Table Of ContentDraft version January 30, 2017
TypesetusingLATEXtwocolumnstyleinAASTeX61
NEUTRAL ISM, LYMAN-ALPHA AND LYMAN-CONTINUUM IN THE NEARBY STARBURST HARO 11
∗
T. Emil Rivera-Thorsen,1,2 Göran Östlin,1,2 Matthew Hayes,1,2 and Johannes Puschnig1,2
1Department of Astronomy, Stockholm University, AlbaNova University Centre, SE-106 91 Stockholm, Sweden
7 2Oscar Klein Centre for Cosmoparticle Physics, Stockholm, Sweden
1
0
(Received November 24, 2016; Revised January 25, 2017; Accepted January 26, 2017)
2
Submitted to ApJ
n
a
J ABSTRACT
7
Star forming galaxies are believed to be a major source of Lyman Continuum (LyC) radiation responsible for
2
reionizing the early Universe. Direct observations of escaping ionizing radiation have however been few and with low
] escape fractions. In the local Universe, only 10 emitters have been observed, with typical escape fractions of a few
A
percent. The mechanisms regulating this escape need to be strongly evolving with redshift in order to account for
G
the Epoch of Reionization. Gas content and star formation feedback are among the main suspects, known to both
. regulate neutral gas coverage and evolve with cosmic time. In this paper, we reanalyze HST-COS spectrocopy of the
h
p first detected local LyC leaker, Haro 11. We examine the connection between LyC leakage and Lyman-α line shape,
- and feedback-influenced neutral ISM properties like kinematics and gas distribution. We discuss the two extremes of
o
an optically thin, density bounded ISM and a riddled, optically thick, ionization bounded ISM, and how Haro 11 fits
r
t into their theoretical predictions. We find that the most likely ISM model is a clumpy neutral medium embedded in
s
a a highly ionized medium with a combined covering fraction of unity and a residual neutral gas column density in the
[
ionized medium high enough to be optically thick to Lyman-α, but low enough to be at least partly transparent to
1 Lyman continuum and undetected in Si II. This suggests that SF feedback and galaxy-scale interaction events play a
v
major role in opening passageways for ionizing radiation through the neutral medium.
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8
0
.
1
0
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Correspondingauthor: T.EmilRivera-Thorsen
[email protected]
BasedonobservationswithHST-COS,programGO13017,obtainedfromtheMikulskiArchiveforSpaceTelescopes(MAST).STScI
∗
isoperatedbytheAssociationofUniversitiesforResearchinAstronomy,Inc.,underNASAcontractNAS5-26555. SupportforMASTfor
non-HSTdataisprovidedbytheNASAOfficeofSpaceScienceviagrantNNX09AF08Gandbyothergrantsandcontracts.
2 T. E. Rivera-Thorsen et al.
1. INTRODUCTION AND OBSERVATIONS BandCarebothverystronginHα,revealingthatthey
also are producing strong Lyα But while knot C is a
Young, star-forming galaxies are believed to be the
strong Lyα emitter, knot B is a strong absorber, indi-
source of a major part of the radiation which reionized
cating that the neutral ISM properties along the line of
the early Universe. It is however unclear which physi-
sight to the two knots are quite different (Hayes et al.
cal conditions can facilitate the escape of the necessary
2009; Östlin et al. 2009). Its complex kinematics indi-
amount of radiation, given that these galaxies contain
catesthatitisundergoingamergerormajorinteraction
largeamountsofneutralgaswhichisopaquetothision-
event (Östlin et al. 2001, 2015; James et al. 2014). It
izing radiation at column densities above logN 17.2
∼ is the first galaxy in the local Universe reported to leak
(Verhammeetal.2015). Searchesforleakinggalaxiesat
redshifts z (cid:38) 1 have yielded few detections (e.g. Iwata LyC (Bergvall et al. 2006). The same dataset has since
beenre-analyzedbyGrimesetal.(2007)whoconcluded
etal.2009;Vanzellaetal.2010,2012;Nestoretal.2013;
that there was no convincing evidence for LyC escape;
Cowie et al. 2009; Siana et al. 2010), with escape frac-
and since by Leitet et al. (2011) who report an escape
tions well below the 20% needed to account for cos-
∼ fraction of 3.3 0.7% based on a new background mod-
mic reionization (Bouwens et al. 2011; Robertson et al.
±
eling. Inthiswork, weassumethatsomeradiationdoes
2013). A population of lower mass and lower luminos-
escape, but it should be kept in mind that this result
ity, star forming galaxies are expected to contribute to
hasbeendisputed. SinceLyαescapeisfavoredbysome
reionization, but high star formation is usually coinci-
ofthesameconditionswhichalsoallowLyCescape(see
dent with higher neutral gas (surface) density, which
e.g. Hayes 2015; Dijkstra 2014; Leitet et al. 2011), the
wouldcauseahigherprobabilityofblockingtheionizing
leaking photons are assumed to originate from knot C,
photons (e.g. Erb 2016; Robertson et al. 2013). In the
an assumption adopted in this work.
local Universe, only 9 leakers have been detected so far
In Rivera-Thorsen et al., submitted to ApJ, we an-
(Bergvalletal.2006;Leitetetal.2011,2013;Borthakur
alyzed optical and NUV nebular emission lines in slit
et al. 2014; Izotov et al. 2016a,b; Leitherer et al. 2016),
spectra from ESO VLT/X-Shooter (see also Guseva
with escape fractions ranging typically between 1-8%,
et al. 2012). We find from kinematics modelling that
with one as high as f 13% (Izotov et al. 2016b).
esc
≈ both knot B and knot C are associated to a compo-
ModelsofISMsurroundingacentralsourceandallow-
nent blueshifted by 50 100 km s 1 relative to the
ingescapeofLymancontinuum(LyC),spantherangeof −
∼ −
mean nebular velocity. This component extends as far
two extremes: In one regime, the surrounding gas is op-
as 200 pc. SE of knot B, and 1.5 kpc S of the
ticallythin,highlyionizedanddensity-bounded(seee.g.
∼ ∼
midpointbetweenthetwoknots. Giventhedensecloud
Jaskot & Oey 2013), allowing escape for at least a frac-
coverage of knot B and the strong star formation ac-
tionofionizingphotons. Intheotherregime,thecentral
tivity in both knots, we conclude that this component
source is surrounded by an optically thick, ionization
is not only approaching but also found in front of the
boundedmediumwiththeneutralmediumsurrounding
starburst regions and thus along the LOS to knot C.
thecentralStrömgrenspherenotcompletelycoveringall
Recently, the galaxy has been observed in 21 cm H I
lines of sight to the background source in what is called
emission with the 100 m. Robert C. Byrd Green Bank
the picket fence model (Conselice et al. 2000; Bergvall
Telescope (Pardy et al. 2016). The authors find that
et al. 2006; Heckman et al. 2011; Zackrisson et al. 2013)
Haro 11 has a low gas mass and a very low neutral
ortheriddled ionization bounded medium byVerhamme
gas to stellar mass fraction, and that it contains around
et al. (2015).
twice as much ionized as neutral Hydrogen. Interest-
The latter paper presents modeling of the imprints
ingly, the authors also find that the bulk of the neutral
of these two extreme scenarios on the observed spectral
gasisredshiftedrelativetothesystemicvelocitydefined
signature of Lyman-α emission lines, and suggests how
fromnebularemissionfromtheHIIregionssurrounding
these can help point to candidate LyC leakers. The au-
the main starbursts, signifying that the majority of the
thors compare their theoretical predictions to sample of
neutral gas reserves are detached from the star forma-
Lyman-α profiles, including a section of the spectrum
tion activity, possibly a tidal arm being flung outwards
treated in this work.
as a part of the ongoing merger event.
The galaxy treated in this study is Haro 11, a well-
Inthispaper,were-analyzetheHST-COSspectrumof
studied Blue Compact Galaxy at low redshift (z =
Haro 11 C acquired as part of HST program GO 13017,
0.021). Morphologically, it is dominated by three major
PI Timothy Heckman. It was first published in Alexan-
star-formingknots, calledknotA,BandCrespectively,
droff et al. (2015) and Heckman et al. (2015) as part
followingtheterminologyofVaderetal.(1993)(seealso
of a sample of 22 Lyman Break Analog (LBA) galax-
Kunth et al. 1998; Hayes et al. 2007, and fig. 1). Knots
Neutral ISM, Ly-Alpha and LyC in Haro 11 3
six pixels of the extracted spectrum per resolution ele-
-33:33:12.0
ment. We have therefore binned the data by a factor of
6 to minimize noise while not losing information. The
14.0 resolution, however, is generally lower than this for ex-
tended sources; for a uniformly filled aperture, it is as
0) 16.0 low as R ≈ 2,000. For more morphologically complex
0 B sources, the effective resolution depends on the exact
0
J2 shape and angular size of the target.
ec ( 18.0 C To estimate the effective resolution for this target, we
D
proceeded as follows. We used imaging data of Haro 11
20.0 A in the UV-continuum from Östlin et al. (2009); Hayes
et al. (2009), since the resolution of absorption lines is
22.0 determined by the resolution of the background contin-
uum source. We then extracted a circular image at the
5" 2.15 kpc
≈ same position and radius as the COS aperture, which
52.80 52.50 52.20 0:36:51.90
was rotated to match the orientation of the COS as
RA (J2000)
given in the spectrum headers. We modelled vignetting
Figure 1. Approximate position of the COS aperture, in the COS aperture by multiplying each pixel in the
shown on HST imaging data of Hayes et al. (2009); Östlin circular UV-continuum image with an interpolation of
et al. (2009), encoding UV continuum in green, Hα in red, the values given in the throughput grid in Fig. 7 in
andcontinuumsubtractedLymanαinblue. Nisup,Eisto
Goudfrooij et al. (2010). Finally, the aperture cutouts
the left.
were collapsed along the cross-dispersion direction, and
the FWHM of the flux distribution along the dispersion
ies analyzed individually and as a stack. These authors
direction is reported as the effective resolution of this
mainly focus on three indirect indicators of LyC leak-
observation. This is converted from arc seconds to COS
age (Overzier et al. 2009; Heckman et al. 2011): Resid-
resolution elements by the factor 0.171 arcsec/resel re-
ualfluxinthetroughofsaturatedISMabsorptionlines,
portedintheCOSinstrumenthandbook(Hollandetal.
blueshiftedemissioninLyman-α,andweakoptical[SII]
2014)
emission lines. Details about observation and data re-
The procedure is illustrated in Fig. 3. Here, the up-
duction are described in depth in Alexandroff et al.
per left panel shows Haro 11 in UV-continuum using
(2015); we point the reader there for further informa-
HSTimagingdatafromÖstlinetal.(2009);Hayesetal.
tion about these. The Lyα profile in this spectrum is,
(2009), with the COS aperture coverage from this ob-
in addition to Heckman et al. (2011), also discussed in
servation overlaid in blue. The upper right panel shows
Verhamme et al. (2015).
thegalaxyastheCOSsawit,withtheregioncoveredby
Wemeasureanumberofkinematicpropertiesforboth
the aperture cut out, rotated and vignetting-corrected
theneutral(Low-IonizedState, LIS)andionized(High-
as described above. Finally, the lower panel shows the
Ionized State, HIS) phase. We apply the apparent opti-
fluxprofileintheaperture,collapsedalongthecrossdis-
caldepthmethod(AOD, Savage&Sembach1991;Pet-
persion direction. The FWHM of this distribution has
tini et al. 2002; Quider et al. 2009; Jones et al. 2013),
been adopted as the effective resolution for this obser-
with the implementation being as described in Rivera-
vation. We find R = 0.3 = 1.75 spectral resolution
Thorsen et al. (2015) (RT15) to infer geometric proper- eff (cid:48)(cid:48)
elements as defined in the COS instrument handbook
ties of the neutral medium. Under the assumtion that
Holland et al. (2014), which at the wavelength of ob-
theLyCphotonsdoindeedescapefromknotC,wecon-
served Lyα corresponds to 31 km s-1. This value might
strain the column density of neutral hydrogen covering
however be slightly underestimated, since the flux pro-
the background source.
file is not well described by a Gaussian profile, but has
Fig.2givesanoverviewoftheCOSspectrumofHaro
stronger wings due to the morphology of the target in
11 C treated in this work. Some important features are
the aperture, meaning that a larger fraction of the total
marked in blue (internal to Haro 11) or orange (Milky
flux is outside the FWHM.
Way features).
1.1. Effective resolution
2. ANALYSIS
For a point source, the resolution of the Cosmic Ori-
2.1. Individual lines
gins Spectrograph is R =20,000, which corresponds to
4 T. E. Rivera-Thorsen et al.
1]− SiIISiII SiIII αLy SiII OISiII CII SiIV SiIV SiII
˚A
1
− 60
s
2
−
m 40
c
g
r 20
e
5
1
− 0
0
1 Haro11C
[ 20
x−
u 1200 1300 1400 1500
Fl
Rest-frame wavelength [˚A]
Figure 2. OverviewoftherelevantregionsoftheCOSspectrumofHaro11C,showninrestframewavelengthsoftheobject.
Some key spectral lines are marked; Milky Way features in orange, features of Haro 11 in blue.
1.4
Haro11C
1.2
ux1.0
fl SiII1304
Relative00..68 SSiiIIII11522660
0.4 OI1302
0.2 CII1334
0.0
1.4
1.2
1.2
Haro 11 C ux1.0
fl
ary units]01..80 H ACarOlcfSs-l eiRgce:h s0te .w3ls0i:d 1th.7: 5 Relative000...468 SiIV1393
bitr0.6 0.2 SiIV1402
r 0.0
ux [a0.4 −1000 −500 Velocity0[kms−1] 500 1000
Fl0.2
Figure 4. The Si II and other LIS (upper) and Si IV
0.0 (lower) profiles included in this study. Dashes denote data
1.0 0.5 0.0 0.5 1.0
Angle ["] points masked out due to foreground contamination. Data
in this figure has been smoothed to better show the line
Figure 3. Effective resolution estimate. Upper left: Haro shapes, but all computations have been carried out on the
11 in UV-continuum, data from Östlin et al. (2009); Hayes unsmootheddata. Notethehigh-velocitycomponentinboth
et al. (2009). Data is shown on a square root scale and cut SiIV lines, extending out to 850 km s−1.
∼
levels set for best detail. Inset in blue is the COS aperture
covering the region surrounding knot B in the terminology
somewhat shallower than Si II λ1260. The former two
ofVaderetal.(1993). Nisup,Eisleft. Upper right: The
lineshavecomparableoscillatorstrengths,bothabouta
extracted, throughput-corrected in-aperture image, rotated
so horizontal corresponds to the dispersion direction of the factorof10lowerthanthatofλ1260. Itisthusclearthat
spectrograph. Lower panel: The collapsed flux profile in we do not find ourselves in the optically thin regime, in
the aperture, with the shaded region showing the width of which the latter line should be correspondingly around
the effective resolution element, which is also given in arc 10 times stronger; on the other hand, it is possible that
seconds and resolution elements/bins.
the two weak lines are not completely saturated. In the
lowerpanel, thetwoSiIVlineshaveoscillatorstrengths
Figure 4 shows the individual profiles of the transi- within a factor of 2 of each other. They are thus at first
tions included in our analysis; the upper panel shows glance consistent with a medium that is not completely
transitionsofSiII,lowerpanelofSiIV.Itisplainlyvis- opaque, but not with an optically thin one, and within
ible that the ionization fraction is high, with the Si IV uncertainties consistent with the optically thick. The
curves being considerably deeper than the low-ionized stronger absorption in Si IV reveals a high level of ion-
lines. Lookingattheupperpanel,SiIIλλ1304,1526are ization of the medium covering the background source.
Neutral ISM, Ly-Alpha and LyC in Haro 11 5
Table 1. Measured properties
1.4
1.2
Quantity Haro 11 C
z a 0.02043 0.00002 x1.0
± u
z [km s-1] a 6126 7 fl0.8
v e
± v
∆vLvISLIS 481049±157 Relati0.6
int − ± 0.4
vLIS 421 12
95% − ± 0.2
vLyα 158 0.8
peak ±
0.0
log10(NSiII)v=0 12.1±0.2 −400 −200 0 200
aFrom Sandberg et al. (2013) v−v0[kms−1]
1.0
0.9
0.8
2.2. NSi and fC ux0.7
fl
0.6
FollowingthemethoddescribedinRT15,wehaveper- e
v
formed fits for column density and covering factor in ati0.5
each velocity bin, for both the high- and low-ionization Rel0.4
state. Here we shall briefly summarize the method, but 0.3
refer to RT15 and references therein for a detailed ex-
0.2
planation.
0.1
Inanygivenvelocitybin,theresiduallineintensityin 0 500 1000 1500
terms of the continuum intensity is given as fλ
I
=1 f (1 e τ), (1) Figure 5. Illustration of the Apparent Optical Depth com-
C −
I0 − − putations. Upper panel: the three SiII lines in this anal-
with the optical depth τ given as: ysis shown together, with the zero-velocity bin marked with
a vertical red line. Lower panel: From this bin, the three
πe2 N measuredrelativeintensitiesareplottedagainstfλasblack
τ =fλ N =fλ (2)
m c 3.768 1014 dots, with vertical error bars showing the error spectrum
e
×
from the COS pipeline. The best-fit function I/I (fλ) as
Here, f is the oscillator strength of a given transition, 0
defined above is drawn in magenta. Also shown for illustra-
λ is its rest frame wavelength in Å, N = N(v) is the
tion are the two extreme examples of a very high column
column density of the relevant ion within the given ve-
densityand unchangedcovering fraction, and of avery high
locity bin, and fC is the covering fraction of neutral gas covering fraction but significantly lower column density.
in same velocity bin. When multiple absorption lines
are present which arise from the same ground state; the
twoparametersinfluenceitsshape. One,indottedgray,
population of this state is the same for all transitions,
shows what happens to the best-fit function when N is
andtheirrelativestrengthsaregovernedsimplybytheir
raised strongly, while f is kept unchanged, while the
C
oscillator strengths, and with two or more such transi-
other in gray dashes shows the curve for a f 1 and
C
tions, fC and N can be inferred from knowledge of fλ N is 0.5dexbelowthebest-fitvalue. Thisisof≈interest
and measured values of I/I0. to the discussion of radiative transfer effects below in
ThemethodisillustratedinFig.5: Intheupperpanel
sect. 3.4.
is shown the line profiles of the three Si II transitions TheresultingvaluesofNSiiiandfC areshowninfig.6.
included in the analysis. The red vertical line marks Theupperpanelsshowthepseudo-reducedχ2asdefined
the zero-velocity bin. From this bin, the three relative in RT15 ( = χ2/(DOF+1)) for each bin, middle pan-
fluxesareplottedinthelowerframeagainsttheirwave-
els show the inferred column density in each bin, with
length scaled oscillator strengths fλ on the x axis. In surrounding shaded columns showing the confidence in-
magenta is shown the best fit of the function I/I0(fλ) tervals. In the lower panels, the mean LIS line profile
describedabove. Alsoshownaretwoexamplesofdiffer-
is shown in black with gray shaded uncertainty inter-
ent parametrizations of this function, to show how the
6 T. E. Rivera-Thorsen et al.
vals. On these are overlaid the best-fit values of f as This suggests that the NaD is tracing only the densest
C
coloreddots, withsurroundingshadedbarsshowingthe and/or dustiest regions which are more slowly acceler-
confidence intervals. We again caution that f is the ated by star formation feedback than the surrounding,
C
covering fraction of neutral gas within the given velocity moredilutemedium. Infig.1,dustyregionsintheaper-
bin, and hence only provides a lower limit for the to- tureareapparentEandWofknotC.Seeninthebetter
tal, geometric neutral gascovering fraction, sincegas at resolutionoffig. 1inAdamoetal.(2010), theseregions
different velocities generally does not occupy the same seemliketheymightbeconnectedbyanarrowdustlane
projected area. partially covering the background source. We speculate
thattheNaDabsorptionofSandbergetal.(2013)might
3. DISCUSSION AND CONCLUSIONS be associated with this.
3.1. Lyman-α and ISM absorption profiles Also the absorption feature in the Lyman α profile
in the upper panel seems to morphologically follow the
In fig. 7, we show the neutral and ionized absorption
shapeofthemetallines,indicatingthatradiativetrans-
profile as in fig. 6 together with the profile of Lyα on a
fer effects are modest, indicative of a fairly low column
common velocity scale.
density of H I around line center. We find a Lyman α
The picture is what we would expect from a LyC peakvelocityofvLyα =158 1kms-1relativetothesys-
leaker: The neutral absorption features are weak, and peak ±
temic velocity found by Sandberg et al. (2013), derived
the ionized features are strong, revealing a highly ion-
fromnebularemissionlinesintheregionaroundknotC.
ized medium in front of the central cluster . This is
This velocity, as it is also discussed in Verhamme et al.
also in good agreement with the findings of Pardy et al.
(2015), is just consistent with their theoretical predic-
(2016), who conclude that the galaxy has about twice
tionsforadensity-bounded,low-columndensitysystem,
as much ionized as neutral gas. In addition to this, it
albeit on the upper limit of their allowed range.
is interesting to note the close similarity in shape be-
tween the Si II and Si IV line profiles, indicating that 3.2. Metal and HI column density
they likely represent two different phases in the same
Lyman α escape is mainly governed by gas at or near
higher density regions. These regions will likely be ion-
systemic velocity. In the middle panel of fig. 6 (a) is
ized on the side facing the central cluster, being pho-
shownthebest-fitcolumndensityN foreachvelocity
toionizedbyitshot,massivestars. Thisalsoimpliesthat SiII
bin. The value at systemic velocity is log (N ) =
thenebularemissionoutflowfoundinRivera-Thorsenet 10 SiII
12.1 0.2. We adapt the value for local starbursts of
al, submitted, at least partially traces the same gas as ±
log(Si/O) = 1.59 0.07 from Garnett et al. (1995),
the Si IV absorption here and, by extension, also the − ±
and use this to estimate the column density of neutral
neutral medium. Looking at fig. 4, we see that maxi-
Hydrogen in front of the light source in the same way
mum absorption and thus the largest amount of gas in
asinPuschnigetal. (submittedtoMNRAS)asfollows.
a single velocity bin for both phases is found at around
Guseva et al. (2012) found a metallicity of Haro 11 C of
v 50 km s-1. Interestingly, a component was also
≈ − 12+log (O/H)=8.1. Withlog Si/O= 1.59 0.07,
found in nebular emission at this velocity in the obser- 10 10 − ±
thisleadstoaSi/HratiointheneutralmediumofSi/H
vationsofRivera-Thorsenetal.,submitted. Theoutflow
=3.24+0.5710 6, leading to a Hydrogen column density
velocity v is at 149 7 km s-1, fully consistent with 0.48 −
int ± of 6.2+−0.9 1017 cm-2 for just one velocity bin. Since
the findings of Heckman et al. (2015) and Alexandroff H I ge−t1s.1o×paque to ionizing radiation at logN 17.2
etal.(2015). Interestingly,Sandbergetal.(2013)found ∼
(Verhammeetal.2015),thisrangeisnotconsistentwith
from the neutral Sodium resonance absorption doublet
the low- optical depth, density bounded scenario.
λλ5889.95,5895.92 (NaD) an overall, weak redshift of
Furthermore, while Lyα radiative transfer is domi-
v = 32 km s-1. From e.g. fig. 7, it is evident that while
nated by gas of v v , LyC is sensitive to H I at all
neutral gas is present at these velocities, the velocity ∼ 0
velocities. The total column density of SiII depends on
is at odds with the integrated velocity of v = 149
− the configuration of the neutral clouds. Assuming that
km s-1 found in this work. We note that Na I has a
theareacoveredbygasineachvelocitybiniscompletely
very low ionization potential of only 5 eV, meaning
∼ randomlylocated,alowerlimittothetotalcolumnden-
that these atoms may well only be present in the dens-
sity is:
estand/ordustiest regions, inwhichSodiumisshielded
(cid:88)
Nmean = N f (3)
fromionization. Thisisinagreementwiththefindingof SiII HI,i C,i
Sandbergetal.(2013)thatNaD,despitebeingastrong i
transition,showsabsorptionofonly 95%ofcontinuum Summing this over 450 km s-1 < v < 150 km s-1,
∼ −
levelandismostlyfoundinsmall,opticallythickclouds. the velocity range over which the column densities can
Neutral ISM, Ly-Alpha and LyC in Haro 11 7
2χ 80 2χ 5
d. 60 d. 4
o-re 40 o-re 23
ud 20 ud 1
e e 0
s 0 s
NPSiII 11112345 NSiIVP11112345
log 1101 log 1101
n n
flux 1.0 0.0 actio flux 1.0 0.0 actio
Relative 00000.....02468 HafSrcoi1I1IC1260 00001.....24680 Coveringfr Relative 00000.....02468 HarfIoc/1I10C,HIS 00001.....24680 Coveringfr
800 600 400 200 0 200 400 600 800 600 400 200 0 200 400 600
− − − − − − − −
v[km/s] v[km/s]
(a)LIS (b)SiIV
Figure 6. Upper panels: Pseudo-reduced χ2 as described in RT15. Middle panels: Best-fit ion column density with
confidenceintervalsinshadedgray. Lower panels: SiII1260/meanSiIVprofileshownasblacksteps,withinferredfC shown
with yellow dots. Lighter shaded columns show confidence intervals for both.
be reasonably well determined (and removing the un-
12
Haro11C physically high value in the bin at v 222 km s-1),
10 ≈
yields a lower limit of logN = 13.52 0.15 and
SiII
8 logN 19.01 0.17, corresponding to ±64 optical
Lyα HI
± ≈
6 depths in LyC, strongly incompatible with an optically
thin, density-bounded scenario.
4
However, the strong riddled ionization bounded sce-
2
nario – i.e. consisting of only a fully neutral and a fully
flux 0 ionized phase – is easily ruled out since the Lyα pro-
e
v file does not have any appreciable emission component
ati
el at zero velocity. We therefore expect a residual neutral
R1.0 0.0
)
v fractiontoremainintheionizedphase;afractionwhich
(
0.5 fC (SiII) Avg. LIS 0.5fc has a column density high enough to block Lyα effi-
0.0 1.0
cientlyatlinecenter, butlowenoughtobeatleastpart
transparent to LyC radiation and undetected in SiII.
1.0 0.0
(v) We can estimate the lowest detectable NSiII by not-
0.5 fC (SiIV) Avg. SiIV 0.5fc ing that the relative errors for the Si II lines in our
0.0 1.0 spectrum are 0.05. Assuming a covering fraction
−1000 −500 v v 0[km/s] 500 1000 of unity for th∼e dilute neutral component, and adopt-
− 0 ing the oscillator strength of the strongest of the Si II
transitions, fλ = 1486.8, we find in the limit that
Figure 7. Upper panel: Lyα profile of Haro 11 C, in ap- 1260
I/I =e τ =0.95, and eq. 2 becomes:
proximate units of the surrounding continuum level. The 0 −
full line indicates the measured values smoothed by a 5 3.768 1014
px. flat kernel; surrounding shading encloses the ±1σ er- NSmiIiIn =−loge(0.95) f×λ =1010.1cm−2, (4)
ror band (mostly comparable in width to the line width).
Middle panel: BlackstepsshowtheaveragedLISlinepro- which with the adopted metallicity for Haro 11 C cor-
file, smoothed by a 5px kernel. Surrounding gray shading responds to a minimum Hydrogen column density of
denotes the 1σ confidence band. Lower panel: Same as Nmin 4.0 1015 cm-2. This leaves around 2 orders
middle panel±, but for the SiIV transitions. HI ∼ ×
of magnitude in N , in which the gas is not detected
HI
in Si II and is optically thick to Lyα while translucent
to LyC. If we require the gas to be detected in at least
8 T. E. Rivera-Thorsen et al.
two of the lines included in this analysis, the limiting
hydrogen column density becomes Nmin 4.9 1016 2.0×10−13
HI ∼ ×
cm-2, adding another order of magnitude to the allowed
range, but we adopt the lower value as a conservative
1.5
estimate. ˚-1A]
The existence of a diffuse neutral component being -2m
c
present in the ionized medium seems consistent with -1s1.0
g
what is found in Lyman Break Analogs and Green Pea er
[
x
galaxies(Heckmanetal.2011;Henryetal.2015). These Flu SiII1260
0.5 Observeddata
galaxieshavesometimesverylowLISabsorptiondepths,
SiII*bestfit
which is usually indicative of low covering fractions and
RT-corr.data
high porosity of the neutral medium - and yet they find 0.0
1282 1284 1286 1288 1290 1292
covering fractions of near unity of H I from Lyman β Wave[A˚]
absorption, indicating column densities of N (cid:38) 1016
Figure 8. SiIIλ1260anditsfluorescentlineatλ1265,with
cm 2. Like in the case of Haro 11, this indicates that
− the observed data shown in blue. The orange curve shows
a non-negligible neutral component must be present in a fit to the fluorescent emission, and in gray is shown the
the ionized phase, although it may very well be of such worst-case absorption profile corrected for radiative transfer
lowdensitiesandmetallicitythatmetalabsorptionfrom effects re-filling the absorption trough.
this medium is undetectable.
The column densities we derive for Si II in this work couldbemixed,orhaveanunseenLOSdistancecompo-
are generally between 12.0(cid:46)logN (cid:46)12.5, column den- nentlargerthantheknotseparation,drawingintoques-
sities around which the transitions involved become op- tionwhichisthebettervaluetoassumeforthisgas. We
tically thick: For λλ1260,1526 and 1304, τ becomes 1 base our conclusions on the value found for knot C, but
at logN 11.3,12.2 and 12.5, respectively. Off the re- note that using the metallicity of 12+log(O/H) = 8.3
∼
gions of strongest absorption, and in particular at v0, foundforknotBbyGusevaetal.(2012),theinferredHI
only λ1260 seems to be optically thick, and it seems columndensitiesare3.9 0.6 1017 cm 2. However,the
−
± ×
the column densities arrived at can be trusted. But at questionoftheexactmetallicityoftheoutflowinggasis
line center, around v v , the column densities found complicated. Gas closer to the star forming regions is
int
∼
are so close to the limit that they are most likely to be expected to be more strongly enriched than gas further
interpretedaslowerlimits. Theremightalsobesystem- away, which would imply that the H I column density
atics in the determination of the continuum around the is larger than inferred from Si II above. To this can
lines which may lead to the lines at λλ1304,1526 to be be added the further complication stemming from the
falsely seen as shallower than λ1260, meaning that all merger event that the galaxy is currently undergoing,
computedcolumndensitiesarereallylowerlimitsrather which may have mixed gas of different metal contents.
than actual values. The confidence intervals in fig. 6 do In any case, though, we would expect the regions near-
not reflect this possible saturation, but only the formal esttothestarburststogenerallyhavehighermetallicity
errors from the best approximation to the residual in- than the surrounding cool gas, such that the HI column
tensity, they do not include systematics. density inferred above is more likely to be underesti-
Ifthecomputedcolumndensitiesareinfactlowerlim- mated than overestimated.
its,thismeanstheinferredHIcolumndensitiesarealso
lower limits. This would strengthen the modified rid-
3.4. Radiative transfer effects
dled,ionization-boundedmediumscenariothattheISM
The conclusions drawn by the AOD method rest on
on the line of sight to Haro 11 C consists of dense, neu-
theassumptionthattheobservedlinesareclosetobeing
tralclumpswithanionizedinterclumpmediumcontain-
pureabsorptionlines,withredistributionofphotonsdue
ing a dilute neutral component.
to radiative transfer effects being modest. Modelling
work by e.g. Prochaska et al. (2011) and Scarlata &
3.3. Neutral gas metallicity
Panagia (2015) has shown that in certain conditions,
The exact value of the metallicity is however uncer- radiativetransfercanre-fillabsorptionfeaturesinaway
tain. Thevaluesfoundfromnebularrecombinationlines thatcanmakeanisotropic,opticallythinmediummimic
byGusevaetal.(2012)aremeasuredmainlyinthecen- the observational fingerprints of a system of optically
tral H II regions around the clusters; and differ by 0.2 thick clumps. We therefore need to investigate whether
dex between knot B and C. The neutral, outflowing gas our conclusions could be generated by such effects.
Neutral ISM, Ly-Alpha and LyC in Haro 11 9
Because the ground level in Si II is a doublet with
a short-lived upper level, each of the Silicon lines in- 1.4
cluded in the analysis has a fluorescent emission com-
1.2
panion with practically no absorption component. The
fluorescent line can be used to constrain the possible ef- 1.0
fect of radiative infilling of the absorption trough. If ux 0.8
fl
eachphotoninthefluorescenttransitiononaveragehas d
e
scatteredonceintheneutralmedium, theratioofemis- aliz 0.6
m
sion in this line and re-filling of the resonant absorp- Nor 0.4
tionlineissimplythatoftheirEinsteincoefficientsA .
ki
0.2
Foreachsubsequentscattering,morephotonsescapedi- CII1334+1335
rectly through the fluorescent channel, and the infilling 0.0 SiII1260
of the absorption line is further suppressed. ModelCII
0.2
In fig. 8, the observed line profile is shown in blue, − 800 600 400 200 0 200 400 600 800
along with the best fit to Si II shown in orange. Since − − − − v−v0[kms-1]
∗
the fluorescent and resonant reemission originate in the Figure 9. C II 1334, Si II 1260, along with a C II line
sameenvironments,theshapeoftheresonantreemission synthesized from SiII (see text for details).
profile can be completely determined from the shape
and center of the fluorescent line and the assumption of system of origin, especially those in photo dissociation
one scattering. It is therefore simple to construct this regions.
theoreticalreemissionline,subtractitfromtheobserved Fig.9showsaquickback-of-the-envelopecheckofthis
absorption line and find the limiting depth of the pure hypothesis. Assuming the ions reside in the same phys-
absorption line without reemission; this modeled line is ical regions, we used the SiII and a second contribution
shown in gray in the figure. It is readily seen that the created by shifting the original profile to the red by the
differencebetweenworst-caseintrinsicandobservedline appropriate velocity ( 260 km s-1) and multiplying it
∼
is so small that it cannot affect our conclusions about by a free parameter, which was chosen by eye to give a
the covering factors and column densities of the neutral reasonable replication of the observed C II; the chosen
gas significantly. value shown in the figure is 0.7 times the original line.
We have not performed similar measurements for the It is clear from the figure that the reproduction of the
linesatλλ1304,1526,butnotethatifsignificantrefilling observed CII line is very reasonable, consideringthat it
is present here, this would mean the column densities isgeneratedfromadifferentspecieswithaverydifferent
had been underestimated in our analysis, which would ionization potential.
strengthentheconclusionsofaclumpymediumoptically Whetherthisisaphysicallyreasonablestrengthisdif-
thick in Si II 1260, but part transparent in the much ficult to say with precision, but we note that to repro-
weaker other lines. ducethefeatureobserved,λ1334mustbeopticallythick
and λ1335 part transparent. This leaves open a degen-
3.5. CII λ1334 absorption eracy of relative level populations and ion abundance,
suchthatthechoiceofthisvalueonlyweaklyconstrains
Looking back at Fig. 4, the profile of C II λ1334 is
these quantities and thus it is compatible with a wide
clearly deeper than SiII λ1260, which we had otherwise
range of scenarios.
concludedisopticallythickandthusprovidesalimitfor
It is also worth noting that C II has a significantly
the velocity binned covering fractions. We believe the
higher ionization potential than SiII, such that the for-
explanationisacontributionfromCIIλ1335.7blending
mer will be more abundant in regions of higher ioniza-
withλ1334. Thistransitionhasanoscillatorstrengthof
tion, with CII in effect tracing slightly different regions
f =0.114,comparedtof =0.127forλ1334(thereis
ik ik thanSiII.Furthermore,intheseregionsofhigherioniza-
athirdlineat1335Å,butthatisanorderofmagnitude
tion,therelativelevelpopulationsoftheCarbonground
weaker), so given a sufficient population of its ground
state can be significantly altered by shifts in e.g. elec-
level,itisstrongenoughtomakeanon-negligiblecontri-
trondensityandradiationfield,whichcanaltertheline
bution to the resulting line profile. One should however
shape further. We make no attempt at mapping these
bear in mind that these lines arise from different lower
complexconditionshere,butnoteonlythatthereexists
fine structure levels; λ1334 arises from 2P , while the
1/2 arangeofnon-exoticphysicaleffectswhichcangenerate
other two arise from 2P . The population of the lat-
3/2 thedeeperandwiderλ1334profileweobservewhilestill
terleveldependssensitivelyonphysicalconditionsinits
10 T. E. Rivera-Thorsen et al.
being compatible with the ISM being optically thick in therefore conclude that the leaked ionizing photons, if
SiII λ1260. originating from this cluster, most likely escaped via
sight lines between the neutral clumps, through an ion-
ized medium which must contain a neutral gas column
3.6. Conclusion density 5 1013 (cid:46) NHIS (cid:46) 4 1015 cm 2. This range
× HI × −
In this work, we have re-analyzed an archival FUV is bounded downwards by the value at which HI be-
HST-COS spectrum of Haro 11 to investigate the kine- comes transparent to Lyman-α, and upwards by the
matics and geometry of both the hot, ionized and cold, detectability of SiII in this observation.
neutral ISM along the line of sight to a strongly star It is interesting to note that Haro 11 is relatively on
forming knot suspected of being the source of the ob- neutral gas poor, and has a very strong ongoing star-
served LyC leakege from the galaxy. We have used burst episode. If we were to see a case in which neu-
the Apparent Optical Depth method to compute col- tral gas depletion and/or ionization provided a density-
umn densities and velocity-binned covering fractions bounded scenario, this would be a likely place to find
of the gas and compared these results to the extreme it. Still, it seems that dynamical effects of feedback and
cases of either an optically thin, density bounded neu- galaxy-scale interaction plays the greater part in peel-
tral medium, or a riddled, optically thick, ionization ingawaytheneutralgasfromthecentralstarburstsand
boundedneutralmedium. AssumingthattheLyCemis- opening passages for Lyα and LyC to escape. Given
sion previously observed from this galaxy indeed does that star formation and merger activity is significantly
originatefromknotC,wefindthattheobservationsare stronger at z (cid:38) 1 than in the local Universe, and that
not compatible with the latter case, since the charac- DarkMatterpotentialwellswereshallower,thesemech-
teristic, bright Lyman-α emission spike at line center anisms could be even more important in that era. one
is absent in this spectrum. Furthermore, the observa- could speculate that this could provide a piece of the
tions are not consistent with an optically thin, density puzzle of which sources have driven the epoch of reion-
bounded neutral medium. ization.
We confirm previous authors concluding that the
ACKNOWLEDGEMENTS
medium is highly ionized with clumps of neutral gas
of low velocity-binned covering fraction, increasing the The authors thank the anonymous referee for con-
probability of finding direct sight lines to the back- structive and insightful comments, which have helped
ground star cluster between these. The clumps have improve the quality of this paper significantly.
HI column densities of gas around v = 0 in the range GÖ & MH acknowledge the support of the Swedish
N = 6.2+0.9 1017 cm 2 given the metallicity of the Research Council, Vetenskapsrådet, and the Swedish
baHcIkgroun−d1.H1×II region,−which is likely a slight un- National Space Board (SNSB). MH is an Academy Fel-
derestimate of N . A conservative estimate of the lowoftheKnutandAliceWallenbergFoundation. This
HI
H I column density integrated over all velocities is project has made extensive use of the Python-based
logNtot = 19.01 0.17 cm 2. There is a possibility packages Numpy (Walt et al. 2011), SciPy (Jones et al.
HI ± −
that the found column densities are in fact lower lim- 2001–), Matplotlib (Hunter 2007), Pandas (McKinney
its, since the found Si II column densities are close to 2010), LMfit (Newville et al. 2014), and Astropy (As-
the limit where this ion becomes optically thick. We tropy Collaboration et al. 2013).
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