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Register a new userThe ALMaQUEST Survey: V. The non-universality of kpc-scale star formation relations and the factors that drive them
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Using a sample of ~15,000 kpc-scale star-forming spaxels in 28 galaxies drawn from the ALMA-MaNGA QUEnching and STar formation (ALMaQUEST) survey, we investigate the galaxy-to-galaxy variation of the `resolved Schmidt-Kennicutt relation (rSK; Sigma_H2 - Sigma_SFR), the `resolved star forming main sequence (rSFMS; Sigma_* - Sigma_SFR) and the `resolved molecular gas main sequence (rMGMS; Sigma_* - Sigma_H2). The rSK relation, rSFMS and rMGMS all show significant galaxy-to-galaxy variation in both shape and normalization, indicating that none of these relations is universal between galaxies. The rSFMS shows the largest galaxy-to-galaxy variation and the rMGMS the least. By defining an `offset from the average relations, we compute a Delta_rSK, Delta_rSFMS, Delta_rMGMS for each galaxy, to investigate correlations with global properties. We find the following correlations with at least 2 sigma significance: the rSK is lower (i.e. lower star formation efficiency) in galaxies with higher M_*, larger Sersic index and lower specific SFR (sSFR); the rSFMS is lower (i.e. lower sSFR) in galaxies with higher M_* and larger Sersic index; the rMGMS is lower (i.e. lower gas fraction) in galaxies with lower sSFR. In the ensemble of all 15,000 data points, the rSK relation and rMGMS show equally tight scatters and strong correlation coefficients, compared with a larger scatter and weaker correlation in the rSFMS. Moreover, whilst there is no correlation between Delta_rSK and Delta_rMGMS in the sample, the offset of a galaxys rSFMS does correlate with both of the other two offsets. Our results therefore indicate that the rSK and rMGMS are independent relations, whereas the rSFMS is a result of their combination.
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The ALMaQUEST (ALMA-MaNGA QUEnching and STar formation) survey is a program with spatially-resolved $^{12}$CO(1-0) measurements obtained with the Atacama Large Millimeter Array (ALMA) for 46 galaxies selected from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) DR15 optical integral-field spectroscopic survey. The aim of the ALMaQUEST survey is to investigate the dependence of star formation activity on the cold molecular gas content at kpc scales in nearby galaxies. The sample consists of galaxies spanning a wide range in specific star formation rate (sSFR), including starburst (SB), main-sequence (MS), and green valley (GV) galaxies. In this paper, we present the sample selection and characteristics of the ALMA observations, and showcase some of the key results enabled by the combination of spatially-matched stellar populations and gas measurements. Considering the global (aperture-matched) stellar mass, molecular gas mass, and star formation rate of the sample, we find that the sSFR depends on both the star formation efficiency (SFE) and the molecular gas fraction ($f_{rm H_{2}}$), although the correlation with the latter is slightly weaker. Furthermore, the dependence of sSFR on the molecular gas content (SFE or $f_{rm H_{2}}$) is stronger than that on either the atomic gas fraction or the molecular-to-atomic gas fraction, albeit with the small HI sample size. On kpc scales, the variations in both SFE and $f_{rm H_{2}}$ within individual galaxies can be as large as 1-2 dex thereby demonstrating that the availability of spatially-resolved observations is essential to understand the details of both star formation and quenching processes.
We present a new characterization of the relations between star-formation rate, stellar mass and molecular gas mass surface densities at different spatial scales across galaxies (from galaxy wide to kpc-scales). To do so we make use of the largest sample combining spatially-resolved spectroscopic information with CO observations, provided by the EDGE-CALIFA survey, together with new single dish CO observations obtained by APEX. We show that those relations are the same at the different explored scales, sharing the same distributions for the explored data, with similar slope, intercept and scatter (when characterized by a simple power-law). From this analysis, we propose that these relations are the projection of a single relation between the three properties that follows a distribution well described by a line in the three-dimension parameter space. Finally, we show that observed secondary relations between the residuals and the considered parameters are fully explained by the correlation between the uncertainties, and therefore have no physical origin. We discuss these results in the context of the hypothesis of self-regulation of the star-formation process.
The origin of the star forming main sequence ( i.e., the relation between star formation rate and stellar mass, globally or on kpc-scales; hereafter SFMS) remains a hotly debated topic in galaxy evolution. Using the ALMA-MaNGA QUEnching and STar formation (ALMaQUEST) survey, we show that for star forming spaxels in the main sequence galaxies, the three local quantities, star-formation rate surface density (sigsfr), stellar mass surface density (sigsm), and the h2~mass surface density (sigh2), are strongly correlated with one another and form a 3D linear (in log) relation with dispersion. In addition to the two well known scaling relations, the resolved SFMS (sigsfr~ vs. sigsm) and the Schmidt-Kennicutt relation (sigsfr~ vs. sigh2; SK relation), there is a third scaling relation between sigh2~ and sigsm, which we refer to as the `molecular gas main sequence (MGMS). The latter indicates that either the local gas mass traces the gravitational potential set by the local stellar mass or both quantities follow the underlying total mass distributions. The scatter of the resolved SFMS ($sigma sim 0.25$ dex) is the largest compared to those of the SK and MGMS relations ($sigma sim$ 0.2 dex). A Pearson correlation test also indicates that the SK and MGMS relations are more strongly correlated than the resolved SFMS. Our result suggests a scenario in which the resolved SFMS is the least physically fundamental and is the consequence of the combination of the SK and the MGMS relations.
Using a sample of 11,478 spaxels in 34 galaxies with molecular gas, star formation and stellar maps taken from the ALMA-MaNGA QUEnching and STar formation (ALMaQUEST) survey, we investigate the parameters that correlate with variations in star formation rates on kpc scales. We use a combination of correlation statistics and an artificial neural network to quantify the parameters that drive both the absolute star formation rate surface density (Sigma_SFR), as well as its scatter around the resolved star forming main sequence (Delta Sigma_SFR). We find that Sigma_SFR is primarily regulated by molecular gas surface density (Sigma_H2) with a secondary dependence on stellar mass surface density (Sigma_*), as expected from an `extended Kennicutt-Schmidt relation. However, Delta Sigma_SFR is driven primarily by changes in star formation efficiency (SFE), with variations in gas fraction playing a secondary role. Taken together, our results demonstrate that whilst the absolute rate of star formation is primarily set by the amount of molecular gas, the variation of star formation rate above and below the resolved star forming main sequence (on kpc scales) is primarily due to changes in SFE.
In the era of large spectroscopic surveys, massive databases of high-quality spectra provide tools to outline a new picture of our Galaxy. In this framework, an important piece of information is provided by our ability to infer stellar ages. We aim to provide empirical relations between stellar ages and abundance ratios for a sample of solar-like stars. We investigate the dependence on metallicity, and we apply our relations to Gaia-ESO samples of open clusters and field stars. We analyse high-resolution and high-S/N HARPS spectra of a sample of solar-like stars to obtain precise determinations of their atmospheric parameters and abundances through differential spectral analysis and age through isochrone fitting. We investigate the relations between ages and abundance ratios. For the abundance ratios with a steeper dependence on age, we perform multivariate linear regressions, including the dependence on metallicity. We apply our relations to a sample of open clusters located in 4<R$_{GC}$<16 kpc. Using them, we are able to recover the literature ages only for clusters located at R$_{GC}$>7 kpc. In these clusters, the content of s-elements is lower than expected from chemical evolution models, and consequently the [s/$alpha$] are lower than in clusters of the same age located in the solar neighbourhood. With our chemical evolution model and a set of empirical yields, we suggest that a strong dependence on the star formation history and metallicity-dependent yields of s-elements can substantially modify the slope of the [s/$alpha$]-[Fe/H]-age relation in different regions of the Galaxy. Our results point towards a non-universal relation [s/$alpha$]-[Fe/H]-age, indicating the existence of relations at different R$_{GC}$ or for different star formation history. A better understanding of the s-process at high metallicity is necessary to fully understand the origin of these variations.
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