The Southern Photometrical Local Universe Survey (S-PLUS): searching for metal-poor dwarf galaxies

TL;DR

This work develops a photometric pathway to identify metal-poor dwarf galaxies in the Local Universe using the S-PLUS 12-band system, followed by spectroscopic confirmation with GMOS-S. By applying color cuts and SED fitting, the authors assemble a sample of ~47–50 metal-poor dwarf candidates and confirm their low gas-phase metallicities with direct-method abundances in six galaxies, finding $12 + olog( ext{O/H})$ in the range $7.28$–$7.82$. Five of six show elevated N/O at fixed O/H, and several targets are outliers in both mass-metallicity and luminosity-metallicity relations, suggesting external gas inflow or dwarf-dwarf interactions as drivers of dilution. Morphological diagnostics reveal that about half the sample shows perturbed or merging features, supporting interaction-driven metal dilution theories. The results highlight the value of wide-field multi-band photometry for assembling metal-poor dwarf samples and motivate extending spectroscopic follow-up to build a statistical view of chemical evolution in low-mass galaxies.

Abstract

The metal content of a galaxy's interstellar medium reflects the interplay between different evolutionary processes such as feedback from massive stars and the accretion of gas from the intergalactic medium. Despite the expected abundance of low-luminosity galaxies, the low-mass and low-metallicity regime remains relatively understudied. Since the properties of their interstellar medium resemble those of early galaxies, identifying such objects in the Local Universe is crucial to understand the early stages of galaxy evolution. We used the DR3 catalog of the Southern Photometric Local Universe Survey (S-PLUS) to select low-metallicity dwarf galaxy candidates based on color selection criteria typical of metal-poor, star-forming, low-mass systems. The final sample contains approximately 50 candidates. Spectral energy distribution fitting of the 12 S-PLUS bands reveals that $\sim$ 60% of the candidates are best fit by models with low stellar metallicities. We obtained long-slit observations with the Gemini Multi-Object Spectrograph to follow-up a pilot sample and confirm whether these galaxies have low metallicities. We find oxygen abundances in the range $7.28<$ 12 + log(O/H) $< 7.82$ (4% to 13% of the solar value), confirming their metal-poor nature. Most targets are outliers in the mass-metallicity relation, i.e. they display a low metal content relative to their observed stellar masses. In some cases, perturbed optical morphologies might give evidence of dwarf-dwarf interactions or mergers. These results suggest that the low oxygen abundances may be associated with an external event causing the accretion of metal-poor gas, which dilutes the oxygen abundance in these systems.

Figures (15)

• Figure 1: Transmission curves of the 12 S-PLUS filters overlaid on the SDSS spectrum of one of the targets of this work (DR4_3_SPLUS-n03s22_0021537). The H$\alpha$ emission line falls within the wavelength range of the J0660 filter, illustrating the importance of the narrow-band filters in identifying star-forming dwarf galaxy candidates.
• Figure 2: Legacy $grz$ cutouts of the six metal-poor dwarf galaxy candidates selected in this work. Legacy images are deeper than S-PLUS allowing to infer more details about the galaxies morphology and structure. The cutout size is 55$^{\prime\prime} \times 55^{\prime\prime}$. See Table 1 for the definition of the ID numbers given at the top-left and bottom-left corners of each postage stamps.
• Figure 3: Example of SED fitting with CIGALE. Best-fit result for ID 1 ($\chi_r^2 = 0.7$, top) and ID 4 ($\chi_r^2 = 0.8$, bottom). The best-fit stellar metallicity is 1/30th and 1/20th the solar value, respectively. Diamonds represent the observed fluxes, while crosses and hexagons indicate the model fluxes for the wide-band and narrow-band filters, respectively.
• Figure 4: Top panels: Legacy images of the four dwarf galaxies observed with GMOS with the position of the slit overlaid. Central panels: S-PLUS $J0660$ continuum-subtracted images of the four targets, illustrating the location of the slit over the main star-forming regions. Bottom panels: 2D long-slit spectra of the four galaxies with the observed wavelength on the horizontal axis and the spatial position on the vertical axis. The image shows the H$\alpha$ and the [N ii] doublet emission from the different star forming knots included in the slits. The gray lines delimit the aperture chosen for the extraction of the 1D-spectra of Fig. \ref{['fig:specs']}.
• Figure 5: Gemini-S/GMOS spectra of DR4_3_SPLUS-s27s07_0011404 (ID 1, top) and DR4_3_SPLUS-n14s01_0028263 (ID 2, bottom) obtained with the B600 grating. The [O iii]$\lambda$4363 and the [N ii], [O ii] doublets are highlighted in the insets. The red lines show the best-fit model of the spectrum.