[Paper Review] The formation of cluster elliptical galaxies as revealed by extensive star formation

J. A. Stevens, R. J. Ivison+7 more|arXiv (Cornell University)|Sep 17, 2003

Galaxies: Formation, Evolution, Phenomena25 references4 citations

TL;DR

This study reveals that cluster elliptical galaxies form through synchronous, extensive star formation in high-redshift radio galaxies (HzRGs) and their dense environments. Using deep submillimetre mapping with SCUBA on the JCMT, the authors detect extended dust emission and luminous submillimetre companions aligned with HzRG radio jets, indicating that massive galaxies form coevally in proto-cluster structures at z ≈ 3–4, with star formation peaking at separations of 30–250 kpc.

ABSTRACT

The most massive galaxies in the present-day Universe are found to lie in the centres of rich clusters. They have old, coeval stellar populations suggesting that the bulk of their stars must have formed at early epochs in spectacular starbursts - luminous phenomena at submillimetre wavelengths. The most popular model of galaxy formation predicts that these galaxies form in proto-clusters at high-density peaks in the early Universe. Such peaks are signposted by massive high-redshift radio galaxies. Here we report deep submillimetre mapping of seven high-redshift radio galaxies and their environments. These data confirm not only the presence of spatially extended massive star-formation activity in the radio galaxies themselves, but also in companion objects previously undetected at any wavelength. The prevalence, orientation, and inferred masses of these submillimetre companion galaxies suggest that we are witnessing the synchronous formation of the most luminous elliptical galaxies found today at the centres of rich galaxy clusters.

Motivation & Objective

• To investigate the spatial extent and environmental context of star formation in high-redshift radio galaxies (HzRGs), which are thought to be progenitors of massive cluster ellipticals.
• To determine whether star formation in HzRGs and their surroundings is spatially extended and coeval, as predicted by hierarchical galaxy formation models.
• To test whether the alignment between HzRG radio jets and submillimetre emission or companion galaxies is statistically significant and indicative of physical association in large-scale structures.
• To assess whether the observed submillimetre companions are physically associated with HzRGs or random background sources, using source density and alignment statistics.

Proposed method

• Deep 850 µm submillimetre imaging of seven high-redshift radio galaxies (z = 2.2–4.3) using the Submillimetre Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT).
• Two-dimensional Gaussian fitting to deconvolve the beam response and quantify the spatial extent of dust emission, with beam FWHM ≈ 14.4′′.
• Statistical analysis using the Kolmogorov-Smirnov test to assess the significance of alignments between radio jet position angles and submillimetre source positions.
• Comparison of observed submillimetre source counts with blank-field surveys to evaluate excess source density in HzRG fields.
• N-body ΛCDM simulations to model dark matter halo distributions around HzRGs, estimating the expected number of massive haloes and companions within 3-arcmin fields.
• Use of the quasar BR 1202–0725 as a calibration source to verify that observed extended morphologies are not instrumental artifacts.

Experimental results

Research questions

• RQ1 Is the submillimetre emission from high-redshift radio galaxies spatially extended beyond their radio structures, and on what physical scales does it occur?
• RQ2 Are the luminous submillimetre sources detected near HzRGs physically associated with them, or are they random background sources?
• RQ3 Do the radio jets of HzRGs align with the submillimetre emission and with the positions of the brightest submillimetre companions, and is this alignment statistically significant?
• RQ4 Can the observed alignment and high companion source density be explained by selection effects in massive, high-density peaks in the early universe?
• RQ5 Do the observed star formation rates and spatial extents in HzRGs and companions support the formation of massive elliptical galaxies within <1 Gyr?

Key findings

• The dust emission from the central HzRGs is spatially extended, with half-light radii ranging from 30 to 150 kpc, matching the sizes of local brightest cluster galaxies.
• In five of seven HzRGs, the submillimetre emission is resolved, with extents from 50 to 250 kpc, indicating massive, extended star formation regions.
• The survey detected 10 submillimetre companions with S850 > 5–6 mJy, significantly exceeding the expected blank-field source density of ~1 per field.
• Four out of seven HzRGs show significant alignment (p < 0.05) between their radio jet position angles and the direction to the brightest submillimetre companion, suggesting physical association.
• The alignment between radio jets and submillimetre emission is also significant (p = 0.04), indicating that jets may trace the densest gas regions in proto-cluster environments.
• Simulations confirm that the observed number of massive haloes (M > 10^12 M☉) within 3-arcmin of HzRGs is consistent with ΛCDM models, supporting the idea that HzRGs reside in dense proto-clusters.