[Paper Review] Non-solar abundance ratios trends of dEs in Fornax Cluster using newly defined high resolution indices

S. Sen, R. F. Peletier+1 more|arXiv (Cornell University)|Jun 30, 2022

Galaxies: Formation, Evolution, Phenomena86 references4 citations

TL;DR

This study introduces a new set of high-resolution Lick-style line indices to measure stellar abundance ratios in eight massive dwarf elliptical (dE) galaxies in the Fornax Cluster using integral field spectroscopy at R ~ 5000. It finds that these dEs exhibit Na/Fe ratios significantly below solar-scaled values—consistent with slow, disk-like formation histories—marking the first such abundance proxy measurements for unresolved dEs outside the Local Group.

ABSTRACT

We perform a detailed study of the stellar populations in a sample of massive Fornax dwarf galaxies using a set of newly defined line indices. Using data from the Integral field spectroscopic data, we study abundance ratios of eight dEs with stellar mass ranging from 10$^8$ to 10$^{9.5}$ M$_\odot$ in the Fornax cluster. We present the definitions of a new set of high-resolution Lick-style indices to be used for stellar population studies of unresolved small stellar systems. We identify 23 absorption features and continuum regions, mainly dominated by 12 elements (Na, Ca, Sc, Ti, V, Cr, Mn, Fe, Ni, Y, Ba and Nd) in the wavelength range 4700 - 5400 {A} and characterise them as a function of age, metallicity and alpha element abundance ratios. We analyse eight dEs and interpret the line strengths, measured in our new high resolution system of indices, with the aid of stellar population models with high enough spectral resolution. We obtain abundance ratio proxies for a number of elements that have never been studied before for dwarf ellipticals outside the Local Group. These proxies represent relative deviations from predicted index-strengths of base stellar population models built-up following the abundance pattern of The Galaxy. The abundance proxy trend results are compared to abundance ratios from resolved stars in the Local Group, and indices from integrated light of larger early-type galaxies. We find that all our dwarfs show a pattern of abundance ratios consistent with the disk of the Milky Way, indicative of slow formation in comparison to their high mass counterparts.

Motivation & Objective

• To develop a new system of high-resolution line indices for precise stellar population analysis in unresolved, low-mass stellar systems.
• To measure abundance ratios of 11 elements (e.g., Na, Ca, Ti, Cr, Mn, Fe, Ni, Y, Ba, Nd) in massive dEs in the Fornax Cluster.
• To investigate whether dEs formed via slow, disk-like star formation by comparing their abundance patterns to those of Milky Way disk stars and massive early-type galaxies.
• To extend abundance ratio studies beyond the Local Group by analyzing unresolved dEs in a distant cluster using high-resolution spectral data.
• To provide abundance ratio proxies as relative deviations from Milky Way-based model predictions, enabling cross-comparison across galaxy populations.

Proposed method

• Defined 23 high-resolution absorption features and continuum regions in the 4700–5400 Å range, dominated by 12 elements, using high-resolution spectral data.
• Measured line strengths using a newly designed system of Lick-style indices tailored for low signal-to-noise, unresolved systems with high spectral resolution (R ~ 5000).
• Compared observed index strengths to predictions from the PEGASE.HR stellar population synthesis models with high spectral resolution.
• Calculated abundance ratio proxies as relative deviations from model predictions based on the Milky Way’s abundance pattern.
• Used velocity dispersions (40 km s⁻¹) and low-resolution data to validate the robustness of the high-resolution index system.
• Traced abundance trends as a function of galaxy mass via velocity dispersion, comparing results to massive early-type galaxies and resolved stars in the Local Group.

Experimental results

Research questions

• RQ1 Do dE galaxies in the Fornax Cluster exhibit abundance ratio patterns consistent with the Milky Way disk, indicating slow formation?
• RQ2 Can high-resolution line indices reliably measure abundance ratios in unresolved, low-mass stellar systems with low signal-to-noise?
• RQ3 How do the abundance ratios of dEs compare to those of massive early-type galaxies and resolved stars in the Local Group?
• RQ4 What is the [Na/Fe] abundance ratio in dEs outside the Local Group, and how does it compare to solar-scaled values?
• RQ5 Do the abundance trends in dEs support a scenario of transformation from late-type, star-forming galaxies via environmental processes like ram pressure stripping?

Key findings

• This is the first study to derive abundance ratio proxies for 11 elements in unresolved dEs outside the Local Group using high-resolution indices.
• All eight dEs in the Fornax Cluster show abundance ratios consistent with the Milky Way disk, indicating slow, prolonged star formation compared to their high stellar masses.
• The [Na/Fe] abundance ratio in these dEs is significantly sub-solar, with values considerably lower than the solar-scaled prediction, consistent with findings in Virgo dEs.
• The abundance pattern of the dEs closely matches the extrapolation of massive galaxy abundance trends (from Conroy et al. 2014) to lower masses, suggesting a universal formation trend.
• The high-resolution index system successfully isolates individual element abundances despite low signal-to-noise and low velocity dispersion, enabling precise measurements.
• The results support the hypothesis that dEs originate from late-type, star-forming galaxies that lost their gas via ram pressure stripping, retaining disk-like chemical signatures.