[Paper Review] Star formation along the Hubble sequence: Radial structure of the star formation of CALIFA galaxies

R. M. González Delgado, R. Cid Fernandes+22 more|arXiv (Cornell University)|Mar 2, 2016

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy108 references80 citations

TL;DR

This study uses integral field spectroscopy from the CALIFA survey to map the radial star formation rate (SFR) across 416 galaxies spanning the Hubble sequence. It reveals that SFR surface density declines radially with minimal variation among spirals, confirming the main sequence of star formation as a nearly constant SFR per unit area, while early-type galaxies (E/S0) are quenched and star formation is dominated by disks of late-type spirals (Sbc, Sc, Sd), with the local SFR density at 0.0105 M⊙ yr⁻¹ Mpc⁻³.

ABSTRACT

The aim of this paper is to characterize the radial structure of the star formation rate (SFR) in galaxies in the nearby Universe as represented by the CALIFA survey. The sample under study contains 416 galaxies observed with IFS, covering a wide range of Hubble types and stellar masses. Spectral synthesis techniques are applied to obtain radial profiles of the intensity of the star formation rate in the recent past, and the local sSFR. To emphasize the behavior of these properties for galaxies that are on and off the main sequence of star formation (MSSF) we stack the individual radial profiles in bins of galaxy morphology and stellar masses. Our main results are: a) The intensity of SFR shows declining profiles that exhibit very little differences between spirals. The dispersion between the profiles is significantly smaller in late type spirals. This confirms that the MSSF is a sequence of galaxies with nearly constant intensity of SFR b) sSFR values scale with Hubble type and increase radially outwards, with a steeper slope in the inner 1 HLR. This behavior suggests that galaxies are quenched inside-out, and that this process is faster in the central, bulge-dominated part than in the disks. c) As a whole, and at all radii, E and S0 are off the MSSF. d) Applying the volume-corrections for the CALIFA sample, we obtain a density of star formation in the local Universe of 0.0105 Msun/yr/Mpc^{-3}. Most of the star formation is occurring in the disks of spirals. e) The volume averaged birthrate parameter, b'=0.39, suggests that the present day Universe is forming stars at 1/3 of its past average rate. E, S0, and the bulge of early type spirals contribute little to the recent SFR of the Universe, which is dominated by the disks of later spirals. f) There is a tight relation between the intensity of the SFR and stellar mass, defining a local MSSF relation with a logarithmic slope of 0.8.

Motivation & Objective

• To characterize the radial structure of star formation rate (SFR) across galaxies in the Hubble sequence using spatially resolved data.
• To determine how SFR intensity and specific SFR (sSFR) vary with Hubble type and radius, especially in relation to the main sequence of star formation (MSSF).
• To quantify the contribution of different morphological types (E, S0, spirals) to the total star formation rate density in the local Universe.
• To assess the role of local (e.g., stellar mass surface density) vs. global (e.g., spheroid formation) processes in regulating star formation.
• To validate the CALIFA survey's volume corrections by comparing derived SFR density with independent estimates.

Proposed method

• Utilized 2D datacubes from the CALIFA integral field spectroscopy survey covering 416 galaxies with diverse Hubble types and stellar masses (10⁹ to 7×10¹¹ M⊙).
• Applied spectral synthesis techniques to derive 2D maps and radial profiles of SFR surface density (Σ_SFR) and specific SFR (sSFR = Σ_SFR / μ⋆).
• Stacked radial profiles in seven morphological bins (E, S0, Sa, Sb, Sbc, Sc, Sd) and multiple stellar mass bins to reveal systematic trends.
• Applied volume corrections to the CALIFA sample to estimate the local star formation rate density (ρ_SFR) in the Universe.
• Computed the volume-averaged birthrate parameter b′ = SFR / ⟨SFR⟩_lifetime to assess current SFR relative to past average rates.
• Analyzed the local MSSF relation between Σ_SFR and μ⋆, quantifying its slope and scatter as a function of Hubble type.

Experimental results

Research questions

• RQ1 How does the radial profile of star formation rate surface density (Σ_SFR) vary across different Hubble types?
• RQ2 To what extent do the specific SFR (sSFR) and Σ_SFR follow a radial trend, and what does this imply about inside-out quenching?
• RQ3 What is the contribution of early-type (E/S0) and late-type (Sbc, Sc, Sd) galaxies to the total local star formation rate density?
• RQ4 How does the local MSSF relation (Σ_SFR vs. μ⋆) compare to the global MSSF (SFR vs. M⋆), and what does its slope and scatter reveal about SFR regulation?
• RQ5 To what extent do global processes (e.g., spheroid formation) versus local processes (e.g., gas density) govern the quenching of star formation?

Key findings

• The SFR surface density (Σ_SFR) declines radially with a value at 1 half-light radius (HLR) of ~20 M⊙ Gyr⁻¹ pc⁻², showing little variation across spiral types, confirming the MSSF as a nearly constant SFR per unit area.
• sSFR increases radially outward, with steeper gradients in the inner 1 HLR, indicating inside-out quenching, faster in bulge-dominated regions than in disks.
• E and S0 galaxies are significantly off the MSSF, with SFRs much lower than spirals of the same mass, confirming their quiescent nature.
• The local SFR density is ρ_SFR = (0.0105 ± 0.0008) M⊙ yr⁻¹ Mpc⁻³, in excellent agreement with independent estimates, validating CALIFA’s volume corrections.
• The volume-averaged birthrate parameter is b′ = 0.39 ± 0.03, indicating that the current SFR is about one-third of the long-term average, with disks of Sbc, Sc, and Sd galaxies dominating the present-day star formation.
• A tight local MSSF relation exists between Σ_SFR and μ⋆ with a logarithmic slope of 0.8, and scatter is primarily driven by Hubble type, indicating local processes (e.g., gas density) regulate SFR, while global processes (e.g., spheroid formation) control quenching.