Local Group

M33 (Credit: John Corban & the ESA/ESO/ NASA )

M33 (Credit: John Corban & the ESA/ESO/ NASA )

The Local Group of galaxies is a group of relatively few galaxies that lacks very massive ones (e.g. large ellipticals). Despite this, it is an excellent laboratory for studies of galaxy evolution because it provides us with a wide range of different galaxy types in a variety of environments. Similar to many other nearby groups, the mass and the luminosity of the Local Group are dominated by two large spirals, the Milky Way (MW) and M31. Most of the other Local Group members are dwarf galaxies, and the majority of them are found in close proximity to the two large spirals. J-PLUS not only will allow a homogeneous study of the Local Group galaxies but also it will be deep enough to probe regions well beyond where the surface brightness profiles of these galaxies are no longer described by a single exponential profile. J-PLUS will open up a new and important window in this field and in particular will shed light into the following scientific cases.

Spiral Galaxies in the Local Group: The special case of M33

The process of galaxy formation and evolution is one of the most active research areas in modern astrophysics. It is widely accepted that galaxy evolution occurs in the framework of the Lambda-Cold Dark Matter (Λ-CDM) model. Within this context of hierarchical clustering theory, large disk galaxies, like the Milky Way (MW) and M31, derive from the merger and accretion of many smaller subsystems. As the progenitor disrupts, its stellar components are incorporated into the halo. Therefore, there is a deep connection between the ages and chemical compositions of the system parent-progenitor. Tidal debris are deposited in both and can maintain spatial and kinematic coherence for many gigayears. Accordingly, stellar halos of galaxies under this context should possess significant spatial and metallicity substructure in the form of disrupted satellites. It is less clear how accurate the Λ-CDM framework is at lower galaxy masses and if we should expect disrupted dwarf galaxies or tidal tails in the vicinity of a dwarf spiral galaxy. With these questions in mind, J-PLUS will study M33, the nearest example of a dwarf spiral galaxy.

M33, also called the Triangulum galaxy, is the third most massive galaxy in the Local Group and is gravitationally bound to M31. At a distance of 870 kpc, it is the only nearby late-type spiral galaxy, and it provides a notable connection between the population of earlier-type spirals and the numerous nearby later-type dwarf galaxies. With a total mass ∼ 20 times lower than the MW and only ∼ 2 times greater than the Magellanic Clouds, M33 provides a unique opportunity to test the Λ-CDM framework predictions in a regime different from the MW. Taking advantage of the IFU-like capabilities of J-PLUS, this study will determine the properties of the spatially resolved and unresolved components of the galaxy. In particular, it will perform a 2-D analysis of the underlying population as well as a detailed study of M33 star cluster system.This study will provide key insights into the star formation history and composition of low-mass galaxies as well as place M33 within the context of galaxy formation process.

Dwarf Galaxies in the Local Group

The Andromeda galaxy (M31) and its dE satellites M32 and M110

The Andromeda galaxy (M31) and its dE satellites M32 and M110 (Credit: Bill Schoening, Vanessa Harvey/REU program/AURA/NSF/NOAO)

Dwarf galaxies (dEs/dSphs and dIrr) are low-luminosity galaxies that form the dominant population of nearby galaxy clusters. In fact, dEs exceed in number (by a factor of 6) the high luminosity galaxies in the Local Group. These objects are often defined as the building blocks of massive galaxies in hierarchical frameworks of galaxy formation, so clearly the study of these objects will provide important clues on the main processes involved in galaxy assembly and evolution.

In spite of their large number and their importance in our understanding of galaxy evolution, the low-luminosity of these systems has always prevented detailed and extensive studies. On one hand, major photometric surveys, while a good source for identifying candidates, are often not able to map the properties of these galaxies far out in radius. In fact, determining the integrated photometric and structural properties of these galaxies is very challenging. Many Local Group dwarfs are so extended that few telescope/detector combinations can survey the entire extent of the system in one or even several exposures. When available, the integrated properties of these objects are commonly based on observations of only a small fraction of the galaxy. However studies of the luminosity function of the Local Group rely on these values. Spectroscopic studies, on the other hand, are based on a rather limited number of dwarf galaxies and are typically restricted to a single aperture measurement or a short long-slit profile. Integral-field spectroscopic studies, while providing a wealth of detailed spectral information, are still scarce.

The multi-band observing strategy of the J-PLUS survey will allow us, for the first time, to produce a very detailed study of the stellar populations of dwarf galaxies well into their outskirts. The analysis of their star formation histories at different radii will reveal whether star formation takes place in an inside-out fashion (i.e. as most ordinary galaxies exhibit) or if on the contrary secular evolutionary processes dominate their evolution. It will also reveal the importance of environmental processes in those dwarfs living in clusters.