Given their low surface brightness, it is hard to determine high-quality spectra of dwarf galaxies outside the Local Group, where it is not possible to study the individual stars. Although several studies exist that use absorption line lengths to study the stellar populations of dwarf galaxies, studies of the Star Formation Histories in these galaxies are scarce. Those, in principle, could provide important information on how the stars in dwarfs are being built up.

In this talk I present a study of Star Formation Histories of 31 dwarf galaxies in the Fornax Cluster, with stellar mass ranging from 10^7 to 10^9 M_Sun, as presented in Romero-Gomez et al. (2024). These are based on ultra-deep multi-IFU observations from the SAMI instrument at the AAT at high resolution R=5000, a resolution comparable to the velocity dispersion of the galaxies. Using full spectral fitting, with the PPXF code (Cappellari & Emsellem 2004) and the E-MILES library, we are able, for the first time in the literature, to produce accurate Star Formation Histories for these galaxies.

To be able to interpret these results well, we added a sample of massive early-type galaxies in the Virgo-Cluster from the ATLAS-3D survey (McDermid et al. 2015), for which we determine the Star Formation Histories ourselves in the same way as for the dwarf galaxies, and a sample of fainter galaxies, consisting of early-type dwarfs in the Local Group, from 10^4 - 10^8 M_Sun from Weisz et al. (2014), where the Star Formation Histories have been determined using Col-Mag studies of individual stars.

We find that star formation is the slowest in galaxies from 10^7 -10^8 M_Sun, becoming faster for both more massive and smaller galaxies. In this sense, we find that the Star Formation Histories of some Ultra Faint dwarfs of 10^5 M_Sun galaxies are similar to very massive galaxies of 10^12 M_Sun.

Our study shows that the evolution of massive galaxies is primarily governed by their internal properties, with time-scales that do not depend on their environmental location. In contrast, dwarf galaxies below 10^8 M_Sun can be significantly affected by dense environments, such as the inner regions of a cluster, or a massive nearby galaxy, that severely quench the galaxies before the assembly of their 50 per cent present-day mass. The galaxies from from 10^7 -10^9 M_Sun are too massive to lose all their gas by the environment, so they can continue forming stars for a very long time. Their star formation histories are similar to those of Star Forming dwarf galaxies, except that star formation in these objects finished recently. They are also very different from massive galaxies, which form their stars very fast. These results are confirmed by the behavior of alpha/Fe, which is similar to solar in these objects, while it is enhanced in both very massive and very faint galaxies (Romero-Gomez et al. 2023). The result confirm models in which star forming

Our study highlights and corroborates a critical threshold around 10^9 M_Sun in galaxy evolution from previous studies, separating more massive galaxies minimally impacted by the environment from those less massive galaxies quenched by it.

References:

Cappellari, M. & Emsellem, E. 2004, PASP, 116, 138.

McDermid, R.M., Alatalo, K., Blitz, L., et al. 2015, MNRAS, 448, 3484.

Romero-Gomez, J., Aguerri, J.A.L., Peletier, R.F., et al. 2024, MNRAS, 527, 9715.

Romero-Gomez, J., Peletier, R.F., Aguerri, J.A.L., et al. 2023, mnras, 522, 130.

Weisz, D.R., Dolphin, A.E., Skillman, E.D., et al. 2014, apj, 789, 147.