Galaxy Zoo

A sample of blue early-type galaxies at low redshift

Kevin Schawinski, Chris Lintott, Daniel Thomas, Marc Sarzi, Dan Andreescu, Steven P. Bamford, Sugata Kaviraj, Sadegh Khochfar, Kate Land, Phil Murray, Robert C. Nichol, M. Jordan Raddick, An Že Slosar, Alex Szalay, Jan Vandenberg, Sukyoung Yi

Research output: Contribution to journalArticle

97 Citations (Scopus)

Abstract

We report the discovery of a population of nearby, blue early-type galaxies with high star formation rates (0.5 < SFR < 50 M yr -1). They are identified by their visual morphology as provided by Galaxy Zoo for Sloan Digital Sky Survey Data Release 6 and their u - r colour. We select a volume-limited sample in the redshift range 0.02 < z < 0.05, corresponding to luminosities of approximately L* and above and with u - r colours significantly bluer than the red sequence. We confirm the early-type morphology of the objects in this sample and investigate their environmental dependence and star formation properties. Blue early-type galaxies tend to live in lower density environments than 'normal' red sequence early-types and make up 5.7 ± 0.4 per cent of the low-redshift early-type galaxy population. We find that such blue early-type galaxies are virtually absent at high velocity dispersions above 200 km s -1. Our analysis uses emission line diagnostic diagrams and we find that ∼25 per cent of them are actively star forming, while another ∼25 per cent host both star formation and an active galactic nucleus (AGN). Another ∼12 per cent are AGN. The remaining 38 per cent show no strong emission lines. When present and uncontaminated by an AGN contribution, the star formation is generally intense. We consider star formation rates derived from Hα, u band and infrared luminosities, and radial colour profiles, and conclude that the star formation is spatially extended. Of those objects that are not currently undergoing star formation must have ceased doing so recently in order to account for their blue optical colours. The gas-phase metallicity of the actively star-forming blue early-types galaxies is supersolar in all cases. We discuss the place of these objects in the context of galaxy formation. A catalogue of all 204 blue early-type galaxies in our sample, including star formation rates, emission line classification is provided.

Original languageEnglish
Pages (from-to)818-829
Number of pages12
JournalMonthly Notices of the Royal Astronomical Society
Volume396
Issue number2
DOIs
Publication statusPublished - 2009 Jun 1

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zoo
galaxies
star formation
star formation rate
active galactic nuclei
color
population type
luminosity
diagram
stars
galactic evolution
metallicity
catalogs
gas
rate
diagrams
vapor phases
profiles

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

Schawinski, Kevin ; Lintott, Chris ; Thomas, Daniel ; Sarzi, Marc ; Andreescu, Dan ; Bamford, Steven P. ; Kaviraj, Sugata ; Khochfar, Sadegh ; Land, Kate ; Murray, Phil ; Nichol, Robert C. ; Raddick, M. Jordan ; Slosar, An Že ; Szalay, Alex ; Vandenberg, Jan ; Yi, Sukyoung. / Galaxy Zoo : A sample of blue early-type galaxies at low redshift. In: Monthly Notices of the Royal Astronomical Society. 2009 ; Vol. 396, No. 2. pp. 818-829.
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abstract = "We report the discovery of a population of nearby, blue early-type galaxies with high star formation rates (0.5 < SFR < 50 M ⊙ yr -1). They are identified by their visual morphology as provided by Galaxy Zoo for Sloan Digital Sky Survey Data Release 6 and their u - r colour. We select a volume-limited sample in the redshift range 0.02 < z < 0.05, corresponding to luminosities of approximately L* and above and with u - r colours significantly bluer than the red sequence. We confirm the early-type morphology of the objects in this sample and investigate their environmental dependence and star formation properties. Blue early-type galaxies tend to live in lower density environments than 'normal' red sequence early-types and make up 5.7 ± 0.4 per cent of the low-redshift early-type galaxy population. We find that such blue early-type galaxies are virtually absent at high velocity dispersions above 200 km s -1. Our analysis uses emission line diagnostic diagrams and we find that ∼25 per cent of them are actively star forming, while another ∼25 per cent host both star formation and an active galactic nucleus (AGN). Another ∼12 per cent are AGN. The remaining 38 per cent show no strong emission lines. When present and uncontaminated by an AGN contribution, the star formation is generally intense. We consider star formation rates derived from Hα, u band and infrared luminosities, and radial colour profiles, and conclude that the star formation is spatially extended. Of those objects that are not currently undergoing star formation must have ceased doing so recently in order to account for their blue optical colours. The gas-phase metallicity of the actively star-forming blue early-types galaxies is supersolar in all cases. We discuss the place of these objects in the context of galaxy formation. A catalogue of all 204 blue early-type galaxies in our sample, including star formation rates, emission line classification is provided.",
author = "Kevin Schawinski and Chris Lintott and Daniel Thomas and Marc Sarzi and Dan Andreescu and Bamford, {Steven P.} and Sugata Kaviraj and Sadegh Khochfar and Kate Land and Phil Murray and Nichol, {Robert C.} and Raddick, {M. Jordan} and Slosar, {An Že} and Alex Szalay and Jan Vandenberg and Sukyoung Yi",
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Schawinski, K, Lintott, C, Thomas, D, Sarzi, M, Andreescu, D, Bamford, SP, Kaviraj, S, Khochfar, S, Land, K, Murray, P, Nichol, RC, Raddick, MJ, Slosar, AŽ, Szalay, A, Vandenberg, J & Yi, S 2009, 'Galaxy Zoo: A sample of blue early-type galaxies at low redshift', Monthly Notices of the Royal Astronomical Society, vol. 396, no. 2, pp. 818-829. https://doi.org/10.1111/j.1365-2966.2009.14793.x

Galaxy Zoo : A sample of blue early-type galaxies at low redshift. / Schawinski, Kevin; Lintott, Chris; Thomas, Daniel; Sarzi, Marc; Andreescu, Dan; Bamford, Steven P.; Kaviraj, Sugata; Khochfar, Sadegh; Land, Kate; Murray, Phil; Nichol, Robert C.; Raddick, M. Jordan; Slosar, An Že; Szalay, Alex; Vandenberg, Jan; Yi, Sukyoung.

In: Monthly Notices of the Royal Astronomical Society, Vol. 396, No. 2, 01.06.2009, p. 818-829.

Research output: Contribution to journalArticle

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T1 - Galaxy Zoo

T2 - A sample of blue early-type galaxies at low redshift

AU - Schawinski, Kevin

AU - Lintott, Chris

AU - Thomas, Daniel

AU - Sarzi, Marc

AU - Andreescu, Dan

AU - Bamford, Steven P.

AU - Kaviraj, Sugata

AU - Khochfar, Sadegh

AU - Land, Kate

AU - Murray, Phil

AU - Nichol, Robert C.

AU - Raddick, M. Jordan

AU - Slosar, An Že

AU - Szalay, Alex

AU - Vandenberg, Jan

AU - Yi, Sukyoung

PY - 2009/6/1

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N2 - We report the discovery of a population of nearby, blue early-type galaxies with high star formation rates (0.5 < SFR < 50 M ⊙ yr -1). They are identified by their visual morphology as provided by Galaxy Zoo for Sloan Digital Sky Survey Data Release 6 and their u - r colour. We select a volume-limited sample in the redshift range 0.02 < z < 0.05, corresponding to luminosities of approximately L* and above and with u - r colours significantly bluer than the red sequence. We confirm the early-type morphology of the objects in this sample and investigate their environmental dependence and star formation properties. Blue early-type galaxies tend to live in lower density environments than 'normal' red sequence early-types and make up 5.7 ± 0.4 per cent of the low-redshift early-type galaxy population. We find that such blue early-type galaxies are virtually absent at high velocity dispersions above 200 km s -1. Our analysis uses emission line diagnostic diagrams and we find that ∼25 per cent of them are actively star forming, while another ∼25 per cent host both star formation and an active galactic nucleus (AGN). Another ∼12 per cent are AGN. The remaining 38 per cent show no strong emission lines. When present and uncontaminated by an AGN contribution, the star formation is generally intense. We consider star formation rates derived from Hα, u band and infrared luminosities, and radial colour profiles, and conclude that the star formation is spatially extended. Of those objects that are not currently undergoing star formation must have ceased doing so recently in order to account for their blue optical colours. The gas-phase metallicity of the actively star-forming blue early-types galaxies is supersolar in all cases. We discuss the place of these objects in the context of galaxy formation. A catalogue of all 204 blue early-type galaxies in our sample, including star formation rates, emission line classification is provided.

AB - We report the discovery of a population of nearby, blue early-type galaxies with high star formation rates (0.5 < SFR < 50 M ⊙ yr -1). They are identified by their visual morphology as provided by Galaxy Zoo for Sloan Digital Sky Survey Data Release 6 and their u - r colour. We select a volume-limited sample in the redshift range 0.02 < z < 0.05, corresponding to luminosities of approximately L* and above and with u - r colours significantly bluer than the red sequence. We confirm the early-type morphology of the objects in this sample and investigate their environmental dependence and star formation properties. Blue early-type galaxies tend to live in lower density environments than 'normal' red sequence early-types and make up 5.7 ± 0.4 per cent of the low-redshift early-type galaxy population. We find that such blue early-type galaxies are virtually absent at high velocity dispersions above 200 km s -1. Our analysis uses emission line diagnostic diagrams and we find that ∼25 per cent of them are actively star forming, while another ∼25 per cent host both star formation and an active galactic nucleus (AGN). Another ∼12 per cent are AGN. The remaining 38 per cent show no strong emission lines. When present and uncontaminated by an AGN contribution, the star formation is generally intense. We consider star formation rates derived from Hα, u band and infrared luminosities, and radial colour profiles, and conclude that the star formation is spatially extended. Of those objects that are not currently undergoing star formation must have ceased doing so recently in order to account for their blue optical colours. The gas-phase metallicity of the actively star-forming blue early-types galaxies is supersolar in all cases. We discuss the place of these objects in the context of galaxy formation. A catalogue of all 204 blue early-type galaxies in our sample, including star formation rates, emission line classification is provided.

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