What are these mysterious Low Surface Brightness galaxies ?
1er mai 2003
Because of their low surface brightness, a whole population of galaxies (aptly named "Low Surface Brightness galaxies", or LSBs) has gone almost unnoticed in early extragalactic studies. During the last few decades however they have received more and more attention as the observational data on them increased, and as it has been realized that they may play an important role in the processes of formation and evolution of galaxies, and in the chemical evolution of the universe. A team of astronomers, led by Paris Observatory researchers, has just studied a large sample of LSB objects selected from the recently completed 2MASS near-infrared all-sky survey. They have built evolution models of disc galaxies, and shown that LSB galaxies can best be explained when their dark matter halos have a larger specific angular momentum than classical spirals. The (rather rare) red LSBs are accounted for by short-duration events perturbing their star formation history. In the past few decades, has been discovered the existence of galaxies with a central disc surface brightness well below the “Freeman value” of 21.65 mag / arcsec2 in blue light, typical of “classical” spiral galaxies (cf Hubble Atlas). In recent years a considerable body of data has been collected concerning these Low Surface Brightness galaxies (LSBs), and it was realized that they are crucial for studies of galaxy formation and evolution, of cosmic chemical evolution, and of the high redshift universe (especially because of their relatively large number). There is a large variety among the LSBs identified to date, which range from dwarf Ellipticals to giant spirals. Although LSB disc galaxies show a large range in size, surface brightness and absolute magnitude (figure 2), they are in practice often selected using a simple generic criterion, namely a blue central surface brightness lower than 22 mag / arcsec2. To compare with models of properties and evolution of LSBs, 5 samples of LSBs were used, which together cover wide ranges in, e.g., central surface brightness, luminosity, colour and bulge-to-disc ratio. The Digital Sky Survey images of six of these objects (shown in Figure 1) show the difficulty of detecting them in imaging surveys ; one is actually a giant galaxy and one has a prominent bulge and an LSB disc component. It should be noted, however, that there may still be uncharted LSBs with extreme properties which lie beyond the present detection limits.Figure 1. Digital Sky Survey blue images of 6 of the LSB galaxies used to compare with evolutionary models. Each image is 5 x 5 arcmin in size. Click on the figure to enlarge Figure 2. Some properties of disc Low Surface Brightness (LSB) galaxies, (scale-length and central surface brightness of the disc as a function of its absolute magnitude, in the B band). The different symbols indicate various samples of LSBs, which together probe different population of disc-LSB (dwarfs, giants...) ; the green symbols represent galaxies from the present sample of near-infrared selected 2MASS LSBs. The curve marked with stars indicates the position of models made for classical spirals. The other curves are models with larger angular momentum, aimed at modelling LSBs. Click on the figure to enlarge The similarity of this wide range of global properties with those of “classical” High Surface Brightness (HSB) spiral galaxies may provide a clue about their nature and evolution. Models of the formation of dark matter halos (in which galaxies are thought to settle and build up) show that such halos can form with various angular momenta. In this scenario, models of the evolution of disc galaxies can be constructed as function of angular momentum, from low values corresponding to “classical” spirals to higher values representing LSB systems. Since the central surface density of a disc scales as the inverse of the square of the “spin parameter” (a measure of the specific angular momentum of its dark halo), LSB galaxies can be regarded as the extension of classical spirals towards larger angular momentum systems. We therefore extrapolated evolutionary models we had already successfully tested on the Milky Way galaxy and on HSB spirals, by increasing the spin parameter.The predictions of these models on a wide variety of properties of LSBs are in accordance with what is observed for, e.g., disc scale-length, central disc surface brightness and absolute disc magnitude (see figure 2), the relation between mass and luminosity, gaseous content, and (see figure 3) measurements of oxygen abundances in ionized (HII) regions. oxygen.jpg
Figure 3. Oxygen abundance in HII regions of LSB disc galaxies (dots). Classical HSB spirals exhibit a clear relation between absolute disc magnitude and abundances (not shown). This is less clear in LSBs because the observations are made in HII regions at various galactocentric radii. The grey-shades and contours show the abundances expected in models of LSBs with large spin parameters. Click on the figure to enlarge
Figure 3. Oxygen abundance in HII regions of LSB disc galaxies (dots). Classical HSB spirals exhibit a clear relation between absolute disc magnitude and abundances (not shown). This is less clear in LSBs because the observations are made in HII regions at various galactocentric radii. The grey-shades and contours show the abundances expected in models of LSBs with large spin parameters. Click on the figure to enlarge Some LSBs have surprisingly red colours, however, contrary to what is predicted by the simple large-angular momentum models, which have a star formation rate changing smoothly with time . A way to explain this discrepancy is the occurrence of small bursts and truncations in the star formation process, which would affect mainly the colours of the galaxies (as illustrated in figure 4). Such events are relatively more likely to occur in LSB galaxies than in HSB spirals because of the lower densities that characterize them ; indeed, the gas density in LSBs is lower than the “threshold” value for star formation observed in nearby spirals. Under these conditions, the star formation in LSBs is not expected to proceed smoothly, but rather to be subject to local perturbations and density waves. colors.jpg Figure 4. Colour index (B-I) as a function of absolute disc magnitude for LSB galaxies (dots) and models (cyan curves). The models, which have a smoothly changing star formation history, lie on the blue edge of the data. A way to explain the discrepancy is to add small starbursts (dark curve) or truncations of the star formation history (arrow) to these simple models. Click on the figure to enlarge As mentioned earlier, there is a great variety of known disc LSBs and hitherto all of them were selected in the optical. Using the recently completed 2MASS near-infrared all-sky survey we defined the first large, homogenous sample of LSBs in this wavelength domain, and studied subsets of them in the 21 cm HI line at Nançay and Arecibo and with multi-colour optical CCD surface photometry. A first analysis of the data (cf PhD Thesis of D. Monnier Ragaigne), shows the rarity of red LSBs, the presence of objects with extremely high HI masses (more than 1010 solar masses) and a Tully-Fisher relation between absolute magnitude and rotation velocity similar to that of classical High Surface Brightness (HSB) spiral galaxies. This sample is also the first to cover the transition zone between LSB and HSB galaxies, as defined above in blue light.
References : "Chemical and spectrophometric evolution of Low Surface Brightness galaxies" Boissier, Monnier Ragaigne, Prantzos, van Driel, Balkowski, O’Neil, 2003, accepted for publication in MNRAS. (preprint:astro-ph/0304313) "A search for Low Surface Brightness galaxies in the near-infrared I. Description of the sample" Monnier Ragaigne, van Driel, Schneider, Jarrett, Balkowski, 2003, accepted for publication in A&A (preprint : astro-ph/0304380)
Contact :
Wim van Driel (Observatoire de Paris, GEPI) Chantal Balkowski (Observatoire de Paris, GEPI) Delphine Monnier Ragaigne (Observatoire de Paris, GEPI) Samuel Boissier (Carnegie Observatories, Pasadena)
Dernière modification le 4 mars 2013