arXiv subjects

William M. Baker

Publications and source records attributed to William M. Baker.

96 records · Page 6Linked to original sources

arXiv2023

Stellar mass, not dynamical mass nor gravitational potential, drives the mass-metallicity relationship

The widely known relation between stellar mass and gas metallicity (mass-metallicity relation, MZR) in galaxies is often ascribed to the higher capability of more massive systems to retain metals against the action of galactic outflows. In this scenario the stellar mass would simply be an indirect proxy of the dynamical mass or of the gravitational potential. We test this scenario by using a sample of more than one thousand star-forming galaxies from the MaNGA survey for which dynamical masses have been accurately determined. By using three different methods (average dispersion, Partial Correlation Coefficients, Random Forest) we unambiguously find that the gas metallicity depends primarily and fundamentally on the stellar mass. Once the dependence on stellar mass is taken into account, there is little or no dependence on either dynamical mass or gravitational potential (and, if anything, the metallicity dependence on the latter quantities is inverted). Our result indicates that the MZR is not caused by the retention of metals in more massive galaxies. The direct, fundamental dependence of metallicity on stellar mass suggests the much simpler scenario in which the MZR is just a consequence of the stellar mass being proportional to the integral of metals production in the galaxy.

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arXiv2023

Evolution in the orbital structure of quiescent galaxies from MAGPI, LEGA-C and SAMI surveys: direct evidence for merger-driven growth over the last 7 Gy

We present the first study of spatially integrated higher-order stellar kinematics over cosmic time. We use deep rest-frame optical spectroscopy of quiescent galaxies at redshifts z=0.05, 0.3 and 0.8 from the SAMI, MAGPI and LEGA-C surveys to measure the excess kurtosis $h_4$ of the stellar velocity distribution, the latter parametrised as a Gauss-Hermite series. Conservatively using a redshift-independent cut in stellar mass ($M_\star = 10^{11}\,{\rm M}_\odot$), and matching the stellar-mass distributions of our samples, we find 7 $σ$ evidence of $h_4$ increasing with cosmic time, from a median value of 0.019$\pm$0.002 at z=0.8 to 0.059$\pm$0.004 at z=0.06. Alternatively, we use a physically motivated sample selection, based on the mass distribution of the progenitors of local quiescent galaxies as inferred from numerical simulations; in this case, we find 10 $σ$ evidence. This evolution suggests that, over the last 7 Gyr, there has been a gradual decrease in the rotation-to-dispersion ratio and an increase in the radial anisotropy of the stellar velocity distribution, qualitatively consistent with accretion of gas-poor satellites. These findings demonstrate that massive galaxies continue to accrete mass and increase their dispersion support after becoming quiescent.

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arXiv2023

Different higher-order kinematics between star-forming and quiescent galaxies based on the SAMI, MAGPI and LEGA-C surveys

We present the first statistical study of spatially integrated non-Gaussian stellar kinematics spanning 7 Gyr in cosmic time. We use deep, rest-frame optical spectroscopy of massive galaxies (stellar mass $M_\star > 10^{10.5} {\rm M}_\odot$) at redshifts z = 0.05, 0.3 and 0.8 from the SAMI, MAGPI and LEGA-C surveys, to measure the excess kurtosis $h_4$ of the stellar velocity distribution, the latter parametrised as a Gauss-Hermite series. We find that at all redshifts where we have large enough samples, $h_4$ anti-correlates with the ratio between rotation and dispersion, highlighting the physical connection between these two kinematic observables. In addition, and independently from the anti-correlation with rotation-to-dispersion ratio, we also find a correlation between $h_4$ and $M_\star$, potentially connected to the assembly history of galaxies. In contrast, after controlling for mass, we find no evidence of independent correlation between $h_4$ and aperture velocity dispersion or galaxy size. These results hold for both star-forming and quiescent galaxies. For quiescent galaxies, $h_4$ also correlates with projected shape, even after controlling for the rotation-to-dispersion ratio. At any given redshift, star-forming galaxies have lower $h_4$ compared to quiescent galaxies, highlighting the link between kinematic structure and star-forming activity.

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arXiv2022

The metallicity's fundamental dependence on both local and global galactic quantities

We study the scaling relations between gas-phase metallicity, stellar mass surface density ($Σ_*$), star formation rate surface density ($Σ_{SFR}$), and molecular gas surface density ($Σ_{H_2}$) in local star-forming galaxies on scales of a kpc. We employ optical integral field spectroscopy from the MaNGA survey, and ALMA data for a subset of MaNGA galaxies. We use Partial Correlation Coefficients and Random Forest regression to determine the relative importance of local and global galactic properties in setting the gas-phase metallicity. We find that the local metallicity depends primarily on $Σ_*$ (the resolved mass-metallicity relation, rMZR), and has a secondary anti-correlation with $Σ_{SFR}$ (i.e. a spatially-resolved version of the 'Fundamental Metallicity Relation', rFMR). We find that $Σ_{H_2}$ is less important than $Σ_{SFR}$ in determining the local metallicity. This result indicates that gas accretion, resulting in local metallicity dilution and local boosting of star formation, is unlikely to be the primary origin of the rFMR. The local metallicity depends also on the global properties of galaxies. We find a strong dependence on the total stellar mass ($M_*$) and a weaker (inverse) dependence on the total SFR. The global metallicity scaling relations, therefore, do not simply stem out of their resolved counterparts; global properties and processes, such as the global gravitational potential well, galaxy-scale winds and global redistribution/mixing of metals, likely contribute to the local metallicity, in addition to local production and retention.

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arXiv2022

The Molecular-Gas Main Sequence and Schmidt-Kennicutt relation are fundamental, the Star-Forming Main Sequence is a (useful) byproduct

We investigate the relationship between the star formation rate (SFR), stellar mass ($M_*$) and molecular gas mass ($M_{H_2}$) for local star-forming galaxies. We further investigate these relationships for high-z (z=1-3) galaxies and for the hosts of a local sample of Active Galactic Nuclei (AGN). We explore which of these dependencies are intrinsic and which are an indirect by-product by employing partial correlation coefficients and random forest regression. We find that for local star-forming galaxies, high-z galaxies, and AGN host galaxies, the Schmidt-Kennicutt relation (SK, between $M_{H_2}$ and SFR), and the Molecular Gas Main Sequence (MGMS, between $M_{H_2}$ and $M_*$) are intrinsic primary relations, while the relationship between $M_*$ and SFR, i.e. the Star-Forming Main Sequence (SFMS), is an indirect by-product of the former two. Hence the Star-Forming Main Sequence is not a fundamental scaling relation for local or high-redshift galaxies. We find evidence for both the evolution of the MGMS and SK relation over cosmic time, where, at a given stellar mass, the higher the redshift, the greater the molecular gas mass and the star formation efficiency. We offer a parameterisation of both the MGMS and SK relation's evolution with redshift, showing how they combine to form the observed evolution of the SFMS. In addition, we find that the local AGN host galaxies follow an AGN-MGMS relation (as well as a AGN-SK relation), where the MGMS is offset to lower $M_{H_2}$ for a given $M_*$ compared to local SF galaxies.

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arXiv2022

The ALMaQUEST Survey IX: The nature of the resolved star forming main sequence

We investigate the nature of the scaling relations between the surface density of star formation rate ($Σ_{\rm SFR}$), stellar mass ($Σ_*$), and molecular gas mass ($Σ_{\rm H_2}$), aiming at distinguishing between the relations that are primary, i.e. more fundamental, and those which are instead an indirect by-product of the other relations. We use the ALMaQUEST survey and analyse the data by using both partial correlations and Random Forest regression techniques. We unambiguously find that the strongest intrinsic correlation is between $Σ_{\rm SFR}$ and $Σ_{\rm H_2}$ (i.e. the resolved Schmidt-Kennicutt relation), followed by the correlation between $Σ_{\rm H_2}$ and $Σ_*$ (resolved Molecular Gas Main Sequence, rMGMS). Once these two correlations are taken into account, we find that there is no evidence for any intrinsic correlation between $Σ_{\rm SFR}$ and $Σ_*$, implying that SFR is entirely driven by the amount of molecular gas, while its dependence on stellar mass (i.e. the resolved Star Forming Main Sequence, rSFMS) simply emerges as a consequence of the relationship between molecular gas and stellar mass.

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