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:
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:


GLASS Survey Paper and First Data Release

Today we released the GLASS survey paper together with the first data product of the survey.
The paper gives a thorough overview of GLASS and its science goals.
We utilize the Hubble Frontier Field cluster MACSJ0717, the first cluster to be observed by GLASS, to illustrate the data quality and the high-level data products we will be releasing for each of the 10 GLASS clusters.
From a visual inspection with GiG and GiGz of each spectrum brighter than H_AB = 23 we measure the redshift when sufficient information is present in the spectra.
Furthermore, we conduct a thorough search for emission lines through all the GLASS WFC3 spectra with the aim of measuring redshifts for sources with continuum fainter than H_AB = 23.
As a product of these efforts, we present a catalog of 139 emission-line based spectroscopic redshifts for extragalactic sources, including three new redshifts of multiple imaged systems.

The GLASS survey paper:
The GLASS data products:
The GLASS inspection software:

An improved mass reconstruction of Abell 2744 from GLASS and HFF

Abell 2744 is the first cluster for which both the GLASS spectroscopy and the Hubble Frontier Field (HFF) imaging are complete. In Wang et al. (2015), we perform targeted and blind searches of faint emission lines in the GLASS data and thus compile a sample of 55 galaxies with high-confidence spectroscopic redshift, 5 of which are multiply lensed and 3 of which belong to the same system. We also measure photometric redshifts for this sample from the HFF 7-filter exposures. A reasonable agreement between these two sorts of redshift estimation is seen, when nebular emission is considered in photometric redshift fitting.

Modeling gravitational potential of galaxy clusters is known as a notorious difficulty, partly because of the inclusion of mis-identified multiple image systems in lens modeling. In this work, we develop a stringent and rigorous procedure to screen off insecure image systems, based upon colors derived from HFF, image morphology and spectroscopy from GLASS. This leads to a sample of 25/72 multiple arc systems/images as secure out of a total of 57/179 candidate systems/images. The resulting total mass map is only given by the secure set of arcs.

We also derive a stellar mass map from the Spitzer Frontier Field data and find out that the stellar to total mass ratio varies substantially across the cluster field, ranging from 0 to 5%, and the light-traces-mass assumption is not valid in this merging cluster.

Gas-Phase Metallicity Gradients at z~2

The broad spectral coverage and high angular resolution of GLASS are ideal for detailed studies of galaxies at intermediate redshifts. In Jones et al. 2015 our team analyzed the spatially resolved stellar mass and gas-phase metallicity in a system of three strongly lensed galaxies at redshift z=1.855, drawn from the first complete GLASS data set (MACS0717).

These initial GLASS data reveal strong, spatially extended emission lines in all three galaxies. We find no significant radial variation in metallicity, possibly as a result of gravitational interaction among the galaxies.

These results represent the first constraint of metallicity gradient evolution using HST. Our analysis also extends previous studies of the mass-metallicity relation to an order of magnitude lower stellar mass, confirming that GLASS is able to characterize dwarf galaxies at intermediate redshift. The mass-metallicity relation and metallicity gradient evolution are both valuable diagnostics of how gas cycles in and out of galaxies over cosmic time.

For more details go to

GLASS Data Reveals The First Multiply Imaged Strongly Lensed SN

In GLASS data taken of the MACSJ1149.6+2223 cluster (z=0.54) in November, we discovered the first strongly lensed supernova visible in multiple images.  The supernova occurred in the spiral arm of a galaxy at redshift z=1.49 lensed by a foreground early-type cluster galaxy, whose gravitational potential forms four separate, strongly magnified images of the supernova.  With the Frontier Fields SN team, we describe the exciting system in Kelly et al. 2014.

Our team is measuring the time delays and relative magnifications between the separate images of the evolving supernova, by comparing the phase and brightness of the multiple light curves. These delays and magnifications will place tight constraints on the cosmic expansion rate, as well as the distribution of luminous and dark matter in the galaxy lens and the cluster.

The MACSJ1149.6+2223 galaxy cluster furthermore lenses the spiral host galaxy of the supernova itself into multiple images, and models of the cluster predict that the supernova will appear in the future at additional locations in the cluster field.

MACS1423.8+2404 Complete

As of April 15 2014 the GLASS observations of the cluster MACS1423.8+2404 have been completed, i.e., 14 orbits split between imaging in F105W and F140W and grism spectroscopy in G102 and G141 as well as 14 orbits of imaging in F814W and grism spectroscopy in G800L.

Hence, the observations of 2 out of the 10 GLASS clusters are complete… and counting.

MACS0717.5+3745 Complete

As of 2014 February 19 the GLASS observation of MACS0717 are complete. Hence, the full GLASS data set on the first of the 10 targeted clusters is available and ready for exploration.

The dataset includes 4 orbits of HST WFC3 G141 grism spectroscopy, 10 orbits of HST WFC3 G102 grism spectroscopy, and 2×7 orbits of HST ACS G800L grism spectroscopy. The WFC3 data are all on the core of the cluster taken at two different position angles to minimize the effect of contamination. The ACS spectroscopy are on the two parallel fields.