Linking Star Formation to Cluster Environments with GLASS

GLASS Paper VII: The Spatial Distribution of Star Formation
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Using GLASS data, Vulcani et al. (2016a) analyzed the spatial distribution of star formation in 76 galaxies in 10 clusters and 85 galaxies in the field at 0.3< z <0.7. The samples are well matched in stellar mass (10^8-10^11 M⊙) and star formation rate (0.5-50 M⊙/yr). The authors have visually classified galaxies in terms of broad-band morphology, Hα morphology and likely physical process acting on the galaxy. They found that most Hα emitters have a spiral morphology (41±8% in clusters, 51±8% in the field), followed by mergers/interactions (28±8%, 31±7%, respectively) and early-type galaxies (remarkably as high as 29±8 in clusters and 15±6% in the field). A diversity of Hα morphologies have been detected, suggesting a diversity of physical processes. In clusters, 30±8% of the galaxies present a regular morphology, mostly consistent with star formation diffused uniformly across the stellar population (mostly in the disk component, when present). The second most common morphology (28±8%) is asymmetric/jellyfish, consistent with ram pressure stripping or other non-gravitational processes in 18±8% of the cases. Ram pressure stripping appears significantly less prominent in the field (2±2%), where the most common morphology/mechanism appears to be consistent with minor gas rich mergers or clump accretion.
Comparing the position of the peak of the Hα emission to that of the continuum, as traced by the F475W filter, the author found that in both environments, for most of the galaxies the displacement is smaller than 1.5 kpc and the average offset is ~0.5 kpc. The existence of the offset suggests that current star formation is not generally colocated with recent star formation, perhaps as the result of accretion of satellites or gas, or non gravitational interactions such as ram pressure stripping affecting the spatial distribution of the cold gas.
The emerging picture is that Hα emitters are a very heterogeneous population, characterized by a range of morphologies, sizes and SFRs, therefore, a simple explanation can not describe all the observations. Even though some small systematic differences between galaxies in the field and in clusters emerge, both populations present very mixed morphologies and experience a variety of processes.
This work demonstrates that the effects of cluster- specific mechanisms on galaxy evolution are detectable in GLASS unprecedented data. However, they are both subtle and complex. They are subtle in the sense that no dramatic trend is found between the morphology of the current star formation and the environment or other properties of the galaxy. Every trend  is weak and there are always exceptions. A full understanding of this complexity requires larger samples and detailed and spatially resolved physical models.

GLASS Paper VIII: The Correlation Between Star Formation and Cluster Environment

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Extending the study presented in Vulcani et al. (2016a) described above, Vulcani et al. (2016b) used the same GLASS data and the results from Vulcani et al. (2016a) to characterize the spatial distribution of star formation in 76 galaxies in 10 clusters at 0.3< z <0.7. They correlated the properties of Hα emitters to a number of tracers of the cluster environment to investigate its role in driving galaxy transformations. Hα emitters are found in the clusters out to 0.5 virial radii, the maximum radius covered by GLASS. The peak of the Hα emission is offset with respect to the peak of the UV-continuum. Decomposing this offsets into a radial and tangential component, the radial component points away from the cluster center in 60% of the cases, with 95% confidence. The decompositions agree with cosmological simulations, i.e. the Hα emission offset correlates with galaxy velocity and ram pressure stripping signatures. Our clusters span a wide range of morphologies. Trends between Hα emitters properties and surface mass density distributions and X-ray emissions emerge only for unrelaxed clusters.
The most statistically significant result is that galaxies with asymmetric Hα distribution, interpreted as signatures of recent ram pressure stripping, are preferentially found within 0.3 r500, at higher local density conditions and higher X-ray counts and have a negative radial projected offset, i.e. the peak of the Hα emission is pointing away from the cluster center with respect to the continuum emission.
The lack of strong correlations with the global environment does not allow to identify a unique environmental effect originating from the cluster center. In contrast, correlations between Hα morphology and local number density emerge.
This work concludes that local processes, such as ram pressure, strangulation and galaxy-galaxy interactions, rather than processes taking place on large scale, are the most important, or at least the most easily detectable, drivers of environmental evolution.

Further Details

For further details on the above studies please refer to:

GLASS Reveals Dual-Role Environments in Galaxy Clusters

Using the GLASS spectroscopic and very deep imaging data, Morishita et al. (2016) found that galaxy clusters are dual-role environments, which both accelerate and curtail galaxy evolution without affecting their sizes and structures.

Morishita et al. derived structural parameters of more than 3900 galaxies by using the GLASS grism spectroscopy combined  with the imaging from the Hubble Frontier Fields. The sample is unique in terms of the low masses it proves, reaching log(M*/Msun)~7.8, a factor of 10-100 lower than previous studies at the epoch of the Universe observed in this study, about 5 Gyr ago.

The paper studied galaxy size—stellar mass relations in different environments,  i.e. the cluster center (the most crowded environment in the universe) and the normal field, to study the impact of the environment on galaxy sizes. The four figures above show the size—stellar mass relations for four populations: blue and red galaxies in clusters and the field.

The origin of low-mass red galaxies is a key topic in galaxy evolution, and the target of this study. By considering structural parameters, color, and the difference in sizes between red and blue galaxies, Morishita et al. found a strong connection between low-mass red galaxies and blue galaxies of similar masses — evidence that blue galaxies are transformed by ram pressure stripping and starvation in cluster environments.

Yet, about 20% of the low-mass red galaxies could not be explained in the same scenario — that population is too old and dense (consistent with more massive log(M*/Msun) > 10 galaxies), though it also seems only to exist in cluster environments.

The authors concluded the cluster environment has two roles on low-mass galaxies evolution; one is killing galaxies (environmental quenching) without significant structural transformations; the other is an acceleration of the evolution phase, which forms high surface density galaxies more than 10 Gyr ago, spanning most masses.

For more information see Morishita et al. (2016).

The catalog used in this study will soon be available through the GLASS website.

MACS0416 lens model and surface mass density map from GLASS spectroscopy

In Hoag et al. (2016), the GLASS team used the GLASS spectroscopy combined with the deep imaging from the Hubble Frontier Fields (HFF) to produce a gravitational lens model of the galaxy cluster MACSJ0416.1-2403 (MACS0416), following the approach by Wang et al. (2015) who modeled Abell 2744. The GLASS data allowed the team to measure the spectroscopic redshifts of galaxies that are multiply imaged by the cluster, significantly improving the constraints on the lens model. Figures showing the spectroscopic confirmations of multiply-imaged galaxies with the GLASS data are available on this page. The above figure on the left shows a co-added F105W image from the HFF, Cluster Lensing and Supernova survey with Hubble (CLASH), and the GLASS direct imaging. Overlaid on the figure is the critical curve from the lens model (dark green line) with the multiple images (multi-colored circles) that have been discovered in the cluster field. The Gold circles enclose spectroscopically confirmed multiple images from the GLASS spectroscopy and previous spectroscopic programs. All of the other colored circles lack spectroscopy, so they were vetted more carefully. We only use the Gold and Silver objects to constrain the lens model as they are the most trustworthy multiple image candidates.

From the lens model, the team derived a map of the total surface mass density throughout the cluster. Using deep mid-infrared data from the Spitzer Frontier Fields program, the team obtained a map of the stellar surface mass density in the cluster. The figure on the right shows the projected stellar to total mass ratio (f*) throughout the central ~500 kpc of the cluster. The f* varies significantly with distance from the two BCGs (black dots). The mean projected stellar to total mass ratio is <f*> = 0.009 +/- 0.003 (stat.), using a diet-Salpeter IMF, in agreement with other measurements of <f*> in massive cluster environments.

For more information see Hoag et al. (2016).

Completion of GLASS NIR data release v001

With the release of the NIR data products and redshift catalogs for the clusters A370, MACS0416, MACS0744, the GLASS NIR v001 data release is now complete! All data products including individually extracted grism spectra in 1D and 2D as well as redshift catalogs for all 10 GLASS clusters are available on the GLASS MAST webpage: https://archive.stsci.edu/prepds/glass/.

If you find any of the GLASS products useful, please cite:

And in particular, if you used the MACS0416 redshift catalog, please also cite:

Hoag et al. (2016), ApJ, submitted, arxiv 1603.00505

GLASS Fall 2015 release available

The GLASS data products, including the GLASS redshift catalogs, for the HFF clusters A2744 and MACS1149 are now available on the GLASS MAST webpage: https://archive.stsci.edu/prepds/glass/. Similar to the redshift catalog released for MACS0717 presented in the GLASS survey paper Treu et al. (2015), the cluster over-density is clearly detected in the redshift distributions (See above figure).

Note that the full redshift catalog for MACS1149 also includes redshifts from VLT-MUSE, Keck-DEIMOS, and the deep G141 HST grism spectroscopy from HST-GO-14041.

If you find any of the GLASS products useful, please cite:

If you used the full MACS1149 redshift catalog, please also cite:

GLASS presents 24 potential Lyα emitters at z ≥ 7

In the most recent paper by the GLASS collaboration, Schmidt et al. (2015), we present a sample of 24 objects with emission lines consistent with being Lyα at redshifts around or above 7, at the heart of the epoch of reionization. The figure shows the redshift distribution of these line emitters. Taking advantage of the GLASS 1σ flux limits of 5 x 10-18 erg/s/cm2 (not corrected for lensing magnification) in each of the GLASS spectra, the Lyα emitters were assembled via visual inspection of the GLASS spectra of a sample of more than 150 photometrically selected Lyman break galaxies, in the first 6 completed GLASS clusters.

The discovered fraction of Lyα emitters is consistent with the number of detections, within the uncertainties, expected from the conditionally probability of Lyα emission measured from the ground at ~ 7. Deep high-resolution ground-based follow-up spectroscopy is needed to confirm the Lyα emitters, as exemplified by the independent Keck-DEIMOS confirmation of one of the sources presented by Huang et al. (2015).

A stack of the most promising Lyα emitters with a mean redshift of 7.2 allowed us to study the spatial extent of the Lyα emission. We found it to be consistent with the spatial extent of the UV continuum. Extended Lyα emission, if present, is below the surface brightness detection threshold in the GLASS Lyα emitter stack.

For more information please see Schmidt et al. (2015) 

The Spatial Extent of Hα in Cluster Galaxies from GLASS Pilot Study

The GLASS data yield spatially resolved Hα fluxes for all star-forming galaxies in the core (< 1 Mpc) of the clusters. In addition, each cluster is observed at two different position angles. These two orientations allow us to mitigate the impact of contamination from overlapping spectra, and reliably measure for the first time the relative position of the Hα emission with respect to the continuum.
In Vulcani et al. (2015) we present a pilot study characterizing the spatial distribution of the Hα emission in cluster galaxies beyond the local universe based on WFC3-IR data.
We analyze two of the ten clusters in the GLASS sample. We select MACS0717.5+3745  and MACS1423.8+2404 because they are at similar redshift (z~0.55), so as to minimize evolutionary effects and differences in the sensitivity/selection function, and are in very different dynamical states, so as to span the range of expected environments. We use use foreground and background galaxies as field control sample.
Both in clusters and in the field, Hα is more extended than the rest-frame UV continuum in 60% of the cases, consistent with diffuse star formation and inside out growth. In ∼ 20% of the cases, the Hα emission appears more extended in cluster galaxies than in the field, pointing perhaps to ionized gas being stripped and/or star formation being enhanced at large radii. The peak of the Hα emission and that of the continuum are offset by less than 1 kpc.
The diversity of morphologies and sizes observed in Hα illustrates the complexity of the environmental process that regulate star formation.

GLASS Summer 2015 data release now available

The GLASS data products, including the GLASS redshift catalogs, for the four clusters MACS1423, MACS2129, RXJ1347, and RXJ2248 are now available on the GLASS MAST webpage: https://archive.stsci.edu/prepds/glass/
Similar to the redshift catalog released for MACS0717 presented in the GLASS survey paper Treu et al. (2015), the cluster over-density is clearly detected in the redshift distributions for these four clusters (See above figure).

If you find any of these GLASS products useful, please cite: