SDSS-IV MaNGA: Star Formation Cessation in Low-redshift Galaxies. I. Dependence on Stellar Mass and Structural Properties

Abstract

We investigate radial gradients in the recent star formation history (SFH) of low-redshift galaxies using a large sample of 1917 galaxies with 0.01 < z < 0.14 and integral-field spectroscopy from the ongoing Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey. For each galaxy, we obtain two-dimensional maps and radial profiles for three spectroscopically-measured parameters that are sensitive to the recent SFH: D n (4000) (the 4000Å break), EW(Hδ A ) (equivalent width of the Hδ absorption line), and EW(Hα) (equivalent width of the Hα emission line). We find the majority of the spaxels in these galaxies are consistent with models with continuously declining star formation rate, indicating that starbursts occur rarely in local galaxies with regular morphologies. We classify the galaxies into three classes: fully star-forming (SF), partly quenched (PQ) and totally quenched (TQ), according to the fraction of quenched area within 1.5 times the effective radius, f Q (1.5R e ). We find that galaxies less massive than a stellar mass of 10 10 M ⊙ present at most weak radial gradients in all the diagnostic parameters. In contrast, massive galaxies with stellar mass above 10 10 M ⊙ present significant gradients in all the three diagnostic parameters if classified as SF or PQ, but show weak gradients in D n (4000) and EW(Hδ A ) and no gradients in EW(Hα) if in the TQ class. This implies the existence of a critical stellar mass, at ∼ 10 10 M ⊙ , above which the star formation in a galaxy gets shutdown from the inside out. Galaxies tend to evolve synchronously from inner to outer regions before their mass reaches the critical value. We have further divided the sample at fixed mass by both bulge-to-total luminosity ratio and morphological type, finding that our conclusions hold regardless of these factors: it appears that the presence of a central dense object like a bulge is not a driving parameter, but rather a byproduct of the star formation cessation process.

On the Elevation and Suppression of Star Formation within Galaxies

“…They further pointed out that the sSFR suppression is not simply due to the contribution of a massive bulge. Consistent with this, Wang et al (2018b) found that massive SF galaxies, with or without significant bulges, show strong gradients of sSFR, as indicated by the 4000Å break and Hα equivalent width. This suggested that the presence of a central dense object is not a main driving parameter of quenching, and that an inside-out cessation of star formation (distinct from the inside-out assembly picture) might be indicated.…”

The Variability of the Star Formation Rate in Galaxies. I. Star Formation Histories Traced by EW(Hα) and EW(HδA)

“…The three diagnostic parameters have long been used to indicate the recent SFHs on different timescales (e.g. Worthey & Ottaviani 1997;Balogh et al 1999;Kauffmann et al 2003;Li et al 2015;Wang et al 2017Wang et al , 2018. In SF galaxies, the Hα emission mainly comes from the recombination of gas ionized by photons from extremely massive stars (>15M ⊙ ), which is therefore expected to trace the SFHs within the lifetime of these massive stars (∼5 Myr).…”

“…The Hα emission is a good tracer of the SFR averaged within the last 5 Myr, the EW(Hδ A ) traces the star formation within the last roughly ∼1 Gyr, and the 4000Å break is sensitive to the lightweighted stellar age within 2 Gyr (e.g. Balogh et al 1999;Kauffmann et al 2003;Li et al 2015;Wang et al 2017Wang et al , 2018. These three parameters can be directly measured from galaxy spectra, and each being measured at a single wavelength they are all, in principle, insensitive to dust attenuation, although dust effects can still enter if different components of the system suffer different extinctions (this is explored in Section 2.6 below).…”

The stellar host in star-forming low-mass galaxies: Evidence for two classes

“…The different ways of selecting these samples, and the physical characteristics of the transition-type galaxies can provide useful clues to place the results of the present paper in a broader context. There is a variety of methods for selecting transition galaxies presented in the literature: Pandya et al (2017) selected galaxies below the the main sequence of star formation, Wang et al (2017) identified SFGs in MaNGA with larger 4000 Å break at 1.5R e than in the center, and Wang et al (2018) In all those studies where the Sérsic index or the radius were measured, the transition samples presented intermediate values between star-forming and quiescent galaxies in both parameters (in the Wang et al (2018) case, the radius is smaller than in other SFGs by design). In some cases Pawlik et al 2018;Maltby et al 2018) the color of the galaxies was measured and presented intermediate values as well.…”

“…The exact nature of the compaction, i.e. whether it is a physical contraction with radial migration of the stars, or a relative accumulation of mass in the inner regions compared to the outskirts (Wang et al 2018) is also a matter of debate.…”