[Paper Review] Witnessing the birth of the red sequence: the physical scale and morphology of dust emission in hyper-luminous starbursts in the early Universe

I. Oteo, R. J. Ivison+23 more|arXiv (Cornell University)|Sep 13, 2017

Astrophysics and Star Formation Studies21 citations

TL;DR

This study uses high-resolution ALMA 870 μm observations of 44 ultrared dusty starburst galaxies at z ≈ 4–6 to investigate their dust emission morphology and physical size. It finds these sources have compact sizes (~1.46 kpc on average), short gas-depletion timescales, and are likely progenitors of massive, compact red-and-dead galaxies, marking the birth of the high-mass end of the red sequence in the early Universe.

ABSTRACT

We present high-spatial-resolution ($\sim 0.12''$ or $\approx 800 \, { m pc}$ at $z = 4.5$) ALMA $870\,μ$m dust continuum observations of a sample of 44 ultrared dusty star-forming galaxies (DSFGs) selected from the H-ATLAS and HerMES far-infrared surveys because of their red colors from 250 to 500 $μ$m: $S_{500} / S_{250} > 1.5$ and $S_{500} / S_{350} > 1.0$. With photometric redshifts in the range $z \sim 4$-6, our sample includes the most luminous starbursting systems in the early Universe known so far, with total obscured star-formation rates (SFRs) of up to $\sim 4,500 \, M_\odot \, { m yr}^{-1}$, as well as a population of lensed, less intrinsically luminous sources. The lower limit on the number of ultrared DSFGs at 870 $μ$m (with flux densities measured from the ALMA maps and thus not affected by source confusion) derived in this work is in reasonable agreement with models of galaxy evolution, whereas there have been reports of conflicts at 500 $μ$m (where flux densities are derived from SPIRE). Ultrared DSFGs have a variety of morphologies (from relatively extended disks with smooth radial profiles, to compact sources, both isolated and interacting) and an average size, $θ_{ m FWHM}$, of $1.46 \pm 0.41\, { m kpc}$, considerably smaller than the values reported in previous work for less-luminous DSFGs at lower redshifts. The size and the estimated gas-depletion times of our sources are compatible with their being the progenitors of the most massive, compact, red-and-dead galaxies at $z \sim 2$-3, and ultimately of local ultra-massive elliptical galaxies or massive galaxy clusters. We are witnessing the birth of the high-mass tail of the red sequence of galaxies.

Motivation & Objective

• To determine the physical size and morphology of dust emission in the most luminous starbursting galaxies in the early Universe.
• To resolve the discrepancy between observed number counts of ultrared DSFGs at 500 μm (SPIRE) and model predictions, by using high-resolution ALMA data.
• To test whether these hyper-luminous, dusty starbursts are the progenitors of massive, compact quiescent galaxies observed at z ≈ 2–3.
• To assess the role of source confusion, beam smearing, and flux boosting in previous far-IR surveys that may have biased number counts.
• To establish the evolutionary link between high-redshift starbursts and the formation of local ultra-massive elliptical galaxies.

Proposed method

• Selected 44 ultrared dusty star-forming galaxies (DSFGs) from H-ATLAS and HerMES surveys using red SPIRE colors (S500/S250 > 1.5, S500/S350 > 1.0), indicating high redshift (z ≈ 4–6).
• Conducted high-spatial-resolution (≈0.12″ or 800 pc at z=4.5) ALMA 870 μm dust continuum observations to measure flux densities and resolve source morphologies.
• Used photometric redshifts and SED modeling to estimate total obscured star-formation rates (SFRs), with individual components reaching up to ~2,400 M⊙ yr⁻¹.
• Compared ALMA-derived number counts at 870 μm with SPIRE-based counts at 500 μm to assess the impact of beam smearing and flux boosting.
• Measured full width at half maximum (FWHM) sizes of dust emission components to infer physical sizes and gas depletion timescales.
• Evaluated the evolutionary link between these compact, luminous starbursts and massive quiescent galaxies at z ≈ 2–3 via size and SFR timescale comparisons.

Experimental results

Research questions

• RQ1 What is the physical size and morphological structure of the dust emission in the most luminous starbursting galaxies at z ≈ 4–6?
• RQ2 Why do number counts of ultrared DSFGs at 500 μm from SPIRE conflict with model predictions, while those at 870 μm from ALMA do not?
• RQ3 Are these hyper-luminous, dusty starbursts the progenitors of massive, compact, quiescent galaxies observed at z ≈ 2–3?
• RQ4 How do the gas depletion timescales of these sources compare to the timescales required for quenching and formation of red-and-dead galaxies?
• RQ5 To what extent do beam smearing and flux boosting in large SPIRE beams affect the reliability of number counts at 500 μm?

Key findings

• The average FWHM size of dust emission in the 44 ultrared DSFGs is 1.46 ± 0.41 kpc, significantly smaller than sizes measured for less luminous DSFGs at lower redshifts.
• The sources exhibit a variety of morphologies, including smooth disks, compact sources, and interacting systems, indicating diverse formation pathways.
• The total obscured star-formation rate across all components reaches up to ~4,500 M⊙ yr⁻¹, with individual components reaching ~2,400 M⊙ yr⁻¹.
• The gas-depletion timescale is estimated to be a few hundred million years, consistent with rapid quenching and the formation of compact, quiescent galaxies.
• ALMA-derived number counts at 870 μm show good agreement with galaxy evolution models, resolving the conflict seen at 500 μm with SPIRE data.
• The observed discrepancy at 500 μm is attributed to beam smearing and flux boosting in large SPIRE beams, effects absent in high-resolution ALMA observations.