Monthly Notices of the Royal Astronomical Society, 2008
We combine Galaxy Evolution Explorer GALEX near-ultraviolet (NUV) photometry with a volume-limite... more We combine Galaxy Evolution Explorer GALEX near-ultraviolet (NUV) photometry with a volume-limited sample of local (0.005 < z < 0.037) Sloan Digital Sky Survey (SDSS) DR4 galaxies to examine the composition and the environmental dependencies of the optical and ultraviolet (UV)-optical colour-magnitude (CM) diagrams. We find that ∼30 per cent of red-sequence galaxies in the optical CM diagram show signs of ongoing star formation from their spectra having EW(Hα) > 2 Å. This contamination is greatest at faint magnitudes (M r > −19) and in field regions where as many as three-quarters of red-sequence galaxies are star forming, and as such has important consequences for following the build-up of the red sequence. The NUV − r colour instead allows a much more robust separation of passively evolving and star-forming galaxies, which allows the build-up of the UV-selected red sequence with redshift and environment to be directly interpreted in terms of the assembly of stellar mass in passively evolving galaxies. In isolated field regions, the number density of UV-optical red-sequence galaxies declines rapidly at magnitudes fainter than M r ∼ −19 and appears completely truncated at M r ∼ −18. These results support the downsizing paradigm whereby the red sequence is assembled from the top down, being already largely in place at the bright end by z ∼ 1, and the faint end filled in at later epochs in clusters and groups through environmentrelated processes such as ram-pressure stripping or galaxy harassment.
We present a study of the spatial distribution and kinematics of star-forming galaxies in 30 mass... more We present a study of the spatial distribution and kinematics of star-forming galaxies in 30 massive clusters at 0.15 < z < 0.30, combining wide-field Spitzer 24 μm and GALEX near-ultraviolet imaging with highly complete spectroscopy of cluster members. The fraction (f SF) of star-forming cluster galaxies rises steadily with clustercentric radius, increasing fivefold by 2r 200 , but remains well below field values even at 3r 200. This suppression of star formation at large radii cannot be reproduced by models in which star formation is quenched in infalling field galaxies only once they pass within r 200 of the cluster, but is consistent with some of them being first pre-processed within galaxy groups. Despite the increasing f SF-radius trend, the surface density of star-forming galaxies actually declines steadily with radius, falling ∼15× from the core to 2r 200. This requires star formation to survive within recently accreted spirals for 2-3 Gyr to build up the apparent over-density of star-forming galaxies within clusters. The velocity dispersion profile of the star-forming galaxy population shows a sharp peak of 1.44 σ ν at 0.3r 500 , and is 10%-35% higher than that of the inactive cluster members at all cluster-centric radii, while their velocity distribution shows a flat, top-hat profile within r 500. All of these results are consistent with star-forming cluster galaxies being an infalling population, but one that must also survive ∼0.5-2 Gyr beyond passing within r 200. By comparing the observed distribution of star-forming galaxies in the stacked caustic diagram with predictions from the Millennium simulation, we obtain a best-fit model in which star formation rates decline exponentially on quenching timescales of 1.73 ± 0.25 Gyr upon accretion into the cluster.
We present a sample of 290 24µm-selected active galactic nuclei (AGNs) mostly at z ∼ 0.3 − 2.5, w... more We present a sample of 290 24µm-selected active galactic nuclei (AGNs) mostly at z ∼ 0.3 − 2.5, within 5.2 deg 2 distributed as 25 × 25 fields around each of 30 galaxy clusters in the Local Cluster Substructure Survey (LoCuSS). The sample is nearly complete to 1 mJy at 24µm, and has a rich multi-wavelength set of ancillary data; 162 are detected by Herschel. We use spectral templates for AGNs, stellar populations, and infrared emission by star forming galaxies to decompose the spectral energy distributions (SEDs) of these AGNs and their host galaxies, and estimate their star formation rates (SFRs), AGN luminosities, and host galaxy stellar masses. The set of templates is relatively simple: a standard Type-1 quasar template; another for the photospheric output of the stellar population; and a far infrared star-formng template. For the Type-2 AGN SEDs, we substitute templates including internal obscuration, and some Type-1 objects require a warm component (T > ∼ 50 K). The individually Herscheldetected Type-1 AGNs and a subset of 17 Type-2 ones typically have luminosities > 10 45 ergs s −1 , and supermassive black holes of ∼ 3 × 10 8 M emitting at ∼ 10% of the Eddington rate. We find them in about twice the numbers of AGN identified in SDSS data in the same fields, i.e., they represent typical high luminosity AGN, not an infrared-selected minority. These AGNs and their host galaxies are studied further in an accompanying paper.
We evaluate the dry merger activity in the Coma cluster, using a spectroscopically complete sampl... more We evaluate the dry merger activity in the Coma cluster, using a spectroscopically complete sample of 70 red sequence (RS) galaxies, most of which (∼ 75%) are located within 0.2R 200 (∼ 0.5 Mpc) from the cluster center, with data from the Coma Treasury Survey obtained with the Hubble Space Telescope. The fraction of close galaxy pairs in the sample is the proxy employed for the estimation of the merger activity. We identify five pairs and one triplet, enclosing a total of 13 galaxies, based on limits on projected separation and line-of-sight velocity difference. Of these systems, none show signs of ongoing interaction and therefore we do not find any true merger in our sample. This negative result sets a 1-sigma upper limit of 1.5% per Gyr for the major dry merger rate, consistent with the low rates expected in present-day clusters. Detailed examination of the images of all the RS galaxies in the sample reveals only one with low surface brightness features identifiable as the remnant of a past merger or interaction, implying a post-merger fraction below two percent.
We present wide-field Herschel/PACS observations of Abell 1689, a massive galaxy cluster at z=0.1... more We present wide-field Herschel/PACS observations of Abell 1689, a massive galaxy cluster at z=0.1832, from our Open Time Key Programme. We detect 39 spectroscopically confirmed 100micron-selected cluster members down to 1.5x10^10 Lsun. These galaxies are forming stars at rates in the range 1-10 Msun/yr, and appear to comprise two distinct populations: two-thirds are unremarkable blue, late-type spirals found throughout the cluster; the remainder are dusty red sequence galaxies whose star formation is heavily obscured with A(Halpha)~2 mag, and are found only in the cluster outskirts. The specific-SFRs of these dusty red galaxies are lower than the blue late-types, suggesting that the former are in the process of being quenched, perhaps via pre-processing, the unobscured star formation being terminated first. We also detect an excess of 100micron-selected galaxies extending ~6 Mpc in length along an axis that runs NE-SW through the cluster centre at >95% confidence. Qualitatively t...
We present a joint analysis of panoramic Spitzer/MIPS mid-infrared and GALEX ultraviolet imaging ... more We present a joint analysis of panoramic Spitzer/MIPS mid-infrared and GALEX ultraviolet imaging of the Shapley supercluster at z=0.048. Combining this with spectra of 814 supercluster members and 1.4GHz radio continuum maps, this represents the largest complete census of star-formation (both obscured and unobscured) in local cluster galaxies to date, reaching SFRs~0.02Msun/yr. We take advantage of this comprehensive panchromatic dataset to perform a detailed analysis of the nature of star formation in cluster galaxies, using several quite independent diagnostics of the quantity and intensity of star formation to develop a coherent view of the types of star formation within cluster galaxies. We observe a robust bimodality in the infrared (f_24/f_K) galaxy colours, which we are able to identify as another manifestation of the broad split into star-forming spiral and passive elliptical galaxy populations seen in UV-optical surveys. This diagnostic also allows the identification of gal...
Monthly Notices of the Royal Astronomical Society, 2010
We present panoramic Spitzer/MIPS mid-and far-infrared (MIR/FIR) and GALEX ultraviolet imaging of... more We present panoramic Spitzer/MIPS mid-and far-infrared (MIR/FIR) and GALEX ultraviolet imaging of the the most massive and dynamically active system in the local Universe, the Shapley supercluster at z=0.048, covering the five clusters which make up the supercluster core. We combine these data with existing spectroscopic data from 814 confirmed supercluster members to produce the first study of a local rich cluster including both ultraviolet and infrared luminosity functions (LFs). This joint analysis allows us to produce a complete census of star-formation (both obscured and unobscured), extending down to SFRs∼0.02-0.05 M ⊙ yr -1 , and quantify the level of obscuration of star formation among cluster galaxies, providing a local benchmark for comparison to ongoing and future studies of cluster galaxies at higher redshifts with Spitzer and Herschel. The GALEX near-ultraviolet (NUV) and far-ultraviolet (FUV) luminosity functions (LFs) obtained have steeper faint-end slopes than the local field population, due largely to the contribution of massive, quiescent galaxies at M F UV -16. The 24µm and 70µm galaxy LFs for the Shapley supercluster instead have shapes fully consistent with those obtained for the Coma cluster and for the local field galaxy population. This apparent lack of environmental dependence for the shape of the FIR luminosity function suggests that the bulk of the star-forming galaxies that make up the observed cluster infrared LF have been recently accreted from the field and have yet to have their star formation activity significantly affected by the cluster environment. We estimate a global SFR of 327 M ⊙ yr -1 over the whole supercluster core, of which just ∼20 per cent is visible directly in the ultraviolet continuum and ∼80 per cent is reprocessed by dust and emitted in the infrared. The level of obscuration (L IR /L F UV ) in star-forming galaxies is seen to increase linearly with L K over two orders of magnitude in stellar mass.
Monthly Notices of the Royal Astronomical Society, 2011
We present a joint analysis of panoramic Spitzer/MIPS mid-and far-infrared (MIR/FIR) and GALEX ul... more We present a joint analysis of panoramic Spitzer/MIPS mid-and far-infrared (MIR/FIR) and GALEX ultraviolet imaging of the most massive and dynamically active structure in the local Universe, the Shapley supercluster at z=0.048, covering the five clusters which make up the supercluster core. Combining this with existing spectroscopic data from 814 confirmed supercluster members and 1.4 GHz radio continuum maps, this represents the largest complete census of star-formation (both obscured and unobscured) in local cluster galaxies to date, extending down to SFRs∼0.02-0.05 M ⊙ yr -1 . We take advantage of this comprehensive panchromatic dataset to perform a detailed analysis of the nature of star formation in cluster galaxies of the kind previously limited to local field galaxy surveys such as SINGS, using several quite independent diagnostics of the quantity and intensity of star formation to develop a coherent view of the types of star formation within cluster galaxies. We observe a robust bimodality in the infrared (f 24µm /f K ) galaxy colours, which we are able to identify as another manifestation of the broad split into star-forming spiral and passive elliptical galaxy populations seen in UV-optical surveys. This diagnostic also allows the identification of galaxies in the process of having their star formation quenched as the infrared analogue to the ultraviolet "green valley" population. The bulk of supercluster galaxies on the star-forming sequence have specific-SFRs consistent with local field specific-SFR-M relations of , and form a tight FIR-radio correlation (0.24 dex) confirming that their FIR emission is due to star formation. We show that 85 per cent of the global SFR is quiescent star formation within spiral disks, as manifest by the observed sequence in the (L IR /F F UV )-(F U V -N U V ) plane -the IRX-β relation -being significantly offset from the starburst relation of , while their FIR-radio colours indicate dust heated by low-intensity star formation. Just 15 per cent of the global SFR is due to nuclear starbursts. The vast majority of star formation seen in cluster galaxies comes from normal infalling spirals who have yet to be affected by the cluster environment.
We study the mid-infrared (MIR) properties of galaxies in 30 massive galaxy clusters at 0.02 z 0.... more We study the mid-infrared (MIR) properties of galaxies in 30 massive galaxy clusters at 0.02 z 0.40, using panoramic Spitzer/MIPS 24 μm and near-infrared data, including 27 new observations from the LoCuSS and ACCESS surveys. This is the largest sample of clusters to date with such high-quality and uniform MIR data covering not only the cluster cores, but extending into the infall regions. We use these data to revisit the socalled Butcher–Oemler (BO) effect, measuring the fraction of massive infrared luminous galaxies (K < K +1.5, LIR > 5×1010 L ) within r200, finding a steady increase in the fraction with redshift from ∼3% at z = 0.02 to ∼10% by z = 0.30, and an rms cluster-to-cluster scatter about this trend of 0.03. The best-fit redshift evolution model of the form fSF ∝ (1+z) has n = 5.7+2.1 −1.8, which is stronger redshift evolution than that of L IR in both clusters and the field. We find that, statistically, this excess is associated with galaxies found at large cluster...
We study the mid-infrared (MIR) properties of galaxies in 30 massive galaxy clusters at 0.02 z 0.... more We study the mid-infrared (MIR) properties of galaxies in 30 massive galaxy clusters at 0.02 z 0.40, using panoramic Spitzer/MIPS 24 μm and near-infrared data, including 27 new observations from the LoCuSS and ACCESS surveys. This is the largest sample of clusters to date with such high-quality and uniform MIR data covering not only the cluster cores, but extending into the infall regions. We use these data to revisit the socalled Butcher-Oemler (BO) effect, measuring the fraction of massive infrared luminous galaxies (K < K +1.5, L IR > 5×10 10 L) within r 200 , finding a steady increase in the fraction with redshift from ∼3% at z = 0.02 to ∼10% by z = 0.30, and an rms cluster-to-cluster scatter about this trend of 0.03. The best-fit redshift evolution model of the form f SF ∝ (1 + z) n has n = 5.7 +2.1 −1.8 , which is stronger redshift evolution than that of L IR in both clusters and the field. We find that, statistically, this excess is associated with galaxies found at large cluster-centric radii, specifically r 500 < r < r 200 , implying that the MIR BO effect can be explained by a combination of both the global decline in star formation in the universe since z ∼ 1 and enhanced star formation in the infall regions of clusters at intermediate redshifts. This picture is supported by a simple infall model based on the Millennium Simulation semianalytic galaxy catalogs, whereby star formation in infalling galaxies is instantaneously quenched upon their first passage through the cluster, in that the observed radial trends of f SF trace those inferred from the simulations. The observed f SF values, however, lie systematically above the predictions, suggesting an overall excess of star formation, either due to triggering by environmental processes, or a gradual quenching. We also find that f SF does not depend on simple indicators of the dynamical state of clusters, including the offset between the brightest cluster galaxy and the peak of the X-ray emission. This is consistent with the picture described above in that most new star formation in clusters occurs in the infall regions, and is thus not sensitive to the details of cluster-cluster mergers in the core regions.
Monthly Notices of the Royal Astronomical Society, 2008
We combine Galaxy Evolution Explorer GALEX near-ultraviolet (NUV) photometry with a volume-limite... more We combine Galaxy Evolution Explorer GALEX near-ultraviolet (NUV) photometry with a volume-limited sample of local (0.005 < z < 0.037) Sloan Digital Sky Survey (SDSS) DR4 galaxies to examine the composition and the environmental dependencies of the optical and ultraviolet (UV)-optical colour-magnitude (CM) diagrams. We find that ∼30 per cent of red-sequence galaxies in the optical CM diagram show signs of ongoing star formation from their spectra having EW(Hα) > 2 Å. This contamination is greatest at faint magnitudes (M r > −19) and in field regions where as many as three-quarters of red-sequence galaxies are star forming, and as such has important consequences for following the build-up of the red sequence. The NUV − r colour instead allows a much more robust separation of passively evolving and star-forming galaxies, which allows the build-up of the UV-selected red sequence with redshift and environment to be directly interpreted in terms of the assembly of stellar mass in passively evolving galaxies. In isolated field regions, the number density of UV-optical red-sequence galaxies declines rapidly at magnitudes fainter than M r ∼ −19 and appears completely truncated at M r ∼ −18. These results support the downsizing paradigm whereby the red sequence is assembled from the top down, being already largely in place at the bright end by z ∼ 1, and the faint end filled in at later epochs in clusters and groups through environmentrelated processes such as ram-pressure stripping or galaxy harassment.
We present a study of the spatial distribution and kinematics of star-forming galaxies in 30 mass... more We present a study of the spatial distribution and kinematics of star-forming galaxies in 30 massive clusters at 0.15 < z < 0.30, combining wide-field Spitzer 24 μm and GALEX near-ultraviolet imaging with highly complete spectroscopy of cluster members. The fraction (f SF) of star-forming cluster galaxies rises steadily with clustercentric radius, increasing fivefold by 2r 200 , but remains well below field values even at 3r 200. This suppression of star formation at large radii cannot be reproduced by models in which star formation is quenched in infalling field galaxies only once they pass within r 200 of the cluster, but is consistent with some of them being first pre-processed within galaxy groups. Despite the increasing f SF-radius trend, the surface density of star-forming galaxies actually declines steadily with radius, falling ∼15× from the core to 2r 200. This requires star formation to survive within recently accreted spirals for 2-3 Gyr to build up the apparent over-density of star-forming galaxies within clusters. The velocity dispersion profile of the star-forming galaxy population shows a sharp peak of 1.44 σ ν at 0.3r 500 , and is 10%-35% higher than that of the inactive cluster members at all cluster-centric radii, while their velocity distribution shows a flat, top-hat profile within r 500. All of these results are consistent with star-forming cluster galaxies being an infalling population, but one that must also survive ∼0.5-2 Gyr beyond passing within r 200. By comparing the observed distribution of star-forming galaxies in the stacked caustic diagram with predictions from the Millennium simulation, we obtain a best-fit model in which star formation rates decline exponentially on quenching timescales of 1.73 ± 0.25 Gyr upon accretion into the cluster.
We present a sample of 290 24µm-selected active galactic nuclei (AGNs) mostly at z ∼ 0.3 − 2.5, w... more We present a sample of 290 24µm-selected active galactic nuclei (AGNs) mostly at z ∼ 0.3 − 2.5, within 5.2 deg 2 distributed as 25 × 25 fields around each of 30 galaxy clusters in the Local Cluster Substructure Survey (LoCuSS). The sample is nearly complete to 1 mJy at 24µm, and has a rich multi-wavelength set of ancillary data; 162 are detected by Herschel. We use spectral templates for AGNs, stellar populations, and infrared emission by star forming galaxies to decompose the spectral energy distributions (SEDs) of these AGNs and their host galaxies, and estimate their star formation rates (SFRs), AGN luminosities, and host galaxy stellar masses. The set of templates is relatively simple: a standard Type-1 quasar template; another for the photospheric output of the stellar population; and a far infrared star-formng template. For the Type-2 AGN SEDs, we substitute templates including internal obscuration, and some Type-1 objects require a warm component (T > ∼ 50 K). The individually Herscheldetected Type-1 AGNs and a subset of 17 Type-2 ones typically have luminosities > 10 45 ergs s −1 , and supermassive black holes of ∼ 3 × 10 8 M emitting at ∼ 10% of the Eddington rate. We find them in about twice the numbers of AGN identified in SDSS data in the same fields, i.e., they represent typical high luminosity AGN, not an infrared-selected minority. These AGNs and their host galaxies are studied further in an accompanying paper.
We evaluate the dry merger activity in the Coma cluster, using a spectroscopically complete sampl... more We evaluate the dry merger activity in the Coma cluster, using a spectroscopically complete sample of 70 red sequence (RS) galaxies, most of which (∼ 75%) are located within 0.2R 200 (∼ 0.5 Mpc) from the cluster center, with data from the Coma Treasury Survey obtained with the Hubble Space Telescope. The fraction of close galaxy pairs in the sample is the proxy employed for the estimation of the merger activity. We identify five pairs and one triplet, enclosing a total of 13 galaxies, based on limits on projected separation and line-of-sight velocity difference. Of these systems, none show signs of ongoing interaction and therefore we do not find any true merger in our sample. This negative result sets a 1-sigma upper limit of 1.5% per Gyr for the major dry merger rate, consistent with the low rates expected in present-day clusters. Detailed examination of the images of all the RS galaxies in the sample reveals only one with low surface brightness features identifiable as the remnant of a past merger or interaction, implying a post-merger fraction below two percent.
We present wide-field Herschel/PACS observations of Abell 1689, a massive galaxy cluster at z=0.1... more We present wide-field Herschel/PACS observations of Abell 1689, a massive galaxy cluster at z=0.1832, from our Open Time Key Programme. We detect 39 spectroscopically confirmed 100micron-selected cluster members down to 1.5x10^10 Lsun. These galaxies are forming stars at rates in the range 1-10 Msun/yr, and appear to comprise two distinct populations: two-thirds are unremarkable blue, late-type spirals found throughout the cluster; the remainder are dusty red sequence galaxies whose star formation is heavily obscured with A(Halpha)~2 mag, and are found only in the cluster outskirts. The specific-SFRs of these dusty red galaxies are lower than the blue late-types, suggesting that the former are in the process of being quenched, perhaps via pre-processing, the unobscured star formation being terminated first. We also detect an excess of 100micron-selected galaxies extending ~6 Mpc in length along an axis that runs NE-SW through the cluster centre at >95% confidence. Qualitatively t...
We present a joint analysis of panoramic Spitzer/MIPS mid-infrared and GALEX ultraviolet imaging ... more We present a joint analysis of panoramic Spitzer/MIPS mid-infrared and GALEX ultraviolet imaging of the Shapley supercluster at z=0.048. Combining this with spectra of 814 supercluster members and 1.4GHz radio continuum maps, this represents the largest complete census of star-formation (both obscured and unobscured) in local cluster galaxies to date, reaching SFRs~0.02Msun/yr. We take advantage of this comprehensive panchromatic dataset to perform a detailed analysis of the nature of star formation in cluster galaxies, using several quite independent diagnostics of the quantity and intensity of star formation to develop a coherent view of the types of star formation within cluster galaxies. We observe a robust bimodality in the infrared (f_24/f_K) galaxy colours, which we are able to identify as another manifestation of the broad split into star-forming spiral and passive elliptical galaxy populations seen in UV-optical surveys. This diagnostic also allows the identification of gal...
Monthly Notices of the Royal Astronomical Society, 2010
We present panoramic Spitzer/MIPS mid-and far-infrared (MIR/FIR) and GALEX ultraviolet imaging of... more We present panoramic Spitzer/MIPS mid-and far-infrared (MIR/FIR) and GALEX ultraviolet imaging of the the most massive and dynamically active system in the local Universe, the Shapley supercluster at z=0.048, covering the five clusters which make up the supercluster core. We combine these data with existing spectroscopic data from 814 confirmed supercluster members to produce the first study of a local rich cluster including both ultraviolet and infrared luminosity functions (LFs). This joint analysis allows us to produce a complete census of star-formation (both obscured and unobscured), extending down to SFRs∼0.02-0.05 M ⊙ yr -1 , and quantify the level of obscuration of star formation among cluster galaxies, providing a local benchmark for comparison to ongoing and future studies of cluster galaxies at higher redshifts with Spitzer and Herschel. The GALEX near-ultraviolet (NUV) and far-ultraviolet (FUV) luminosity functions (LFs) obtained have steeper faint-end slopes than the local field population, due largely to the contribution of massive, quiescent galaxies at M F UV -16. The 24µm and 70µm galaxy LFs for the Shapley supercluster instead have shapes fully consistent with those obtained for the Coma cluster and for the local field galaxy population. This apparent lack of environmental dependence for the shape of the FIR luminosity function suggests that the bulk of the star-forming galaxies that make up the observed cluster infrared LF have been recently accreted from the field and have yet to have their star formation activity significantly affected by the cluster environment. We estimate a global SFR of 327 M ⊙ yr -1 over the whole supercluster core, of which just ∼20 per cent is visible directly in the ultraviolet continuum and ∼80 per cent is reprocessed by dust and emitted in the infrared. The level of obscuration (L IR /L F UV ) in star-forming galaxies is seen to increase linearly with L K over two orders of magnitude in stellar mass.
Monthly Notices of the Royal Astronomical Society, 2011
We present a joint analysis of panoramic Spitzer/MIPS mid-and far-infrared (MIR/FIR) and GALEX ul... more We present a joint analysis of panoramic Spitzer/MIPS mid-and far-infrared (MIR/FIR) and GALEX ultraviolet imaging of the most massive and dynamically active structure in the local Universe, the Shapley supercluster at z=0.048, covering the five clusters which make up the supercluster core. Combining this with existing spectroscopic data from 814 confirmed supercluster members and 1.4 GHz radio continuum maps, this represents the largest complete census of star-formation (both obscured and unobscured) in local cluster galaxies to date, extending down to SFRs∼0.02-0.05 M ⊙ yr -1 . We take advantage of this comprehensive panchromatic dataset to perform a detailed analysis of the nature of star formation in cluster galaxies of the kind previously limited to local field galaxy surveys such as SINGS, using several quite independent diagnostics of the quantity and intensity of star formation to develop a coherent view of the types of star formation within cluster galaxies. We observe a robust bimodality in the infrared (f 24µm /f K ) galaxy colours, which we are able to identify as another manifestation of the broad split into star-forming spiral and passive elliptical galaxy populations seen in UV-optical surveys. This diagnostic also allows the identification of galaxies in the process of having their star formation quenched as the infrared analogue to the ultraviolet "green valley" population. The bulk of supercluster galaxies on the star-forming sequence have specific-SFRs consistent with local field specific-SFR-M relations of , and form a tight FIR-radio correlation (0.24 dex) confirming that their FIR emission is due to star formation. We show that 85 per cent of the global SFR is quiescent star formation within spiral disks, as manifest by the observed sequence in the (L IR /F F UV )-(F U V -N U V ) plane -the IRX-β relation -being significantly offset from the starburst relation of , while their FIR-radio colours indicate dust heated by low-intensity star formation. Just 15 per cent of the global SFR is due to nuclear starbursts. The vast majority of star formation seen in cluster galaxies comes from normal infalling spirals who have yet to be affected by the cluster environment.
We study the mid-infrared (MIR) properties of galaxies in 30 massive galaxy clusters at 0.02 z 0.... more We study the mid-infrared (MIR) properties of galaxies in 30 massive galaxy clusters at 0.02 z 0.40, using panoramic Spitzer/MIPS 24 μm and near-infrared data, including 27 new observations from the LoCuSS and ACCESS surveys. This is the largest sample of clusters to date with such high-quality and uniform MIR data covering not only the cluster cores, but extending into the infall regions. We use these data to revisit the socalled Butcher–Oemler (BO) effect, measuring the fraction of massive infrared luminous galaxies (K < K +1.5, LIR > 5×1010 L ) within r200, finding a steady increase in the fraction with redshift from ∼3% at z = 0.02 to ∼10% by z = 0.30, and an rms cluster-to-cluster scatter about this trend of 0.03. The best-fit redshift evolution model of the form fSF ∝ (1+z) has n = 5.7+2.1 −1.8, which is stronger redshift evolution than that of L IR in both clusters and the field. We find that, statistically, this excess is associated with galaxies found at large cluster...
We study the mid-infrared (MIR) properties of galaxies in 30 massive galaxy clusters at 0.02 z 0.... more We study the mid-infrared (MIR) properties of galaxies in 30 massive galaxy clusters at 0.02 z 0.40, using panoramic Spitzer/MIPS 24 μm and near-infrared data, including 27 new observations from the LoCuSS and ACCESS surveys. This is the largest sample of clusters to date with such high-quality and uniform MIR data covering not only the cluster cores, but extending into the infall regions. We use these data to revisit the socalled Butcher-Oemler (BO) effect, measuring the fraction of massive infrared luminous galaxies (K < K +1.5, L IR > 5×10 10 L) within r 200 , finding a steady increase in the fraction with redshift from ∼3% at z = 0.02 to ∼10% by z = 0.30, and an rms cluster-to-cluster scatter about this trend of 0.03. The best-fit redshift evolution model of the form f SF ∝ (1 + z) n has n = 5.7 +2.1 −1.8 , which is stronger redshift evolution than that of L IR in both clusters and the field. We find that, statistically, this excess is associated with galaxies found at large cluster-centric radii, specifically r 500 < r < r 200 , implying that the MIR BO effect can be explained by a combination of both the global decline in star formation in the universe since z ∼ 1 and enhanced star formation in the infall regions of clusters at intermediate redshifts. This picture is supported by a simple infall model based on the Millennium Simulation semianalytic galaxy catalogs, whereby star formation in infalling galaxies is instantaneously quenched upon their first passage through the cluster, in that the observed radial trends of f SF trace those inferred from the simulations. The observed f SF values, however, lie systematically above the predictions, suggesting an overall excess of star formation, either due to triggering by environmental processes, or a gradual quenching. We also find that f SF does not depend on simple indicators of the dynamical state of clusters, including the offset between the brightest cluster galaxy and the peak of the X-ray emission. This is consistent with the picture described above in that most new star formation in clusters occurs in the infall regions, and is thus not sensitive to the details of cluster-cluster mergers in the core regions.
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