Re-assessing the stellar population scaling relations of the galaxies in the Local Universe

Abstract

Local galaxies follow scaling relations between mass and stellar population properties such as age and metallicity, which encode key information on their evolutionary histories. We revise these relations using the largest spectroscopic dataset from SDSS DR7 (0.005<z<0.22), improved Stellar Population Synthesis (SPS) models, aperture-corrections, and statistical weights to account for selection biases. In a Bayesian framework, we estimate stellar masses, mean ages, and metallicities by comparing spectral indices and photometry with composite SPS models. We adopt updated prescriptions for Star Formation Histories (SFH) and Chemical Enrichment Histories (CEH), while also testing different models and priors. We measure light-weighted ages for 354,977 galaxies (SNR>10) and metallicities for 89,852 galaxies (SNR>20), analyzing their dependence on stellar mass. Key findings include: i) A revised bimodal mass-age distribution, with a young sequence at low mass and an old sequence at high mass, partly overlapping in mass and transitioning at 10^10.8 solar masses. ii) A Mass-Metallicity Relation (MZR) shifted upwards by 0.2 dex relative to previous works. Aperture corrections lower masses, ages, and metallicities in a mass-dependent way, enhancing the young sequence and steepening the MZR. iii) Using Halpha-based SFRs, we found that while star-forming/young and quiescent/old correspondences generally hold, exceptions exist for many galaxies. Quiescent galaxies show a flatter, less scattered MZR than star-forming ones, with convergence at high mass. iv) SPS assumptions strongly affect our results, particularly SFHs and CEHs. These revised relations provide new benchmarks for galaxy evolution studies and simulations. Systematic uncertainties of 0.15 dex may arise from aperture biases and SPS modelling choices, highlighting the need for consistent assumptions when comparing observations and models.

Figures (15)

• Figure 1: Colour Magnitude Diagram (CMD) for the galaxy sample with $\mathrm{SNR}\geq10$. The top left panel displays the volume completeness (i.e. the inverse of the volume-weights). The top right panel displays the completeness fractions (i.e. the inverse of the completeness-weights). The bottom left panel displays the statistical weights obtained combining the volume and completeness statistical weights, multiplied by the factor $N=4.89\times10^{-9}~h^3\,\mathrm{Mpc}^{-3}$, to convert number counts to the number volume-density of galaxies. The bottom right panel displays the number density of galaxies in the CMD applying the completeness corrections. The green contours enclose $16\%$, $50\%$, $84\%$, and $97.5\%$ of the galaxy sample, and have been obtained on the distribution smoothed with a Gaussian KDE. Similar plots are applicable to the $\mathrm{SNR}\geq20$ sample.
• Figure 2: Distribution of the galaxies in the H$\delta_{\rm A}$+H$\gamma_{\rm A}$ vs $\rm D4000_n$ and H$\delta_{\rm A}$+H$\gamma_{\rm A}$ vs $\rm [MgFe]^\prime$ planes before and after the implementation of the statistical weights and the aperture corrections. The left panels display the distribution of the original data, i.e. simple number counts for uncorrected index values. The central panels display the distributions of galaxies weighted by the volume and completeness statistical corrections, for uncorrected index values. The right panels display the distributions obtained implementing both the statistical weights and the corrections for aperture effects. The error bars correspond to the median uncertainties calculated in four bins of $\rm D4000_n$ and $\rm [MgFe]^\prime$, for samples with $\mathrm{SNR}\geq20$ (red) and $\mathrm{SNR}\geq10$ (black, only top panels).
• Figure 3: Number density of galaxies for the SDSS DR7 in the mass-age and mass-metallicity planes, obtained using both the volume and completeness statistical weights. The left and central panels display the distributions obtained with light-weighted mean ages and metallicities, without the implementation of aperture corrections (left panels) and implementing aperture corrections (central panels). The right panels display the distributions obtained with mass-weighted mean ages and metallicities for the aperture-corrected sample. In the mass-age planes, the coloured black, green, and fuchsia lines identify the density levels enclosing $16\%$, $50\%$, $84\%$, and $97.5\%$, obtained smoothing the corresponding distribution using a Gaussian KDE. The red and blue lines, overplotted on the top of the mass-age distributions in the left and central panels, for uncorrected and aperture-corrected estimates, respectively, identify the divisions between young and old galaxies as defined in Sec. \ref{['subsec:Transition_mass']}. In the mass-metallicity planes, the coloured black and green solid and dashed lines identify the median and $16^\mathrm{th}$ and $84^\mathrm{th}$ percentiles of the corresponding distributions in bins of mass. The red lines overplotted on top of the mass-metallicity distribution identify the median relation from Gallazzi2005. The dotted horizontal lines identifies the solar metallicity $Z_\odot\equiv0.02$.
• Figure 4: Number density of galaxies for the SDSS DR7 in the mass-age and mass-metallicity planes, obtained using both the volume and completeness statistical corrections. Following the same colour-code of Fig. \ref{['fig:Scaling_relations_comparison']}, the left panels display the distributions obtained without aperture corrections, and the right panels display the distributions obtained from aperture-corrected indices. The red arrows in the left panels represent the median shift of galaxies in each bin due to the implementation of aperture corrections. The dotted horizontal lines identify the solar metallicity $Z_\odot\equiv0.02$.
• Figure 5: Mass-age (top row) and mass-metallicity (bottom row) distributions of galaxies with $\mathrm{SNR}\geq20$, colour-coded by the uncertainties $\delta$ in mass (left panels), light-weighted mean age (central panels), and light-weighted mean metallicity (right panels) estimates. The green lines identify the number density levels enclosing $16\%$, $50\%$, $84\%$, and $97.5\%$ of the statistically-weighted distributions. The dotted horizontal lines identifies the solar metallicity $Z_\odot\equiv0.02$.