VLT directly observes the formation of present-day spirals

VLT directly observes the formation of present-day spirals Astronomers have long been intrigued by a population of luminous and compact galaxies that are observed in large numbers in the distant Universe. Although they have sizes comparable to those of dwarf galaxies, they emit 10 to 100 times more energy at all wavelengths than present-day dwarf galaxies. They also constitute the population of galaxies that has most evolved during the last two-thirds of the universe’s existence : while they represent the majority of all galaxies about 7 to 10 Gyr ago, they are almost non-existent today. In particular, they are mainly responsible for the reported decline of the global star formation since these epochs. An international team of astronomers from the Observatoire de Meudon (Francois Hammer and Nicolas Gruel) and Saclay (Hector Flores) in France, the University of Virginia (Trinh Xuan Thuan) in the USA and the Catholic University (Leopoldo Infante) in Chile have undertaken a detailed study of these intriguing luminous compact galaxies, using a very efficient spectrograph (FORS1) at the focus of the VLT 8-meter telescope ANTU based at Paranal and operated by ESO. They have obtained spectra of unprecedented quality, detecting for the first time at such large distances, important lines such as iron lines and other metallic absorption lines. Figure : (on the right) is represented a sketchy sequence of compact galaxy morphologies (HST imagery). From the top picture to the bottom closely interaction are shown, and then formation of stars in bulges and finally in the disk. (on the left) are shown two spectra of compact galaxies observed with VLT (6 hours) revealing strong emission lines and strong metal absorption lines. The similarity of their spectral properties along the sequence suggests a common origin. These data have shed a new light on the stellar and gaseous content of luminous compact galaxies and their formation history. They reveal that these galaxies contain an evolved stellar population of old stars with a metal content similar to or higher than that of the Sun. On the other hand, they are also the site of intense star formation, forming young stars at a rate some 40 times higher than in the Milky Way. Analysis of their spectra also shows that they are very dusty galaxies, with a large fraction of their emission being reprocessed and reemitted by dust at infrared wavelengths. This result is confirmed by the detection of a number of them by the Infrared Space Observatory (ISO). From these observations, the following new picture has emerged. Luminous compact galaxies are evolved galaxies which are rapidly forming new young stars. The star formation is triggered and enhanced by interactions with nearby companions and galaxy mergers. Hubble Space Telescope high resolution images of some of them do indeed often reveal the presence of closeby companions with tidal distortions. These observations provide a spectacular confirmation of the hierarchical scenario of galaxy formation, in which galaxies are formed by the fusion of smaller building blocks. Luminous compact galaxies show a wide variety of morphologies, from mergers to systems with an already developed disk structure. The similarities between their spectral properties and those of bulges in present-day spiral galaxies have led the team to propose that they are the progenitors of today’s bulges of massive spirals, and not of dwarf galaxies as previously thought. Thus, by observing luminous compact galaxies, we are witnessing a key period in the formation of a galaxy, that of its bulge prior to the formation of stars in the disk, such as the Sun. If the above scenario holds for the majority of luminous compact galaxies, it is estimated that a large fraction of present-day spiral galaxies have formed the bulk of their stars at relatively recent epochs, during the last 8-9 Gyr of the universe’s existence.