Two types of galaxy activity
A galaxy is said to have an "active nucleus" when its nucleus emits broad high-excitation lines, which are attributed to the presence of an accretion disk fueling a massive black hole in the centre of the galaxy. The famous Seyfert galaxies are the prototypes of galaxies with active nuclei — often refered to simply as "active galaxies". However, the word "activity" can also describe galaxies that are forming stars. This activity is revealed through emission lines as well, but this time they are produced by gas that is ionized by newborn stars.
How can we tell apart these two types of activity ?
Active nuclei produce higher-energy photons than massive stars. Those photons are therefore able to heat the gas more efficiently. In 1981, Baldwin, Phillips and Terlevich proposed a variety of diagrams based on emission line ratios which could clearly separate those two types of excitation. The most famous among them, usually called the BPT diagram, shows [OIII/Hbeta as a function of [NII]/Halpha. In galaxies hosting active nuclei, the [OIII] and [NII] lines, being excited by thermal collisions, are stronger with respect to hydrogen recombination lines than in star-forming galaxies.
The two groups of emission-line galaxies in the Sloan Digital Sky Survey
The Sloan Digital Sky Survey (SDSS), which has observed a quarter of the sky and has collected more than 700 000 galaxy spectra, has revolutionized our knowledge about the local Universe in many ways. Concerning the emission lines, the BPT diagram revealed that about 30% of galaxies lie in the zone of active nuclei ! This result strongly contradicts the idea that we had about the population of galaxies hosting an active nucleus, which previous studies estimated to be 1-2%.
Moreover, in the BPT diagram applied to SDSS data, emission-line galaxies are distributed along two sequences evoking the wings of a flying seagull (Fig. 1). One of them, the left wing, has long been known as the sequence of star-forming galaxies. But the other one, attributed to galaxies hosting active nuclei, was unexpected !
Taking a better look at it, it was found that the right wing is composed of two branches, respectively identified by Kewley and collaborators to Seyfert galaxies and LINERs — "Low Ionization Nuclear Emission Regions", a term proposed in 1980 to designate galactic nuclei that seemed to have a faint evidence of non-stellar activity. This term was then applied to galaxies whose spectra looked like LINERs. The emission lines in these objects are not very strong, and they can only be detected after a detailed modelling of the stellar continuum. Many hypotheses were made about the origin of activity in LINERs.
Active, passive and ..... retired galaxies
All galaxies, at one time or another in theirs lives, have formed stars. After consuming their gas — or, more accurately, the molecular clouds where stars are formed — they follow a "passive" evolution : Their stellar populations simply get older and older. However, such galaxies can still exhibit faint emission lines, and, what is more, these lines have ratios similar to those of LINERs. This is what a French-Brazilian team has recently shown, dubbing these galaxies as "retired". A large proportion of the LINERs revealed by the SDSS could indeed be retired galaxies !
The evolution of the ionizing radiation field of a galaxy
The most massive stars —those that ionize the gas in star-forming galaxies— are those who die first. After 100 million years, the number of ionizing photons emitted by those stars decreases by more than 4 orders of magnitude. But the stars of mass lower than 8-10 solar masses have just started evolving beyond the main sequence. During this phase, they will become nuclei of planetary nebulae, and then white dwarfs, reaching much higher temperatures than massive stars. The photons which they emit, not so numerous but more energetic (see Fig. 2), are thus able to heat the gas to the same extent as photons emitted by accretion disks around massive black holes. In fact, contrary to common sense, the temperature of an ionized gas does not depend on the quantity of ionizing photons, but only on their energy.
- Figure 2 : A gauche : évolution temporelle du nombre de photons QHI capables d’ioniser l’hydrogène, pour un ensemble d’étoiles nées au même moment. A droite : évolution du rapport QHeI/ QHI, qui illustre la température caractéristique du rayonnement ionisant. Cliquer sur l’image pour l’agrandir
It remains to show that galaxies in the LINERs branch could truely be retired galaxies. The optical continuum of each SDSS galaxy was analysed and decomposed into stellar populations by the STARLIGHT code (http://www.starlight.ufsc.br). This allowed to estimate the far UV radiation emitted by those stellar populations and plug it into a code computing photoionization models to calculate the intensity of lines arising from the residual gas of the galaxy (especially the gas from stellar winds).
It results that the old stellar populations can explain the observed Halpha luminosities for a significant fraction of LINERs (30% and may be more). Regarding the ratios of emission lines, they cover the entire LINER domain in the BPT diagram, and even beyond.
Why do the SDSS galaxies draw a seagull in the BPT diagram ?
One should not forget that the BPT diagram is also the result of an observational bias. The SDSS is magnitude-limited. Consequently, the least massive —and least metal-rich— galaxies are not seen in the diagram except during their most luminous phase, that is, during their star-forming phase. In order to appear in the BPT diagram, the galaxies must also be visible in 4 emission lines : [OIII], Hbeta, [NII] and Halpha. The bottom part of the right wing is absent because the lines are too faint to be detected, even in luminous galaxies.
Not all galaxies are equal with respect to retirement
The most massive galaxies eat up their gas faster to form stars and become more metal-rich when they retire. This is one of the reasons why the sequence of star-forming galaxies does not contain objects with metallicities greater than 2 or 3 times solar, whereas such metal-rich galaxies can be found in the LINER branch.
- Can retired galaxies mimic active galaxies ? Clues from the Sloan Digital Sky Survey Stasinska, G. (LUTH) ; Asari, N. V. (LUTH-UFSC) ; Cid Fernandes, R. (UFSC) ; Gomes, J. M. (GEPI-UFSC) ; Schlickmann, M. (UFSC) ; Mateus, A. (LAM) ; Schoenell, W. (UFSC) ; Sodré, L., Jr.(IAG), (the SEAGal collaboration), 2008, MNRAS, in press Semi-empirical analysis of Sloan Digital Sky Survey galaxies - III. How to distinguish AGN hosts Stasinska, G. ; Cid Fernandes, R. ; Mateus, A. ; Sodré, L. ; Asari, Natalia V., 2006, MNRAS, 371, 972
- Grazyna Stasinska (Observatoire de Paris, LUTH et CNRS
Dernière modification le 27 novembre 2013