[Paper Review] Low-redshift analogs of submm galaxies: a diverse population

I. Oteo, Ian Smail+12 more|arXiv (Cornell University)|Jul 17, 2017

Astronomy and Astrophysical Research3 citations

TL;DR

This study identifies 21 low-redshift (z < 0.5) analogs of high-redshift submillimeter galaxies (SMGs) based on matching far-IR luminosity and dust temperature. Using IRAM-30m observations, it reveals a diverse population with highly variable CO line ratios, indicating that extrapolating molecular gas masses from J≥2 CO transitions is highly uncertain, while confirming a tight dust-gas luminosity correlation and average molecular gas mass of ~1.6×10¹⁰ M☉, with gas depletion timescales of ~100 Myr.

ABSTRACT

We have combined the wide-area Herschel-ATLAS far-IR survey with spectroscopic redshifts from GAMA and SDSS to define a sample of 21 low--redshift ($z_{ m spec} &lt; 0.5$) analogs of submm galaxies (SMGs). These have been selected because their dust temperatures and total IR luminosities are similar to those for the classical high-redshift SMG population. As well as presenting the sample, in this paper we report $^{12}$CO(2-1) and $^{12}$CO(1-0) observations of 16 low-redshift analogs of SMGs taken with the IRAM-30m telescope. We have obtained that low-redshift analogs of SMGs represent a very diverse population, similar to what has been found for high-redshift SMGs. A large variety in the molecular gas excitation or $^{12}$CO(2-1)/$^{12}$CO(1-0) line ratio is seen, meaning that extrapolations from $J \geq 2$ CO lines can result in very uncertain molecular gas mass determinations. Our sources with $^{12}$CO(1-0) detections follow the dust--gas correlation found in previous work at different redshifts and luminosities. The molecular gas mass of low-redshift SMGs has an average value of $M_{ m H_2} \sim 1.6 imes 10^{10}\,M_\odot$ and will be consumed in $\sim 100 \, { m Myr}$ . We also find a wide range of molecular gas fractions, with the highest values being compatible with those found in high-redshift SMGs with $^{12}$CO(1-0) detections, which are only the most luminous. Low-redshift SMGs offer a unique opportunity to study the properties of extreme star formation in a detail not possible at higher redshifts.

Motivation & Objective

• To identify low-redshift analogs of high-redshift submillimeter galaxies (SMGs) with similar far-IR luminosities and dust temperatures, enabling detailed study of extreme star formation in a more accessible redshift regime.
• To investigate the molecular gas content and excitation in these low-redshift SMG analogs using CO(1–0) and CO(2–1) line observations, addressing uncertainties in gas mass estimation from high-J CO transitions.
• To test the universality of the dust-gas correlation and gas depletion timescale in extreme star-forming systems by comparing low-redshift analogs to high-redshift SMGs.
• To explore the morphological and dynamical diversity of low-redshift SMG analogs and its implications for interpreting high-redshift SMG properties, particularly double-peaked CO line profiles.

Proposed method

• Selected 21 low-redshift (z < 0.5) SMG analogs from the Herschel-ATLAS survey using spectroscopic redshifts (GAMA, SDSS), matching total IR luminosity and dust temperature to high-redshift SMGs.
• Conducted 12 CO(1–0) and 12 CO(2–1) line observations with the IRAM-30m telescope on 16 of the 21 analogs to measure molecular gas content and excitation.
• Calculated molecular gas masses using standard luminosity-to-mass conversion, with corrections for CO excitation based on observed 12CO(2–1)/12CO(1–0) line ratios.
• Analyzed CO line profiles to identify kinematic signatures of isolated disks or interacting systems, linking morphology to gas kinematics.
• Compared observed CO luminosities with IR luminosities and dust emission to test the existence and scatter of the L′_CO–L_IR and L′_CO–L_850 correlations.
• Used near-IR and optical imaging to classify morphologies and assess interactions, linking structural features to CO line profiles.

Experimental results

Research questions

• RQ1 To what extent do low-redshift SMG analogs reproduce the diversity of morphologies and kinematics seen in high-redshift SMGs?
• RQ2 How variable is the CO excitation (12CO(2–1)/12CO(1–0) line ratio) in low-redshift SMG analogs, and what are the implications for molecular gas mass estimation from high-J CO transitions?
• RQ3 Do low-redshift SMG analogs follow the same dust-gas luminosity correlation observed in high-redshift SMGs and other star-forming galaxies?
• RQ4 What is the typical molecular gas mass and depletion timescale in low-redshift SMG analogs, and how do they compare to high-redshift SMGs?
• RQ5 Can double-peaked CO line profiles in low-redshift analogs be reliably interpreted as merger signatures, or do isolated rotating disks also produce such profiles?

Key findings

• Low-redshift SMG analogs exhibit a wide range of morphologies, including isolated star-forming disks, interacting systems, and groupings of red and blue sources, confirming their diversity similar to high-redshift SMGs.
• The 12CO(2–1)/12CO(1–0) line ratio varies significantly across the sample, indicating highly uncertain molecular gas mass estimates when extrapolating from J≥2 CO transitions without 12CO(1–0) detections.
• The average molecular gas mass in the sample is ~1.6×10¹⁰ M☉, with individual masses ranging from ~0.9 to ~2.5×10¹⁰ M☉, consistent with the most massive SMGs at high redshift.
• The observed 12CO(1–0) detections follow a tight dust-gas luminosity correlation (L′_CO–L_850), supporting a universal link between dust and molecular gas in extreme starbursts.
• Despite similar IR luminosities, CO luminosities vary by nearly an order of magnitude, indicating a not-so-tight correlation between total IR and CO luminosities, with significant scatter.
• Gas depletion timescales are estimated at ~100 Myr, indicating that the current star formation in these systems is short-lived and consistent with intense, burst-like activity.