[Paper Review] Nearby early-type galaxies with ionized gas.II. Line-strength indices for 18 additional galaxies

F. Annibali, A. Bressan+2 more|ArXiv.org|Aug 24, 2005

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy53 references21 citations

TL;DR

This paper extends the Lick IDS line-strength index database for early-type galaxies by measuring 25 indices across 7 luminosity-weighted apertures and 4 radial gradients in 18 additional galaxies, including three with repeated observations for consistency checks. The study confirms high reliability of measurements by comparing with literature data, particularly Trager et al. (1998), González (1993), and Denicolò et al. (2005), and establishes a consistent Mg2–σ relation across the full sample, reinforcing the utility of the dataset for stellar population and ISM evolution studies in early-type galaxies.

ABSTRACT

Rampazzo et al. 2005 (Paper I) presented a data-set of line-strength indices for 50 early-type galaxies in the nearby Universe. The galaxy sample is biased toward galaxies showing emission lines, located in environments corresponding to a broad range of local galaxy densities, although predominantly in low density environments. The present addendum to Paper I enlarges the above data-set of line-strength indices by analyzing 18 additional early-type galaxies (three galaxies, namely NGC 3607, NGC 5077 and NGC 5898 have been already presented in the previous set). As in Paper I, we measured 25 line-strength indices, defined by the Lick IDS "standard" system (Trager et al. 1998; Worthey & Ottaviani 1997), for 7 luminosity weighted apertures and 4 gradients of each galaxy. This paper presents the line-strength data-set and compares it with the available data in the literature.

Motivation & Objective

• Expand the existing database of line-strength indices for early-type galaxies with ionized gas to include 18 additional galaxies.
• Ensure internal consistency by re-measuring three galaxies (NGC 3607, NGC 5077, NGC 5898) already in the original sample.
• Provide high-precision, Lick IDS-standardized line-strength indices corrected for emission infilling, velocity dispersion, and instrumental effects.
• Enable detailed comparison of stellar populations across radial gradients and apertures to study galaxy formation and evolution.
• Establish a reliable reference dataset for future studies of intermediate- and high-redshift early-type galaxies.

Proposed method

• Obtained spectroscopic data from the European Southern Observatory (La Silla) for 18 early-type galaxies.
• Measured 25 line-strength indices using the Lick IDS standard system, including 21 original indices and 4 from Worthey & Ottaviani (1997).
• Analyzed 7 luminosity-weighted apertures (from 1.5″ to re/2) and 4 radial gradients (0–re/16, re/16–re/8, re/8–re/4, re/4–re/2), corrected for galaxy ellipticity.
• Corrected indices for emission-line infilling using the observed emission-line fluxes and velocity dispersion effects via standard empirical corrections.
• Transformed indices into the Lick IDS system using standard transformation equations and applied instrumental corrections.
• Validated results by comparing with published data from Trager et al. (1998), González (1993), Longhetti et al. (1998a), Beuing et al. (2002), and Denicolò et al. (2005).

Experimental results

Research questions

• RQ1 How consistent are the line-strength index measurements for the same galaxies when re-observed, confirming the reliability of the data reduction pipeline?
• RQ2 To what extent do the measured line-strength indices in the new galaxies align with those from previous studies (e.g., Trager et al. 1998, González 1993)?
• RQ3 What is the radial trend of key indices (e.g., Mg2, Hβ, Fe5335) across different apertures and gradients in the new sample?
• RQ4 How does the Mg2–σ relation derived from the extended sample compare with previous fits (e.g., Trager et al. 1998, Bernardi et al. 1998)?
• RQ5 To what extent does the extended dataset improve the statistical robustness of stellar population modeling in early-type galaxies with ionized gas?

Key findings

• The internal consistency check on NGC 3607, NGC 5077, and NGC 5898 shows index differences within 3σ of the measurement error, confirming measurement reliability.
• The Mg2–σ relation derived from the full sample (50 galaxies) yields a slope of 0.195 and a value of 0.335 at σ = 300 km s⁻¹, consistent with the Trager et al. (1998) fit and within 1σ of the Bernardi et al. (1998) fit.
• The comparison with literature data (González 1993, Trager 1998, Longhetti et al. 1998a, Beuing et al. 2002, Denicolò et al. 2005) shows excellent agreement, with most data points lying within the one-to-one relation.
• Radial trends of Mg2, Hβ, and Fe5335 indices across apertures and gradients are well resolved, showing systematic variations consistent with stellar population gradients.
• The extended dataset now includes 50 early-type galaxies with emission lines, significantly enhancing the statistical power for studying stellar population and ISM co-evolution.
• The final data set is publicly available and serves as a robust reference for future studies of stellar populations in early-type galaxies at intermediate and high redshift.