Project ALCATRAZ: Archival Lyman-Continuum andTheoretical Reionization Analysis vs. z: Where, When, & HowMuch Does LyC Escape from Galaxies & AGN?

Abstract Project ALCATRAZ will study Lyman continuum (LyC) radiation escaping from galaxies and weak AGN atz=2.3-4, vastly expanding our systematic study of the Early Release Science (ERS) field to include ArchivalUV images in HUDF and GOODS/CANDELS. Each field was imaged in 2-3 WFC3 UV-filters and 6-9 ACS +WFC3/IR filters. SED-fits for objects with secure spectroscopic redshifts will provide luminosity, stellar mass,age, SFR, extinction, and escape fraction estimates. With very careful attention to systematics, stacking 6-15 orbit native depth for many 100s of objects will reachUV-depths of 100s-1000s of orbits. ALCATRAZ will reveal where, when, how, and how much LyC escapes,and if galaxies started and AGN maintained cosmic reionization. Its science goals are: 1. HOW MUCH LyC escapes? First results from separately stacking 50 galaxies and 14 weak AGN at z=2.3-4suggest m(LyC) ~ 29-30.5 mag (>3-4 sigma). With robust 8-12 filter SEDs, HST rejection of foregroundinterlopers, and 115 new ground-based spectra, ALCATRAZ will increase current statistics 7x and depth 2.7x. 2. WHERE and HOW does LyC escape? HST stacking will measure LyC light-profiles at radii r<0.7", whichare likely shallow if LyC escapes through an ISM that gets more porous at larger radii, which we will constrain. 3. WHEN did LyC escape? We will constrain how LyC escape fractions evolved with epoch for galaxies &weak AGN, and if this followed the cosmic star-formation rate. 4. WHY do escape fractions evolve so strongly with epoch? We will study for LBGs, Lya-galaxies, dusty star-forming galaxies, and weak AGN with outflows how dust and gas accumulating over time may have suppressedthe escaping LyC.

Scientific Category: Galaxies

Scientific Keywords: Dust, Extra-Galactic Legacy & Deep Fields, Galaxy Formation And Evolution,Spectral Energy Distributions, Star-Formation Histories

Alternate Category: Massive Black Holes and their Hosts

Budget Size: Regular

UV Initiative: Yes

Hubble Space Telescope Cycle 24 AR Proposal 49

Investigators:

Dataset Summary:

Investigator Institution Country* R Bielby Durham Univ. GBR

S Cohen Arizona State University USA/AZ

* C Conselice University of Nottingham GBR

* M Dijkstra University of Oslo NOR

B Frye University of Arizona USA/AZ

A Inoue Osaka Sangyo University, College of GeneralEducation,

JPN

R Jansen Arizona State University USA/AZ

A Koekemoer Space Telescope Science Institute USA/MD

J MacKenty Space Telescope Science Institute USA/MD

B Smith Arizona State University USA/AZ

R Windhorst Arizona State University USA/AZ

Number of investigators: 11* ESA investigators: 3

Instrument No. of Datasets Retrieval Method Retrieval PlanACS 3076 FTP FTP at 100 Gb/week or one STScI visit

WFC3 2170 FTP FTP at 100 Gb week or one STScI visit

Project ALCATRAZ: Archival Lyman-Continuum and Theoretical Reionization Analysis vs. z:Where, When, & How Much Does LyC Escape from Galaxies & AGN?

Scientific Justification

A. Scientific Background: Cosmic Reionization was the second major phase transition ofhy-drogen in the universe, following recombination. It began at z<

∼9–20 (Planck 2015), and was

completed by z≃6–7 (e.g., Mesinger & Haiman 2004; Becker et al. 2001, 2015; Fan et al. 2002,McGreer et al. 2014). Bubbles of ionized hydrogen formed around UV-bright objects, expanded,and merged until the intergalactic medium (IGM) became fully ionized. Lyman-continuum (LyC;λ≤912A) photons can be produced by massive stars in star-forming (SF) galaxies, by hot accretiondisks in Active Galactic Nuclei (AGN), or by more exotic sources. Neutral hydrogen and dust areopaque in the far-UV, so LyC can only escape where the HI column density (NH) and dust extinc-tion (AV ) are low. For a fraction of LyC photons to escape (fesc), pathways need to be cleared inthe object’s interstellar medium (ISM) and its surroundingIGM, which both SNe winds and AGNoutflows can do (Fujita et al. 2003; Silk & Norman 2009).

Fig. 1 compares the rest-frame far-UV spectra of QSOs (z≃0.1–4; van den Berk et al. 2001)and Lyman Break Galaxies (LBGs; z≃2–4; Shapley et al. 2003; Bielby et al. 2013), illustrating thatAGN can produce more hard UV radiation than LBGs. LBGs may also by selection have lowerLyC escape fractions. Because AGN are much more rare than galaxies and their density decreasesat z>

∼2.5 (Madau et al. 1999, 2015; Fan et al. 2001; Hunt et al. 2004;Siana et al. 2007), they likely

did not reionize the IGM at z>∼

3. However, AGN contributed much of the LyC background fromtheir peak at z≃2 until today, maintaining the current ionization of the IGM(Cowie et al. 2009).

No significant escaping LyC flux was detected in rest-frame far-UV data of SF galaxies at0.5<

∼z<

∼2 (Siana et al. 2007, 2010; Grimes et al. 2009; Cowie et al. 2009; Bridge et al. 2010).

Ground-based spectra yielded evidence for escaping LyC in SF galaxies at 3<∼

z<∼

4 (f relesc≃1–16%;

Shapley et al. 2006; Cooke et al. 2014; de Barros et al. 2015), and images indicatef relesc≃2–24%

(Iwata et al. 2009; Nestor et al. 2011; Vanzella et al. 2010, 2012; Boutsia et al. 2011; Mostardiet al. 2013; Borthakur et al. 2014), despite the higher IGM opacity at higher z (Haardt & Madau1996, 2012). HST UV spectra (Bridge et al. 2010) detected oneAGN with significantf rel

esc≃15% atz≃0.7. Siana et al. (2015) and Vanzella et al. (2015) give a careful assessment of the contributionof contaminating flux to LyC detections in previous ground-based work. Because AGN activitypeaked late (Micheva et al. 2016) and galaxies outnumber AGN, the combined far-UV output from(dwarf) galaxies may have started reionization at z>

∼8, completed and then maintained it at z<

∼6,

until AGN started to dominate at z<∼

2–3. Using the best Archival HST UV-data and 115 newground-based spectra, Project ALCATRAZ will study where, when, how, and how much LyCflux escapes from galaxies and weak AGN at 2.3<

∼z<

∼4.

B. Available Data for Project ALCATRAZ, Assessing and Correcting Systematics

Smith et al. (2016; S16) analyzed WFC3/UVIS UV-images in the Early Release Science field(ERS; Windhorst et al. 2011; W11). The F225W, F275W, and F336Wfilters can capture LyCemission at z>

∼2.26, z>

∼2.47, and z>

∼3.08 (Fig. 1). The higher IGM opacity at z>

∼4.5 makes it much

more difficult to detect LyC (Inoue et al. 2014, I14; Fig. 2). Through SED-fitting, 8–12 band objectcatalogs in CANDELS/GOODS, ERS, and HUDF (Teplitz et al. 2013) yielded luminosity/stellar

1

mass, age, star-formation rate (SFR) and extinction (AV ) for each object.S16 showed that, bycarefully correcting for systematics (§ D), stacking images with a native 2-orbit depth for 64objects can reach sensitivities equivalent to>

∼100 UV-orbits. ALCATRAZ will expand this to

stacking 6–15 orbit images for 360 objects, equivalent to LyC depths of 1000s of orbits.

C. Science Goals of Project ALCATRAZ at 2.3<∼

z<∼

4

Goal 1. HOW MUCH? What level of escaping LyC radiation can we measure reliably throughstacking?: The WFC3 ERS data of S16 (Fig. 3–6; Table 1) suggest that in each of three redshiftsbins from z≃2.3 to z≃4, LyC flux is detected atmAB≃29–30.5 mag (>

∼3–4σ) in 50 stacked galax-

ies and 14 weak AGN (Fig. 3). The spectroscopically selectedsample with reliable redshifts at2.26<

∼z<

∼4 shows that galaxies without and withweakAGN haveL∗ luminosities (〈MAB 〉≃–20.8

±0.8 mag). ALCATRAZ will analyze new UV-images in the HUDF and GOODS-N+S, adding>∼

250 new objects at 2.3<∼

z<∼

4 with firm spectroscopic redshifts over deeper and wider HST fields(Table 2). Using accurate 8–12 filter HST images, we can reliably mask all foreground interlopers(Fig. 3), reducing the faint interloper fraction to well below <

∼3% (S16). We will add ground-

based spectra with LBT, MMT, Magellan, and VLT for 115 new objects with 10-band photo-z’sat 2.3<

∼zphot

<∼

4 to AB<∼

24.5 mag (Fig. 6) to confirm their current lower-quality spectroscopicredshifts (Fig. 3, bottom row).ALCATRAZ will significantly improve upon LyC detectionswith rigorous treatment of low-level systematics in the WFC3 UVIS stacks (§ D), increasingstatistics by 7× to >

∼360 objects at 2.3<

∼z<

∼4, and the stacking depth by 2.7×.

Goal 2. WHERE and HOW? What does the radial dependence of escaping LyC radiationtell us about where and how LyC escapes?:Fig. 3c shows a stack of 50 galaxies in the highlyreliable spectroscopic sample of S16 (§ D.2a). Fig. 4 shows that their stacked LyC light-profilesare extended with respect to the PSFs,andmuch flatter than non-ionizing UV-continuum (UVC;1500A) profiles for radii r<

∼0.′′7, beyond which sky-subtraction errors become substantial. This

holds clues as to how ionizing photons may escape along a few random sight-lines through aporous ISM and surrounding gaseous, dusty material. With atmost a few clear sight-lines pergalaxy, the likeliest LyC escape paths may be on averagesomewhat offsetfrom a galaxy center(Fig. 3dh). This could arise naturally in a porous ISM in which the covering factor of neutralgas decreases with increasing galacto-centric radius. To illustrate this quantitatively, we considerthe transfer of UV and LyC continuum photons through simplified models with a multi-phase ISM(Dijkstra & Kramer 2012), adapted for LyC scattering (light-blue models in Fig. 4).ALCATRAZwill carefully measure and model stacked LyC light-profilesfor LBGs, Ly α galaxies, dustystar-forming galaxies, and AGN with outflows, and constrainthe radial dependence of theirISM porosity from the escaping LyC light-profiles.

Goal 3. WHEN? How does the LyC escape fraction evolve with epoch for both galaxies andweak AGN? Doesfesc(z) follow the cosmic star-formation rate?:Estimating the escape fractionof LyC photons requires modeling of the intrinsic LyC flux andthe wavelength-dependent frac-tion of LyC photons transmitted through the IGM at redshift z. Fig. 5 summarizes publishedfesc

results as blue triangles, all converted tof absesc with 1σ errors (see S16). With SED-fitting of the

UVC longwards of Lyα, the observed LyC fluxes correspond toaverage absoluteLyC escape frac-tions (orange points) that seem to rapidly increase fromf abs

esc ≃0.2% at〈z〉≃2.37–2.67 to∼20% at

2

〈z〉≃3.5 (Table 1). Several authors (Inoue et al. 2006; Kuhlen & Faucher-Giguere 2012; Finlatoret al. 2012; Becker & Bolton 2013; Dijkstra et al. 2014) suggested thatfesc may increase signifi-cantly with redshift, possibly as steeply as∝(1+z)3. The combined data in Fig. 5 suggest a trendin thefesc of galaxies with redshift that may not be a simple power-law in (1+z). The violet-shadedregion indicates:f abs

esc ≃ (0.6 ± 0.2) × (1 + z)1.5±0.7%. More than half of the 49 independentdata points in Fig. 5 deviates bymore than1σ from either one of the three power-laws,henceno single (1+z)-regression fits all thefesc-data for galaxies. Hence,a more sudden increase offesc with redshiftmay instead have occurred, indicated by the greentanh(1+z)function in Fig. 5.Each of the four free parameters in this tanh-fit has a straightforward meaning. For galaxies, thesteepest drop infesc occurs atzo≃3 — i.e., right around the peak in the cosmic star-formationhistory — over an interval less than±0.8 Gyr in cosmic time. The pivot point for galaxies atzo≃3occurs at∼2%, andf abs

esc may have dropped by a factor of nearly∼100 fromf absesc ∼20% at z>

∼3.5

to f absesc ∼0.3% at z<

∼2.5. The UVC SEDs of galaxies with weak AGN are dominated by their stellar

population, since their luminosities are〈MAB 〉 ≃–20.8±0.8 mag, although the LyC of weak AGNoutshines that ofL∗ -galaxies by∼1.2 mag (S16).Scaling by the numbers in Table 1, galaxiesthen appear to dominate the reionizing flux at z>

∼3, while (weak) AGN take over at z<

∼3.

The transition from galaxy–dominated to weak-AGN–dominated reionization may have oc-curred at z<

∼2.5–3, i.e., right after the peak in the cosmic SFR (Madau et al. 1995), where the

universe transitioned from infall- and SF-driven to more passively evolving galaxies. This may re-sult in gas and dust rapidly accumulating in the central bulges and disks of forming galaxies (Fig.6), combined with a SN-rate that has progressively less impact on removing gas from galaxieswhich are steadily growing in mass over time. This could cause the escape fraction torapidly dropover a relatively narrow interval of cosmic time. When AGN-outflows started to ramp up afterthe peak in the cosmic star-formation rate at z≃3 (Hopkins et al. 2006), they may have clearedenough paths in the ISM of host galaxies to enhance the fraction of escaping LyC radiation, result-ing in AGN starting to dominate reionization at z<

∼2.5–3.ALCATRAZ will estimate the relative

contribution to reionization from both ∼L∗ galaxies and weak AGN vs. epoch for 2.3<∼

z<∼

4.

Goal 4. WHY do LyC escape fractions evolve so strongly with epoch?:We will divide the newdata in Table 1 into large enough sub-samples tostack escaping LyC flux vs. redshift, extinction(Fig. 6), age, SFR, or luminosity/mass. We will do the same for(weak) AGNselected from deepChandra X-ray data (Xue et al. 2016), E-VLA radio images (Miller et al. 2013), variability, oremission-line spectra (e.g., Cohen et al. 2006; Windhorst & Cohen 2010). The current samples arevery small, and clearly need confirmation through much larger fesc-samples for both galaxies andweak AGN. Fig. 6 shows that the extinction values (Calzetti 2000) for our LyC sample as a functionof spectroscopic redshift are consistent with those in the entire ERS sample of 6500 galaxies toAB <

∼27 mag with accurate 10-band SEDs and photo-z’s (W11).The median extinction of galaxies

appears to increase from z≃4 to z≃2.3, and our directfesc-error calculations will incorporate that.ALCATRAZ will estimate the gradual increase in extinction AV (z) from z≃4 to z≃2.3, whenstellar populations have aged and produced more dust. We will study for for LBGs, Lya-galaxies, dusty star-forming galaxies, and weak AGN how dust and gas increasing aroundthe peak in the cosmic star-formation rate may have suppressed the escaping LyC flux.

3

Fig. 1 [LEFT] : Composite rest-frame spectra of SDSS QSOs at z≃1.3 (van den Berk et al. 2001 [blue]) andof LBGs at z≃2–4 (Bielby et al. 2013 [greenandorange]; Shapley et al. 2003 [red]). The F225W, F275W,and F336W transmission curves capture LyC (λ<912A) at z≥2.26, z≥2.47, and z≥3.08. The averagez≃1.3 QSO produces a much stronger LyC signal than an z≃3 LBG, while its IGM is only 1.5× moretransparent.ALCATRAZ will carefully compare the LyC escaping from AGN to LyC escaping fromgalaxies.Fig. 2 [RIGHT] : Inoue et al. (2014) IGM transmission model used forfesc calculations. Red is themedianand grey the 68% range, based on a Monte Carlo (MC) simulation of IGM attenuation vs. redshift.ALCATRAZ will use any significant LyC detections at z>

∼4 to help improve IGM transmission models

(see the Analysis Plan in§ D).

<_ <_

smoothed

smoothede) All Galaxies f) Galaxies w/ AGN g) Galaxies w/o AGN h) Galaxies w/o AGN,

a) All Galaxies b) Galaxies w/ AGN c) Galaxies w/o AGN d) Galaxies w/o AGN,

Super-Stacks of LyC Fv at 2.3 z 4

N=50N=14N=64

N=133 N=18 N=115

Bes

t spe

ctra

+N

ext b

est s

pect

ra

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Isophotal Semi − Major Axis [arcsec]

22

24

26

28

30

32

SurfaceBrightn

ess

[magABarcsec−2]

F275W PSF

〈z〉=2.38

〈z〉=2.68

〈z〉=3.49

Fig. 3 [LEFT]: Stacks ofall LyC objects at 2.3<∼

z<∼

4 of S16; [Top Row]: objects with the best spectraand secure redshifts; [Bottom Row]: objects with additional spectra, whose redshifts we propose to se-cure through this ALCATRAZ project (images are 6.′′4×6.′′4); Panels(a, e)All objects at 2.3<

∼z<

∼4; (b, f)

Galaxies with (weak) AGN;(c, g) Galaxies without AGN;(d, h) Galaxies without AGN smoothed by 1pixel. Blue and green circles have radii of 8 and 13 pixels (0.′′72 and 1.′′17), and yield S/N≃7–13 for these“super-stacks”. The very faint LyC emission from the average of 50 (115) galaxies has a rather flat sur-face brightness (SB) distribution.These LyC-stacks are equivalent in depth to>

∼100 HST UV-orbits.

Fig. 4 [RIGHT]: Radial SB-profiles of the non-ionizing UVC signal (solid curves) and of the LyC signal(dashed curves) in the stacks of Fig. 3c. The horizontal dashed line indicates the 1σ SB-limit for stackedLyC (see S16). All UVC-profiles (solid) and LyC-profiles (dashed) are extended with respect to the WFC3PSF (dotted blue). The LyC-profiles are also clearlymuch flatterthan the UVC-profiles, as predicted byour scattering model for z≃2.68 (light-blue curves).ALCATRAZ will include new HUDF andGOODS/CANDELS UV-samples, add 115 new ground-based spectra, increasing the totalsample∼7×, and provide ∼2.7× the current stacking S/N on LyC light-profiles.

4

1 2 3 4 5 6 7 8 91 + z

0.1

1.0

10.0

100.0〈f

abs

esc〉(%

)

ThisARproject:7×more objects+115 new spectraat 2.3<z<4

〈fabs

esc〉≃ ∆

2·tanh([(1+z)−(1+z0)]/δ)+f0

≃ F0·(1+z)κ

13 11 9 7 5 3 1Age of the Universe (Gyr)

1 2 3 4 5 6 7 8 9

0

1

2

3

Fig. 5 [LEFT] : Absoluteescape fractions for various galaxy samples vs. redshift. Orange circles areERS data for S16’s sample with reliable spectroscopic redshifts, withfabs

esc -errors from MC simulations ofthe IGM transmission using the Inoue+ (2014) code. Blue triangles are published data (§ A). The violetregion is bounded by:fabs

esc ≃(0.6±0.2)×(1+z)1.5±0.7%. The truefesc(z) relation may not be a power-lawin (1+z). A tanh(1+z) function (green curve) may better capture the possibly rather sudden change infesc

with redshift around z>∼

3, fromfabsesc ≃0.3% at z<

∼2.5 tofabs

esc>∼

20% at z>∼

3. ALCATRAZ will confirm thesudden rise in escape fraction at z>

∼3 with ∼7× better LyC statistics in HST UV-fields and >

∼2.7× the

depth, plus ground-based spectra for 115 new objects with 2.3<∼

z<∼

4 to AB<∼

24.5 mag, and so betterconstrain how galaxies dominated reionization for z>

∼3, while AGN took over at z<

∼3. Fig. 6 [RIGHT] :

Distribution of dust extinction (AV ) vs. redshift from best-fit SEDs for all 6500 galaxies in the 10-band ERSdata (W11; small black dots), compared to the LyC objects of S16 (open circles & asterisks indicate galaxies& AGN, resp.). Note the extinction (AV ) steadily increases from z≃7 to z≃3 at the peak in the cosmic star-formation rate, and then declines towards lower redshifts, where lower-luminosity galaxies are sampled.ALCATRAZ will address how dust and gas rapidly accumulating over cosmic time around the peakin the cosmic star-formation rate may have shut down the escape fraction in L∗ -galaxies at z<

∼3.

Analysis Plan (§ D)

(0) Building/Reducing Database: Table 2 shows a total of 2170 WFC3 images and 3076 ACSimages. We already have the full ERS data set. At∼128–164 Mb per image, there are>

∼280 Gb of

WFC3 and 400 Gb of ACS data, a total of 680 Gb. To resolve remainingastrometric and subtle biaslevel (DC) and flat-field systematics (see below), we may also need to retrieve the individual dataframes. A student will retrieve these from MAST at the rate of50–100 Gb a day, which will takeabout one week. The student may re-drizzle the 90 orbit HUDF UV and the GOODS/CANDELSUV-data, if needed. All raw data are public, as are the v1 UVUDF mosaics.

(1) Assess UVIS mosaic quality:We will make a rigorous assessment of the GOODS and HUDFUV images that goes well beyond the normal use of these data toAB≃26–29 mag for point sourcedetection. Our ERS study thus far (Smith et al. 2016; S16) hasshown some very low-level system-atics in the WFC3 UVIS image stacks that we must address first. These are:

(1a) Remaining astrometric offsets:The 2009 calibration of the WFC3 ERS mosaics had as-trometric offsets>

∼5 pixels (W11), which have been addressed with new geometrical distortion

corrections (GDCs; Kozhurina-Platais 2013) to within〈∆(X,Y )〉<∼

0.′′017±.′′05 (0.19±0.56 pix).

5

Table 1. Summary of LyC Stacking thus far, and Proposed New UV-Samples to be AnalyzedFilter z-range 〈z〉 Nobj mLyC SNLyC mUV C SNUV C f1500/fLyC AV med fabs

esc (MC)(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11)

ERS GALAXIES WITH AGN (SMITH ET AL . 2016)+ 12 new AGN spectra:F225W 2.291–2.291 2.291 1+3 >

∼30.12 <

∼2.34 27.90 7.85 3.45+0.12

−0.11 0.90+0.14−0.14 TBD (use AGN SEDs)

F275W 2.470–3.008 2.697 7+5 28.92 8.77 25.00 156.9 2.98+0.08−0.07 1.23+1.14

−1.13 TBD (use AGN SEDs)F336W 3.217–3.474 3.326 4+4 29.53 5.17 24.52 102.9 11.4+0.2

−0.2 0.10+0.14−0.10 TBD (use AGN SEDs)

ERS GALAXIES WITHOUT AGN (SMITH ET AL . 2016)+ 80 new galaxy spectra:F225W 2.302–2.450 2.380 14+30 29.98 5.64 24.43 237.5 3.45+0.12

−0.11 0.55+0.70−0.44 0.10+1.97

−0.06%F275W 2.559–3.076 2.682 11+25 30.09 5.71 24.51 192.2 2.98+0.08

−0.07 0.58+0.89−0.40 0.14+1.46

−0.07%F336W 3.132–3.917 3.494 10+25 30.79 3.91 24.96 111.6 11.4+0.2

−0.2 0.18+0.64−0.12 17.6+31.3

−9.8 %PROPOSED HUDF-UV Galaxies + weak AGN for analysis, including newspectra (estimated):F225W 2.26–2.47 2.38 12+3 ∼29.8 >

∼9 ∼24.5 >

∼1100 3.74+0.12

−0.10 0.5+0.7−0.4 ∼ 0.2+0.9

−0.02%?F275W 2.47–3.08 2.68 15+3 ∼29.5 >

∼12 ∼24.6 >

∼900 3.25+0.06

−0.06 0.6+0.9−0.4 ∼ 0.3+0.6

−0.03%?F336W 3.08–4 3.49 20+5 ∼30.0 >

∼9 ∼24.7 >

∼1250 4.33+0.34

−0.34 0.2+0.6−0.1 ∼ 20.+19.

−6.2%?PROPOSED GOODS-N+S–UV Galaxies + weak AGN for analysis, including new spectra (estimated):F275W 2.47–3.08 2.68 75+7 ∼29.5 >

∼20 ∼24.6 >

∼360 3.25+0.06

−0.06 0.6+0.9−0.4 ∼ 0.3+0.5

−0.02%?F336W 3.08–4 3.49 75+7 ∼30.0 >

∼12 ∼24.7 >

∼290 4.33+0.34

−0.34 0.2+0.6−0.1 ∼ 20.+11.

−3.4%?TOTAL New objects: 2.3–4 ∼360 (+115 new spectra)

Table columns:(1) WFC3 filter; (2) Redshift range used in LyC/UVC stacks (seeS16); (3) Average redshift of stack; (4) Number of galaxies withreliable spectroscopic redshifts; (5) Observed total LyC AB-mag; (6)S/N ratio of LyC stack; (7) Observed total UVC AB-mag; (8)S/N ratio ofUVC stack; (9) Averageintrinsic model flux ratiof1500/fLyC and its±1σ range; (10) Median dust extinction AV and±1σ range from 10-bandSED fits; (Average IGM transmission〈TIGM〉 is ∼1/e×T med

IGM from Fig. 2; see I14 and S16); (11) Modalfabsesc derived from MC modeling of

IGM transmission using the Inoue+ (2014) code, plus±1σ range, usingfabsesc ≡ fLyC,obs/fLyC,⋆. These ERS UV-stacks are equivalent in depth

to >∼

100 HST orbits.ALCATRAZ will increase these samples∼7× with ∼2.7× the current stacking depth. All the proposed numbers of newobjects are indicated in violet.For the HUDF and GOODS/CANDELS-UV, the corresponding numbers in black are theexpectedvalues resultingfrom Project ALCATRAZ (see§ D).

Table 2. Building the Database:Our ALCATRAZ project will use the following data sets:====================================================================================================Table 2: ----GOODS-N+S/CANDELS Data---- ---------HUDF Data---------- ----------ERS Data-----------Instrum Nfilt*NexpxNfld Nexp Norb Nfilt*Nepoch*Nexp Nexp Norb Nflt*NexpxNfld=Nexp Norb Nexp----------------------------------------------------------------------------------------------------WFC3 UVIS: 2*(6x8+6x8)+(9x5+8x5)= 277 181 3*2*(2x15) = 180 090 4x8+4x8+3x8 = 88 40 545ACS/WFC: (6+10+10+20)*(3x3x2) = 828 234 164+286+460+362+700=1972 494 (6+10+10+20)x6 =276 78 3076WFC3 IR: 3*4*(2x5)x(2x2) = 480 240 248+289+122+366 =1025 254 3*4*(2x5) =120 60 1625Grand Tot 1585 655 3177 838 484 178 5246----------------------------------------------------------------------------------------------------

For accurate UV-stacking of all data in Table 2, we must remove these across all mosaicsat thesub-pixel level, by cross-correlating the UVIS images with the deepestχ2-images.

(1b) CTE effects and their correction: The 2-orbit WFC3 ERS images (W11) taken in Sept.2009 do not suffer significantly from Charge Transfer Efficiency (CTE) losses compared to the2012 HUDF (2×15 orbits per UV-filter) or GOODS-UV (2–9 orbits per UV-filter) data. The latterwere post-flashed with∼6–12e− to fill in CCD-traps that accumulated over time. Following S16,we will verify that the (post-flashed) LyC fluxes are not affected by CTE-degradation by sub-dividing all images in one-half closer to the A/D converters(i.e., suffering less CTE-degradation),and one-half further away. In the ERS, the LyC photometry of the data-half further from the A/Dconverters was not significantly fainter than the data-halfthat is closer (S16).

(1c) Subtle DC or Dark-Current Offsets, Residual Sky-Gradients, and Sky-Subtraction Er-rors: Some subtle DC offsets are seen in the>

∼100 orbit LyC stacks in the ERS in Fig. 3. These may

be due to subtle variations in the dark-current between exposures, residual flat-field errors, or theway sky-subtraction was done in each image during drizzling. The original WFC3/UVIS thermalvacuum flat-fields (Sabbi 2009) left residual gradients of 5–10% on top of Zodi sky (which itself is

6

AB∼25.5 mag arcsec−2 ; W11). This has significantly improved with on-orbit “delta-flats” (Macket al. 2013), reducing residual gradients in the sky to∼3% of Zodi (S16). Such dim gradients arevery hard to remove, but they appear to be mostly linear across the entire CCD. Following S16,subtle DC offsets and these residual flat-field gradients amount to alocal sky-subtraction errorof∼32.3 mag arcsec−2 across our 6.′′39×6.′′39 LyC sub-images (Fig. 3). We will map any low-levelresidual gradients across all images in Table 2 as a 2D-surface, and correct the individual imagesafter super-darkframe subtraction, and (if needed) again after flat-fielding and before drizzling, asin S16. This may require an additional step, where all cosmic-rays (CRs) and objects are firstidentified and masked in each image to stabilize the 2D surface-fits.

(1d) Red-Leak and Filter-Pinhole Corrections:Fig. 1 and S16 predict the LyC red-leak fractionfor LBGs at z≃2–6 relative to their UVC flux as 0.0030%–0.0001% —i.e., <

∼1% of theactual

measured LyC flux — in line with the∼10−5 red-leak wings of these three filters. Redleak fractionsare thus very small compared to our relativefesc-values.

(2) Panchromatic SED fitting: We will use panchromatic SED-fitting (Coe et al. 2006; Dahlenet al. 2013) to estimate the following galaxy properties: zphot, total flux, luminosity, stellar mass,stellar population age, star-formation rate (SFR), and extinction (AV , e.g., Windhorst et al. 2010).

(2a) Spectroscopic Redshifts, Sample Selection, Reliability & Completeness:About >∼

250 newobjects with available firm spectroscopic redshifts (zspec) at 2.3<

∼z<

∼4 will be studied in more and

deeper HST fields (Table 2). To this, we will addLBT, MMT, Magellan or VLTspectra for 115objects to AB<

∼24.5 mag with 10-band photo-z’s at 2.3<

∼z<

∼4 (Fig. 6) to confirm their currently

lower-quality spectroscopic redshifts (Fig. 3, bottom row). We will inspect all available spectra,and assign a binary flag to each redshift: “highly reliable”,or “not reliable enough” for this study.

(2b) Photometric Redshifts and Removal of Interlopers:To remove foreground interlopers, wewill use the 8-12 filter P(zphot) distribution to maximize the probability that each nearest neighboror potential interloping object belongs to the LyC candidate to be stacked. The galaxy countsshow<

∼5×105 galaxiesdeg2 to J<

∼27.5 AB-mag (W11), so the fraction of contaminating objects in

the 0.′′50 radius definition-apertures is<∼

3%. All contaminating neighbors visible in the deepest 8–12-filter χ2-stacks will be masked in the LyC apertures and surrounding sky using SEXTRACTOR

“segmentation” maps, further reducing the foreground interloper fraction (S16). This worked verywell for UVC stacking of HUDF objects at z≃4–6 (Hathi et al. 2008) and the LyC stacking of S16.

(3) LyC Stacks and Quality Checks:We will perform various quality checks (see S16), verifyingthat the LyC flux remains present (to within the errors in Table 1) when the images are rotated byrandom multiples of 90◦, or when the first and second independent data-halves are stacked. Wewill also verify that no spurious fluxis seen when stacking an equal number ofrandom emptysky-boxeswithout known objects, confirming the point-source and SB-sensitivity limits in Table 1.For the ERS, all these checks indeed confirmed the LyC photometry presented in S16.

(4a) Theoretical Modeling — Scattering Models:Dijkstra will predict LyC SB-profiles for a gridof wind model parameters, and investigate what constraintscan be placed from either detectionsand/or upper limits. He will perform full MC simulations of asub-set of models to accountfor multiple scattering events, and to test the accuracy of analytic models. These models will be

7

compared to the observed UVC and LyC light-profiles (see the light blue lines Fig. 4), and theISM scattering models will be adjusted as needed.

(4b) Theoretical Modeling — Improved Constraints on IGM Tran smission Models:Inouewill investigate if any significant z>

∼4 LyC detections require updates of IGM transmission mod-

els, which predictsmedianIGM transmission valuesTmedIGM

<∼

0.25 at z>∼

4 (Fig. 2). The I14 modelis based on the redshift (dn/dz) and column density distributions from Lyα Forest (LAF), Ly-man Limit Systems (LLS), and Damped Lyα Absorbers (DLAs), and best reproduces Lyman limitmean-free-path (MFP) measurements at z<

∼5 by Worseck et al. (2014), which averages over∼50

QSO-sightlines, but leaving room for a∼0.3 dex systematic uncertainty between estimation meth-ods. Hence, we will also check ACS/WFC F435W images to trace escaping LyC in objects at red-shifts just over z∼4.35, since there may be some LyC flux in GOODS B-band images for objectsat z∼4.4 (see S16). These are not plotted in Fig. 4–5, but will helpconstrain IGM transmissionmodels at z>

∼4 (Fig. 2), if such samples are large enough.

An 0.3 dex difference in MFP results in a 1–2 mag difference inAIGM , which has a largeimpact onTIGM . Moreover, the dn/dz and column density distribution measurements of LLS andDLAs — which mainly determine the LyC transmission — are still sufficiently sparse at z>

∼4, so

it may be possible to find plausible IGM transmission models that allow us to interpret LyC detec-tions at z≃4–4.5. Another possibility to avoidfesc

>∼

100% is mechanisms that produce higher LyCemissivity than normal stellar population models do. For example, rapid stellar rotation (Leithereret al. 2014) and massive binary stars (Eldridge & Stanway 2009) can emit more LyC. The escapingnebular bound-free continuum can boost LyC emission (Inoue2010), and so can top-heavy IMFand extreme metal-poor/zero-metal stellar populations. These possibilities will be studied in detail.

Management Plan

We have the following personnel: Grad Student Assistant (GS; Mr. Brent Smith); Windhorst(RAW); Cohen (SHC); Jansen (RAJ); Brenda Frye (BLF), Koekemoer(AMK); MacKenty (JM);Dijkstra (MD); Inoue (AI); Bielby (RB), Conselice (CC). Our Analysis Plan requires as FTEs:

(0) Database+Reduction:3.0 mo GS + 0.5 mo AMK +0.5 mo JM;

(1) UVIS CTE/gradient-removal/quality: 3.0 mo GS + 1.0 mo RAJ/SHC/RAW; [namely: (1a)Astrometric offsets: 1 mo GS + 0.5 mo RAJ;(1b) CTE effects:1 mo GS + 1 mo SHC;(1c) DCoffsets:0.5 mo GS + 0.5 mo RAJ;]

(2) SED fitting and photo-z’s:1.0 mo GS + 1.0 mo SHC;

(2a) New spectra and redshifts (LBT, MMT, Magellan, VLT): 2.0 mo GS + 1.0 mo BLF(RB+CC are funded by the UK);

(2b) Contamination correction: 1.0 mo GS + 0.25 mo RAW;

(3) LyC stacking + checks:2.0 mo GS + 1 mo RAJ + 0.25 mo RAW + BLF;

(4) Theoretical modeling:2.0 mo MD & AI (funded by Norway & Japan);

(5) Team papers (1 Method, 1 Results, 1 Theory):3.0 mo GS + 1 mo RAW, helped by co-I’s.In summary: we need 15 mo GS. RAW and RAJ will each spend 1–2 mo each of their academicresearch time on this project.

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