Molecular gas and star formation in the red-sequence counter-rotating disc galaxy NGC 4550

Alison F. Crocker, Hyunjin Jeong, Shinya Komugi, Francoise Combes, Martin Bureau, Lisa M. Young, Sukyoung Yi

Research output: Contribution to journalArticle

34 Citations (Scopus)

Abstract

We present observations of the CO(1-0) emission in the central 750 pc (10 arcsec) of the counter-rotating disc galaxy NGC 4550, obtained at the Institut de Radioastronomie Millimétrique (IRAM) Plateau de Bure Interferometer. Very little molecular gas is detected, only 1×10 7 M , and its distribution is lopsided, with twice as much molecular gas observed at positive relative velocities than at negative relative velocities. The velocity gradient in the CO(1-0) emission shows that the molecular gas rotates like the thicker of the two stellar discs, which is an unexpected alignment of rotations if the thinner disc was formed by a major gas accretion event. However, a simulation shows that the gas rotating like the thicker disc naturally results from the coplanar merger of two counter-rotating disc galaxies, demonstrating the feasibility of this scenario for the formation of NGC 4550. We investigate various star formation tracers to determine whether the molecular gas in NGC 4550 is currently forming stars. Ultraviolet (UV) imaging data and optical absorption line strengths both suggest a recent star formation episode; the best-fitting two-population model to the UV-optical colours yields a mass of young stars of 5.9×10 7 M with an age of 280 Myr. The best information on the current star formation rate is a far-infrared-based upper limit of only 0.02 M yr -1. We are thus witnessing NGC 4550 either in a dip within a bursty star formation period or during a more continuous low-level star formation episode.

Original languageEnglish
Pages (from-to)1255-1264
Number of pages10
JournalMonthly Notices of the Royal Astronomical Society
Volume393
Issue number4
DOIs
Publication statusPublished - 2009 Mar 1

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disk galaxies
rotating disks
molecular gases
star formation
counters
gas
stars
star formation rate
gases
tracers
plateaus
optical absorption
interferometers
alignment
interferometer
color
gradients
merger
dip
tracer

All Science Journal Classification (ASJC) codes

  • Space and Planetary Science
  • Astronomy and Astrophysics

Cite this

Crocker, Alison F. ; Jeong, Hyunjin ; Komugi, Shinya ; Combes, Francoise ; Bureau, Martin ; Young, Lisa M. ; Yi, Sukyoung. / Molecular gas and star formation in the red-sequence counter-rotating disc galaxy NGC 4550. In: Monthly Notices of the Royal Astronomical Society. 2009 ; Vol. 393, No. 4. pp. 1255-1264.
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Molecular gas and star formation in the red-sequence counter-rotating disc galaxy NGC 4550. / Crocker, Alison F.; Jeong, Hyunjin; Komugi, Shinya; Combes, Francoise; Bureau, Martin; Young, Lisa M.; Yi, Sukyoung.

In: Monthly Notices of the Royal Astronomical Society, Vol. 393, No. 4, 01.03.2009, p. 1255-1264.

Research output: Contribution to journalArticle

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T1 - Molecular gas and star formation in the red-sequence counter-rotating disc galaxy NGC 4550

AU - Crocker, Alison F.

AU - Jeong, Hyunjin

AU - Komugi, Shinya

AU - Combes, Francoise

AU - Bureau, Martin

AU - Young, Lisa M.

AU - Yi, Sukyoung

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N2 - We present observations of the CO(1-0) emission in the central 750 pc (10 arcsec) of the counter-rotating disc galaxy NGC 4550, obtained at the Institut de Radioastronomie Millimétrique (IRAM) Plateau de Bure Interferometer. Very little molecular gas is detected, only 1×10 7 M ⊙, and its distribution is lopsided, with twice as much molecular gas observed at positive relative velocities than at negative relative velocities. The velocity gradient in the CO(1-0) emission shows that the molecular gas rotates like the thicker of the two stellar discs, which is an unexpected alignment of rotations if the thinner disc was formed by a major gas accretion event. However, a simulation shows that the gas rotating like the thicker disc naturally results from the coplanar merger of two counter-rotating disc galaxies, demonstrating the feasibility of this scenario for the formation of NGC 4550. We investigate various star formation tracers to determine whether the molecular gas in NGC 4550 is currently forming stars. Ultraviolet (UV) imaging data and optical absorption line strengths both suggest a recent star formation episode; the best-fitting two-population model to the UV-optical colours yields a mass of young stars of 5.9×10 7 M ⊙ with an age of 280 Myr. The best information on the current star formation rate is a far-infrared-based upper limit of only 0.02 M ⊙ yr -1. We are thus witnessing NGC 4550 either in a dip within a bursty star formation period or during a more continuous low-level star formation episode.

AB - We present observations of the CO(1-0) emission in the central 750 pc (10 arcsec) of the counter-rotating disc galaxy NGC 4550, obtained at the Institut de Radioastronomie Millimétrique (IRAM) Plateau de Bure Interferometer. Very little molecular gas is detected, only 1×10 7 M ⊙, and its distribution is lopsided, with twice as much molecular gas observed at positive relative velocities than at negative relative velocities. The velocity gradient in the CO(1-0) emission shows that the molecular gas rotates like the thicker of the two stellar discs, which is an unexpected alignment of rotations if the thinner disc was formed by a major gas accretion event. However, a simulation shows that the gas rotating like the thicker disc naturally results from the coplanar merger of two counter-rotating disc galaxies, demonstrating the feasibility of this scenario for the formation of NGC 4550. We investigate various star formation tracers to determine whether the molecular gas in NGC 4550 is currently forming stars. Ultraviolet (UV) imaging data and optical absorption line strengths both suggest a recent star formation episode; the best-fitting two-population model to the UV-optical colours yields a mass of young stars of 5.9×10 7 M ⊙ with an age of 280 Myr. The best information on the current star formation rate is a far-infrared-based upper limit of only 0.02 M ⊙ yr -1. We are thus witnessing NGC 4550 either in a dip within a bursty star formation period or during a more continuous low-level star formation episode.

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