CONSTRAINING THE LOW-MASS SLOPE OF THE STAR FORMATION SEQUENCE AT 0.5 <z< 2.5

Abstract

We constrain the slope of the star formation rate (log Ψ) to stellar mass (log M ⋆ ) relation down to log(M ⋆ /M ⊙ ) = 8.4 (log(M ⋆ /M ⊙ ) = 9.2) at z = 0.5 (z = 2.5) with a mass-complete sample of 39,106 star-forming galaxies selected from the 3D-HST photometric catalogs, using deep photometry in the CANDELS fields. For the first time, we find that the slope is dependent on stellar mass, such that it is steeper at low masses (log Ψ ∝ log M ⋆ ) than at high masses (log Ψ ∝ (0.3 − 0.6) log M ⋆ ). These steeper low mass slopes are found for three different star formation indicators: the combination of the ultraviolet (UV) and infrared (IR), calibrated from a stacking analysis of Spitzer/MIPS 24µm imaging; β-corrected UV SFRs; and Hα SFRs. The normalization of the sequence evolves differently in distinct mass regimes as well: for galaxies less massive than log(M ⋆ /M ⊙ ) < 10 the specific SFR (Ψ/M ⋆ ) is observed to be roughly self-similar with Ψ/M ⋆ ∝ (1 + z) 1.9 , whereas more massive galaxies show a stronger evolution with Ψ/M ⋆ ∝ (1 + z) 2.2−3.5 for log(M ⋆ /M ⊙ ) = 10.2 − 11.2. The fact that we find a steep slope of the star formation sequence for the lower mass galaxies will help reconcile theoretical galaxy formation models with the observations.

THE SFR–M* RELATION AND EMPIRICAL STAR FORMATION HISTORIES FROM ZFOURGE AT 0.5 < z < 4*

“…In agreement with recent results, we find that SFRs are roughly proportional to stellar mass at low masses (10 10.2 M e ), but that this trend flattens at higher masses (see also Whitaker et al 2014;Lee et al 2015;Schreiber et al 2015;Tasca et al 2015). Furthermore, although the evolution of the SFR-M * relation is still predominantly in normalization, the slope at high masses (M *  10 10.2 M e ) is also changing.…”

“…This result is at odds with our findings for individually FIR-detected galaxies (see Figure 2) implying that galaxies with low L IR have different infrared SEDs than galaxies with high L IR . The middle panel shows a similar systematic trend for an external comparison of our UV+IR SFRs (24-160 μm) to those of 3D-HST (Whitaker et al 2014) which only utilize 24 μm photometry in the IR. Finally, the panel on the right shows the comparison of 24 μm only UV+IR star formation rates.…”

“…Papovich et al (2015) find similar results in their analysis of the progenitors of galaxies with present-day masses of the Milky Way and M31 galaxies. Finally, we have repeated this comparison using the SFR-M * parameterizations provided by Whitaker et al (2014) and Schreiber et al (2015) and in both cases we find similar disagreements. This suggests that the discrepancies between the star formation rates and mass evolution that were previously reported by Leja et al (2015) have not been completely resolved using the more up-to-date data sets.…”

“…Unfortunately however, imaging used to probe obscured star formation (typically far-IR and radio) rarely ever reach complementary depths. Thus, many studies over the past several years have turned to measuring SFRs from stacked data in order to compensate for this disparity (e.g., Dunne et al 2009;Rodighiero et al 2010;Karim et al 2011;Whitaker et al 2014;Schreiber et al 2015). However it is important to keep in mind that the interpretation of stacked results may be complicated by the fact that the intrinsic distribution of SFRs may not be unimodal or symmetric.…”

“…Evaluating completeness limits for Ψ UV+IR is complicated since the depth of the far-IR imaging in CDF-S is deeper than in COSMOS and UDS. Furthermore, the ratio of IR to UV flux (infrared excess: IRX ≡ L IR /L UV ) is strongly correlated with mass (e.g., Papovich et al 2006;Whitaker et al 2014), therefore, the completeness in Ψ UV+IR will also be a function of stellar mass. Thus, to provide a visual guide in Figure 3 we plot the 1σ MIPS 24 μm flux uncertainty converted to Ψ IR as horizontal dashed lines.…”