Dwarf Galaxies Point to a Fundamental Flaw in Our Understanding of How Galaxies Form and Evolve

6a00d8341bf7f753ef0120a67bc7f8970c.jpg "if big galaxies are supposed to be formed from numerous mergers of smaller systems, how come the properties of the small galaxies we see today don't match the larger galaxies? You may start to wonder if there isn't a fundamental flaw in some aspect of our current understanding of how galaxies form and evolve, and exactly how important mergers are in the history of galaxies."

Eline Tolstoy, astrophysicist at the University of Groningen in the Netherlands.

The smallest dwarf galaxies are the most straight forward objects in which to study star formation processes on a galactic scale. They are typically single cell star forming entities, and as small potentials in orbit around a much larger one they are unlikely to accrete extraneous matter during their lifetime (either intergalactic gas, or galaxies) because they will typically lose the competition with the much larger galaxy.

We can utilise observations of stars of a range of ages to measure star formation and enrichment histories back to the earliest epochs. The most ancient objects we have ever observed in the Universe are stars found in and around our Galaxy. Their proximity allows us to extract from their properties
detailed information about the time in the early Universe into which they were born.

A currently fashionable conjecture is that the earliest star formation in the Universe took place in the smallest dwarf galaxy sized objects.

One of the fundamental pillars of current structure formation models is that small galaxies are the building blocks of larger ones. Thus, the dwarf galaxies around our Galaxy are arguably the remnants of the formation of the Milky Way and as such provide a unique laboratory for the detailed study of generic galactic assembly processes.

While the  CDM model has been quite successful at modelling clusters of galaxies and large-scale
structure it currently faces problems on the small, dwarf galaxy, scale. It appears to over-predict the number and the mass spectrum of satellites seen around galaxies such as our own (e.g., Moore et al. 1999) and there also appear to be inconsistencies with regard to the timescale of the build up of larger galaxies, and the differences in the stellar populations of large and small galaxies.

CDM  (Lambda Cold Dark Matter) is frequently referred to as the standard model of big bang cosmology, since it attempts to explain: the existence and structure of the cosmic microwave background; the large scale structure of galaxy clusters; the distribution of hydrogen, helium, lithium, oxygen; the accelerating expansion of the universe observed in the light from distant galaxies and supernovae.

These problems might arise only because we have not yet made detailed enough studies of our neighbours; there are still quite a number of uncertainties in our interpretation of current observations. However, results to date provide some realm challenges  to the current standard implementation of CDM on small scales

All of the dSph galaxies, without exception, have an ancient stellar population. Even the oldest and simplest have had a complex star formation history, and these systems, withvirtually identical SFHs, still have different CMDs from each other.

Tolstoy suggests that perhaps this is a result of different environmental influences The evolution of dSph must be influenced, maybe strongly, by the presence of our Galaxy. The dynamical friction of their orbits may have a strong (and varying) influence on the rate of star formation (SFR). No dSph around our Galaxy currently has an (obviously) associated interstellar medium.

There are many different approaches to studying the evolutionary history of a galaxy and the most reliable is to look directly at the detailed properties of individual stars. This is, however, restricted to galaxies
in the very nearby Universe. Although there is much to be learnt from studies of integrated properties of more distant dwarf galaxies.

Dwarf spheroidal galaxy (dSph) is a term in astronomy applied to low luminosity galaxies that are companions to the Milky Way and to the similar systems that are companions to the Andromeda Galaxy M31.

In Cold Dark Matter cosmology, the total amount of mass inferred from the motions of stars in dwarf spheroidals is many times that which can be accounted for by the mass of the stars themselves is seen as a sure sign of dark matter, and the presence of dark matter is often cited as a reason to classify dwarf spheroidals (dSph) as a different class of object from globular clusters (which show little to no signs of dark matter). Because of the extremely large amounts of dark matter in these objects, they may deserve the title "most dark matter-dominated galaxies"

The dSph around the Milky Way are among the few galaxies in the Universe for which we have accurate main sequence turnoff (MSTO)ages going back to the epoch of earliest star formation. MSTOs are the most accurate measurementsof the age distribution of a stellar population and even so there are problems in converting turnoff luminosities and colors into accurate absolute ages, and thus CMDs into SFHs.

Image at top of page: Our Milky Way Galaxy is part of a gathering of about 25 galaxies known as the Local Group. Members include the Great Andromeda Galaxy (M31), the Large Magellanic Cloud, the Small Magellanic Cloud, Dwingeloo 1, several small irregular galaxies, and many dwarf elliptical and dwarf spheroidal galaxies. Pictured on the lower right is one of the dwarf ellipticals: NGC 205. The Andromeda Galaxy and the Milky Way are approaching one another at a speed of 100 to 140 kilometres per second. The collision is predicted to occur in about 2.5 billion years, the two merging to form a giant elliptical galaxy.

The Daily Galaxy via Eline Tolstoy

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Image Credit & Copyright: Jean-Charles Cuillandre (CFHT) & Giovanni Anselmi (Coelum Astronomia)

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