[Paper Review] EROs found behind lensing clusters. II. Stellar populations and dust properties of optical dropout EROs and comparison with related objects

D. Schaerer, A. Hempel+7 more|arXiv (Cornell University)|Mar 15, 2007

Astronomy and Astrophysical Research4 citations

TL;DR

This study analyzes lensed extremely red objects (EROs) behind galaxy clusters using multi-wavelength photometry from Hubble, VLT, Spitzer, and Chandra to determine their redshifts, stellar populations, and dust properties. Despite photometric redshifts favoring high-z (z~6–7), the objects' bright absolute magnitudes, number density, and long-wavelength data strongly indicate low-redshift (z~1–3) dusty starbursts with typical stellar masses of 0.5–5×10¹⁰ M☉ and star formation rates of 1–18 M☉ yr⁻¹, with one extreme LIRG at z~3 showing SFR~1000 M☉ yr⁻¹.

ABSTRACT

We determine the nature, redshift, stellar populations and dust properties of optically faint or non-detected extremely red objects (ERO) found from our survey of the lensing clusters A1835 and AC114. We perform SED fitting and use deep optical, HST, VLT, Spitzer data, and for some objects and sub-mm data. For most of the lensed EROs we find photometric redshifts showing a strong degeneracy between "low-z" (z~1-3) and high-z (z~6-7). Although formally best fits are often found at high-z, their resulting bright absolute magnitudes, the number density of these objects, and in some cases Spitzer photometry or longer wavelength observations, suggest strongly that all of these objects are at "low-z". The majority of these objects are best fitted with young (

Motivation & Objective

• To determine the true nature, redshift, stellar populations, and dust properties of optically faint or non-detected EROs found behind strong lensing clusters A1835 and AC114.
• To resolve the ambiguity in photometric redshifts that show degeneracy between low-z (z~1–3) and high-z (z~6–7) solutions for these EROs.
• To compare the lensed EROs with related high-redshift candidates, including the z~6.5 post-starburst galaxy HUDF-J2 and IRAC-selected EROs from Yan et al. (2004).
• To predict the mid-IR to sub-mm SEDs of the dusty EROs for future observations with APEX, Herschel, and ALMA.
• To re-evaluate the nature of the z~6.5 post-starburst candidate HUDF-J2, arguing it is more plausibly a dusty starburst at z~2.3–2.6.

Proposed method

• SED fitting using an updated version of the Hyperz code with a comprehensive library of synthetic, semi-empirical, and empirical spectral templates.
• Photometry from deep optical (ACS/HST), near-IR (VLT/ISAAC), mid-IR (Spitzer/IRAC), and longer-wavelength data (Spitzer/MIPS 24 µm and sub-mm from Chandra) were used for SED modeling.
• Photometric redshifts were derived by fitting observed broad-band magnitudes to template SEDs, with careful assessment of degeneracy between low- and high-redshift solutions.
• Stellar masses and star formation rates (SFRs) were estimated from bolometric luminosities derived via template fitting, corrected for gravitational lensing magnification.
• The analysis included comparison with IRAC-selected EROs from Yan et al. (2004) and re-evaluation of the z~6.5 post-starburst candidate HUDF-J2 using updated photometry and spectral templates.
• Predictions of the full IR to sub-mm SEDs were generated for the most dusty objects to guide future observations with APEX, Herschel, and ALMA.

Experimental results

Research questions

• RQ1 Are the lensed EROs behind clusters A1835 and AC114 truly at high redshift (z~6–7), or do they lie at lower redshift with strong dust extinction?
• RQ2 What are the intrinsic stellar population properties (age, metallicity, mass) and dust content (A_V) of these EROs, and how do they compare to known high-redshift candidates?
• RQ3 Can the observed 24 µm emission from the z~6.5 post-starburst candidate HUDF-J2 be explained by a dusty starburst at lower redshift rather than a high-z post-starburst?
• RQ4 How do the properties of lensed EROs compare with those of IRAC-selected EROs from Yan et al. (2004), and do they share similar stellar population and dust characteristics?
• RQ5 What are the predicted SEDs of these dusty EROs in the mid-IR to sub-mm range, and how detectable are they with upcoming facilities like Herschel and ALMA?

Key findings

• Most lensed EROs have photometric redshifts suggesting z~6–7, but their bright absolute magnitudes, number density, and long-wavelength photometry (Spitzer and sub-mm) strongly favor a low-redshift origin at z~1–3.
• The majority of the EROs are best fitted with young (≤0.5–0.7 Gyr) and dusty starburst populations, indicating recent or ongoing intense star formation.
• Three EROs show strong extinction with A_V ≈ 2.4–4, and the most extreme object, SMMJ14009+0252, is a LIRG at z~3 with a star formation rate of ~1000 M☉ yr⁻¹ and stellar mass of ~6×10¹¹ M☉.
• The z~6.5 post-starburst candidate HUDF-J2 is more plausibly a dusty starburst at z~2.3–2.6, which naturally explains its 24 µm emission and matches the SED of other lensed EROs.
• IRAC-selected EROs from Yan et al. (2004) show very similar SEDs and properties to the lensed EROs, with reasonable fits obtained using young, dusty starburst templates rather than composite populations.
• Predicted SEDs for the most dusty EROs show strong emission in the mid-IR to sub-mm, making them prime targets for detection with APEX, Herschel, and ALMA.