Clusters as Laboratories for the Study of Galaxy Evolution Alan DresslerCarnegie Observatories Ringberg 2005 “Distant Clusters of Galaxies”
“Clusters…are laboratories for the study of galaxy evolution and may become as useful as star clusters are in the study of stellar evolution” -- Dressler, 1984 Annual Reviews Back in 1984, (and still?) the prevalent view was that clusters were themselves agents that influenced the evolution of galaxies, so that the processes that produced different morphological types, for example, could be observed at work in (today’s) rich clusters. This notion is very different than the case of star clusters, where we exploit the coeval populations of stars to understand how stars evolve, but do not expect that the evolution of these stars has been influenced by the cluster environment. Given this difference, my “laboratories” statement would have been an imperfect analogy, at best. In this talk I will review evidence that clusters of galaxies may indeed be a good place to study galaxy evolution, but, not primarily because of the present-day cluster environment – just the like star clusters.
Topics to be covered in this talk: • Investigate the evidence that nearly all elliptical galaxies (and some S0 galaxies) in today’s rich clusters were formed very early and their properties are basically independent of the cluster environment. Look at some counter-evidence, too. • How is the evolution of disk galaxies affected by the cluster environment, or is that too a second-order effect? Star formation is more enhanced in cluster galaxies at z>0.3 compared to z=0, but it is always less than in the field at the same-epoch. A morphology-density relation holds for the z > 0.3 clusters, and starbursts are quite common. What mechanisms are responsible for these effects – do they operate in rich clusters or in the infall to rich clusters? • Describe an instrument well suited for studying the outskirts of galaxy clusters: IMACS on the Magellan-Baade Telescope, and introduce the IMACS Cluster-Building Survey)
Present-epoch morphology/density relation (Dressler 1980) At the Nearly Normal Galaxies II meeting, Sandy Faber showed this morphology-density relation and remarked that it influenced all of us to take seriously the idea that elliptical galaxies are formed by mergers (following Toomre & Toomre) I was very surprised – we all know that ellipticals in today’s cluster cores are in environments that suppress mergers (high ΔV’s). The alternate point of view is that cluster environment has nothing to do with it, at least not today’s environment, but that the prevalence of E’s in rich clusters tells us about the environment in which these galaxies were born, at z > 2.
Let’s look at the evidence that suggests that cluster elliptical galaxies are very old, zf > 2. If this is true, E’s formed in a very different environment from the cores of today’s clusters, in a less-deep potential well (lower velocity dispersion), without a pervasive hot ICM and without strong tidal effects from the cluster core.
from Gladders et al. Color-magnitude relations (CMR) as a tool for determining the stellar ages of elliptical galaxies *Bower R. G., Lucey J. R., Ellis R., 1992, MNRAS, 254, 589 were the first to do this well, in Virgo and Coma, concluding that the small scatter of the red sequence implied formation ages z > 2. Extending this to higher redshift increases the “leverage” on the formation age of the red galaxies: “The Homogeneity of Spheroidal Populations in Distant Clusters” – Ellis et al. (Morphs), 1997 ApJ, 483, 582. “The evolution of early-type galaxies in distant clusters” -- Stanford, Eisenhardt, & Dickson, 1998 ApJ, 492, 461. “The Slope of the Cluster Elliptical Red Sequence: A Probe of Cluster Evolution” – Gladders et al, 1998 ApJ, 501, 571. z=0 z=1.2
“Advanced Camera for Surveys Photometry of the Cluster RDCS 1252.9-2927: The Color-Magnitude Relation at z = 1.24” -- Blakeslee et al., 2003 ApJ 596, L143 “Simulations using the latest stellar population models indicate an age scatter for the ellipticals of about 34%, with a mean age τL>~2.6 Gyr (corresponding to zL>~2.7, and the last star formation occuring at zend>~1.5. …Thus, elliptical galaxies were already well established in X-ray-luminous clusters when the universe was a third of its present age.” Deep HST/ACS imaging – galaxies selected by morphological classification, (3.5 mag of LF) including distinguishing E’s from S0’s. 52 galaxies: σ = 0.029 mag! or 0.024 mag for 30 ellipticals Slope (top) and scatter (bottom) of the rest-frame (U-B) CM relation as a function of redshift. The filled circle is from the present work; the open circles show results from, in order of increasing redshift, Bower, Lucey, & Ellis (1992); van Dokkum et al. (1998); Ellis et al. (1997); van Dokkum et al. (2000); and van Dokkum et al. (2001). The dotted lines indicate the average values.
No dependence on zero-point or slope of CMR with regard to richness and x-ray luminosity “The Environmental Dependence of Galaxy Colors in Intermediate-Redshift X-Ray-selected Clusters” – Wake et al., 2005 ApJ 627, 186 “We find that the “red” galaxies form a tight color-magnitude relation (CMR) and that neither the slope nor zero point of this relation changes significantly over the factor of 100 in X-ray luminosity covered by our sample.” – neither does the blue fraction, but both “depend significantly with cluster radius…It thus appears that the global cluster environment, in the form of cluster mass (Lx), has little influence on the properties of the bright cluster galaxies, whereas the local environment, in the form of galaxy density (radius), has a strong effect. Also, “Homogeneity of early-type galaxies across clusters” – Andreon, 2003 A&A, 409, 37. SDSS investigated – no dependence of red sequence on group or cluster richness.
z=0 “Hubble Space Telescope Photometry and Keck Spectroscopy of the Rich Cluster MS 1054-03: Morphologies, Butcher-Oemler Effect, and the Color-Magnitude Relation at Z = 0.83” – van Dokkum et al., 2000 ApJ, 541, 95. van Dokkum and collaborators have argued that this tight CMR red-sequence at z~1 is an illusion. Progenitor bias – you only see on the red sequence galaxies where star formation has ceased, and this could be recent, and the progenitors of the next bunch of red-sequence galaxies can be actively forming stars and blue only 1-2 Gyr earlier. z=1.2
The quantitative effects of progenitor bias: “The elliptical galaxy colour magnitude relation as a discriminant between the monolithic and merger paradigms” – Kaviraj et al., 2005 MNRAS, 360, 60. “We use a cold dark matter hierarchical merger model of galaxy formation to investigate the existence and redshift evolution of the elliptical galaxy CMR in the merger paradigm. We show that the SFH of cluster ellipticals predicted by the model is quasi-monolithic, with only 10 per cent of the total stellar mass forming after z = 1…[The CMR] is not a meaningful tool for constraining the SFH in the merger paradigm, since a progressively larger fraction of the progenitor set of present-day cluster ellipticals is contained in late-type star-forming systems at higher redshift, which cannot be ignored when deriving the SFHs.
“Spectroscopic Confirmation of Multiple Red Galaxy-Galaxy Mergers at z < 1” -- Tran et al. 2005 ApJ627,L25 Identified as a special epoch in the cluster’s life, or significant subcluster merging (relative velocities still low), before virialization The apparent mergers are real…R<14 kpc, ΔV < 165 km s-1. “…these bound pairs must evolve into E/S0 members by z = 0.7…most if not all, of its early-type members evolved from (passive) galaxy-galaxy mergers at z < 1”
Can progenitor bias “save the day” for CDM hierarchical manufacture of massive elliptical galaxies?: • Problems to overcome: • The scatter in the CMR does not seem to increase with z, as predicted • (2) The luminosity function of clusters does not shift as predicted. • (3) The fundamental plane is not populated differently at z~1 • (4) The morphology-density relation at z~1 shows the same fraction of luminous ellipticals at a given density – it should instead contain the progenitors, which would include late-types. • (5) The high Mg/Fe ratios of luminous ellipticals seem to argue for prompt enrichment, i.e., early star formation, with later (Pop II supernoave) suppressed (Trager et al. 2000 AJ, 119, 1645) Kaviraj et al.
“Evolution of Cluster Elliptical Galaxies” – Barrientos & Lilly, 2003 ApJ, 596, 129 A study of 8 clusters at z ~ 0.45 “We find that the luminosity function for red galaxies with -25.6 < MV < -17.6… are well described by a Schechter function for all clusters. In spite of the fact that the clusters span a wide range in richness and X-ray luminosities, all the cluster luminosity functions are consistent with a single value for their characteristic magnitude (M*). The value of M* at z = 0.45 appears brighter than that observed in local clusters ( M = -0.94 ± 0.41), in agreement with models of passive evolution and with studies of surface brightness and fundamental plane at these redshifts.”
“Ks-band luminosity function of the z = 1.237 cluster of galaxies RDCS J1252.9-2927”, Toft et al., 2004 A&A, 422, 29. “The brightening of the characteristic magnitude, and lack of evolution in the shape of the bright end of the LF to redshift z=1.237 is consistent with a simple formation scenario in which the massive elliptical galaxies that dominate the bright end of the K-band LF are passively evolving systems assembled at high redshift…also in agreement with the observed properties of the CM-relation of elliptical galaxies in RDCS J1252.9-2927, … consistent with old populations of stars formed at 2.7 < zf < 3.6... From the evolution of the CM relationship alone it is not possible to distinguish between formation scenarios where the old stars are formed in monolithic collapse…[or] in the disks of less massive late-type galaxies which later merge to form the ellipticals, as long as the merging does not trigger significant star formation. From the lack of evolution in the shape of the bright end of the K-band LF we can however deduce that if the massive ellipticals in clusters formed through merging, it took place at higher redshifts (z >> 1 ) than is predicted by current semi- analytical models.” p.s. van Dokkum and Stanford, 2003 ApJ, 585, 78, have measured velocity dispersions for 3 of the bright galaxies in this cluster, which confirm the large mass M > 10^11 solar masses for two of these. The third is a factor of two less massive with strong Balmer lines in its spectrum. All three fall into the same “extremely red object class.” “…our results show that it is hazardess to use simple models for converting luminosity to mass for these objects.” See also Holden et al. 2005 ApJ, 620, L83. “Thus, even in the centers of massive clusters, there appears to have been significant star formation in some massive (> 10^11 Msun) galaxies at z ~ 1.5” Deep VLT/ISAAC K-band (plus optical imaging for photo-z’s) of a very distant cluster “…bright end of the LF was already in place at z=1.237, while the flattening of the faint end slope suggests that clusters at z~1 contain relatively smaller fractions of low mass galaxies than clusters in the local universe.”
The Buildup of Red Sequence in Galaxy Clusters since z ~ 0.8, De Lucia et al, ApJ, 610, L77. Uses 4 clusters from EDisCS (VLT) project, z=0.7-0.8 to study the evolution of the red sequence in earlier epoch clusters. Comparing to the Coma cluster, they find that the luminous L/L* > 0.4 galaxies are “in place,” and old, passively evolving population. In contrast, the less luminous galaxies 0.1 > L/L* > 0.4 are under-represented. They conclude that many of these galaxies are arrived more recently, and that star formation continued for them after z = 0.8.
From Toft et al., a summary of K-band cluster photometry – the evolution of K* -- and its comparison with models of Kodama & Arimoto (1997)
“On the evolutionary status of early-type galaxies in clusters at z ~ 0.2 I. The Fundamental Plane” – Fritz et al. 2005 MNRAS, 358, 233 Photometry, morphology, and spectra for galaxies in A2390 & A2218 “For the total sample of 34 E+S0 cluster galaxies which enter the FP we deduce only a mild evolution with a zero-point offset [from Coma] of 0.10 ± 0.06, corresponding to a brightening of 0.31 ± 0.18 mag. Elliptical and lenticular galaxies are uniformly distributed along the FP with a similar scatter of 0.1 dex. Within our sample we find little evidence for differences between the populations of elliptical and S0 galaxies… Our results can be reconciled with a passive evolution of the stellar populations and a high formation redshift for the bulk of the stars in early-type galaxies. However, our findings are also consistent with the hierarchical formation picture for rich clusters, if ellipticals in clusters had their last major merger at high redshift.” Two projections of the fundamental plane:
“The Detailed Fundamental Plane of Two High-Redshift Clusters: MS 2053-04 at z=0.58 and MS 1054-03 at z=0.83” – Wuyts et al., 2004 ApJ, 605, 677 “To avoid a bias induced by the magnitude limit of our sample, we focus on the high-mass end, where selection effects are less relevant. Applying a mass cut at M > 10^11 M to all four considered clusters below z = 0.6 and at M > 10^11.5 Msun to the two higher redshift clusters at z = 0.83 and 1.27 increases the best-fitting formation redshift from zform = 2.26 to 2.95.”
“The Fundamental Plane of Cluster Elliptical Galaxies at z=1.25” -- Holden et al., 2005 ApJ 620, 83 Velocity dispersion of 4 cluster members, plus 3 members of another cluster at z=1.27, gives characteristic age of 3.0 Gyr, or z* = 3.4 +/- 0.5. Including progenitor bias lowers these numbers to 2.1 Gyr and z* = 2.3 – still quite old! High-z cluster members are red dots and green crosses; Coma cluster ellipticals are “stars” “Two lower mass galaxies in our z=1.25 sample have much lower M/L values, implying significant star formation close to the epoch of observation. Thus, even in the centers of massive clusters, there appears to have been significant star formation in some massive, M~10^11 Msun, galaxies at z~1.5.”
Kuntschner & Davies “The Ages and Metalicities of early-type galaxies in the Fornax Cluster (1998 MNRAS, 295, L33) “…we find that the Fornax ellipticals follow the locus of fixed age in Worthey’s models…the lenticular [S0] galaxies, however, exhibit a substantial spread to younger luminosity-weighted ages, indicating a more extended star formation history. cluster field
unpublished work of Trager, Faber, and Dressler Result so far: elliptical galaxies in clusters 5 Gyr ago look younger, but old even then: ages of > 4 Gyr 5 Gyr ago Black points – Coma; Red points A851 (z = 0.41) Squares = ellipticals, triangles are S0’s
Before we leave these kinds of studies, we can briefly ask the question, are field ellipticals really all that different (in age) from cluster E’s?
“Evolved Galaxies at z>1.5 from the Gemini Deep Deep Survey: The Formation Epoch of Massive Stellar Systems”, McCarthy et al. (GDDS), 2004 ApJ, 614, L9 “Conservative age estimates for 20 galaxies with z > 1.3…give a median age of 1.2 Gyr and zf = 2.4. One-quarter of the galaxies have inferred zf > 4. Models restricted to [Fe/H] ~0 give median ages and zf of 2.3 Gyr and 3.3, respectively. These galaxies are among the most massive and contribute 50% of the stellar mass density at 1 < z < 2. …Our results point toward early and rapid formation for a significant fraction of present-day massive galaxies.” Can feedback with hierarchical CDM explain? See Cirasuolo et al, 2005 ApJ, 629, 816. “Both composite spectra show strong Mg II 2800, Mg I 2852 absorption and broad spectral features due primarily to Fe II absorption. Overlaid in red is a single-burst Bruzual & Charlot spectral synthesis model with an age of 2 Gyr, solar abundances, and a Salpeter IMF cutoff at 120 Msun” “Spectra of evolved GDDS galaxies with z > 1.3. The SDSS LRG composite has been overlaid on each spectrum, and an offset has been applied to each, in steps of 10^-18 ergs cm-2 Å-1”
The DEEP Groth Strip Survey. IX. Evolution of the Fundamental Plane of Field Galaxies – Gebhardt et al., 2003 ApJ 537, 239. Velocity dispersions and optical photometry for 21 “early-type” galaxies in the Groth strip. “The difference in the degree of evolution between our field sample and published cluster galaxies suggests a more recent formation epoch, around z=1.5 for field galaxies compared to z>2.0 for cluster galaxies. The magnitude difference implies that the field early-type galaxies are about 2 Gyr younger than the cluster ellipticals using standard single-burst models. However, the same models imply a significant change in the rest-frame U-B color from then to the present, which is not seen in our sample. Continuous low-level star formation, however, would serve to explain the constant colors over this large magnitude change. A consistent model has 7% of the stellar mass created after the initial burst, using an exponentially decaying star formation rate with an e-folding time of 5 Gyr.”
Put this together with the claim that evolution in the cluster galaxies at z ~ 1 is luminosity dependent, and you find another related result: “Mass-to-Light Ratios of Field Early-Type Galaxies at z ~ 1 from Ultradeep Spectroscopy: Evidence for Mass-dependent Evolution” – Van der Wel et al., 2005 ApJ, 145, 162 27 early-type field galaxies 0.6 < z < 1.15, VLT spectra, HST photometry “For galaxies with masses M > 2 × 10^11 Msun , we find no significant difference between the evolution of field and cluster galaxies: ln(M/LB) = (-1.20 ± 0.18)z for field galaxies and ln(M/LB) = (-1.12 ± 0.06)z for cluster galaxies.” “corrected” for cluster galaxy evolution
Wilmer et al. 2005, astro-ph 506041, The DEEP2 Redshift Survey: The Galaxy Luminosity Function to z ~ 1
Faber et al. 2005 astro-ph 506044, Galaxy Luminosity Functions to z~1: DEEP2 vs. COMBO-17 and Implications for Red Galaxy Formation Quenching star formation in spheroids? Alternatively, are we seeing the end of star formation in the spiral disks that become S0’s? Faber says no, because these are the brightest early-types, which are usually E’s.
Summing up part 1: • A good deal of evidence points to an early formation epoch, z > 2, for elliptical galaxies in rich clusters, with no significant star formation after z = 1.5. • Progenitor bias could mean that some much younger systems became ellipticals and that they experienced ongoing star formation to z < 1, but there is no clear evidence that this actually happened often and contributed significantly to the population of ellipticals in rich clusters. Various tests, LF, FP, σ(CMR) seem to suggest progenitor bias may not be a big effect. • 3) More sensitive measurements of age and metallicity in stellar populations at 0.5 < z < 1.3 should decisive in distinguishing between these two possibilities in the near future.
Implications of part 1: 1) For those ellipticals that formed at z=2 or earlier, mergers are a likely formation mechanisms, as hierarchical clustering suggests, and they are likely to be mergers of gas rich systems. However, the systems that are merging would not have been very much like the giant spiral galaxies we see today, which we believe formed over a much longer timescale. 2) The places where this happened were destined to be the cores of rich clusters, but they too were unlike the rich clusters we see today. A better picture is that of a modern group environment, but with a higher density of galaxies than we see in today’s groups and very gas rich systems. 3) Most elliptical galaxies are not influenced by the environment of today’s rich clusters. 4) Questions: How different was the origin of most field ellipticals? What becomes of the famous examples of disk-disk mergers we see today? S0’s outnumber field E’s by 2:1.
Present-epoch morphology/density relation (Dressler, 1980 ApJ, 236, 1) Commonly cited as evidence for ram pressure stripping in the conversion of spirals to S0’s …but not by me. I suggested that a relationship like this, such a slow function of density, must have arisen at early times when the density contrast was much smaller. And, most S0 galaxies are in the field!
In Dressler et al (1997) the Morphs claimed that there is a striking deficiency of S0 galaxies in intermediate-redshift compared to environments in present-epoch. If confirmed, this would be critical for understanding the fate of the infalling, star-forming galaxies. Present-epoch morphology/density relation …Morphs clusters at z ~ 0.5. Comparing at the same galaxy density (which is important), ellipticals represent an equal (or even greater) percentage of the cluster population. The big difference in the 2 diagrams is the paucity of S0 galaxies compared to today’s clusters.
“Evolution since z = 1 of the Morphology-Density Relation for Galaxies” -- Smith et al., 2005 ApJ, 620, 78 “The Morphology-Density Relation in z ~ 1 Clusters” -- Postman et al., 2005 ApJ, 623, 721 “The evolution in the MDR is confined to densities > 40 galaxies Mpc^-2 and appears to be primarily due to a deficit of S0 galaxies and an excess of Sp+Irr galaxies relative to the local galaxy population. The f(E)-density relation exhibits no significant evolution between z = 1 and 0. We find mild evidence to suggest that the MDR is dependent on the bolometric X-ray luminosity of the intracluster medium.”
Environmental influences on galaxies • (Galaxy-galaxy collisions stripping, a la Baade & Spitzer) • Ram pressure stripping – Gott & Gunn, Abadi, Moore, & Bower… • ram pressure induced star formation? -- Dressler & Gunn, Bekki & Couch 2003 • Evaporative gas removal – Cowie & Songaila, Nulsen • Starvation (removing gas-rich halos) – Larson, Tinsley, & Caldwell • Bekki et al. • Mergers, accretions, tidal interactions – Toomre, Mihos • (driving secular evolution? – see Zhang 1999 ApJ 518, 613) • Harassment (tidal, galaxy & cluster) – Moore et al. • In the 1970’s, global processes like ram pressure stripping were nearly exclusively cited for environmental changes such as the rarity of spirals in clusters. There is now good evidence that stripping does occur, but that it is not the primary mechanism of S0 production. In fact, these mechanisms associated with the dense cluster core may not be the most important for turning spiral galaxies into S0’s.
“A Wide-Field Hubble Space Telescope Study of the Cluster Cl 0024+16 at z = 0.4. I. Morphological Distributions to 5 Mpc Radius” – Treu et al., 2003 ApJ, 591, 53 Discusses the efficacy of different environmental processes and concludes lower-density “long-range” processes are dominant, not cluster-core processes. “The Environmental Dependence of Galaxy Colors in Intermediate-Redshift X-ray Selected Clusters” – Wake et al., 2005 ApJ, 627, 186. “It thus appears that the global cluster environment, in the form of cluster mass (LX), has little influence on the properties of the bright cluster galaxies, whereas the local environment, in the form of galaxy density (radius), has a strong effect. The range of ~100 in LX corresponds to a factor of ~40 in ram pressure efficiency, thus suggesting that ram pressure stripping or other mechanisms that depend on cluster mass, like tidal stripping or harassment, are unlikely to be solely responsible for changing the galaxy population from the “blue” star-forming galaxies, which dominate low-density environments, to the “red” passive galaxies, which dominate cluster cores.”
gas density gas entropy Courtesty of Andrey Kravstov: Accretion, Mergers Shocks, & Turbulence when subclusters of galaxies coalesce gas density gas entropy (slice) gas entropy Norman & Bryan 1998 Nagai, Kravtsov & Kosowsky 2003 Sunyaev, Norman & Bryan 2003 10 Mpc Also thanks to Jaqueline van Gorkum
Courtesy of A.Vikhlinin: Chandra images of z = 0 and z = 0.6 clusters nearby clusters distant clusters (z ~ 0.6)
“The Evolution of Population Gradients in Galaxy Clusters: The Butcher-Oemler Effect and Cluster Infall” – Ellingson, Lin, Yee, & Carlberg, 2001 ApJ, 547, 609 Spectroscopic version of morph-density relation: “It appears that the cluster can be modeled as the sum of two components: a virialized component of older galaxies, and a younger component which, while it is probably in quasi-equilibrium with the cluster potential…has fallen in more recently and which may eventually be expected to transform both spectrally and dynamically to blend with the older population. Galaxies in the midst of this transformation (e.g., K+A galaxies, or the "PSF" PCA component) appear to inhabit an intermediate spatial distribution.” CFHT Spectroscopy of CNOC1 sample of x-ray luminous clusters 0.18 < z < 0.55. PCA classification of spectra a good step forward. Confirm Butcher-Oemler effect but note that, for this x-ray luminous sample, considering only galaxies within 0.5 R200 would result in much smaller evolution. “The main difference in these curves is then a simple vertical shift, and these results can thus be interpreted as a straightforward decline in the infall rate of new galaxies into the cluster. “…implying that about 6% of the stellar mass in the cluster has recently fallen into the cluster at an observed epoch of z ~ 0.45. Thus, the Butcher-Oemler effect illustrates the sensitivity of galaxy populations in tracing even a very small fraction of infall into the cluster. …further suggests that the infall rate has declined further by a factor of 3, by z ~ 0.2.”
Gradient in C-M scatter with distance from cluster center: “The intrinsic scatter in the rest-frame B-V CM relation of the elliptical galaxies is very small, ~0.022 mag. The CM relation of the ellipticals does not depend significantly on the distance from the cluster center. In contrast, the CM relation for the S0 galaxies does depend on radius: the S0's in the core follow a CM relation similar to that of the ellipticals, but at large radii [R > 0.5 h Mpc] the S0's are systematically bluer and the scatter in the CM relation approximately doubles, to ~0.043 mag.” “The Color-Magnitude Relation in CL 1358+62 at Z = 0.33: Evidence for Significant Evolution in the S0 Population” – van Dokkum et al., 1998 ApJ, 500, 714. This study uses spectroscopically-confirmed cluster members, and HST morphologies, to select a sample of E’s and S0’s. With HST multi-color photometry, the scatter in the color properties are compared, and also studied as a function of radius: 4.6’ ~1 Mpc –not very far, but still… The authors go on to model the continuing star formation in the S0’s in contrast to the E’s and conclude that “…~15% of the population in present-day early-type galaxy population in rich clusters was accreted between z=0.33 and z=0.”
“MS 2053 is a classic Butcher-Oemler cluster: 24% of its members are blue galaxies, and an even higher fraction (44%) are star-forming. However, more than half of the blue/star-forming members belong to the infalling structure (MS 2053-B). Unlike previous studies that have found only indirect evidence for the link between the B-O effect and galaxy infall, our unique data set enables us to show conclusively that this is the case in MS 2053.” from the abstract: “MS 2053's deficit of S0 galaxies combined with its overabundance of blue spirals implies that many of these late-type galaxies will evolve into S0 members…Our observations show that most of MS 2053's blue cluster members, and ultimately most of its low-mass S0 galaxies, originate in the field.” “Infall, the Butcher-Oemler Effect, and the Descendants of Blue Cluster Galaxies at z~0.6” – Tran et al., 2005 ApJ, 619, 134. Extensive HST imaging and Keck spectroscopy (149 confirmed cluster members!) of MS 2053-04, a massive, X-ray luminous cluster at z = 0.59. “We find two pronounced redshift peaks, one at z = 0.5840 (MS 2053-A) and a smaller one at z = 0.5983 (MS 2053-B)” – ΔV ~ 2700 km/s “More than half of the blue/late-type cluster members are in MS 2053-B.”
What seems to be emerging from all these studies of the radial dependence of cluster populations is that the effects of environment on disk galaxies begin at surprisingly low galaxy densities. This is actually an old result. Notice how, in the morphology-density relation the population gradients in this relation begin at very low densities, orders-of-magnitude lower than cluster cores.
Postman & Geller 1984, ApJ, 281, 95 Extended the morphology-density relation outside of rich clusters of galaxies out into low-to-moderate density groups. The trends were seen in these environments, which are very unlike the cores of rich clusters.
Pisces-Perseus Supercluster Giovanelli, Haynes, & Chincarini 1984 ApJ, 300, 72
Distribution of all morphological types E,S0,S0a Sc, S Later than Sc Sa,Sab, Sb,Sbc
Sloan Digital Sky Survey result Critical Density: 1 galaxy per h2 Mpc-2 Morphological fraction 75th SFR-density Morph-radius Sc 25th C4 Clusters median SDSS sees a dramatic decrease in the strongly star-forming Sc galaxies: in-fall regions of clusters Surface density (10th NN) in a redshift slice (pseudo 3D density) Mpc-2
There is universal agreement among researchers that dense environments “quench” star formation, and the evidence I’ve just shown suggests it doesn’t take much of a rise in density to see the effect. In most studies, [O II] is the sole indicator of star formation activity. It requires higher resolution, and higher-S/N spectra to see the Balmer absorption lines, which record the history of starbursts. My colleagues and I believe that starbursts, which are much more prevalent in earlier-epoch clusters than today, are an important clue as to what mechanisms are actually at work in the “quenching,” and the accompanying conversion of spirals to S0’s
…a few slides to demonstrate the prevalence of starbursts at z > 0.3 “Studying the Star Formation Histories of Galaxies in Clusters from Composite Spectra” – Dressler et al., 2004 ApJ, 617, 867. • The Morphs have co-added, cluster by cluster, the 30-60 spectra in each of 8 clusters. These composite spectra, high S/N, enable accurate measurements of line strengths. • K-type spectral features (metal lines) show the fractional representation of “old” (τ > 3 Gyr) stars • [O II] strength (or better, Hα emission – if you have it) measures the rate of ongoing star formation (timescale: τ ~ 10-100 Myr) • Balmer absorption, specifically Hδabsorption, reveals the presence of a starburst – the fraction of intermediate age stars (A-F stars), enhanced over normal levels compared to galaxies with continous (quiescent) star formation (timescale: τ ~ 1 Gyr)
Composite spectra for 12 present-epoch clusters and 8 intermediate-redshift clusters – typically 30-60 galaxies coadded per cluster Spectra are normalized, then luminosity-weighted and summed [O II] Hδ 5 clusters (8 total) z ~ 0.5 Morphs sample 5 clusters (12 total) z ~ 0 Dressler-Shectman sample
...a remarkable change in the amount and character of star formation in the recent past. In rich clusters of galaxies, but even in “the field,” the mode of star formation appears to shift to a much greater fraction which are starbursts (20-30%) The disappearance of starbursts appears to be cosmic evolution rather than cluster specific (no evidence for variation cluster-to-cluster).
Recipe for a cluster at z = 0.5 (from the Morphs cookbook) Starbursts! About 30% of the population!