Knowing how galaxies grow as they transform gas into successive generations of stars is a complex problem: We need detailed physical information about a significant sample of galaxies, and data bases that allow “rewinding” and extracting the history of stellar formation inside each one of them. The first part has been possible only recently, thanks to the CALIFA (Calar Alto Legacy Integral Field Area) survey, and it has been deduced that massive galaxies not only grow faster than lighter ones, but also follow that growth from inside out, i.e., developing first their inner regions.
Extragalactic surveys have been traditionally based whether on direct images, that give detailed information on the structure of galaxies, or on spectroscopy, that reveals the physical properties of these objects (composition, temperature, age…) but not specifying those values for specific regions. “The most widely used survey up to now, SDSS, provides one spectrum per galaxy, what induces an observational bias”, says Enrique Pérez, scientist at the Astrophysical Institute of Andalusia (IAA-CSIC) and head of this research. “On the contrary, CALIFA obtains one thousand spectra per galaxy, what allows finally mapping the history of complete galaxies.”
These researchers applied to CALIFA data a technique known as “fossil record method”, what lead them to define the history of star formation at each one among the one hundred thousand regions analyzed inside a total of one hundred and five galaxies.
“A part of one galaxy can be considered as the sum of a population of stars with different ages, masses and metallicities (or proportion of elements heavier than hydrogen and helium), and all this information is codified in its spectrum”, explains Enrique Pérez. This way, from each spectrum, and provided a data base that takes into account all the possible evolutionary paths for the stars, we are able to invert the history of the galaxy and figure out how much mass was transformed into stars at each time, and what kind of stellar populations were then present.
When establishing the spatial and time evolution of the sample of galaxies, the researchers realised that more massive galaxies not only grow faster than lighter ones, but they also grow from inside out, in such a way that central parts are formed first (observations show that those areas are much older than the outer parts.)
Also, they computed the star forming rate at specific regions, in comparison with the average of each individual galaxy, and they found an interesting fact: “We saw that at all galaxies and all regions the rate is very similar, except from the inner parts of those most massive, where stars are born at a pace twice as large as the average,” underlines Enrique Pérez. “However, this maximum is produced when the galaxy reaches a mass of a few tens of billions of solar masses and, afterwards, the star forming rate falls down again for massive galaxies.” This star forming peak, that this research dates between five and seven thousand million years ago, had been described in theoretical studies, but it had never been observed before.
These results fit well what we observe in our Galaxy (that belongs to the not massive kind) and its neighbour, the Andromeda galaxy, more massive and showing an aged central region. The researchers propose that the differences may be due to the fact that, in the past, massive galaxies have undergone processes of fusion with other galaxies, what accelerated the growing of the internal zones, while low-mass galaxies experienced a more steady evolution.
CALIFA project is still in progress at Calar Alto Observatory, operated by the Astrophysical Institute of Andalusia (IAA-CSIC, Granada, Spain) and the Max Planck Institute for Astronomy (MPIA-MPG, Heidelberg, Germany). This survey uses the integral field spectrograph PMAS attached to the 3.5 m Zeiss reflecting telescope.
The sample of galaxies of this study. Each galaxy is represented with a color code that indicates the surface density at its different zones, according to the code shown in the vertical bar at right. The positions of the galaxies inside the graph is related with their real mean colour (redder up, bluer down) and with their intrinsic brightness (brighter left, fainter right). An animated version of this diagram showing the time evolution of the stellar mass buildup is available at this link.
The colour-magnitude diagram for 151 galaxies observed by CALIFA. For each galaxy it is shown a true-colour image created using the B (blue), V (green) and R (red) band reconstructed images extracted from the CALIFA datacubes. In this diagram, the galaxies get fainter towards the left and brighter towards the right; at the same time, they are cooler (redder) at the top, and warmer (bluer) towards the bottom of the image. (2.59 MB)
The 3.5 m Zeiss reflector at Calar Alto Observatory.
The 3.5 m Zeiss reflector at Calar Alto Observatory, with the PMAS spectrograph attached.