Calar Alto Legacy Integral Field Area Survey

Project

The CALIFA project addresses a number of open questions in galaxy evolution, among them:

1. The chemical evolution of galaxies: how, when, and where metals are produced in galaxies
2. Galaxy masses from different tracers: how much mass there is in stars, gas, and dark matter, and how it is distributed
3. Galaxy assembly as traced from the kinematic structure: what the motions of stars and gas tell us about the structure of the galaxies
4. Galaxy assembly as traced through the stellar population content: how, when, and where did stars form throughout the history of the galaxies

CALIFA data are made public through regular releases (DR). DR1, containing 200 data cubes of 100 galaxies, was released 1 November 2012. DR2, containing 400 data cubes of 200 galaxies, was released on 1 October 2014.

The most relevant science results unveiled by CALIFA during the period 2012-2014 are the following:

1. Spatially resolved Star formation History (SFH): the spectra of galaxies encode their history of star formation: how many stars, of what mass and metal content, formed at which time along the evolution of the galaxy. Stars of different mass evolve at very different rates: while stars of 1 solar mass live for ten billion years (10 Gyr), those of 10 times the solar mass live only 32 million years (32 Myr). These wildly different rates of evolution are encoded in the spectra of galaxies, so we can use our knowledge of stellar evolution to unfold how galaxies formed their stars along time in a sort of galactic archeology. For this purpose we use several numerical codes, such as STARLIGHT; and because with CALIFA we can make these studies in 3D –for each of the two spatial coordinates we have the time evolution–, we have developed the PyCASSO analysis pipeline (Cid Fernandes et al., 2013, 2014). We have found that: stellar mass assembly in typical massive galaxies happens from inside-out (Pérez et al., 2013). We also found that the SFH of bulges and early-type galaxies are fundamentally related to the total stellar mass, while for disk galaxies it is more related to the local stellar mass density, and how these manifest in the mass-metallicity relation for stars and for the gas (González Delgado et al., 2014a, 2014b).
2. Chemical enrichment in galaxies: When new hot massive stars are born they radiate very energetic photons that ionize the remnant parent gaseous nebula that has not formed stars; this craddle of new stars and ionized gas is known as an HII region. HII regions are very important because they are very luminous, so they can be used to study the star formation in far away galaxies. Also, their emission line spectrum can be used to computed the chemical abundances of the different atomic elements, thus tracing the chemical evolution of galaxies in their most recent epochs. We have developed new tools to detect and extract the spectroscopic information from HII regions (Sánchez et al., 2012b), building the largest catalog currently available (~10.000 HII regions and aggregations). This catalog has been used to define a new oxygen abundance (the abundance of oxygen is used as a reference in HII regions) calibrator anchored with electron temperature measurements (Marino et al., 2013). From these, we explored the proposed dependence of the Mass-Metallicity relation with the star formation rate (SFR; Sánchez et al., 2013), and the local Mass-Metallicity relation (Rosales-Ortega et al. 2012). We found that all galaxies in our sample present an abundance radial gradient with a common slope (when normalized to the effective radius; Sánchez et al., 2014); this agrees with the proposed inside-out scenario for galaxy growth. This characteristic slope is independent of the properties of the galaxies, and in particular of the presence or absence of dynamical structures such as stellar bars. This result has been confirmed by the analysis of the stellar abundance gradient in the same sample (Sánchez-Blázquez et al., 2014).
3. Origin of the low intensity ionized gas: in many spectroscopic studies since the last century, the ratio of intensities of some emission lines in the spectra of ionized gaseous nebulae have been used to study some properties of the sources of ionizing photons (such as massive stars in HII regions, or quasars/AGN in the centers of some galaxies, for example). One such emission line intensity ratio involves the singly ionized nitrogen to hydrogen [NII]6584/H6563. High values of this ratio have been regularly found in the nuclei of galaxies and interpreted as produced by either shocked gas or a quasar like ionization; this is known as LINER emission. Several articles using CALIFA data have explored the origin of this ionization, both in early exploratory studies with a few galaxies (Kehrig et al., 2012), and using larger number of galaxies. On one hand, we found that the so-called LINER emission is not always related with a galactic central ionizing source, i.e., an AGN (Singh et al., 2013), and can be found at any galactocentric distance, being most probably related with ionization produced by evolved stars (so called post-AGB stars). The analysis of this nebular emission and the escape of Lyman continuum photons have found evidence that this escape can explain the weak ionization seen in a fraction of elliptical galaxies with evidence of AGN activity in radio continuum and/or X-ray wavelengths (Papaderos et al. 2013); this may introduce an observational bias in this kind of selections based on optical emission line ratios. A larger sample is currently under analysis (Gomes et al. 2014).
4. Aperture and resolution effects on the data: in studying the structure and evolution of galaxies, the distribution of their mass, their stellar populations, and the gaseous content, it is very important that we can observed most of the extent of galaxies. Although this is routinely achieved with imaging techniques, it is much more difficult when observing with spectroscopic techniques. For example, the Sloan Digital Sky Survey (SDSS, that has revolutionized for the last one and a half decades the study of the universe) only provides one spectrum per galaxy, covering the central 3 arcseconds. The currently ongoing IFS surveys, like MaNGA or SAMI, provide spatially resolved spectroscopy, but only out to a fraction of the extent of the galaxies. Thus, when reporting conclusions from the scientific analysis, it is important to take into account the possible biases introduced by a limited spatial coverage of the properties. CALIFA gives the community a unique opportunity to understand the aperture and resolution effects, because it provides a balanced combination of galaxy coverage (out to more than 2.5 effective radii), and spatial resolution (~1 kpc). We have explored the effects of the dilution of the signal in different gas and stellar population properties (Mast et al., 2014), which will be useful in the interpretation of the data provided by IFS surveys of lower spatial resolution. On the other hand, we have proposed an new empirical aperture correction for the SDSS data (Iglesias-Páramo et al., 2013, 2014), that will allow to re-calibrate the satr formation rate and chemical abundances derived from SDSS.
5. Kinematic properties of the galaxies: the study of how stars and gas move (their kinematics) provides important clues about the structure and evolution of galaxies. With CALIFA we can measure the kinematics of the gas and of the stars in galaxies of all morphological types (this is the first IFS survey that allows this) with enough spectroscopic resolution to study: (i) the detailed 2D kinematic maps of the ionized gas (García-Lorenzo et al., 2014), (ii) the perturbations produced by interacting galaxies in different stages (Barrera-Ballesteros et al., 2014), (iii) the motions of stellar bars (their so-called pattern speeds) in spiral galaxies (Aguerri et al., 2014), (iv) the angular momentum of galaxies in previously unexplored ranges of morphology and ellipticity (Falcón-Barroso et al., 2014), (v) and the dynamical mass, to derive the spatially resolved Mass-to-Light relation (an important parameter of the structure of galaxies; Lyubenova et al., 2014).
6. Technical publications: the scientific results obtained from the analysis of observed data, particularly in legacy surveys like CALIFA, are robust and have a significant impact on the advance of our knowledge of the universe if they are based on sound and rigorous processes of sample definition, and data acquisition and calibration. Because CALIFA data are released to the community at large, we have invested many resources to make the data of the highest possible quality. These processes are described in a series of more technical publications, that include the survey presentation and details on the depth and quality of the data (Sánchez et al., 2012a), the first public Data Release (DR1, Husemann et al., 2013), new analysis techniques (HIIexplorer, Sánchez et al., 2012b; PyCOSMIC, Husemann et al., 2013), the CALIFA sample of galaxies definition and main general properties (Walcher et al., 2014), and the second public Data Release (García Benito et al., 2014).