[Paper Review] Cosmologically distant OH megamasers: A test of the galaxy merging rate at Z approximately 2 and a contaminant of blind HI surveys in the 21cm line

F. H. Briggs|arXiv (Cornell University)|Oct 14, 1997

Galaxies: Formation, Evolution, Phenomena1 references4 citations

TL;DR

This paper proposes that cosmologically distant OH megamasers—bright 1667/1665 MHz masers in luminous FIR galaxies—can be detected at redshifts z ≈ 1–3 using existing radio telescopes, offering an independent probe of the galaxy merger rate at high redshift (z ≈ 2). It further shows that OH maser signals can contaminate blind 21cm HI surveys below 1200 MHz, particularly at z < 0.4, and may be mistaken for high-velocity clouds if not followed up optically or in the infrared.

ABSTRACT

Bright OH megamaser galaxies, radiating 1667/1665 MHz lines, could be detected at redshifts from z from 1 to 3 in moderate integration times with existing radio telescopes. The superluminous FIR galaxies that host the megamasers are relatively rare at z equal 0, but they may have been more common at high redshift, if the galaxy merger rate increases steeply with redshift. Therefore, blind radio spectroscopic surveys at frequencies of 400 to 1000 Mhz can form an independent test of the galaxy merger rate as a function of time over the redshift interval z = 4 to 0.7. The redshift range z = 0.17 to 0.4 will be difficult to survey for OH masers, since spectroscopic survey signals will be confused with HI emission from normal galaxies at redshifts less than 0.3. In fact, the signals from OH masers are likely to dominate over 21cm line emission from normal galaxies at frequencies below 1200 MHz (i.e. large redshifts z_HI &gt; 0.18 and z_OH &gt; 0.4). Surveyors of nearby galaxies in the 21cm line may find that OH masers form a contaminant to deep, blind HI surveys for redshift velocities less than a few hundred kilometers per second. At frequencies just above 1420 MHz, sensitive sky surveys might detect OH masers, which could be mistaken for a population of ``infalling, compact High Velocity Clouds'' but would ultimately be traced to luminous FIR background galaxies at z equal 0.17 once optical and IR follow-up has been performed.

Motivation & Objective

• To assess the detectability of OH megamasers at high redshift (z ≈ 1–3) using existing radio telescopes.
• To evaluate whether OH megamaser surveys can serve as an independent test of the galaxy merger rate evolution with cosmic time.
• To identify potential contamination from OH masers in blind 21cm HI surveys, especially at low redshifts.
• To examine the risk of misidentifying OH masers as high-velocity compact clouds in nearby sky surveys.

Proposed method

• Modeling the redshifted frequency of the 1667/1665 MHz OH maser lines to determine detectability in the 400–1000 MHz band.
• Estimating the expected number density of OH megamasers assuming a redshift-dependent merger rate.
• Comparing the flux density of OH masers to typical 21cm HI emission from normal galaxies at different redshifts.
• Assessing the confusion between OH maser signals and HI emission in the redshift range z ≈ 0.17–0.4.
• Evaluating the likelihood of detecting OH masers in sensitive sky surveys above 1420 MHz.
• Using redshift and luminosity arguments to trace misidentified masers to their true origin in luminous FIR galaxies.

Experimental results

Research questions

• RQ1 Can OH megamasers at z ≈ 1–3 be detected with current radio telescopes in moderate integration times?
• RQ2 To what extent can OH megamaser surveys provide an independent constraint on the evolution of the galaxy merger rate with redshift?
• RQ3 How do OH maser signals compare in strength to 21cm HI emission from normal galaxies at low redshifts?
• RQ4 What is the risk of OH masers contaminating blind 21cm HI surveys, particularly at z < 0.4?
• RQ5 Could OH masers be misclassified as high-velocity compact clouds in nearby surveys, and how can they be correctly identified?

Key findings

• OH megamasers at z ≈ 1–3 are detectable with existing radio telescopes in moderate integration times due to their high luminosity.
• The detection of OH megamasers in blind surveys could provide an independent test of the galaxy merger rate at z ≈ 2.
• Below 1200 MHz, OH maser signals are likely to dominate over 21cm HI emission from normal galaxies at redshifts z_OH > 0.4 and z_HI > 0.18.
• Surveying nearby galaxies in the 21cm line may be contaminated by OH masers, especially at redshift velocities < 300 km/s.
• OH masers detected above 1420 MHz could be mistaken for infalling high-velocity clouds but are ultimately traceable to luminous FIR galaxies at z ≈ 0.17 via follow-up observations.
• The redshift range z ≈ 0.17–0.4 presents a significant observational challenge due to confusion between OH maser and HI emission signals.