Herschel Lensing Survey

Overview:

The Herschel Lensing Survey (HLS) is a large extensive imaging survey of massive galaxy clusters in the far-infrared and submillimeter using the Herschel Space Observatory.  Its main scientific goals are the following:

1. To detect and study infrared/submillimeter galaxies that are below the nominal confusion limit of Herschel by taking advantage of the strong gravitational lensing power of massive galaxy clusters.
   
2. To discover exceptionally bright (Speak>100 mJy) lensed infrared/submillimeter galaxies that will allow a variety of detailed multi-wavelength follow-up observations.
   
3. To examine infrared/submillimeter properties of galaxies in dense environment (i.e., cluster members).
4. To investigate the Sunyaev-Zel'dovich (SZ) effect through the detection of the increment signal.

In terms of design, the HLS consists of the following two types of surveys:

• HLS-deep: Deep PACS (100/160 µm; 7'x7') and SPIRE (250/350/500 µm; 15'x15') imaging of 54 X-ray-luminous (i.e., massive) clusters (including 24/25 CLASH clusters).
• HLS-snapshot: Shallow (but nearly confusion-limited) SPIRE (250/350/500 µm; 15'x15') imaging of 527 X-ray- or SZ-selected massive clusters (including 148 SPT clusters).

The following table provides more detailed information on individual Herschel programs:

	Cycle	Title	Tobs (hrs)	#clusters
HLS-deep I	0	"The Herschel Lensing Survey" Open-Time Key Program (OTKP)	318.2	44
HLS-deep II	2	"Herschel Lensing Survey II": Completing the Herschel Legacy with the HST/MCT CLASH Sample"	48.2	10
HLS-snapshot I	1	"SPIRE Snapshot Survey of Massive Galaxy Clusters"	27.3	279
HLS-snapshot II	2	"SPIRE Snapshot Survey II: Using SPT/CODEX Massive Clusters as Powerful Gravitational Lenses"	25	248
		Total	418.7	581

History:

In its original design, the Herschel Lensing Survey (HLS) was conceived as a deep PACS and SPIRE imaging survey of ~40 massive clusters (HLS-deep I).  However, the discovery of an exceptionally bright cluster-lensed submillimeter galaxy (the Eyelash) by Swinbank et al. (2010) suggested that surveying a much larger cluster sample would likely find similarly spectacular lensed galaxies.  Therefore, we started a second survey, conducting shallower SPIRE-only imaging of ~280 clusters selected from the ROSAT Millennium Cluster Sample constructed by H. Ebeling (HLS-snapshot I).  The success of these two programs motivated us to expand the  surveys even further in Cycle 2 by (i) completing the deep PACS and SPIRE coverage for the HST/MCT CLASH sample by observing the remaining 10 clusters (HLS-deep II), and (ii) continuing the snapshot survey by observing a sample of another ~250 clusters at higher redshift (z>0.3) selected from the SPT and CODEX clusters (HLS-snapshot II).  Unfortunately, we ended up missing one CLASH cluster because the CLASH team later replaced A963 with A1423, the latter of which has not been targeted by Herschel.

To make it simple, we refer to the combination of our four Herschel programs as the Herschel Lensing Survey (HLS), and define two sub-programs, HLS-deep and HLS-snapshot, as described above.  We keep the distinction between the surveys I's and II's only interenally since the corresponding Co-I's and data-sharing policies may differ.