Galaxies at z~6: I-drop Photometric Selection and the GLARE Project

1. Galaxies at z~6 I - drop Photometric Selection and the GLARE Project Elizabeth Stanway IoA, Cambridge, UK Galaxies at z~6: I- drop Photometric Selection and the GLARE Project STScI May Symposium 2004

2. Collaborators Andy Bunker, Exeter Richard McMahon, Cambridge Richard Ellis, Caltech Tomasso Treu, UCLA/Caltech GLARE = Gemini Lyman Alpha at Reionisation Era PI: Karl Glazebrook, JHU Roberto Abraham, Toronto Brian Boyle, ATNF Andrew Bunker, Exeter Matthew Colless, AAO Warrick Couch, New South Wales David Crampton, NRC Isobel Hook, Oxford Inger Jørgensen, Gemini Sangeeta Malhotra, STScI Patrick McCarthy, OCIW Rick Murowinski, NRC James Rhoads, STScI Kathy Roth, Gemini Sandra Savaglio, JHU Elizabeth Stanway, Cambridge Zlatan Tsetanov, JHU Galaxies at z~6: I-drop Photometric Selection and the GLARE Project STScI May Symposium 2004

3. The I - Drop Technique • UV luminous, star forming galaxies at high redshift show a strong spectral break at rest=1216Å (Lyman ) • This leads to a large drop in magnitude between bands bracketing this wavelength which can be detected using a multicolour photometric technique. • The Lyman Break Method (Guhathakurta et al 1990, Steidel et al 1995) Starburst at z=6 f-2.0 For galaxies at 5.6<z<7.0, i'- z'>1.3 Galaxies at z~6: I-drop Photometric Selection and the GLARE Project STScI May Symposium 2004

4. Possible Contaminants in an I - Drop Selection Some objects at lower redshift may have similar i' - z' colours to galaxies at z~6, namely: • Low mass Galactic stars (classes M, L, T) • High redshift AGN • Extremely Red galaxies (EROs) at z~2 Of these, both stars and AGN are expected to be unresolved although care must be taken as a number of the high redshift galaxies identified to date are barely resolved with HST. EROs are more difficult to eliminate unless near-IR data is available. Galaxies at z~6 EROs Galaxies at z~6: I-drop Photometric Selection and the GLARE Project STScI May Symposium 2004

5. Photometric Data from HST/ACS The Hubble Ultra Deep Field (HUDF) lies within the shallower but larger GOODS - S field. These surveys have made the high quality i' and z' photometry necessary for i'-drop selection publicly available at: http://www.stsci.edu/hst/udf http://www.stsci.edu/science/goods The central portion of the HUDF also has deep NICMOS UDF near-IR imaging. GLARE HUDF GOODS-S Tilescheme Galaxies at z~6: I-drop Photometric Selection and the GLARE Project STScI May Symposium 2004

6. I-Drops in the HST/ACS data We have identified I-drop samples satisfying i’-z’>1.3 in both the GOODS v1.0 data (z’AB<27.5) and the UDF data release (z’AB<28.5) working well above the limits of the data. In both north and south GOODS fields, number counts of galaxies appear significantly lower than expected if compared to the z=3 LBGs. Many I-drops in the UDF are found in multiple systems. Their near-IR colours suggest very young starbursts – possibly triggered by mergers.

7. Multicolour Images (Constructed from v, i', z' GOODS v1.0 data) GLARE 3001 GLARE 3011 GLARE 1042 These candidates are by far the reddest objects in this field.

8. The Star Formation History Diagram Previous z~6 Estimates UDF I-drops We observe a decline in the number density of LBGs and little evolution in the shape of the LF  a declining star formation density The ‘Madau-Lilly’ plot as derived from LBGs

9. GLARE: Spectroscopic Follow Up • Interpretation of a Lyman break population can depend on a knowledge of its spectroscopic properties, in particular the redshift distribution. Individual galaxies have been spectroscopically confirmed by different groups. • Hence GLARE aims to obtain high quality spectra of a sample of z~6 galaxies in and around the Hubble Ultra Deep Field using extremely long exposures (approx 100 hours) on a single slitmask. • To measure the luminous properties of faint 5.5<z<7 galaxies and hence to shed light on the end of the reionisation period at z~6. • To do so using the GMOS instrument on Gemini South in Nod & Shuffle mode to optimise sky subtraction. The project was allocated a total of 125 hours, of which 29% was observed (to 24th Jan 2004). Galaxies at z~6: I-drop Photometric Selection and the GLARE Project STScI May Symposium 2004

10. Candidate Selection for GLARE The candidates for GMOS-S spectroscopy were selected: i) From the list of 100 reddest objects in the HUDF pre-released by STScI (October 2003) in the central region. These objects were required to have (i'-z')AB>1.5 (20 slits) ii) From GOODS-S v1.0 data in the outlying regions. Selection criteria of (i'-z')AB>1.3 and z'AB>27.5 were applied. (8 slits) In addition, 5 slits were placed on candidate 4.8<z<5.6 objects (see Bremer et al 2003) although an order blocking filter was used to remove light blueward of 7800Å (Lyman  at z=5.4), in order to allow the maximum number of z~6 slitlets.The remaining space on the slitmask was used for a blank-sky survey (21 slits). Galaxies at z~6: I-drop Photometric Selection and the GLARE Project STScI May Symposium 2004

11. GLARE Results Analysis of the GLARE observations taken to date (a total of approx 25.5 hours on source) is ongoing. Three Lyman-alpha emitting galaxies at z=5.8 - 6 were identified in the first 7.5 hours of observations (Stanway et al 2004) and a number of other line emitters have been observed in the deeper data now available. The three objects observed in the first epoch of data all have spectroscopic properties consistent with other Lyman break galaxies at z~6 which have been spectroscopically confirmed (e.g Bunker et al 2003, Dickinson et al 2004). Their fluxes are all in the range 0.75 - 1.0 x 10-17 ergs cm2 s-1 and the emission lines show evidence of assymetry (expected of high redshift galaxies) even at our relatively low spectral resolution (3.5 Å/pix). Galaxies at z~6: I-drop Photometric Selection and the GLARE Project STScI May Symposium 2004

12. GLARE Object 1042 Stacked Data Smoothed Data Ly  at z=5.83 Contiuum z'AB=25.5 i'AB=27.0 Wrest=20Å Galaxies at z~6: I-drop Photometric Selection and the GLARE Project STScI May Symposium 2004

13. Keck Spectroscopy of a z=5.83 LBG 5.5 hour Keck/Deimos Spectrum of the previously confirmed z=5.83 galaxy in the UDF (Stanway et al 2004a,b, Dickinson et al 2004)

14. Conclusions I- Drop photometric selection techniques can efficiently identify candidate z~6 galaxies, as confirmed by spectroscopic follow- up. The GLARE project has obtained deep and high quality spectra of some of the most distant galaxies yet identified. We are beginning to systematically assess the spectroscopic properties of the Lyman break population at z~6 If we are able to obtain full 100 hour spectra, the continuum properties of the z~6 population are now obtainable. The GLARE proposal has been resubmitted for semester 2004B and aims to obtain further time on two slitmasks, redesigned to make optimum use of the excellent HST/ACS UDF data. Galaxies at z~6: I-drop Photometric Selection and the GLARE Project STScI May Symposium 2004