[Paper Review] Spectrophotometric properties of galaxies at intermediate redshifts (z ~ 0.2--1.0) I. Sample description, photometric properties and spectral measurements

F. Lamareille, T. Contini+4 more|arXiv (Cornell University)|Nov 4, 2005

Galaxies: Formation, Evolution, Phenomena56 references22 citations

TL;DR

This study presents a spectroscopic and photometric analysis of 141 emission-line galaxies at intermediate redshifts (0.2 < z < 1.0), using medium-resolution VLT and Keck spectra to measure fluxes, equivalent widths, and absolute magnitudes. The platefit software, adapted for lower signal-to-noise and resolution, enables robust line measurements and spectral classification, identifying 115 star-forming galaxies, 7 Seyfert 2 AGNs, and 16 candidate star-forming galaxies.

ABSTRACT

We present the spectrophotometric properties of a sample of 141 emission-line galaxies at redshifts in the range $0.2

Motivation & Objective

• To characterize the spectrophotometric properties of 141 emission-line galaxies at intermediate redshifts (z ~ 0.2–1.0), focusing on photometric and spectroscopic measurements.
• To test and adapt the platefit software—originally for SDSS—for use with lower signal-to-noise and resolution spectra typical of deep surveys like VVDS.
• To classify galaxies using diagnostic diagrams, particularly addressing the challenge of Hα redshifting out of the optical window at high z.
• To ensure reliable measurement of emission line fluxes and equivalent widths, especially for blended lines and in the presence of dust extinction.
• To provide a foundation for subsequent studies on chemical abundances, stellar populations, and galaxy evolution using this well-calibrated dataset.

Proposed method

• Acquired medium-resolution (R ~ 500–600) optical spectra using the VLT and Keck telescopes for 141 emission-line galaxies, with redshifts ranging from z = 0.2 to z = 1.0.
• Applied the platefit software—originally developed for SDSS—to automatically measure emission line fluxes and equivalent widths after fitting and subtracting the stellar continuum.
• Validated platefit results against manual measurements, demonstrating improved accuracy for lines with strong Balmer absorption (e.g., Hα, Hβ, [O II] λ3727).
• Tested robustness of measurements by resampling high-resolution spectra to lower resolution (R ~ 250), simulating VVDS-like data, and confirming stability of key line ratios like [N II] λ6584/Hα.
• Corrected for dust extinction using the Hα/Hβ Balmer decrement (assuming Seaton 1979 extinction law and theoretical ratio of 2.87), and compared dust-corrected fluxes with equivalent width ratios.
• Classified galaxies using standard and 'blue' diagnostic diagrams (e.g., [O II] λ3727/Hβ vs. [O III] λ5007/Hβ), with the latter preferred for high-z galaxies where Hα is redshifted out of band.

Experimental results

Research questions

• RQ1 Can the platefit software be reliably adapted to low signal-to-noise and medium-resolution spectra typical of deep extragalactic surveys?
• RQ2 How accurate are flux and equivalent width measurements of emission lines—especially blended lines like [N II] λ6584 and Hα—when using automated fitting in low-SNR conditions?
• RQ3 To what extent does dust extinction differentially affect the [O II] λ3727 and Hβ emission lines, and can equivalent width ratios mitigate this effect?
• RQ4 How do diagnostic diagrams perform for classifying galaxies at high redshift (z > 0.6), where Hα is redshifted out of the optical window?
• RQ5 What is the distribution of galaxy types (star-forming, AGN, candidate) in a representative sample of intermediate-redshift galaxies using multi-wavelength data?

Key findings

• The platefit software provides reliable and consistent measurements of emission line fluxes and equivalent widths, with improved accuracy over manual measurements for lines with strong Balmer absorption features.
• Flux and equivalent width measurements remain stable when spectra are degraded to R ~ 250, confirming robustness for use in low-resolution surveys like VVDS and zCOSMOS.
• The [N II] λ6584/Hα line ratio is insensitive to spectral resolution changes, remaining within measurement error margins, which supports its use as a diagnostic tool at low resolution.
• Dust correction via the Hα/Hβ Balmer decrement yields consistent results when compared to equivalent width ratios, with a residual scatter of only 0.10 dex, indicating minimal differential extinction effects.
• Of the 141 galaxies, 115 are classified as star-forming, 7 as Seyfert 2 AGNs, and 16 as 'candidate' star-forming galaxies due to ambiguous positions in diagnostic diagrams.
• The 'blue' diagnostic diagrams (using [O II] λ3727 and Hβ) are effective for classifying high-redshift galaxies (z > 0.6), where Hα is no longer observable in the optical band.