Sc type with 1/3 of the mass of the primary galaxies model. These galaxies are set up on different orbits list below:
- G00: symmetric prograde-prograde orbit;
- G07: retrograde-prograde orbit;
- G13: retrograde-retrograde orbit
Here we displayed the star formation histories (SFHs) of the merger simulations. The SFHs demonstrate that sharp quenching is only achieved in particular circumstances: progenitor galaxies with similar mass, approaching each other in either prograde-prograde or retrograde-prograde orbits. Neither unequal mass merger nor retrograde-retrograde merger can reproduce fast quenching. We also find that strong kinetic AGN feedback is required.
3. Galaxy properties
Post-starburst galaxies can be identified with spectral continuum indices which describe the 4000Å break and Balmer absorption line strengths. Here we display the 2xSc_07 simulation. In this simulation, after the starburst is shut down at 12.5 Gyr, these spectral indices evolve as the galaxy becomes dominated by A-type stars due to the ageing of the post-starburst stellar population.
We create mock datacubes for the simulated post-starburst galaxies following the MaNGA observing strategy and data reduction pipeline. We find that the radial gradient starting out positive, flattening and then becoming increasingly negative with time. The range of gradients observed in MaNGA PSB galaxies can be reproduced in the single merger simulations, which indicates that the range of gradients observed in MaNGA PSB galaxies is simply due to different times of observation rather than different underlying processes. The strong time evolution in the radial gradients of the spectral indices that can be used to estimate the age of the starburst without reliance on detailed star formation histories from spectral synthesis models.
To interpret the origin of the gradient, we check the star formation history at different radii of the galaxy. The star formation rates peak at roughly the same time at all the radii but have stronger burst at the galaxy centre. The co-eval starburst hypothesis is supported by our simulations.
4. Conclusion
The range of gradients observed in MaNGA PSB galaxies is simply due to different times of observation rather than different underlying processes.
The radial gradients in the Balmer absorption line strengths in these PSBs are consistent with simulations with a single co-eval burst which was stronger in the central regions.