Marco Sirianni

The Morphology-Density Relation in z ~ 1 Clusters

We measure the morphology-density relation (MDR) and morphology-radius relation (MRR) for galaxies in seven z ~ 1 clusters that have been observed with the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope. Simulations and independent comparisons of our visually derived morphologies indicate that ACS allows one to distinguish between E, S0, and spiral morphologies down to z850 = 24, corresponding to L/L* = 0.21 and 0.30 at z = 0.83 and 1.24, respectively. We adopt density and radius estimation methods that match those used at lower redshift in order to study the evolution of the MDR and MRR. We detect a change in the MDR between 0.8 < z < 1.2 and that observed at z ~ 0, consistent with recent work; specifically, the growth in the bulge-dominated galaxy fraction, fE+S0, with increasing density proceeds less rapidly at z ~ 1 than it does at z ~ 0. At z ~ 1 and Σ ≥ 500 galaxies Mpc-2, we find fE+S0 = 0.72 ± 0.10. At z ~ 0, an E+S0 population fraction of this magnitude occurs at densities about 5 times smaller. The evolution in the MDR is confined to densities Σ 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 fE-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. Implications for the evolution of the disk galaxy population in dense regions are discussed in the context of these observations.

Age Determination of Six Intermediate-Age Small Magellanic Cloud Star Clusters with Hst/acs

We present a photometric analysis of the star clusters Lindsay 1, Kron 3, NGC 339, NGC 416, Lindsay 38, and NGC 419 in the Small Magellanic Cloud (SMC), observed with the Hubble Space Telescope Advanced Camera for Surveys (ACS) in the F555W and F814W filters. Our color-magnitude diagrams (CMDs) extend ~3.5 mag deeper than the main-sequence turnoff points, deeper than any previous data. Cluster ages were derived using three different isochrone models: Padova, Teramo, and Dartmouth, which are all available in the ACS photometric system. Fitting observed ridgelines for each cluster, we provide a homogeneous and unique set of low-metallicity, single-age fiducial isochrones. The cluster CMDs are best approximated by the Dartmouth isochrones for all clusters, except for NGC 419 where the Padova isochrones provided the best fit. Using Dartmouth isochrones we derive ages of 7.5 ± 0.5 Gyr (Lindsay 1), 6.5 ± 0.5 Gyr (Kron 3), 6 ± 0.5 Gyr (NGC 339), 6 ± 0.5 Gyr (NGC 416), and 6.5 ± 0.5 Gyr (Lindsay 38). The CMD of NGC 419 shows several main-sequence turnoffs, which belong to the cluster and to the SMC field. We thus derive an age range of 1.2-1.6 Gyr for NGC 419. We confirm that the SMC contains several intermediate-age populous star clusters with ages unlike those of the Large Magellanic Cloud and the Milky Way. Interestingly, our intermediate-age star clusters have a metallicity spread of ~0.6 dex, which demonstrates that the SMC does not have a smooth, monotonic age-metallicity relation. We find an indication for centrally-concentrated blue straggler star candidates in NGC 416, while these are not present for the other clusters. Using the red clump magnitudes, we find that the closest cluster, NGC 419 (~50 kpc), and the farthest cluster, Lindsay 38 (~67 kpc), have a relative distance of ~17 kpc, which confirms the large depth of the SMC. The three oldest SMC clusters (NGC 121, Lindsay 1, and Kron 3) lie in the northwestern part of the SMC, while the youngest (NGC 419) is located near the SMC main body.

Evolution of the Color-Magnitude Relation in High-Redshift Clusters: Early-Type Galaxies in the Lynx Supercluster at z ~ 1.26

Color-magnitude relations (CMRs) have been derived in two high-redshift clusters, RX J0849+4452 and RX J0848+4453(withredshiftsofz ¼ 1:26and1.27,respectively),thatlieinthehighestredshiftclustersuperstructure known today, the Lynx Supercluster. The CMR was determined from ACS imaging in the WFC F775W (i775 )a nd F850LP (z850) filters combined with ground-based spectroscopy. Early-type cluster candidates have been identified according to the Postman et al. morphological classification. In both clusters the bright red early-type population defines atightCMR verysimilarin color,althoughthe two clusters presentdifferentX-rayluminositiesand shapes. The elliptical galaxy CMRs in RX J0849+4452 and RX J0848+4453 show an intrinsic (i775 � z850) color scatter of 0:026 � 0:012 and 0:024 � 0:015 mag, respectively, within 2 0 (� 1M pc atz ¼ 1:26) from the cluster X-ray emission centers. Simple modeling of the scatters using stellar population models from Bruzualand Charlotgives a meanluminosity-weightedaget > 2:5Gyr(zf > 2:75)andt > 2:6Gyr(zf > 2:8)forellipticalsinRXJ0849+4452 andRX J0848+4453,respectively.S0 galaxiesfollowtheellipticalCMR;theyshowlargerscattersabouttheCMR. The intrinsic scatter decreases and the CMR slopes are steeper at smaller radii, within both clusters. Within 1 0 from the cluster X-ray emission centers, elliptical CMR scatters imply a mean luminosity-weighted age t > 3: 2G yr (zf > 3:7). We conclude that old stellar populations in cluster elliptical galaxies are already in place at z ¼ 1:26, both in the more evolved cluster RX J0849+4452 and in its less evolved companion RX J0848+4453. Even at a look-back time of 9 Gyr, in the early merging and buildup of massive clusters, the bulk of the stellar content of the bright elliptical galaxy population was in place—apparently formed some 2.5 Gyr earlier at z � 3. Subject headingg galaxies: clusters: individual (RX J0848+4453, RX J0849+4452) — galaxies: elliptical and lenticular, cD — galaxies: evolution

Evolution of the Color-Magnitude Relation in High-Redshift Clusters: Blue Early-Type Galaxies and Red Pairs in RDCS J0910+5422

The color-magnitude relation has been determined for the RDCS J0910+5422 cluster of galaxies at redshift z = 1.106. Cluster members were selected from HST Advanced Camera for Surveys (ACS) images, combined with ground–based near–IR imaging and optical spectroscopy. Postman et al. (2005) morphological classifications were used to identify the early-type galaxies. The observed early–type color–magnitude relation (CMR) in (i775 z850) versus z850 shows an intrinsic scatter in color of 0.060±0.009 mag, within 1 ′ from the cluster X–ray emission center. Both the ellipticals and the S0s show small scatter about the CMR of 0.042 ± 0.010 mag and 0.044± 0.020 mag, respectively. From the scatter about the CMR, a mean luminosity–weighted age t > 3.3 Gyr (zf > 3) is derived for the elliptical galaxies, assuming a simple stellar population modeling (single burst solar metallicity). This is consistent with a previous study of the cluster RDCS1252.9-292 at z=1.24 (Blakeslee et al.). Strikingly, the S0 galaxies in RDCS J0910+5422 are systematically bluer in (i775 z850) by 0.07 ± 0.02 mag, with respect to the ellipticals. The blue S0s are predominantly elongated in shape; the distribution of their ellipticities is inconsistent with a population of axisymmetric disk galaxies viewed at random orientations, suggesting either that they are intrinsically prolate or there is some orientation bias in the S0 classification. The ellipticity distribution as a function of color indicates that the face-on S0s in this particular cluster have likely been classified as elliptical. Thus, if anything, the offset in color between the elliptical and S0 populations may be even more significant. The color offset between S0 and E corresponds to an age difference of � 1 Gyr, for a singleburst solar metallicity model. Alternatively, it could be the result of a different star formation history; a solar metallicity model with an exponential decay in star formation will reproduce the offset for an age of 3.5 Gyr, i.e. the S0s have evolved gradually from star forming progenitors. The color offset could also be reproduced by a factor of �2 difference in metallicity, but the two populations would each need to have very small scatter in metallicity to reproduce the small scatter in color. The early–type population in this cluster appears to be still forming. The blue early-type disk galaxies in RDCS J0910+5422 likely represent the direct progenitors of the more evolved S0s that follow the same red sequence as ellipticals in other clusters. Thirteen red galaxy pairs are observed and the galaxies associated in pairs constitute �40% of the CMR galaxies in this cluster. This finding is consistent with the conclusions of van Dokkum and Tran et al. that most of the early–type galaxies grew from passive red mergers.

Past and present star formation in the SMC: NGC 346 and its neighborhood

In the quest to understand how star formation occurs and propagates in the low-metallicity environment of the Small Magellanic Cloud (SMC), we acquired deep F555W (~V) and F814W (~I) Hubble Space Telescope ACS images of the young and massive star-forming region NGC 346. These images and their photometric analysis provide us with a snapshot of the star formation history of the region. We find evidence for star formation extending from ?10 Gyr in the past until ?150?Myr in the field of the SMC. The youngest stellar population (~3 ? 1?Myr) is associated with the NGC 346 cluster. It includes a rich component of low-mass pre-main-sequence stars mainly concentrated in a number of subclusters spatially colocated with CO clumps previously detected by Rubio and coworkers. Within our analysis uncertainties, these subclusters appear coeval with each other. The most massive stars appear concentrated in the central subclusters, indicating possible mass segregation. A number of embedded clusters are also observed. This finding, combined with the overall wealth of dust and gas, could imply that star formation is still active. An intermediate-age star cluster, BS 90, formed ~4.3 ? 0.1?Gyr ago, is also present in the region. Thus, this region of the SMC has supported star formation with varying levels of intensity over much of the cosmic time.

The Transformation of Cluster Galaxies at Intermediate Redshift

We combine imaging data from the Advanced Camera for Surveys (ACS) with VLT/FORS optical spectroscopy to study the properties of star-forming galaxies in the z = 0.837 cluster Cl 0152-1357. We have morphological information for 24 star-forming cluster galaxies, which range in morphology from late-type and irregular to compact early-type galaxies. We find that while most star-forming galaxies have r625 - i775 colors bluer than 1.0, eight are in the red cluster sequence. Among the star-forming cluster population, we find five compact early-type galaxies that have properties consistent with their identification as progenitors of dwarf elliptical galaxies. The spatial distribution of the star-forming cluster members is nonuniform. We find none within R ~ 500 Mpc of the cluster center, which is highly suggestive of an intracluster medium interaction. We derive star formation rates from [O II] λ3727 line fluxes and use these to compare the global star formation rate of Cl 0152-1357 to other clusters at low and intermediate redshifts. We find a tentative correlation between integrated star formation rates and TX, in the sense that hotter clusters have lower integrated star formation rates. Additional data from clusters with low X-ray temperatures are needed to confirm this trend. We do not find a significant correlation with redshift, suggesting that evolution is either weak or absent between z = 0.2 and 0.8.

EVOLUTION IN THE CLUSTER EARLY-TYPE GALAXY SIZE-SURFACE BRIGHTNESS RELATION AT z 1

We investigate the evolution in the distribution of surface brightness, as a function of size, for elliptical and S0 galaxies in the two clusters RDCS J1252.9-2927 (z = 1.237) and RX J0152.7-1357 (z = 0.837). We use multicolor imaging with the Advanced Camera for Surveys on the Hubble Space Telescope to determine these sizes and surface brightnesses. Using three different estimates of the surface brightnesses, we find that we reliably estimate the surface brightness for the galaxies in our sample with a scatter of <0.2 mag and with systematic shifts of 0.05 mag. We construct samples of galaxies with early-type morphologies in both clusters. For each cluster, we use a magnitude limit in a band that closely corresponds to the rest-frame B, to magnitude limit of MB = -18.8 at z = 0, and select only those galaxies within the color-magnitude sequence of the cluster or by using our spectroscopic redshifts. We measure evolution in the rest-frame B surface brightness and find -1.41 ± 0.14 mag from the Coma Cluster of galaxies for RDCS J1252.9-2927 and -0.90 ± 0.12 mag of evolution for RX J0152.7-1357, or an average evolution of (-1.13 ± 0.15)z mag. Our statistical errors are dominated by the observed scatter in the size-surface brightness relation, σ = 0.42 ± 0.05 mag for RX J0152.7-1357 and σ = 0.76 ± 0.10 mag for RDCS J1252.9-2927. We find no statistically significant evolution in this scatter, although an increase in the scatter could be expected. Overall, the pace of luminosity evolution we measure agrees with that of the fundamental plane of early-type galaxies, implying that the majority of massive early-type galaxies observed at z 1 formed at high redshifts.

Discovery of a population of pre-main-sequence stars in NGC 346 from deep Hubble space telescope acs images

We report the discovery of a rich population of low mass stars in the young, massive star forming region N66/NGC346 in the Small Magellanic Cloud, from deep V, I and H alpha images taken with the HST/ACS. These stars have likely formed together with the NGC346 cluster, ~3-5 Myr ago. Their magnitude and colors are those of pre-main sequence stars in the mass range 0.6-3 Mo, mostly concentrated in the main cluster, but with secondary subclusters spread over a region across ~45 pc. These subclusters appear to be spatially coincident with previously known knots of molecular gas identified in ground based and ISO observations. We show that N66/NGC346 is a complex region, being shaped by its massive stars, and the observations presented here represent a key step towards the understanding of how star formation occurred and has progressed in this low metallicity environment.

THE LUMINOSITY FUNCTION OF EARLY-TYPE FIELD GALAXIES AT Z 0.75

We measure the luminosity function of morphologically selected E/S0 galaxies from z = 0.5 to 1.0 using deep high-resolution Advanced Camera for Surveys (ACS) imaging data. Our analysis covers an area of 48 arcmin2 (8 times the area of the Hubble Deep Field North) and extends 2 mag deeper (I ~ 24 mag) than was possible in the Deep Groth Strip Survey (DGSS). Our fields were observed as part of the ACS Guaranteed Time Observations. At 0.5 1.7, E/S0 galaxies at brighter luminosities (MB < -20.1), but are increasingly different at fainter magnitudes, where blue galaxies are both smaller and have lower Sersic parameters. We find differences in both the size-magnitude relation and the photometric plane offset for red and blue E/S0s, although neither red nor blue galaxies give a good fit to the size-magnitude relation. Fits of the colors to stellar population models suggest that most E/S0 galaxies have short star formation timescales (? < 1 Gyr), and that galaxies have formed at an increasing rate from z ~ 8 until z ~ 2, after which there has been a gradual decline.

Progressive Star Formation in the Young SMC Cluster NGC 602

NGC 602 is a young stellar cluster located in a peripheral region of the Small Magellanic Cloud (SMC) known as the wing. Far from the main body of the galaxy and abutting the Magellanic Bridge, the SMCs wing is characterized by low gas and stellar content. With deep optical imaging from the Advanced Camera for Surveys (ACS) aboard the Hubble Space Telescope (HST), we have discovered an extensive pre-main-sequence (PMS) population, with stellar masses in the range 0.6-3 M☉. These low-mass PMS stars formed coevally with the central cluster about 4 Myr ago. Spitzer IRAC images of the same region also reveal a population of young stellar objects, some of which are still embedded in nebular material and most of which likely formed even more recently than the young stars detected with HST ACS imaging. We infer that star formation started in this region ~ 4 Myr ago with the formation of the central cluster and gradually propagated toward the outskirts where star formation is presently ongoing.