A characteristic oxygen abundance gradient in galaxy disks unveiled with CALIFA

Abstract: We present the largest and most homogeneous catalog of HII regions and associations compiled so far. The catalog comprises more than 7000 ionized regions, extracted from 306 galaxies observed by the CALIFA survey. We describe the procedures used to detect, select, and analyse the spectroscopic properties of these ionized regions. In the current study we focus on the characterization of the radial gradient of the oxygen abundance in the ionized gas, based on the study of the deprojected distribution of HII regions. We found that all galaxies without clear evidence of an interaction present a common gradient in the oxygen abundance, with a characteristic slope of alpha = -0.1 dex/re between 0.3 and 2 disk effective radii, and a scatter compatible with random fluctuations around this value, when the gradient is normalized to the disk effective radius. The slope is independent of morphology, incidence of bars, absolute magnitude or mass. Only those galaxies with evidence of interactions and/or clear merging systems present a significant shallower gradient, consistent with previous results. The majority of the 94 galaxies with H ii regions detected beyond 2 disk effective radii present a flattening in the oxygen abundance. The flattening is statistically significant. We cannot provide with a conclusive answer regarding the origin of this flattening. However, our results indicate that its origin is most probably related to the secular evolution of galaxies. Finally, we find a drop/truncation of the oxygen abundance in the inner regions for 26 of the galaxies. All of them are non-interacting, mostly unbarred, Sb/Sbc galaxies. This feature is associated with a central star-forming ring, which suggests that both features are produced by radial gas flows induced by resonance processes.

• The paper demonstrates a universal oxygen abundance gradient of -0.1 dex/re in non-interacting galaxy disks, highlighting common evolutionary patterns.
• The study employs spectral analysis on over 7000 HII regions from 306 galaxies to derive accurate radial oxygen gradients using empirical calibrators.
• Results indicate that galaxy morphology, bars, and mass exert minimal influence on the gradient, while interactions and secular processes cause significant deviations.

Overview of the Study on Oxygen Abundance Gradients in Galaxy Disks

The paper investigates the oxygen abundance gradients in galaxy disks through an extensive catalog of \ion{H}{ii} regions, utilizing the Calar Alto Legacy Integral Field Area (CALIFA) survey. The authors analyze more than 7000 ionized regions from 306 galaxies to determine radial oxygen abundance gradients in ions like \ion{H}{ii}, focusing on the galaxy disk's deprojected distribution. A significant finding is the observation of a common oxygen abundance gradient in non-interacting galaxies. This uniform gradient possesses a characteristic slope of $\alpha_{O/H} = -0.1$ dex/$r_e$ observed between 0.3 to 2 effective radii (r_e) of the disk, showing a scatter aligned with random fluctuations. This gradient is noted to be largely independent of morphological classification, presence of bars, or a galaxy’s absolute magnitude or mass.

Methodology

The study employed spectral data analysis from CALIFA to detect, select, and characterize \ion{H}{ii} regions within galaxy disks. The abundance gradient characterization was conducted by applying empirical calibrators to derive oxygen abundances from the obtained spectral line ratios and ensuing regression analyses on the deprojected radial distances from the measured \ion{H}{ii} regions.

Key Findings

1. Uniform Gradient in Non-Interacting Galaxies: The consistent slope discussed signifies an apparent universal trait among disk galaxies not exhibiting significant interactions or mergers. This contradicts previous claims that abundance gradients are strongly dependent on galaxy morphology or presence of bars.
2. Effects of Interactions: Galaxies engaged in interactions generally exhibit shallower abundance gradients, aligning with prior evidence pointing to flattening effects caused by mergers or interactions, which potentially catalyze a mix of chemical elements throughout the galaxy disk.
3. Abundance Flattening in Outer Regions: In a majority of galaxies analyzed, a flattening of oxygen abundance was observed beyond 2 disk effective radii. The cause of this flattening may be attributed to secular evolutionary processes of the galaxies, potentially involving radial mixing or inflows and outflows of intergalactic matter.
4. Central Drop of Oxygen Abundances: In approximately 26 galaxies, a decrease or truncation in oxygen abundance was noticed within central regions—a phenomenon linked to distinct star-forming rings likely induced by gas movements caused by resonance patterns in the disk.

Implications and Future Directions

This study implies a more simplified picture of galaxy disk evolutionary processes pertaining to chemical compositions than previously assumed, indicating a primarily uniform nature in disk galaxy evolution as evident from the universal slope of abundance gradients. Future work could further explore the underlying factors of this uniformity and its deviation in interacting or peculiar systems to broaden the understanding of galaxy formation and development. Enhanced simulations incorporating both cosmological and environmental factors influencing chemical evolution over time will be valuable to contextualize observational data within a refined theoretical framework.