Planck and Herschel Exclusive Interviews

Suzanne Madden & Maud Galametz

CEA, (French atomic energy commission), Saclay, Paris

Suzanne Madden is the principal investigator of 'The ISM in Low Metallicity Environments: Bridging the Gap Between Local Universe and Primordial Galaxies' Herschel guaranteed time project, and Maud Galametz is a PhD student at CEA dedicated to the Herschel guaranteed time key program on the survey of dwarf galaxies.

Our Milky Way Galaxy is metal-rich. Madden and colleagues will be studying metal-poor galaxies to understand galaxy-forming conditions in the early Universe. Image: NASA/JPL-Caltech.

Could you summarise the main differences in characteristics between metal rich and metal poor galaxies, and how both types evolved?
Metal poor galaxies mainly differ from metal rich ones by their sheer size to number of stars (from one million to several billions of stars while our Galaxy, the Milky Way (MW) is composed of around 100 billions of stars). They usually have a lower metallicity, often sub-solar metallicity, than their bigger and dustier cousins. Furthermore, we think that these conditions of low metallicity should be close to those of the Universe at its early stages. These galaxies are thus perfect laboratories to study environments that are rather different than the more metal-rich ones like our MW Galaxy but also close, physically, to the primordial environments of protogalaxies, the first galaxies formed in the Universe. According to the current hierarchical model of galaxy formation and evolution dwarf galaxies are considered to be building blocks of larger galaxies, although there is evidence that dwarf galaxies in our local universe have stellar populations that are younger than their larger cousins. They constitute the dominant population in the zoo of galaxies in the local Universe. For example, in the Local Group, the Group of ~30 galaxies to which our MW belongs, the population is clearly dominated by dwarf galaxies. In spite of their apparent low abundance of metals, they usually have old stellar populations and apparently little molecular gas H2, out of which stars are born. Thereby, the processes capable of initiating the starbursts visible in some of them are not yet well understood. They are usually considered to be less complex systems due to their small size which implies that a starburst for example can affect the overall galaxy, and not be confined to a region and that its properties, metallicity for example, are rather homogeneous through the galaxy.

What sort of processes occur in the ISM and what is meant by recycling?
Dust can be formed around stars, especially dying ones, where one can find the heavy elements or directly in the ISM. When a star evolves, it can lose its envelope through winds (e.g. AGB stars) or explodes in a supernova. The material is thus expelled in the ISM and re-enriches the medium. The dust grains can thus meet a molecular cloud, very dense and very cold (~-400°F) regions where chemical species are usually in a molecular form. Within them, dust grains will participate in the chemistry and synthesis of more complex molecules. By condensation or gravitational collapse of the molecular clouds, stars can born once again, grow, die, explode, let its material escape etc. This is the continuous recycling of interstellar matter that becomes more and more enriched with metals as the stars within it evolve with subsequent generations.

How will Herschel study the low metallicity environments and what new information are you hoping to gain from the observations?
One of the major aims of the Herschel mission is to study the astrochemistry of the gas and dust to better get a handle on the cycle of stellar and interstellar matter. How are stars born, how do they evolve and re-enrich the medium? Related to this question: what properties of a galaxy as a whole control how the stars and the interetellar matter within the galaxy evolve? Is the dust in metal-poor environments different than in the more metal-rich galaxies? What is the distribution of this dust and its different components through the galaxy (cold/hot dust- big/small grains etc.)? The IR astronomy is still a very young domain but enabled us to find a thousand new galaxies and to study their composition and evolution through the different ages of the universe. Many things have still to be studied. The submillimeter wavelength regime is still unexplored mainly due to our atmosphere attenuation in this band. However, this band brings crucial information on the gas and dust contained in a galaxy and thus on its star formation history, its metallicity and chemistry etc. To study galaxies at this wavelength with enough sensitivity, a space telescope is required.

The galaxy IC 10 is an irregular dwarf galaxy. Dwarf galaxies contain up to several billion stars, compared with galaxies like our own Milky Way which contain several hundred billion stars. Image: NOAO/AURA/NSF.

Is it true that low metallicity environments have not experienced much (or any) recycling through the ISM and therefore represent primordial ISM conditions and star formation? Therefore will Herschel be offering the first chance to look at this aspect of stellar formation in the very early Universe in a way that previous missions have not been able to do?
Whether or not there are actual primordial galaxies in the local Universe has yet to be established. The lowest metallicity galaxies that we have found in our Milky Way Galaxy have a metallicity 1/50 that of our Galaxy – extremely low in metals. It would be easy to interpret all of this if the metallicity that we measure spectroscopically in the gas is related directly to the age of the galaxy. Some of these low metallicity galaxies are indeed very young, but in general, metallicity is not an absolute chronometer. We do know that there is a wide variety of local laboratories of differing metallicities. The design of the Herschel survey of dwarf galaxies is to study the effects of the gas and dust over a broad range of metallicity in a statistical sense, with as much wavelength coverage as possible (using Herschel data with the Spitzer data). The MW itself gives us a very detailed view of how stars form and enrich the interstellar medium, it does not give us the wide range of environmental conditions, including range of metallicity, we can hope to put together the big picture of the effects of metallicity on the gas and dust and thus the star formation processes. Dwarf galaxies have historically been detected via their optical light and simply presumed to have negligible dust content. Life would be easy if you could indeed use the optical fluxes, presume it unobscured, and then determine star formation activity and other galactic parameters from the optical light measured. Even the lowest metallicity galaxies have evidence now for substantial dust and therefore, star formation obscuration. Additionally, there is some evidence for the lowest metallicity galaxies to have older stellar population. Herschel gives us, for the first time, the highest possible sensitivity in the far infrared and submillimetre wavelengths. Sensitivity has always been the limiting factor of previous infrared missions as far as dwarf galaxies are concerned. It has never been possible to do a statistical survey as is planned now for Herschel due to the lack of sensitivity.
With Herschel we will have wavelength coverage over the peak of the energy distribution as well as the until-now unexplored submillimetre wavelength regime. We will be able to trace out the bulk of the luminosity of the galaxy.