[Paper Review] Integrated-light analyses vs. colour-magnitude diagrams - II. Leo A, an extremely young dwarf in the Local Group

T. Ruiz-Lara, Carme Gallart+8 more|arXiv (Cornell University)|May 11, 2018

Stellar, planetary, and galactic studies114 references3 citations

TL;DR

This study compares star formation histories (SFHs) of Leo A, a young Local Group dwarf galaxy, derived from integrated-light spectroscopy (using GTC/OSIRIS and STECKMAP) and from deep color-magnitude diagram (CMD) fitting of HST data. The results show excellent agreement between both methods in reconstructing the time evolution of the star formation rate and overall metallicity, validating high-signal-to-noise integrated spectroscopy as a reliable alternative to resolved stellar photometry for young systems.

ABSTRACT

Context. Most of our knowledge on the stellar component of galaxies is based on the analysis of distant systems and comes from integrated light data. It is important to test whether the results of the star formation histories (SFH) obtained with standard full-spectrum fitting methods are in agreement with those obtained through colour-magnitude diagram (CMD) fitting (usually considered the most reliable approach). Aims. We compare SFHs recovered from both techniques in Leo~A, a Local Group dwarf galaxy whose majority of stars formed during the last 8 Gyrs. This complements our previous findings in a field in the Large Magellanic Cloud bar, where star formation has been on-going since early epochs though at varying rates. Methods. We have used GTC/OSIRIS in long-slit mode to obtain a high-quality integrated light spectrum by scanning a selected region within Leo~A, for which a CMD reaching the old main mequence turn-off (oMSTO) is available from HST. We compared the SFH obtained from the two datasets, using state-of-art methods of integrated light ({ t STECKMAP}) and resolved stellar population analysis. In the case of the CMD, we computed the SFH both from a deep CMD (observed with HST/ACS), and from a shallower one (archival data from HST/WFPC2). Results. The agreement between the SFHs recovered from the oMSTO CMD and from full spectrum fitting is remarkable, particularly regarding the time evolution of the star formation rate. The overall extremely low metallicity of Leo~A is recovered up to the last 2 Gyrs, when some discrepancies appear. A relatively high metallicity found for the youngest stars from the integrated data is a recurring feature that might indicate that the current models or synthesis codes should be revised, but that can be significantly mitigated using a more restrictive metallicity range... [Abridged]

Motivation & Objective

• To test the reliability of integrated-light spectroscopy in recovering star formation histories (SFHs) compared to the gold-standard CMD fitting method.
• To assess whether modern full-spectrum fitting codes can accurately recover SFHs in systems dominated by young stellar populations, such as Leo A.
• To investigate discrepancies in metallicity estimates for young stars between the two methods and evaluate their origin.
• To compare the performance of high-signal-to-noise integrated spectra against shallow CMDs in SFH recovery, especially for systems lacking old main sequence turn-off stars.
• To evaluate the robustness of both SFH derivation techniques under realistic observational and modeling uncertainties.

Proposed method

• Obtained a high signal-to-noise integrated-light spectrum of Leo A's central region using the GTC/OSIRIS instrument in long-slit mode.
• Applied the STECKMAP full-spectrum fitting code to derive the SFH and metallicity from the integrated spectrum.
• Analyzed a deep HST/ACS color-magnitude diagram (CMD) reaching the old main sequence turn-off (oMSTO) using standard CMD fitting techniques.
• Compared results from the deep CMD with those from a shallower HST/WFPC2 CMD to assess the impact of photometric depth on SFH recovery.
• Used state-of-the-art stellar population synthesis models (e.g., Vazdekis et al. 2016) and considered age-metallicity degeneracy effects.
• Constrained the metallicity range in models to test whether discrepancies in young star metallicities could be mitigated.

Experimental results

Research questions

• RQ1 How well do integrated-light spectroscopic analyses reproduce the star formation history of Leo A as derived from a deep CMD?
• RQ2 To what extent do discrepancies in metallicity estimates for young stars between the two methods arise from model limitations or observational systematics?
• RQ3 Can high signal-to-noise integrated spectra provide more reliable SFH estimates than shallow CMDs that do not reach the old main sequence turn-off?
• RQ4 How do the time-resolved star formation rates derived from both methods compare, especially on timescales below 1 Gyr?
• RQ5 What is the impact of restricting the metallicity range in synthesis models on the consistency between integrated-light and CMD-derived SFHs?

Key findings

• The star formation rate (SFR) evolution derived from integrated-light spectroscopy and deep CMD fitting show remarkable agreement, particularly in the time evolution of SFR.
• The overall metallicity of Leo A is consistently recovered by both methods, with the integrated-light analysis matching the CMD-derived average metallicity across the galaxy's history.
• A persistent discrepancy appears in the metallicity of the youngest stars, where integrated-light analysis yields a higher metallicity than CMD fitting, likely due to age-metallicity degeneracy or model limitations.
• This metallicity discrepancy is significantly reduced when using a more restrictive metallicity range in the stellar population synthesis models.
• The SFH derived from a shallow CMD (not reaching the oMSTO) shows notable differences from both the deep CMD and integrated-light results, indicating lower reliability.
• High signal-to-noise integrated spectra are shown to provide SFH estimates that are more consistent with deep CMDs than shallow CMDs, especially for young systems like Leo A.