Overturning the Case for Gravitational Powering in the Prototypical Cooling Lyman-alpha Nebula

2.8 MB·English

by  Moire K. M. Prescott

Draftversion January23,2015
PreprinttypesetusingLATEXstyleemulateapjv.5/2/11
OVERTURNING THE CASE FOR GRAVITATIONAL POWERING IN THE PROTOTYPICAL COOLING Lyα
NEBULA
Moire K. M. Prescott1, Ivelina Momcheva2, Gabriel B. Brammer3, Johan P. U. Fynbo1, Palle Møller4
Draft version January 23, 2015
ABSTRACT
5 The Nilsson et al. (2006) Lyα nebula has often been cited as the most plausible example of a Lyα
1 nebula powered by gravitationalcooling. In this paper, we bring together new data from the Hubble
0 Space Telescope and the Herschel Space Observatorya as well as comparisons to recent theoretical
2 simulations in order to revisit the questions of the local environment and most likely power source
n for the Lyα nebula. In contrast to previous results, we find that this Lyα nebula is associated with
a 6 nearby galaxies and an obscured AGN that is offset by ∼4′′≈30 kpc from the Lyα peak. The local
J region is overdense relative to the field, by a factor of ∼10, and at low surface brightness levels the
1 Lyα emission appears to encircle the position of the obscured AGN, highly suggestive of a physical
2 association. At the same time, we confirm that there is no compact continuum source located within
∼2-3′′≈15-23kpcoftheLyαpeak. Sincethelatestcoldaccretionsimulationspredictthatthebrightest

] Lyαemissionwillbecoincidentwithacentralgrowinggalaxy,weconcludethatthisisactuallyastrong
A argumentagainst, ratherthanfor,the idea thatthe nebula is gravitationally-powered. While we may
G be seeing gas within cosmic filaments, this gas is primarily being lit up, not by gravitational energy,
but due to illumination from a nearby buried AGN.
.
h Subject headings: galaxies: evolution — galaxies: formation — galaxies: high-redshift
p
-
o
1. INTRODUCTION GOODS-S field. Intriguingly, the deep GOODS imaging
r
t Therehasbeenalong-standingdebateaboutthepower showed no associated continuum detections in any
s
band, from the X-rays to the infrared. The photometric
a source responsible for radio-quiet Lya nebulae (or “Lyα
[ blobs”). Unlike extended Lyα halos observed around redshifts available at the time indicated that none of
the nearby galaxies were likely to be at the redshift
many radio galaxies and quasars, radio-quiet Lyα nebu-
1 of the nebula nor would they be able to provide suf-
laedonotalwayshaveanobvioussinglesourceofionizing
v ficient energy to power the nebula even if they were.
photons in their midst. Various explanations have been
2 Based on the lack of any obvious source of ionizing
proposed: for example, shock-heating in galactic super-
1
photons and on the similarity between the observed
3 winds (Taniguchi & Shioya 2000; Taniguchi et al. 2001;
Lyα surface brightness profile and early predictions
5 Mori et al. 2004), photoionization from nearby galax-
from cold accretion models, the authors suggested that
0 ies and AGN (e.g., Cantalupo et al. 2005; Geach et al.
this system could be powered by cooling radiation.
. 2009; Kollmeier et al. 2010; Prescott et al. 2012b;
1 Since then, discussions of the range of power sources
Overzier et al. 2013), and resonant scattering of cen-
0 observed for Lyα nebulae typically include a reference
trally produced Lyα photons (e.g., Møller & Warren
5 to this systemas the clearestobservationalexample of a
1998; Laursen & Sommer-Larsen 2007; Zheng et al.
1 “cooling” Lyα nebula (e.g., Smith et al. 2008; Dijkstra
2011; Steidel et al. 2011; Hayes et al. 2011). One final
: 2009; Adams et al. 2009; Rasmussen et al. 2009;
v explanation is that Lyα nebulae are powered by gravi-
i tational cooling radiation within cold accretion streams. Laursen et al. 2009; Yang et al. 2010; Goerdt et al.
X 2010; Weijmans et al. 2010; Colbert et al. 2011;
This possibility has received substantial theoretical at-
r tention over the past few years, motivated in large part Erb et al. 2011; Cen & Zheng 2013; Laursen et al.
a 2013; Tamura et al. 2013; Yajima et al. 2013).
byevidencefromnumericalsimulationsthatacoldmode
However, a number of things have changed substan-
of accretion could play an important if not dominant
tially since the initial discovery of this Lyα nebula. On
role in fueling galaxy formation (e.g., Keres et al. 2005;
the observationalside, grism spectroscopy, deeper X-ray
Dekel et al. 2009).
catalogs, and additional imaging - spanning the rest-
Against this backdrop, Nilsson et al. (2006) reported
the discovery of a Lyα nebula at z ≈ 3.157 in the frame optical to far-infrared - have become available in
the GOODS-S field thanks to the Early Release Science
1DarkCosmologyCentre, NielsBohrInstitute, Universityof program (Windhorst et al. 2011), the 2 Ms and 4 Ms
Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Den- Chandra Deep Field-South (CDF-S) Surveys (Luo et al.
mark;[email protected] 2008; Xue et al. 2011), GOODS-Herschel (Elbaz et al.
2DepartmentofAstronomy,YaleUniversity,NewHaven,CT
2011), and HerMES (Levenson et al. 2010; Oliver et al.
06511, USA
3Space Telescope Science Institute, 3700 San Martin Drive, 2012), allowing for significant improvements to both
Baltimore,MD21218, USA photometric and spectroscopic redshift measurements
4European Southern Observatory, Karl-Schwarzschildstrasse
in the region. In addition, some observational stud-
2,D-85748GarchingbeiMu¨nchen, Germany
a Herschel is an ESA space observatory with science instruments ies of other Lyα nebulae have shown that the Lyα
provided by European-led Principal Investigator consortia and emission can actually be offset substantially (∼10s of
withimportantparticipationfromNASA.
2 Prescott et al.
kpc) from associated galaxies and obscured AGN at by an array of additional ground- and space-based facil-
the same redshift (Prescott et al. 2009, 2012a,b, 2013; ities. In this work, we make use of WFC3-IR F125W,
Yang et al. 2014). On the theoretical side, simulations F140W, and F160W imaging and the WFC3-detected
of “cooling” Lyα nebulae have been carried out with photometric catalog of the GOODS-S field compiled by
greaterandgreatersophisticationoverthepastfewyears Skelton et al. (2014). The WFC3 imaging represents a
(e.g., Goerdt et al. 2010; Faucher-Gigu`ere et al. 2010; significantimprovementovertheground-basedJ andH-
Rosdahl & Blaizot 2012). In light of the new observa- band data available previously, as it provides high reso-
tional data and more advanced numerical simulations, lution imaging coverage near the critical 4000˚A break,
the time is ripe to return to this system and take an- andenabling much more robustphotometric redshift es-
otherlookatitslocalenvironmentandmostlikelypower timates;seeSkelton et al.(2014)forfurtherinformation.
source. We make use of the photometric redshift catalog from
In this paper, we revisit the following questions: (1) Skelton et al. (2014), which is based on the code EAzY
does the Lyα nebula have any associated galaxies, and (Brammer et al.2008),andmorphologicalmeasurements
(2) is it likely powered by cold accretion? We describe forgalaxiesintheregiontakenfromtheCANDELSmor-
thedatasetsusedinthisworkinSection2,andwestudy phological catalog (van der Wel et al. 2012).
the neighborhood of the Lyα nebula in Section 3. In The Lyα nebula is serendipitously located in the one
Section 4 we discuss the most likely powering mecha- and only grism pointing observed as part of the ERS
nismresponsiblefortheLyαnebula,takingintoaccount program(Windhorst et al.2011, GO/DD:11359,11360;
updated predictions from numerical simulations. Sec- PI:R.O’Connell)Theobservationsweredoneinboththe
tion 5 highlights some implications of this work for the G102andG141grisms,spanning 0.8−1.2µm and1.1−
largerclassofLyαnebulae,andweconcludeinSection6. 1.7 µm, respectively. The two grism provide resolutions
We assume the standard ΛCDM cosmology (ΩM=0.3, of R∼ 210 and 130. The pointing was observed for two
ΩΛ=0.7, h=0.7); the angular scale at z = 3.156 is 7.58 orbitswitheachgrism. InthisworkweuseonlytheG141
kpc/′′. AllmagnitudesareintheABsystem(Oke1974). grismobservations,whichcoversthe[Oii]λ3727emission
line out to z ≈ 3.2, as none of the objects relevant to
2. DATA thisworkhavesignificantdetectionsintheG102spectra.
This paper builds on the large number of existing The grism data were reduced by the 3D-HST team in
datasets and catalogs with coverage of the GOODS-S a manner similar to that described in Brammer et al.
field. (2012). Further details on the extraction and redshift-
fitting methods willbe presentedinMomchevaet al. (in
2.1. Lyα Narrowband Imaging and Spectroscopy prep.).
The original Lyα imaging was obtained as part of a 2.3. Mid- and Far-Infrared Data
survey for Lyα-emitting galaxies using FORS1 on the
The originalpaper presented Spitzer IRAC and MIPS
VLT 8.2 m telescope Antu and a standard narrowband
imaging of the field (Nilsson et al. 2006, their Figure 2).
filter (λ =5055˚A; FWHM=59˚A, corresponding to Lyα
at z =c3.126− 3.174). The nebula emission was con- At longer wavelengths, Herschel PACS and SPIRE cov-
erage of the field now exists from GOODS-Herschel and
firmedtobeLyαatz =3.157viafollow-upspectroscopy
HerMES (Elbaz et al. 2011; Oliver et al. 2012). In ad-
from VLT/FORS1, which showed a single emission line
dition, improvements have been made in extracting ac-
withtheclassicasymmetricprofileexpectedforhighred-
curate IRAC/MIPS photometry properly accounting for
shift Lyα emission and no detections of the other strong
source blending and the complex PSF of these instru-
lines that would have been expected from a low red-
ments, and there has been extensive work on the best
shift interloper. Details on the Lyα imaging and follow-
way to use IRAC/MIPS colors to distinguish AGN from
up spectroscopy are given in the original survey papers
star-forminggalaxies(Stern et al.2005;Lacy et al.2004,
(Nilsson et al. 2006, 2007).
2007; Donley et al. 2008, 2012).
Inthiswork,the mid-andfar-infraredphotometryare
2.2. HST Imaging and Grism Spectroscopy
takenfromtheGOODS-Herschelcatalog,whichincludes
The Lyα nebula is located within the GOODS-S/ERS measurements from the GOODS-Spitzer Legacy Survey
field. The area was originally observed as part of (Spitzer/IRAC+MIPS; P.I. Mark Dickinson) and the
the GOODS South program in the ACS BVRiz fil- GOODS-Herschel Survey (Herschel/PACS; Elbaz et al.
ters (Giavalisco et al. 2004; Dickinson et al. 2004). The 2011), accessed via HeDaM.5 Source positions in the
same region was more recently targeted as part of the GOODS-Herschel catalog were determined first using a
WFC3/ERS observations in Cycle 17 following the in- weighted average of the 3.6 and 4.5 micron images, and
stallationofWFC3duringServicingMission4(GO/DD: then empirical PSFs at these source locations were fit
11359, 11360; PI: R. O’Connell Windhorst et al. 2011). to the mid- and far-infrared maps in order to determine
Images were acquired in the F098M, F125W and the flux for eachobject andaccountfor source blending.
F160W filters, using two orbits in each filter. Addi- FurtherdetailsaregivenintheGOODS-Herschelrelease
tional shallower imaging in the F140W filter was done documentation.6
to provide a direct-image reference for a single point- The field has been observed in the far-infrared with
ing of ERS G141 grism observations (discussed below). Herschel/SPIRE as part of HerMES (Oliver et al. 2012;
TheF140W observationspartiallyoverlapwiththefoot- Levenson et al. 2010; Roseboom et al. 2010, 2012). In
print of the 3D-HST program (GO: 12177; PI: van
Dokkum) which also provides F140W images over a 5 http://hedam.lam.fr
largerarea. TheGOODS-S/ERSfieldhasbeenobserved 6 http://hedam.lam.fr/GOODS-Herschel/goodss-data.php
Overturning the Case for GravitationalPowering in a Lyα Nebula 3
this work, we make use of Herschel/SPIRE photometric ∆z±0.15ofthenebularedshift;thepositionsandphoto-
measurementsfromtheMIPS24µm-matchedcatalogde- metricredshiftsofthese“photometricredshiftmembers”
veloped by the HerMES Team (HerMES Team, private are given in Table 1. We plot the selected photometric
communication). redshift members on the HST imaging (Figure 2, right-
hand panel). Of these 6 sources, photometric redshift
2.4. Chandra X-ray Coverage estimates for two (Source 3 and 8) were given in the
original paper; the new photometric redshifts for these
Since the original paper, the Chandra X-ray coverage
sourcesare consistentwith the previousestimates, given
of GOODS-S has improved substantially from the orig-
the quoted error bars. The photometric redshifts of the
inal Chandra Deep Field-South 1 Ms survey, first by a
remaining sources were previously unknown.
factor of two and then by a factor of four in total ex-
We note that while photometric redshifts will clearly
posure time. The resulting Chandra Deep Field-South 2
be less accurate and more susceptible to biases, espe-
Msand4Mssurveycatalogs(Luo et al.2008;Xue et al.
cially at fainter magnitudes, relative to true spectro-
2011)provideasample740X-raydetectedsourcesdown
tolimitingon-axisfluxesof≈3.2×10−17,9.1×10−18,and scopic redshifts, a recent study of a suite of photo-
5.5×10−17 erg s−1 cm−2 for the 0.5-8 keV full band, the metric redshift codes found that individual codes based
on EAzY (Brammer et al. 2008) do reasonably well in
0.5-2keV soft band, and the 2-8 keV hardband, respec-
terms of their reported errors (i.e., corresponding to the
tively.
width of the reported P(z) peak), underestimating by
3. REVISITINGTHENEIGHBORHOOD about 5-15% (Dahlen et al. 2013). The scatter (σ0 =
rms[∆z/(1+z)]) between the spectroscopic redshift and
Nilsson et al. (2006) listed 8 objects within a 10′′ ra-
the median photometric redshift was found to increase
diusthathaddetectionsinatleast8photometricbands, from σ = 0.04 at m < 24 mag to 0.09 at m ∼ 26
andanadditional8nearbysourcesthatweredetectedin 0 H H
mag,whiletheoutlierfractionincreasedfromabout10%
only a subset of those bands, too few for reliable photo- to 20%. Thus by using a redshift window of ∆z±0.15,
metric redshifts to be estimated. Of the 8 sources with
wecanbe reasonablyconfidentthat weare primarilyse-
derived photometric redshifts, they presented projected
lectinggalaxiesattheredshiftofthenebulaevenatthese
distances and photometric redshifts in their Table 2.
fainter magnitudes.
None appeared to be precisely at the Lyα nebula red-
To test the significance of the observed photometric
shift,althoughfourofthesesourceswereconsistentwith
redshift peak coincident with the nebula, we redid the
itto within the relativelylargequotederrors(Sources 3,
analysisfor 10000randomapertures drawnfrom the en-
5,6,and8). Twoofthesesourceswerenotdiscussedfur- tire GOODS-S field. We find that at z ∼ 3, it is not
ther (Sources 5 and 8). The authors argued that Source
common to find a peak in the photometric redshift dis-
3 was unlikely to power the nebula, even if it were at
tributionthatisasstrongaswhatisseenattheposition
the correct redshift, because it did not appear to emit
of the Lyα nebula: formally there is a 2.0% chance of
enough ionizing photons. Finally, in the case of Source
randomly selecting a regionwith comparable areaunder
6, a mid-infrared source that was detected in the IRAC
the summed P(z) distribution, or a 0.4% chance of ran-
and MIPS bands, they concluded the object was most
domlyselectingaregionwithatleast6galaxieswithbest
likely an unassociated starburst galaxy at high redshift fit photometric redshifts within ∆z±0.15 of the nebula
(z ≈ 5.5) based on comparisons to Spitzer IRAC/MIPS
redshift.
color-colordiagnosticdiagramsfromIvison et al.(2004).
Interestingly, the very faint source visible in the
Using the new and improveddatasets described in the
F160W image that is located closest to the peak of the
previoussection,werevisitthefollowingquestion: isthis Lyα emission turns out to be one of the z ≈ 1 sources
Lyα nebula truly a line-emitting nebula without any as-
(labeled as Galaxy ‘F’ in Figure 1). The measured SED
sociated galaxies?
andP(z)derivedby EAzYforthis sourceshowthatitis
well-described by the EAzY fit at z = 0.91 with a stel-
3.1. Searching for Associated Galaxies lar mass of ≈ 1.2×108 M⊙, and that it is inconsistent
3.1.1. Photometric Redshifts with either an AGN or starburst template at the nebula
redshift (Figures 4-5). Thus, we confirm that there does
Figure 1 shows a stack of the WFC3-IR/F125W,
notappeartobeanyassociatedcontinuumsourcewithin
F140W, and F160W imaging in the vicinity of the Lyα
nebula, shown as contours,7 while Figure 2 shows the ∼2-3′′≈15-23 kpc of the peak of the Lyα emission.
Wenotethatgravitationallensingfromtheforeground
individual bands. Using the 3D-HST/CANDELS pho-
sources at z ≈ 1 should only have a minimal effect on
tometric redshift catalog (Skelton et al. 2014), we select
sources that lie within a 10′′ radius of the Lyα nebula. the nebula. If we take all galaxies in the local vicinity
that have photometric redshifts within ±0.15 of z = 1,
Figure 3 shows the distribution of photometric redshifts
the total stellar mass derived from the SED fitting is
in this region, both as a histogram and as a sum of
the individual galaxy P(z) distributions. The photomet- ≈3×1010 M⊙, with the brighter(andpresumably more
massive) z ≈ 1 sources being located about 5′′ to the
ric redshift distribution shows two main peaks: one at
z ≈ 1 and one around z ≈ 3.1, i.e., roughly the red- northwest of the nebula (Figure 2, middle panel). If the
corresponding total mass of the foreground structure is
shift of the nebula. Formally, 6 sources within this lo-
cal region (R < 10′′) have photometric redshifts within oforder3×1011 M⊙,theexpectedEinsteinradiuswould
be≈0.2′′,insignificantcomparedtothefullextentofthe
7Wenotethatthesurfacebrightnesslevelsforthecontoursplot- Lyα emission, and the magnification at the position of
tedinNilssonetal.(2006)appeartohavebeenstatedincorrectly the Lyα peak would only be about 3%. If we consider
inthecaptionoftheirFigure1. onlythesourcelocatedclosesttothe center(Galaxy‘F’,
4 Prescott et al.
with a stellar mass of 1.2×108 M⊙), the expended Ein- (2004, 2007), and within the more recent IRAC power-
stein radius would be ≈0.05′′. Thus, neither the large law AGN selection box of Donley et al. (2012). From
scale morphology nor the surface brightness of the neb- the mid-infrared colors, Source 6 is therefore very likely
ula is significantly affected by gravitationallensing from an obscured AGN and not a starburst galaxy. The Her-
the z ≈1 sources. schel detections are susceptible to source confusion but
suggest that there may be an additional cool dust com-
3.1.2. Grism Redshifts ponent, e.g., emission from the host galaxy.
It is true that Source 6 was not detected in the deep 1
Wenextsearchedthegrismredshiftcatalogforsources
Ms CDF-S X-ray imaging,as pointed out in the original
with grism redshifts consistent with the nebula. We
paper. Wefindthatthesourceisstillnotdetectedinthe
find that one of the 6 photometric redshift neighbors
more recent 2 Ms and 4 Ms CDF-S catalogs (Luo et al.
(Source3)showsaweak[OII]detection(Figure6). This
2008; Xue et al. 2011). However, a study of power-law
corresponds to a measured grism redshift of z =
grism IRAC selected AGN candidates in the 2 Ms Chandra
3.153+−00..001094 (Table 1), consistent with the photomet- Deep Field-North (CDF-N) X-ray imaging showed 23%
ric redshift (z = 3.206+0.065) and with the nebula (35%)werenotdetectedabove3σ(5σ)inanyX-rayband
phot −0.058
redshift measured from the Lyα emission line (z = (Donley et al. 2007), while stacking of the X-ray non-
Lyα
3.157). Neither the extended nebula itself nor any of detected sources still showed evidence for hard emission
the other galaxy members, which are all much fainter in consistent with AGN rather than star formation power-
F160W continuummagnitudethanSource3,showaline ing(Donley et al.2012). Thus,itappearsthatthelackof
detectionineithertheG102orG141grismspectroscopy. anX-raydetectionfromSource6, evenin deepChandra
data,cannotnecessarilybe usedtorule outthe presence
3.1.3. Source 6 - The IRAC/MIPS Source of an intrinsically luminous AGN that is simply heavily
obscured at X-ray wavelengths.
We take a second look at Source 6, the mid-infrared
Based on the photometry derived from the original
source first identified in the Spitzer IRAC and MIPS
GOODS-S HST and Spitzer/IRAC data and the new
data. We plot the full SED including the original
ERS imaging, we use the photometric redshift code
GOODS-S imaging and Spitzer data, as well as the new
EAzY (Brammer et al. 2008) to estimate a photometric
HST/WFC3-IR imaging and measurements from Her-
redshift for Source 6. Including the IRAC photometric
schel/PACS and SPIRE (Figure 7). The source shows
points and the standardEAzY star-forminggalaxytem-
diffuse emission in the WFC3-IR imaging, but it is be- plates results in a redshift of z ≈ [3.2−5.6], similar to
low the detection limit in the combined WFC3 detec-
whatwasfoundintheoriginalpaper. However,ifSource
tion image used in Skelton et al. (2014); therefore, we
6 is an obscured AGN, as we have argued, the IRAC
measure the photometry in these bands using a 2.4′′ ra-
bands will be dominated by AGN emission, making a
dius aperture at the position of Source 6, as determined
photometricredshiftestimatebasedononlystar-forming
fromtheIRAC3.6and4.5µmimaging. TheIRAC/MIPS
templates highly unreliable. The photometric redshift
photometry and Herschel PACS upper limits are taken
estimate will be skewed to higher redshifts (z & 4) by
from the GOODS-Herschel catalog (Elbaz et al. 2011),
the need to fit the bright 8.0µm point with the bright-
and the Herschel SPIRE measurements are from Her-
est part of the star-forming SED, i.e., the peak located
MES(HerMESTeam,private communication). We note at restframe ∼1.6µm. Adding the Mrk 231 AGN tem-
that Source 6 is not detected in either the WFC3/G102
plate to EAzY and including both the IRAC and MIPS
or G141 grism data.
detections (the Herschel detections and upper limits be-
Using these updated measurements, we derive the fol-
ing excluded due to possible source confusion) yields a
lowing MIR colors (relevant for the Ivison et al. (2004) slightly lower redshift range of z ≈ [2.8,5.2] that strad-
color-color selection): S /S = 9.959 ± 2.792 and
24.0 8.0 dles the redshift of the Lyα nebula. This estimate is not
S /S = 2.160±0.380. While Nilsson et al. (2006)
8.0 4.5 very robust, since fitting the power-law MIR SED with
suggestSource 6 is an obscuredstarburst at an approxi-
an AGN template does not contribute a lot of discrimi-
materedshiftofz ≈5.5basedonitsMIRcolors,theMIR natorypower,butitillustratesthatz ≈3.2isaplausible
color measurements taken from the GOODS-Herschel
redshift for Source 6.
catalog place Source 6 elsewhere in the Ivison et al.
(2004) color space, much higher in S /S and there- 3.2. Spatial Distribution of the Associated Galaxies and
24.0 8.0
fore much closer to the Mrk231 (i.e., AGN) locus. It the Larger Scale Lyα Emission
is possible that the original measurements did not ad-
The spatial distribution of likely members relative to
equately account for source blending and the complex
the Lyα nebula is shown in Figure 2. The Lyα nebula
MIPSPSF,yielding anincorrect24µmflux forthis rela-
lies between 6 galaxies with photometric redshifts con-
tivelyfaintMIRsourcethathappenstolieneartheAiry
sistent with being at the nebula redshift, one of which
ring from a brighter neighboring galaxy.
has a grism redshift as well. In Figure 8, we show the
Furthermore, AGN selection based on MIR colors
fullextentofthe Lyαemissioninthe originalLyαimag-
has been explored in great detail since the time of ing, within three spatial windows (10′′×10′′, 20′′×20′′,
the original paper (Stern et al. 2005; Lacy et al. 2004, and32′′×32′′). Atlowersurfacebrightnessesthanshown
2007; Donley et al. 2008, 2012). The measured MIR
in the discovery paper, it appears that there is a nearly
colors relevant for these color-color diagnostics are:
completebridgeofLyαemissionbetweenthepeakofthe
log(S8.0/S4.5) = 0.335+−00..007804 and log(S5.8/S3.6) = nebula,oneassociatedgalaxytothenorth,andthreeas-
0.242+0.107. These measurements place Source 6 solidly sociated galaxies to the west (including Source 3, the
−0.142
within the AGN color selection window of Lacy et al. source with a consistent grism redshift). At even lower
Overturning the Case for GravitationalPowering in a Lyα Nebula 5
surface brightnesses, the diffuse Lyα emission connects around L∗, assuming that M∗ ≈ −20.8 AB mag at this
all but one of the member galaxies and traces out what redshift(Reddy et al.2008). ThetotalUVluminosityof
appears to be two intersecting filaments, a partial ring, the member galaxies is ≈23.9 mag (AB) in the F606W
or an asymmetric biconical nebula roughly centered on band, or ≈2.3 L∗, suggesting this may be a small group
the position of Source 6. These morphological results in formation. The member galaxies have typical ages of
and the result that Source 6 is an obscured AGN plau- around 108 yr, stellar masses of 109 M⊙, star formation
sibly at the redshift of the nebula (Section 3.1.3) are to- rates of <10 M⊙ yr−1, specific star formation rates of
gether highly suggestive that Source 6 is indeed directly 10−9 yr−1, and extinctions of A ≈ 0.2−0.8 mag. In
V
associatedwith the presence of the Lyα nebula. Further Figure 12, we show the effective radii (R ) and S´ersic
e
observations,e.g.,detectingCOemissionlinesatsubmil- parameter(n) measurementsfrom parametricfits to the
limeterwavelengths,willallowustoconfirmtheredshift F160W imaging for the galaxies in the neighborhood of
of Source 6 and verify that we are seeing a Lyα nebula thenebula(van der Wel et al.2012). Themembergalax-
surrounding an obscured AGN. iesaretypically1-3kpcinradius,withS´ersicparameters
clustered more around n ≈ 1, the value for an exponen-
3.3. The Local Environment tial disk, than around n ≈ 4, the value for a De Vau-
couleursprofile. Two-sampleKolmogorov-Smirnov(KS)
Thus far we have shownthat there are in fact galaxies
tests on all parameters did not revealany statistical dif-
associatedwith the Lyα nebula. Here we assesswhether
ferencesbetweenthemembergalaxiesandthefieldpopu-
theregionisinfactoverdenserelativetothefield. Tobe
lationatthesameredshift(P =0.17,P =0.37,
consistent with previous studies of the environments of LF log(Age)
Lyαnebulaeatz ≈3(Prescott et al.2012b),westartby Plog(Mass) = 0.12, Plog(SFR) = 0.34, Plog(sSFR) = 0.92,
P = 0.57, P = 0.54, P = 0.53). This may be due
plottingthenumbercountsintheF606Wbandmeasured AV Re n
from within the R < 10′′ region around the Lyα neb- to the small sample size, or it could reflect that we are
ula and from random R < 10′′ apertures across the full witnessing an early stage in the formation of a group-
mass system when the individual member galaxies are
GOODS-S field (Figure 9; top panel). From this naive
separatedby large physicaldistances and environmental
approach, there is only mild evidence for an enhance-
effects have yet to become important.
ment in the regionof the nebula; the number counts are
slightly higher than in the field at mF606W ∼ 25 but 4. POWERINGTHELyαNEBULA
consistenttowithinthe errors,i.e.,the scattermeasured
4.1. The Energetics: Revisited
fromtherandomaperturetest. Inaddition,wehavealso
seen from the photometric redshift analysis that there Nilsson et al. (2006) estimated that even if Source 3,
is an overdensity of z ≈ 1 sources overlapping this re- thebrightestneighborgalaxy,wasatthenebularedshift,
gion,whichwillnecessarilyboosttherawnumbercounts. itwas unlikely to be able to power the Lyα emissionun-
Nextwelookatthenumberdensityofsourceswithpho- lesshighly collimatedinthe directionofthe nebula. Ex-
tometric redshifts within ∆z±0.15of the redshift ofthe trapolatingthe HST B and V-bandmeasurements of all
nebula (z = 3.157). In the bottom panel of Figure 9 the member galaxies, and accounting for the angle sub-
we plot the corresponding densities for the region of the tended by the nebula in each case, we find that galaxies
nebula and for random apertures drawn from the full canaccountforatmost≈14%ofthe Lyαemission,even
GOODS-Sfield. The regionofthe nebula showsafactor under the optimistic assumptionof anescape fractionof
of &10 overdensity at mF606W ≈ 25 relative to the field unity. Thus, the only source associated with the nebula
galaxypopulationatthesamephotometricredshift,even that could plausibly contribute enough ionizing photons
whenaccountingforthescattermeasuredfromrandomly is the obscured AGN, i.e., Source 6.
placed apertures. Thus, not only are there galaxies as- The Spitzer SED of Source 6 (Figure 7) is con-
sociated with the Lyα nebula, it is in fact sitting in a sistent with a power-law spectrum of: L ≈ 1.7 ×
ν
galaxy-overdenseregion of the Universe. 1030(ν/1014)−1.75 erg s−1 Hz−1; this corresponds to a
totalinfrared luminosity of around∼1.5×1045 erg s−1,
3.4. Properties of the Associated Galaxies estimated by scaling the template of Mrk 231 to match
the 8µm flux, which is approaching the realm of ultra-
HavingshownthatSource6isanobscuredAGNlikely
associatedwiththe nebula,webriefly turnourattention luminous infrared galaxies (ULIRGs; 1012L⊙ ≈4×1045
to the properties of the other galaxies in this overdense erg s−1). Making the crude assumption that the MIR
region, and whether they are distinctive in any way rel- power-law can be extrapolated into the restframe UV,
ative to the field population. In Figure 10 we show the we estimate the ionizing photon flux of Source 6 to be
measuredphotometryandbest-fitSEDsofthe6galaxies ∼ 3.5 × 1053 photons s−1 between restframe 200 and
that are likely associatedwith the Lyα nebula. We note 912˚A. Producing the observed Lyα luminosity of the
that none of these member galaxies is detected in the nebula (1.09±0.07× 1043 erg s−1) requires a total of
MIPS imaging (Whitaker et al. 2014). Figure 11 shows ∼ 1.0×1054 photons s−1, assuming that two-thirds of
theapproximateluminosityfunction(LF)forthesystem, recombinationsresultinaLyαphoton. Naively,itwould
assuming that all 6 galaxies are at z ≈ 3.157, as well appear that Source 6 can only account for about 35% of
as the ages, stellar masses, star formation rates, specific the Lyα emission, even when ignoring geometrical dilu-
starformationrates,andA estimatesfromstellarpop- tion. However, this extrapolation is very uncertain; for
V
ulation synthesis model fitting using FAST (Kriek et al. example, a somewhat shallower power-law index of -1.5
2009), as reported in the 3D-HST/CANDELS catalog is sufficient to match the required ionizing photon flux.
(Skelton et al. 2014). The galaxies are typically low lu- If we instead take the total infrared luminosity (i.e., 3-
minosity, around ∼ 0.2 L∗, with only one (Source 3) 1000µm)tobemeasureoftheintrinsicUV/opticalemis-
6 Prescott et al.
sion(i.e.,1000˚A-1µm)thatis beingreprocessedbydust, Lyα nebula (e.g., Fardal et al. 2001; Furlanetto et al.
we estimate that the intrisic ionizing photon flux from 2005; Goerdt et al. 2010; Faucher-Gigu`ere et al. 2010;
Source 6 is around ∼1055 photons s−1, more than suffi- Rosdahl & Blaizot 2012). Furthermore, these luminous
cienttopowerthenebula. Giventheuncertaintiesabout Lyαnebulaewillnecessarilybehostedby&1012M⊙ ha-
the geometry of the system, the opening angle of the los (Rosdahl & Blaizot 2012). While it is possible that
AGN, the obscuration of the source, and the true SED such halos go through an initial phase of gas cooling
shortward of the Lyman limit, it seems plausible that preceding any substantial star formation, this cooling-
Source6couldsimply be highly obscuredalongourline- only phase would necessarily have occurred much ear-
of-sight but less obscured in the directions of the diffuse lier, when the halo was ∼ 109 − 1010 M⊙ and when
Lyαemission. Therefore,combinedwiththemorpholog- the corresponding cooling Lyα emission was several or-
icalevidence inSection3.2, itappearslikelythatSource ders of magnitude fainter than observed Lyα nebulae
6isresponsibleforasignificantfractionoftheionization (Furlanetto et al. 2005). According to the most recent
in the Lyα nebula. simulations, stellar continuum emission would therefore
While we have shown that there are several associ- be expected at the position of the Lyα peak, unless
ated galaxies and an obscured AGN in this system that this galactic counterpart has been very effectively hid-
couldcontribute tothe ionizationofthe nebula,wehave den (Rosdahl & Blaizot 2012). For this reason, we sus-
also confirmed the previous result that there is no com- pect that the correct argument is the opposite of what
pact continuum source visible within the brightest part has often been assumed: namely, that the lack of any
of the Lyα nebula itself. The GOODS-S HST imag- associated continuum sources in the Lyα nebula makes
ing provides an upper limit on the amount of star for- it less likely that this system is predominantly powered
mation that could be going on within the nebula: the bygravitationalcooling. Itisdifficulttounderstandhow
3σ limit on the F606W flux (restframe ∼1400˚A) of thecentralgrowinggalaxy,presumablythemostmassive
4.66×10−30 that was quoted in the original paper cor- memberintheregionlocatedatanodewithinthecosmic
responds to a limit on the UV star formation rate of web, has been so effectively hidden at the center of the
SFR(UV) = 1.4×10−28Lν < 57 M⊙ yr−1, assuming Lyα nebula. If it is simply highly obscured and there-
the standard conversion (Kennicutt 1998). High resolu- fore not visible in the restframe optical, then it should
tionmillimeter/submillimeter imagingwillbe important be the brightest source visible in the system at longer
for putting further constraints on the obscured star for- wavelengths. Higherresolutionmillimeter/submillimeter
mation and molecular gas reservoirwithin the nebula. observationswill be importantfor shedding further light
on this issue, but the evidence thus far from the Spitzer
data is that the bulk of the obscured emission in this
4.2. Is the Lyα nebula gravitationally powered?
system is concentrated at the position of Source 6, an
The argument up until now has been that a Lyα neb- obscured AGN that is offset by ∼4′′ ≈ 30 kpc from the
ula without any associated continuum counterparts is brightest part of the Lyα nebula and that is instead lo-
thebestcandidateforasystempoweredbygravitational cated in an apparent “hole” in the fainter, larger scale
coolingradiation. In view ofour improvedobservational Lyα emission. Thus, while it is still possible that the
understandingofthe Nilsson et al.(2006)Lyαnebulaas Lyα emission is tracing gas within cosmic filaments, the
well as updated theoretical simulations, it appears that results of this paper argue strongly that it is being pow-
this line of reasoning should actually be reversed, and ered primarily by illumination from the obscured AGN
thateventhis“bestcandidate”foragravitationallypow- in the region instead of by gravitationalcooling.
ered Lyα nebula is in fact powered by an AGN. We suspect that this Lyα nebula is not an isolated
A number of authors have investigated whether grav- case. The key observational characteristics of this sys-
itational cooling radition within cold accretion streams tem - the small sizes, disk-like morphologies, and low
can explain the observed properties of Lyα nebulae, luminosities of the member galaxies,the evidence for an
initially with analytic arguments (Haiman et al. 2000) overdenseneighborhood,andthepresenceofahighlyob-
and then using numerical simulations (Fardal et al. scuredAGNoffsetby10sofkpc fromthe Lyαpeak that
2001; Furlanetto et al. 2005; Yang et al. 2006; can provide some if not all of the requisite ionizing pho-
Dijkstra et al. 2006; Dijkstra 2009; Goerdt et al. tons - are highly reminiscent of the properties LABd05,
2010; Faucher-Gigu`ereet al. 2010; Rosdahl & Blaizot a more luminous Lyα nebula at z ≈ 2.7 (Prescott et al.
2012). Due to the difficulty in appropriately simulating 2012b). It seems likely that these systems are intrin-
the self-shielding of the gas and the temperature of the sically similar to cases such as Weidinger et al. (2004,
IGM, the maximum predicted luminosities of “cooling” 2005), a quasar at z ≈ 3 with an extended, asymmet-
Lyα nebulae from different numerical treatments have ric biconical Lyα halo that is being viewed through one
ranged from ∼1042 erg s−1, far below that of observed side of the ionization cone of the (unobscured) quasar.
samples, to ∼1044 erg s−1, sufficient to explain even the In the Nilsson et al. (2006) Lyα nebula, we are likely
brightest known systems. Thus it remains a matter of observing a similar phenomenon, an AGN powering an
some debate whether gravitational cooling alone can in extended Lyα nebula, but with a line-of-sight passing
fact explain the large Lyα luminosities of Lyα nebulae. outside the ionization cone. This picture is broadly con-
At the same time, it is becoming clear that because sistent with the much higher obscuration of the AGN
gravitationally powered Lyα emission originates from and the two lobe structure seen at low surface bright-
the extended cold streams that are fueling galaxy for- ness levels in the Lyα emission. Our conclusions are
mation, star formation, and AGN activity, the simu- consistent with the argument of Overzier et al. (2013)
lations generically predict that there will be a grow- that AGN must be the dominant power source for the
ing galaxy at the center of any luminous “cooling”
Overturning the Case for GravitationalPowering in a Lyα Nebula 7
majority (if not all) of the most luminous Lyα nebulae. to the field. At the same time, results from the latest
The Nilsson et al. (2006) Lya nebula is somewhat lower generation of cold accretion simulations show that the
luminosity than the systems studied by Overzier et al. brightest Lyα emission powered by gravitational cool-
(2013), suggesting that even at lower luminosities where ing will be centered on a growing galaxy, which should
less powerful mechanisms (gravitational cooling, winds, be detectable as a counterpart in the continuum. Thus,
starformation)couldinprincipleplayalargerrole,AGN while we confirm the previous finding that there is no
continuetobeadominantpowersourceforLyαnebulae. associated continuum source located within the nebula
itself, we conclude that this is actually a strong argu-
5. IMPLICATIONS ment against, rather than for, the nebula being powered
On a more general level, while gravitational cooling bygravitationalenergy. TheLyαemissionatlowsurface
radiation, as a fundamental physical process, must con- brightness levels appears to trace out a complicated fil-
tributetothetotalLyαemissionobservedfromLyαneb- amentary structure, perhaps consistent with gas within
ulae,ourexperiencewiththisLyαnebulaleadsustocon- coldaccretionstreams,butthis gasismostplausiblybe-
cludethatitwillalwaysbemoredifficulttoprovethata ing lit up due to illumination from a nearby obscured
given nebula is primarily gravitationally powered. Since AGNratherthandue to gravitationalpowering. Follow-
bright Lyα emission from gravitationally cooling corre- upobservationsofsuchemissionlinenebulaeofferanex-
latesstronglywithactivelygrowinggalaxies(andAGN), citingwindowintobothAGNphysicsandthekinematics
alackofobviouspowersourcesisnotsufficientevidence, and physical conditions of the cosmic web surrounding
and the morphology and shape of the surface brightness regions of active galaxy formation.
profile could simply reflect the fact that the gas is being
broughtinbyacoldaccretionstreamwithoutnecessarily
We aregratefulto KimNilssonfor assistancewith the
provingthenebulaislitupbygravitationalcooling. De-
original Lyα data, to Rosalind Skelton for guidance on
terminingwhatfractionofagivenLyαnebulaispowered
using the 3D-HST/CANDELS photometric catalog, to
by gravity as opposed to by embedded sources requires
JulieWardlowandtheHerMESteamforprovidingearly
moredetailedstudiesof,e.g.,thepolarizationoftheLyα
accessto the HerMEScatalogs,andto Lise Christensen,
emissionto constrainthe contributionof scatteredlight,
Kristian Finlator, Sebastian Ho¨nig, Pieter van Dokkum,
as well as comparisons between emission line diagnostic
Natascha F¨orster Schreiber, and the anonymous referee
measurements within the diffuse gas and improved the-
for helpful discussions and comments.
oretical predictions for the expected metal line emission
M.K.M.P. was supported by a Dark Cosmology Cen-
from cold accrection streams.
tre Fellowship. J.P.U.F. acknowledges support from the
Instead, we suspect that extended Lyα emission dis-
ERC-StG grant EGGS-278202. The Dark Cosmology
placed from all associated continuum sources is actu-
ally less likely to be powered by cooling radition and CentreisfundedbyTheDanishNationalResearchFoun-
more likely to be a signature of cosmic gas illuminated dation.
ThisworkisbasedonobservationstakenbytheWFC3
by an offset AGN (leading to either Lyα fluorescence or
Early Release Science program (GO 11359 and 11360)
Lyα resonant scattering), an AGN that may be highly-
and the 3D-HST Treasury Program (GO 12177 and
obscured to the line-of-sight or recently extinguished. It
12328)with the NASA/ESA HST, which is operated by
is not particularly hard to hide an AGN with obscura-
the Association of Universities for Research in Astron-
tionandgeometry(i.e.,ifitisoffsetfromthenebulaand
omy, Inc., under NASA contract NAS5-26555.
obscuredtoourline-of-sightbutunobscuredtowardsthe
The GOODS-Herschel data was accessed through
nebula), or if it is in the process of ramping down in
the Herschel Database in Marseille (HeDaM -
its output, leaving a seemingly-abandoned light echo of
http://hedam.lam.fr) operated by CeSAM and hosted
Lyα emission at largerdistances (and light traveltimes)
by the Laboratoire d’Astrophysique de Marseille.
from the AGN position (e.g., similar to Hanny’s Voorw-
erp, an [Oiii] nebula at lower redshift; Schawinski et al. This research has made use of data from HerMES
project (http://hermes.sussex.ac.uk/). HerMES is a
2010). In systems where we can identify an AGN as
Herschel Key Programme utilising Guaranteed Time
the dominant power source in this way, we have an op-
from the SPIRE instrument team, ESAC scientists and
portunity to use the spatially extended line emission as
a laboratory for studying AGN variability over 104-105 a mission scientist.
year timescales as well as the physical conditions within
theunderlyingcosmicweb,litupinemissionwhileinthe REFERENCES
process of fueling and being impacted by ongoing local
galaxy formation.
Adams,J.J.,Hill,G.J.,&MacQueen,P.J.2009,The
AstrophysicalJournal,694,314
6. CONCLUSIONS Brammer,G.B.,vanDokkum,P.G.,&Coppi,P.2008,The
Inlightofnewobservationsandmoresophisticatednu- AstrophysicalJournal,686,1503
Cantalupo,S.,Porciani,C.,Lilly,S.J.,&Miniati,F.2005,The
merical simulations, we have taken a second look at the
AstrophysicalJournal,628,61
local environment and most likely power source for the
Cen,R.,&Zheng, Z.2013,TheAstrophysicalJournal,775,112
Nilsson et al. (2006) Lyα nebula. We find that the neb- Colbert,J.W.,Scarlata,C.,Teplitz,H.,Francis,P.,Palunas,P.,
ula is associatedwith 6 nearbygalaxiesandanobscured Williger,G.M.,&Woodgate, B.2011, TheAstrophysical
AGN located ∼4′′≈30 kpc away. The Lyα emission is Journal,728,59
Dahlen,T.,etal.2013, TheAstrophysicalJournal,775,93
seen to extend further than previously realized, nearly
Dekel,A.,etal.2009,Nature,457,451
encircling the position of the obscured AGN, and the
Dickinson,M.,etal.2004,TheAstrophysicalJournal,600,L99
local region shows evidence for being overdense relative Dijkstra,M.2009,TheAstrophysicalJournal,690,82
8 Prescott et al.
Dijkstra,M.,Haiman,Z.,&Spaans,M.2006,TheAstrophysical Prescott,M.K.M.,Dey,A.,&Jannuzi,B.T.2009,The
Journal,649,14 AstrophysicalJournal,702,554
Donley,J.L.,Rieke,G.H.,P´erez-Gonz´alez, P.G.,&Barro,G. Prescott,M.K.M.,Dey,A.,&Jannuzi,B.T.2012a, The
2008, TheAstrophysicalJournal,687,111 AstrophysicalJournal,748,125
Donley,J.L.,Rieke,G.H.,PerezGonzalez, P.G.,Rigby,J.R.,& Prescott,M.K.M.,Dey,A.,&Jannuzi,B.T.2013,The
AlonsoHerrero,A.2007,TheAstrophysicalJournal,660,167 AstrophysicalJournal,762,38
Donley,J.L.,etal.2012,TheAstrophysicalJournal,748,142 Prescott,M.K.M.,etal.2012b, TheAstrophysicalJournal,752,
Elbaz,D.,etal.2011, Astronomy&Astrophysics,533,A119 86
Erb,D.K.,Bogosavljevi´c,M.,&Steidel,C.C.2011,The Rasmussen,J.,Sommer-Larsen,J.,Pedersen,K.,Toft,S.,
AstrophysicalJournal,740,L31 Benson,A.,Bower,R.G.,&Grove,L.F.2009,The
Fardal,M.A.,Katz,N.,Gardner,J.P.,Hernquist,L.,Weinberg, AstrophysicalJournal,697,79
D.H.,&Dave,R.2001, TheAstrophysicalJournal,562,605 Reddy,N.A.,Steidel,C.C.,Pettini,M.,Adelberger,K.L.,
Faucher-Gigu`ere,C.-A.,Kereˇs,D.,Dijkstra,M.,Hernquist,L.,& Shapley, A.E.,Erb,D.K.,&Dickinson,M.2008,The
Zaldarriaga,M.2010, TheAstrophysicalJournal,725,633 AstrophysicalJournalSupplementSeries,175,48
Furlanetto,S.R.,Schaye, J.,Springel,V.,&Hernquist,L.2005, Rosdahl,J.,&Blaizot,J.2012, MonthlyNoticesoftheRoyal
TheAstrophysicalJournal,622,7 AstronomicalSociety, 423,344
Geach, J.E.,etal.2009,TheAstrophysicalJournal,700,1 Roseboom,I.G.,etal.2010,MonthlyNoticesoftheRoyal
Giavalisco,M.,etal.2004, TheAstrophysicalJournal,600,L93 AstronomicalSociety, 409,48
Goerdt,T.,Dekel,A.,Sternberg,A.,Ceverino,D.,Teyssier,R., Roseboom,I.G.,etal.2012,MonthlyNoticesoftheRoyal
&Primack,J.R.2010,MonthlyNoticesoftheRoyal AstronomicalSociety, 419,2758
AstronomicalSociety, 407,613 Schawinski,K.,etal.2010,TheAstrophysicalJournal,724,L30
Haiman,Z.,Spaans,M.,&Quataert,E.2000, TheAstrophysical Skelton,R.E.,etal.2014,TheAstrophysicalJournalSupplement
Journal,537,L5 Series,214,24
Hayes,M.,Scarlata,C.,&Siana,B.2011,Nature,476,304 Smith,D.J.B.,Jarvis,M.J.,Lacy,M.,&Mart´ınez-Sansigre,A.
Ivison,R.J.,etal.2004,TheAstrophysicalJournalSupplement 2008,MonthlyNoticesoftheRoyalAstronomicalSociety, 389,
Series,154,124 799
Kennicutt, R.C.1998,TheAstrophysicalJournal,498,541 Steidel,C.C.,Bogosavljevi´c,M.,Shapley,A.E.,Kollmeier,J.A.,
Keres,D.,Katz,N.,Weinberg,D.H.,&Dave,R.2005, Monthly Reddy,N.A.,Erb,D.K.,&Pettini,M.2011, The
Notices oftheRoyalAstronomicalSociety, 363,2 AstrophysicalJournal,736,160
Kollmeier,J.A.,Zheng,Z.,Dav´e,R.,Gould,A.,Katz,N., Stern,D.,etal.2005, TheAstrophysicalJournal,631,163
Miralda-Escud´e,J.,&Weinberg,D.H.2010,TheAstrophysical Tamura,Y.,etal.2013,MonthlyNotices oftheRoyal
Journal,708,1048 AstronomicalSociety, 430,2768
Kriek,M.,vanDokkum,P.G.,Labb´e, I.,Franx,M.,Illingworth, Taniguchi,Y.,&Shioya,Y.2000,TheAstrophysicalJournal,
G.D.,Marchesini,D.,&Quadri,R.F.2009,TheAstrophysical 532,L13
Journal,700,221 Taniguchi,Y.,Shioya,Y.,&Kakazu, Y.2001,TheAstrophysical
Lacy,M.,Petric,A.O.,Sajina,A.,Canalizo,G., Journal,562,L15
Storrie-Lombardi,L.J.,Armus,L.,Fadda, D.,&Marleau, vanderWel,A.,etal.2012,TheAstrophysicalJournal
F.R.2007,TheAstronomicalJournal,133,186 SupplementSeries,203,24
Lacy,M.,etal.2004,TheAstrophysicalJournalSupplement Weidinger,M.,Møller,P.,&Fynbo,J.P.U.2004,Nature,430,
Series,154,166 999
Laursen,P.,Duval,F.,&O¨stlin,G.2013,TheAstrophysical Weidinger,M.,Møller,P.,Fynbo,J.P.U.,&Thomsen,B.2005,
Journal,766,124 AstronomyandAstrophysics,436,825
Laursen,P.,&Sommer-Larsen,J.2007,TheAstrophysical Weijmans,A.-M.,Bower,R.G.,Geach,J.E.,Swinbank,A.M.,
Journal,657,L69 Wilman,R.J.,deZeeuw, P.T.,&Morris,S.L.2010,Monthly
Laursen,P.,Sommer-Larsen,J.,&Andersen,A.C.2009, The NoticesoftheRoyalAstronomicalSociety, 402,2245
AstrophysicalJournal,704,1640 Whitaker,K.E.,etal.2014,TheAstrophysicalJournal,795,104
Levenson, L.,etal.2010,MonthlyNoticesoftheRoyal Windhorst,R.A.,etal.2011, TheAstrophysicalJournal
AstronomicalSociety, 409,83 SupplementSeries,193,27
Luo,B.,etal.2008,TheAstrophysicalJournalSupplement Xue,Y.Q.,etal.2011,TheAstrophysicalJournalSupplement
Series,179,19 Series,195,10
Møller,P.,&Warren,S.J.1998, MonthlyNoticesoftheRoyal Yajima,H.,Li,Y.,&Zhu,Q.2013, TheAstrophysicalJournal,
AstronomicalSociety, 299,661 773,151
Mori,M.,Umemura,M.,&Ferrara,A.2004,TheAstrophysical Yang,Y.,Walter,F.,Decarli,R.,Bertoldi,F.,Weiss,A.,Dey,A.,
Journal,613,L97 Prescott, M.K.M.,&Bdescu,T.2014,TheAstrophysical
Nilsson,K.K.,Fynbo,J.P.U.,Møller,P.,Sommer-Larsen,J.,& Journal,784,171
Ledoux, C.2006,AstronomyandAstrophysics,452,L23 Yang,Y.,Zabludoff,A.,Eisenstein,D.,&Dav´e,R.2010, The
Nilsson,K.K.,etal.2007,AstronomyandAstrophysics,471,71 AstrophysicalJournal,719,1654
Oke,J.B.1974,TheAstrophysicalJournalSupplementSeries, Yang,Y.,Zabludoff,A.I.,Dave,R.,Eisenstein,D.J.,Pinto,
27,21 P.A.,Katz,N.,Weinberg,D.H.,&Barton,E.J.2006,The
Oliver,S.J.,etal.2012,MonthlyNotices oftheRoyal AstrophysicalJournal,640,539
AstronomicalSociety, 424,1614 Zheng,Z.,Cen,R.,Weinberg,D.,Trac,H.,&Miralda-Escud´e,J.
Overzier,R.A.,Nesvadba, N.P.H.,Dijkstra,M.,Hatch,N.A., 2011,TheAstrophysicalJournal,739,62
Lehnert, M.D.,Villar-Mart´ın,M.,Wilman,R.J.,&Zirm,
A.W.2013,TheAstrophysicalJournal,771,89
Polletta,M.,etal.2007,TheAstrophysicalJournal,663,81
O
v
e
r
t
u
r
n
in
g
t
h
Table 1 e
LikelyGalaxyMembers C
a
s
IDa 3D-HST/CANDELS RightAscension Declination mF160W zphot zgrism Notes ef
CatalogID (hours) (degrees) o
r
3 GOODS-S-41192 03:32:14.072 -27:43:03.72 23.54 3.206+−00..006558 3.153+−00..001094 Gr
89 GGOOOODDSS--SS--4411108297 0033::3322::1144..808760 --2277::4433::0035..5247 2246..8036 33..214769+−+−0000....111106074011 -- avita
10 GOODS-S-41047 03:32:13.947 -27:43:04.81 26.02 3.204+−00..320983 - tio
11 GOODS-S-41395 03:32:14.550 -27:42:59.34 25.36 3.036+−00..008855 - na
12 GOODS-S-41673 03:32:14.884 -27:42:54.27 27.26 3.231+−00..128182 - lP
o
w
e
6 03:32:14.606 -27:43:06.14 3.801+−10..758163 - IRACdata/SFtemplates only rin
includedinthephotometricredshiftfit g
3.094+−20..029772 - IRACandMIPSdata/AGNandSFtemplates in
includedinphotometricredshiftfit a
L
y
a TheIDcolumnisbasedonthenumberingschemeusedinNilssonetal.(2006). α
N
e
b
u
la
9
10 Prescott et al.
Figure 1. Stacked WFC3-IR/F125W, F140W,andF160W imagingoftheregionaroundthe Lyαnebula. TheLyαemissionisshown
withblackcontours atLyαsurfacebrightness levelsof[5.3,7.0,8.9]×10−18 ergs−1 cm−2 arcsec−2 aftersmoothingby0.4×0.4′′. Sources
withphotometricredshiftswithin∆z=0.15areshown(redcircles),whileSource3,thegalaxywithaconsistentgrismredshift,isindicated
with a double red circle. The IRAC centroid of Source 6 from the GOODS-Herschel catalog is indicated with a blue cross. The galaxy
locatedclosesttotheLyαpeak,labeled‘F’,isaforegroundgalaxyatz≈1.