Spatially-resolved star formation histories of CALIFA galaxies

Abstract

This paper presents the spatially resolved star formation history (SFH) of nearby galaxies with the aim of furthering our understanding of the different processes involved in the formation and evolution of galaxies. To this end, we apply the fossil record method of stellar population synthesis to a rich and diverse data set of 436 galaxies observed with integral field spectroscopy in the CALIFA survey. The sample covers a wide range of Hubble types, with stellar masses ranging from M ∼ 10 9 to 7 × 10 11 M . Spectral synthesis techniques are applied to the datacubes to retrieve the spatially resolved time evolution of the star formation rate (SFR), its intensity (Σ SFR ), and other descriptors of the 2D-SFH in seven bins of galaxy morphology (E, S0, Sa, Sb, Sbc, Sc, and Sd), and five bins of stellar mass. Our main results are: (a) Galaxies form very fast independently of their current stellar mass, with the peak of star formation at high redshift (z > 2). Subsequent star formation is driven by M and morphology, with less massive and later type spirals showing more prolonged periods of star formation. (b) At any epoch in the past the SFR is proportional to M , with most massive galaxies having the highest absolute (but lowest specific) SFRs. (c) While nowadays Σ SFR is similar for all spirals, and significantly lower in early type galaxies (ETG), in the past Σ SFR scales well with morphology. The central regions of today's ETGs are where Σ SFR reached the highest values (> 10 3 M Gyr −1 pc −2 ), similar to those measured in high redshift star forming galaxies. (d) The evolution of Σ SFR in Sbc systems matches that of models for Milky-Way-like galaxies, suggesting that the formation of a thick disk may be a common phase in spirals at early epochs. (e) The SFR and Σ SFR in outer regions of E's and S0's show that they have undergone an extended phase of growth in mass between z = 2 and 0.4. The mass assembled in this phase is in agreement with the two-phase scenario proposed for the formation of ETG. (f) Evidence of an early and fast quenching is found only in the most massive (M > 2 × 10 11 M ) E galaxies of the sample, but not in spirals of similar mass, suggesting that halo-quenching is not the main mechanism for the shut down of star formation in galaxies. Less massive E and disk galaxies show more extended SFHs and a slow quenching. (g) Evidence of fast quenching is also found in the nuclei of ETG and early spirals, with SFR and Σ SFR indicating that they can be the relic of the "red nuggets" detected at high redshift.

Intermediate-mass Early-type Disk Galaxies in the Virgo Cluster. II. Near-Infrared Spectra and Evidence for Differences in Evolution* † ‡ † ‡

“…Such a radial age gradient is in the opposite sense to that seen in the vast majority of nearby isolated gas-rich disk galaxies (e.g. Gonzalez Delgado et al 2017). While the age gradient in late-type disks is usually attributed to inside-out galaxy formation, the radial age distribution can be altered by secular (e.g.…”

“…mon in galaxy disks (e.g. Gonzalez Delgado et al 2017). In addition to gradients within disks, different structural components in each galaxy are sampled along the F2 slit, and these likely have SFHs that are distinct from each other.…”

“…A caveat is that while the identification of epochs with elevated SFRs is of obvious interest, such events may not always be due to cluster-driven evolution. For example, the SFRs in the disks of 'typical' spiral galaxies experienced a rejuventation ∼ 4 Gyr in the past (Gonzalez-Delgado et al 2017). If the galaxies entered the Virgo environment since that time then evidence for an uptick in the SFR ∼ 4 Gyr ago might then be a relic of their evolution prior to entering the cluster.…”

The submillimetre view of massive clusters at z ∼ 0.8–1.6

“…These massive galaxies typically have little-to-no ongoing star formation, display spheroidal "early-type" morphologies and contain old, metal-rich stellar populations. Detailed "archaeological" studies of the star-formation histories of luminous ellipticals ( L * ) indicate most of their stars were formed 8-11 Gyr ago at z > 1, through a series of bursts of star formation (e.g., Bower et al 1992; Thomas et al 2005;Citro et al 2016;González Delgado et al 2017, see also Johnston et al 2014;Cooke et al 2015). More massive galaxies have been found to have older stellar populations (e.g., Nelan et al 2005), and ellipticals within clusters have older stellar ages than those residing in the field (e.g., Rettura et al 2011).…”

Learning the relationship between galaxies spectra and their star formation histories using convolutional neural networks and cosmological simulations

“…Such objects are also mostly in the local universe, restricting any predictions to this period. However, with ever increasing samples locally, including from from integral field unit (IFU) spectrographs (Bundy et al 2015;González Delgado et al 2017), it may soon be possible to train a machine on high resolution observational data in order to predict the SFH of galaxies with only unresolved data on a larger number of objects.…”