Star formation and the interstellar medium in z>6 UV-luminous Lyman-break galaxies
DDraft version August 20, 2018
Preprint typeset using L A TEX style emulateapj v. 12/01/06
STAR FORMATION AND THE INTERSTELLAR MEDIUM IN
Z >
Chris J. Willott
NRC Herzberg, 5071 West Saanich Rd, Victoria, BC V9E 2E7, Canada
Chris L. Carilli
National Radio Astronomy Observatory, P.O. Box 0, Socorro, NM 87801, USA andCavendish Astrophysics Group, University of Cambridge, Cambridge, CB3 0HE, UK
Jeff Wagg
Square Kilometre Array Organization, Jodrell Bank Observatory, Lower Withington, Macclesfield, Cheshire SK11 9DL, UK
Ran Wang
Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, China
Draft version August 20, 2018
ABSTRACTWe present Atacama Large Millimeter Array (ALMA) detections of atomic carbon line and dustcontinuum emission in two UV-luminous galaxies at redshift 6. The far-infrared (FIR) luminositiesof these galaxies are substantially lower than similar starbursts at later cosmic epochs, indicating anevolution in the dust properties with redshift, in agreement with the evolution seen in ultraviolet (UV)attenuation by dust. The [C ii ] to FIR ratios are found to be higher than at low redshift showing that[C ii ] should be readily detectable by ALMA within the reionization epoch. One of the two galaxiesshows a complex merger nature with the less massive component dominating the UV emission andthe more massive component dominating the FIR line and continuum. Using the interstellar atomiccarbon line to derive the systemic redshifts we investigate the velocity of Ly α emission emerging fromhigh- z galaxies. In contrast to previous work, we find no evidence for decreasing Ly α velocity shiftsat high-redshift. We observe an increase in velocity shifts from z ≈ z ≈
6, consistent with theeffects of increased IGM absorption.
Subject headings: cosmology: observations — galaxies: evolution — galaxies: formation — galaxies:high-redshift INTRODUCTION
Observations from the
Hubble Space Telescope havegiven us a broad-brush picture of the evolution of galax-ies over cosmic time. Galaxies in the reionization epochat redshifts z > α emission (Pentericci et al. 2011; Schenkeret al. 2012; Treu et al. 2013) suggests a rapid change inthe IGM neutral hydrogen fraction at z ∼
7. Some ofthe strength of this evolution could also be accounted
Electronic address: [email protected] for by increased Ly α absorption within galaxies due tolower Ly α velocity shifts (Choudhury et al. 2014). Thefirst measurements of Ly α velocity shifts in two z > z ∼ α kinematics.Several studies using ALMA and other facilities haveresulted in non-detections and claims that the FIR line a r X i v : . [ a s t r o - ph . GA ] M a y Willott et al.and/or continuum fluxes are lower than expected basedon UV SFRs (Walter et al. 2012; Kanekar et al. 2013;Ouchi et al. 2013; Ota et al. 2014; Gonz´alez-L´opez et al.2014; Maiolino et al. 2015; Schaerer et al. 2015). ALMAobservations failed to detect either 1.2 mm continuum orthe fine-structure line of singly-ionized carbon, [C ii ], inthe z > . Himiko and IOK-1 (Ouchi et al.2013; Ota et al. 2014), despite the strong UV continuumindicating SFR ∼ − M (cid:12) yr − . They showed thatthe FIR (cool dust) contributions to the spectral energydistributions are similar to nearby dwarf irregulars, morethan an order of magnitude below the levels expectedfor nearby starbursts/spirals such as M82 and M51. Thephysical interpretation of the lack of dust and [C ii ] emis-sion in these z > z galaxies (Fisher et al. 2014).This picture has brightened recently with several detec-tions of the ISM in distant galaxies. Although Maiolinoet al. (2015) failed to detect three 6 . < z < . ii ] emission at the expected red-shift of BDF3299, offset by 4 kpc from the UV position,which they interpret as an accreting or satellite clumpof gas. Watson et al. (2015) detected a gravitationally-lensed galaxy at z = 7 . SF R
FIR about 3 times greater than
SF R UV ,similar to the ratio typically observed at z ∼ ii ] detection in this galaxy islikely due to an uncertain redshift, as Watson et al. notethe lack of an emission line redshift means the ALMAdata only cover 50% of potential [C ii ] line frequencies.Capak et al. (2015) observed nine LBGs at 5 < z < ii ] emission from all of themand dust continuum in four galaxies. They showed thatthe dust emission in these galaxies is much weaker thanexpected based on similar galaxies at lower redshift. TheCapak LBGs were selected based on interstellar UV linesso likely have higher metallicity than most previously-targeted high- z galaxies, although still lower than at lowredshift.We are carrying out an ALMA program targeting U V -luminous LBGs at z >
6. In this paper we present obser-vations of the first two galaxies observed from our sam-ple. The galaxies were first identified in Canada-France-Hawaii Telescope (CFHT) optical and near-IR imagingand both have spectroscopic redshifts based on a con-tinuum break plus Ly α emission. The galaxy which wename here as CLM 1 was discovered by Cuby et al. (2003)at a redshift of z = 6 .
17. The other galaxy, WMH 5 at z = 6 .
07, was discovered in Willott et al. (2013b). Bothgalaxies have near-IR continuum magnitudes AB ∼ ii ]to FIR continuum and Section 5 the Ly α velocity shiftsobserved. We draw conclusions in Section 6. Cosmologi-cal parameters of H = 67 . − Mpc − , Ω M = 0 . Λ = 0 .
692 (Planck Collaboration et al. 2015) areassumed throughout. OBSERVATIONS
ALMA
Observations were made during ALMA cycle 2, in June2014, with between 29 and 32 antennas, and a maximumbaseline of 650m. A total bandwidth of 7.2 GHz wasemployed in Band 6 using 4 dual-polarization sub-bandsbetween 249 and 272 GHz. One of the sub-bands wascentered at the [C ii ] line (rest frame 1900.5369 GHz) foreach source. The rest of the bands were used for con-tinuum measurements. A total of 95 minutes on-sourceintegration time was obtained for each galaxy.The initial data editing and calibration were performedas part of standard data processing by the ALMA staff.The calibrated visibility data were then re-analyzed, per-forming additional flagging of bad time periods and badchannels. The data were re-imaged using the CASABriggs weighting of the visibility data with Robust = 1 tocreate continuum and channel images. For both sources,the Gaussian restoring beam was close to circular, witha FWHM = 0.50”. Spectral cubes at 15.625 MHz resolu-tion (17 . − ) were synthesized, and smoothed spec-trally for subsequent analysis, as required. The contin-uum was subtracted in the image-plane, using the off-linechannels in the line cube.The spectra were transformed from the observedtopocentric system to the local standard of rest (LSR).The velocity offsets applied were ≈ −
11 km s − .Spectral and spatial Gaussian fitting was performedusing the CASA viewer, CASA fitting tools and customsoftware. Results for total line fluxes, continuum fluxdensities, and parametric source sizes, are given below.Flux uncertainties have 10% added in quadrature for ab-solute flux calibration uncertainty. Near- and Mid-Infrared
To compare with the ALMA data we use near-infrared(NIR) data from the ESO VISTA VIDEO survey (Jarviset al. 2013). Images from Data Release 3 in bands
Z, Y, J, H and Ks were obtained, sampling the rest-frame ultraviolet continua. Astrometric calibration ofthe VIDEO images to the radio reference frame usedby ALMA was performed by matching bright VIDEOsources to the AllWISE catalog (Cutri et al. 2013) andextracting the ALMA phase calibrator positions from theAllWISE catalog. Given the size of the residuals this pro-cess gives an uncertainty on astrometric frame matchingof ≈ . (cid:48)(cid:48)
1, comparable to the positional uncertainties ofthe NIR data due to S/N and seeing. Fluxes in the fiveNIR bands of the two target galaxies were determinedusing aperture photometry with aperture corrections tototal fluxes. Due to the similar shapes, fluxes and S/Nof the data in the three bluest VIDEO bands, a deepercombined
ZY J image of each field was generated. Nei-ther galaxy is significantly spatially resolved in the NIRdata.The two galaxies have been observed with the
SpitzerSpace Telescope in the IRAC 3.6 and 4.5 µ m bands dur-ing the SERVS survey (Mauduit et al. 2012). Due tothe proximity of bright galaxies along the line-of-sightto both z > ZY J
VIDEO image as a prior for the object shapes.Models were fit to the NIR images using GALFIT (Pengtar formation and the interstellar medium in z >
RA (J2000) D e c ( J ) CLM 1 z=6.1657+/-0.0003
RA (J2000) D e c ( J ) CLM 1 z=6.1657+/-0.0003
Fig. 1.—
Left:
The background image is ALMA integrated [C ii ] line map of CLM 1. White contours are the 1.2 mm continuum emissionfrom the three line-free basebands at contour levels 1.5, 2.5 σ beam − , specially chosen to show the low S/N possible continuum detectionclose to the [C ii ] emission. The near-infrared centroid is plotted as a black plus symbol. The restoring beam is shown in yellow. Right:
The background is the zY J
NIR image. Contours show the ALMA [C ii ] emission from the left panel at levels 3, 5, 7 σ beam − . Therest-frame UV continuum and [C ii ] emission are co-spatial. Two foreground galaxies are visible on the right side of the image.
500 0 500 1000Velocity offset (km/s)0.50.00.51.01.5 F l u x - d e n s i t y ( m J y ) CLM 1[CII] z =6.1657+/-0.0003 F l u x - d e n s i t y ( µ J y )
500 0 500 1000
CLM 1Ly α z =6.176+/-0.002 Fig. 2.—
ALMA [C ii ] (upper) and VLT FORS2 Ly α (lower)spectra of CLM 1. The ALMA spectrum is well fit by a singleGaussian emission line, with a marginal red wing excess. The Ly α spectrum shows typical blue-absorbed asymmetry. Vertical blueand red dotted lines mark the [C ii ] Gaussian fit peak and Ly α flux-weighted centroid, respectively. Ly α is offset from [C ii ] by+430 ±
69 km s − . et al. 2010) and subsequent fitting of the IRAC data wasdone with PyGFIT (Mancone et al. 2013). Both z > µ m, al-though with flux uncertainties of ∼ Lyman- α spectroscopy Optical spectroscopy, including the detection of asym-metric Ly α emission lines and continuum breaks, of bothgalaxies were presented in their discovery papers (Cubyet al. 2003; Willott et al. 2013b). In order to comparethese spectra to the ALMA spectra they were correctedfrom observed wavelength to the LSR by applying veloc-ity corrections of +26 km s − and +32 km s − for CLM 1and WMH 5, respectively. The spectrum of CLM 1 wasobtained using FORS2 at the ESO VLT with a spectralresolution of R = 1400 and that of WMH 5 using GMOSat Gemini-North with R = 1000. RESULTS
CLM 1
For both galaxies there are strong detections of the[C ii ] line evident in the datacubes at close to the ex-pected position and velocity. In Figure 1 we present theALMA and NIR imaging of the galaxy CLM 1. The[C ii ] map is made from a sum over all channels showingsignificant line emission. The [C ii ] emission is spatiallyextended along a similar direction as the beam exten-sion. A Gaussian fit to the source gives an observed sizeof 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
42 at position angle east of north (PA) =42, compared with a beam size of 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
44 at PA =55. The CASA fitting software does not provide a de-convolved source size due to the only slightly larger sizethan the beam and outputs that the intrinsic source maybe as large as 0 . (cid:48)(cid:48) × . (cid:48)(cid:48) ii ] line. In contrast to the strong and clear[C ii ] emission the continuum in this source is very weak. Willott et al.There is a marginal continuum detection at the 2 σ levelthat is only considered as plausible true emission be-cause its centroid co-incides to within 0 . (cid:48)(cid:48) ii ]line centroid. There are several other peaks of this mag-nitude within the 6 (cid:48)(cid:48) × (cid:48)(cid:48) image of Figure 1 that maybe due solely to noise. We note that there is an emerg-ing trend in ALMA observations of non-ULIRG high-redshift galaxies that the [C ii ] line emission is detectedat much higher significance than the continuum, despitethe large instantaneous bandwidth of 7.5 GHz (Riech-ers et al. 2014; Willott et al. 2013a, 2015; Capak et al.2015). We discuss the ratio of line to continuum emissionfurther in Section 4.The NIR image in Figure 1 shows the location of therest-frame UV continuum in CLM 1. The Ly α contri-bution to this flux is negligible. The galaxy is indistin-guishable from a point source at the 0 . (cid:48)(cid:48) ii ] and rest-frame UVco-incide to within 0 . (cid:48)(cid:48)
1, suggesting they trace the samestar forming regions of the galaxy, at this resolution.Visible to the east of CLM 1 is a lower redshift ellipti-cal galaxy whose centroid is only 3 (cid:48)(cid:48) from CLM 1. Thepresence of this galaxy combined with the high UV lumi-nosity of CLM 1 raises the possibility that the high lu-minosity is in part due to magnification by gravitationallensing. To determine the potential lensing magnificationwe have analyzed the properties of this elliptical galaxy.We fit galaxy models to 12 band photometry from theCFHT Legacy Survey, VIDEO and AllWISE using theFAST code (Kriek et al. 2009). The spectral energy dis-tribution (SED) is well fit by an old stellar populationat z = 0 .
54 with a stellar mass of 5 × ( M (cid:12) ). Usingthe observed correlation between velocity dispersion andstellar mass (Wake et al. 2012) at z = 0 . z = 0 . z = 0 . − . Assumingan isothermal sphere potential this lensing configurationprovides a magnification factor of only 1.13. Given thisrelatively low magnification we do not make any lensingcorrections to physical values in this paper.Figure 2 plots the [C ii ] spectrum of CLM 1 and theLy α spectrum for comparison. Interpretation of the ve-locity difference between [C ii ] and Ly α is deferred to Sec-tion 5. The [C ii ] spectrum is well fit by a single Gaussianplus a very low level, flat continuum. The fit continuumlevel is consistent with the low S/N continuum detec-tion from the other 3 sub-bands described above. Thereis marginal excess emission in the red wing. From theGaussian peak frequency we determine the systemic red-shift of the galaxy as z = 6 . ± . ±
23 km s − . Table 1 contains furthermeasurements made from the ALMA data.With this velocity and the upper limit on the sourcesize ( < . (cid:48)(cid:48)
55 or 3.2 kpc) we can calculate an upper limitto the dynamical mass. Following the procedure of Wanget al. (2013) we derive M dyn < . × / sin i M (cid:12) where i is the unknown inclination angle.Using photometry from the rest-frame UV, optical andfar-IR we fit galaxy spectral models to understand theevolutionary state of CLM 1. A synthetic ALMA contin-uum filter is generated using the wavelengths covered bythe three line-free sub-bands. We use the Python imple-mentation of the CIGALE package (Roehlly et al. 2012)to fit the SED. This code determines the ultraviolet- -4 -3 -2 -1 F l u x - d e n s i t y ( m J y ) NGC 1266Holmberg I
CLM1 z =6.1657 Model spectrumKINGFISH galaxiesModel fluxesObserved fluxes10 Observed wavelength (nm)1.00.50.00.51.0 R e l a t i v e r e s i d u a l f l u x (Obs-Mod)/Obs Fig. 3.—
Observed-frame optical to far-IR SED of CLM 1 (bluecircles). The best-fit model from
CIGALE is shown as a blackcurve and the model fluxes through the observed filters as red cir-cles. The grey curves are 39 nearby galaxy SEDs with UV to far-IRphotometry from the KINGFISH survey (Dale et al. 2012) rang-ing from dusty infra-red galaxies to dwarf irregulars with very lowthermal dust emission. The lower panel shows the residuals fromthe best-fit model. CLM 1 has a SED most similar to the leastdusty nearby dwarf irregulars.
TABLE 1Millimeter data for the z > LBGs
CLM 1 WMH 5 z Ly α . ± .
002 6 . ± . z [CII] . ± . . ± . a . ± . b FWHM [CII] ±
23 km s − ±
15 km s − ±
13 km s − I [CII] (Jy km s − ) 0 . ± .
031 0 . ± . L [CII] ( L (cid:12) ) (2 . ± . × (6 . ± . × f . ( µ Jy) 44 ±
26 218 ± L FIR ( L (cid:12) ) (2 . ± . × (1 . ± . × L [CII] /L FIR (9 . ± . × − (5 . ± . × − SFR [CII] ( M (cid:12) yr − ) 24 ± ± FIR ( M (cid:12) yr − ) 7 ± ± SED ( M (cid:12) yr − ) 37 ± ± Notes. — a The redshift and FWHM of component ‘A’ only. b The redshift and FWHM of component ‘B’ only.Uncertainties in L FIR and SFR only include measurement uncer-tainties, not the uncertainties in extrapolating from a monochro-matic to integrated luminosity, luminosity-SFR calibrations, or un-certainty in star formation history. optical attenuation and its re-emission in the infrared.Stellar light is modeled with a single Bruzual & Char-lot (2003) stellar population with constant star forma-tion rate, Chabrier initial mass function and metallicity= 0.008. Dust attenuation followed the Calzetti et al.(2000) dependence on wavelength with re-emission in theIR via the SED parameterization of Dale et al. (2014).The highest likelihood parameters are a stellar massof 1 . × M (cid:12) , SFR = 37 M (cid:12) yr − and minimal dustattenuation of E(B-V)=0.01. This very low amount ofdust attenuation is constrained by the blue rest-frameUV slope of β = − . f λ ∝ λ β ) and in an IRremission scenario consistent with the low FIR luminos-ity from the marginal 1.2 mm continuum detection. Wetar formation and the interstellar medium in z > ii ]-emitting gas is nottracing the full dynamical mass of the system.In Figure 3 we show the observed photometry of CLM1 along with the best-fit model photometry, the modelspectrum and, in the lower panel, the residuals fromthe fit. With almost as many free parameters as datapoints, a good fit has been found. The observed IRACfluxes are slightly higher than the model suggesting possi-bly stronger nebular lines than modelled, although thereare large uncertainties in the IRAC fluxes. The graycurves in Figure 3 show redshifted SEDs for nearbygalaxies from the KINGFISH survey, normalized at rest-frame 150 nm. These galaxies have been observed with GALEX, Spitzer Space Telescope, Herschel Space Obser-vatory and in the optical and near-IR (Dale et al. 2007,2012). The galaxies range in total IR luminosity from10 to 10 L (cid:12) . CLM 1 has a SED most similar to the ∼ L (cid:12) galaxies, despite having an IR luminosity of > L (cid:12) , in essence it is a scaled up (by 10 ) versionof the low metallicity dwarf irregulars.It has recently been shown that the [C ii ] luminosityis an effective tracer of the star formation rate in lowredshift starbursts (De Looze et al. 2014; Sargsyan et al.2014). In addition to the far-IR luminosity and SED(largely constrained by UV, rather than FIR, fluxes) thisgives three independent measures of the SFR in this z > ii ] luminosity we use the relationSFR ( M (cid:12) yr − ) = 1 . × − L [CII] ( L (cid:12) ) (Sargsyan et al.2014).We calculate the far-IR luminosity L FIR (integratedover rest-frame 42.5 to 122.5 µ m) from the observed1.2 mm continuum assuming a greybody spectrum withdust temperature, T d = 30 K and emissivity index, β =1 .
6. We use T d = 30 K because that is the dust temper-ature for similar FIR-luminosity galaxies at low-redshift(Symeonidis et al. 2013). We note that a higher dust tem-perature of T d = 45 K would increase the L FIR value bya factor of about three. To convert from L FIR to SFR weuse the relation SFR ( M (cid:12) yr − ) = 1 . × − L FIR ( L (cid:12) )appropriate for a Chabrier IMF (Carilli & Walter 2013).At these redshifts the Cosmic Microwave Background(CMB) has a temperature of 19 K and can potentiallybias measurements of dust continuum luminosity. Thereare two competing effects: (i) a high CMB backgroundagainst which the continuum is measured, and (ii) anincrease in the dust temperature due to heating by theCMB. Since we only have one continuum point and noconstraints on dust temperature, we cannot make an ac-curate correction for these effects, but note that accord-ing to the analysis of da Cunha et al. (2013) the twoeffects are of comparable size and opposite sign for likelydust temperatures at this redshift, so we make no cor-rection.All three SFR estimates are listed in Table 1. For CLM1 we find that SFR SED and SFR [CII] are comparable, butSFR
FIR is lower than SFR [CII] (see Section 4) and lowerthan SFR
SED , as expected from the low dust contribu-tion to the SED in Figure 3. Previous studies have shownthat SFR
FIR may be unreliable as a tracer of the total SFR in very low dust and/or metallicity galaxies (Ouchiet al. 2013; Fisher et al. 2014; Ota et al. 2014). An al-ternative is that a higher dust temperature of T d = 45 Kwould raise SFR FIR to a level comparable with SFR
SED and SFR [CII] . Future observations at shorter wavelengthare critical to constrain the full IR SED of high- z galaxiesand constrain the dust temperature. WMH 5
The ALMA and NIR images for WMH 5 are shownin Figure 4. There is a much more firm 1.2 mm con-tinuum detection in this source at significance 6 σ . Thebeam is more circular for WMH 5 than for CLM 1 andboth the [C ii ] and 1.2 mm continuum are clearly spa-tially extended at PA ≈
80. The [C ii ] emission centroidshows a significant offset from the NIR emission of 0 . (cid:48)(cid:48) α emission is also spatially unresolved along the slitdirection.Figure 5 shows the [C ii ] spectrum of WMH 5. Theline can be split into two Gaussian components. We la-bel the component with higher flux and larger linewidth(FWHM=251 km s − ) as ‘A’ and the other componentas ‘B’ (FWHM=68 km s − ). Assuming the linewidthstrace mass gravitationally we identify the systemic red-shift of the main galaxy WMH 5 with the velocity of‘A’. To determine the nature of this multiple veloc-ity component system we show in Figure 6 a position–velocity diagram along the major axis of [C ii ] emis-sion. The separation of the two components in velocityis clear, but we also find a spatial offset of 0 . (cid:48)(cid:48) ii ] compo-nents are marginally resolved with intrinsic major axissizes of 0 . (cid:48)(cid:48) ± . (cid:48)(cid:48) . (cid:48)(cid:48) ± . (cid:48)(cid:48) ii ]sizes and FWHM of M dyn = 1 . × / sin i M (cid:12) for Aand M dyn = 1 . × / sin i M (cid:12) for B.We carry out SED-fitting using CIGALE for the entireWMH 5 system. The observed and modelled SEDs areshown in Figure 7. As for CLM 1, the photometry is wellfit by a single stellar population whose attenuated UVemission is re-radiated in the FIR. The highest likelihoodparameters are a stellar mass of 2 . × M (cid:12) , SFR =43 M (cid:12) yr − and dust attenuation of E(B-V)=0.05. Thehigher dust attenuation in WMH 5 is constrained by theredder UV spectral slope and consistent with the higher L FIR . However, in this system we know that there is aspatial offset between the UV and FIR, so such a simplescenario of attenuation and re-emission is not physicallyplausible.From this analysis the system appears to be an on-going merger of two galaxies. ‘B’ is apparently lowermass but is spatially coincident with the NIR emission.This is similar to some lower redshift ULIRG mergers(Chapman et al. 2004) with an optically obscured com-ponent that dominates the FIR emission and a lower L FIR component less obscured by dust. Similar casesof most of the [C ii ] associated with optically-faint com-ponents have been reported at z > RA (J2000) D e c ( J ) WMH 5 z=6.0695+/-0.0003
RA (J2000) D e c ( J ) WMH 5 z=6.0695+/-0.0003AB
Fig. 4.—
Left:
The background image is ALMA integrated [C ii ] line map of WMH 5. White contours show the clear 1.2 mm dustcontinuum detection (contour levels 2, 3, 4, 5 σ beam − ). The NIR centroid (black plus symbol) is significantly offset to the east. Right:
The background is the zY J
NIR image. Contours show the ALMA [C ii ] emission from the left panel at contour levels 3, 6, 9 σ beam − .Grey crosses labeled ‘A’ and ‘B’ correspond to the centroids of the two [C ii ] velocity components. The rest-frame UV continuum is consistentwith ‘B’, the component with lower [C ii ] luminosity and velocity width. There is a foreground galaxy further east of the system. F l u x - d e n s i t y ( m J y ) WMH 5[CII] z =6.0695+/-0.0003 AB F l u x - d e n s i t y ( µ J y ) WMH 5Ly α z =6.076+/-0.001 Fig. 5.—
ALMA [C ii ] (upper) and Gemini GMOS Ly α (lower)spectra of WMH 5 (see Figure 2 for details). The [C ii ] data havebeen continuum-subtracted in the image plane. For this galaxythere are two [C ii ] velocity components (‘A’ and ‘B’), that areeach well fit by a Gaussian. The systemic redshift is taken to bethe Gaussian peak of ‘A’ as this is the most luminous and highestwidth component. The Ly α spectrum is asymmetric and the flux-weighted centroid offset by +265 ±
52 km s − from ‘A’ and +504 ±
52 km s − from ‘B’. -600 -400 -200 0 200 400Velocity offset (km/s)-2-1012 P o s i t i o n o ff s e t ( a r c s e c ) NIRABWMH 5 z=6.0695+/-0.0003
Fig. 6.—
Position-velocity map for the [C ii ] emission in WMH5. The NIR galaxy centroid is located approximately along themajor axis of the [C ii ] emission and its position is identified by thesolid black line labelled NIR. The two [C ii ] components, ‘A’ and‘B’ are clearly separated in position and velocity, with ‘B’ close tothe NIR centroid, as seen in Figure 4. ponent ‘A’ also dominates the FIR continuum and has alarge linewidth so is not some peripheral, satellite gascloud. The stellar mass estimate of ∼ × M (cid:12) is > . × / sin i M (cid:12) , unless the inclination angle i < ◦ .Either ‘B’ is a disk viewed extremely face-on or it is notresponsible for all the stellar mass.For WMH 5 the SFR estimates from the far-IR, [C ii ]and SED-fitting in Table 1 are all within a factor of two.This is rather surprising given the complex nature of thesystem. The millimeter and UV SFR have similar values,but their spatial displacement indicates that they are notprobing the same star-forming regions, so the true SFRshould be the sum of both components.tar formation and the interstellar medium in z > -4 -3 -2 -1 F l u x - d e n s i t y ( m J y ) NGC 1266Holmberg I
WMH5 z =6.0695 Model spectrumKINGFISH galaxiesModel fluxesObserved fluxes10 Observed wavelength (nm)1.00.50.00.51.0 R e l a t i v e r e s i d u a l f l u x (Obs-Mod)/Obs Fig. 7.—
Observed-frame optical to far-IR SED of WMH 5 (bluecircles), best-fit
CIGALE model and SEDs of nearby KINGFISHsurvey galaxies (see Figure 3 for details). WMH 5 has a SED mostsimilar to nearby dwarf irregulars. THE [C ii ] – FAR-IR LUMINOSITY RELATION For both [C ii ] and FIR luminosities to act as reli-able star formation rate indicators requires that the ra-tio of the two behave in a predictable manner withouttoo high a scatter. At low-redshift most galaxies haveratios in the range 10 − to 10 − with a fairly linear re-lationship between the two luminosities (De Looze et al.2014; Sargsyan et al. 2014). The exception to this is inthe ULIRG regime where a deficit of [C ii ] luminosity isusually observed, thought to be related to extreme densi-ties and temperatures in circumnuclear starburst regions(Farrah et al. 2013; Magdis et al. 2014; Gonz´alez-Alfonsoet al. 2015). Figure 8 plots the low redshift data withsmall black circles.At higher redshift ( z > ii ] measure-ments were made in very high L FIR sources and showedlow [C ii ]/FIR ratios comparable with nearby ULIRGs(Maiolino et al. 2005; Iono et al. 2006). Subsequent workat redshifts between 1 and 7 revealed a wide range ofratios from 10 − to 10 − . At the low end there are high-luminosity z > . L FIR > L (cid:12) )(Riechers et al. 2013, 2014). At the opposite end of therange, high [C ii ]/FIR ratios up to 10 − have been foundin some massive 1 > z > main-sequence of star-forming galaxies (Stacey et al. 2010;Brisbin et al. 2015). In these galaxies, the star-formationis more spatially extended than in nearby ULIRGs lead-ing to higher [C ii ] luminosities.In Figure 8 we also include recent ALMA observa-tions of L FIR < L (cid:12) objects comprising z > z > z > L FIR < L (cid:12) galaxies without AGN. For z > ii ]/FIR ratios display a broad range at some-what higher values than at low-redshift. Ratios of ∼ − suggest extended star-formation with low metallicity andan intense radiation field.The similarity of high-redshift galaxies hosting quasars L FIR ( L fl ) -4 -3 -2 L [ C II ] / L F I R z < . Galaxies and AGN .
Fig. 8.—
Ratio of [C ii ] to far-IR (42.5 to 122.5 µ m) luminosityversus far-IR luminosity. The two z > ii ] transition at z > z = 7 .
107 (Maiolino et al. 2015)and HFLS 3 at z = 6 .
337 (Riechers et al. 2013). Galaxies at5 < z < L FIR . AGN-hosting galaxies at z > . . < z < z < . z > z > L FIR , somewhat higher than at z < . and without quasars gives us confidence that the starformation properties are only weakly affected by blackhole accretion onto moderate mass black holes of M BH ∼ M (cid:12) , compared to the very low [C ii ]/FIR valuesfound in the more luminous (in both accretion and FIR) z > M BH ∼ M (cid:12) quasars of Wang et al. (2013).We note that the L FIR < L (cid:12) z > f . /I [CII] similar to CLM 1 and WMH5, their lower ratios in this figure being due to the factorof 4 higher L FIR calculated using a dust temperature of T d = 47 K , compared to T d = 30 K for the galaxies. Ly α EMISSION LINE VELOCITY SHIFTS ANDINTERPRETATION OF RAPID EVOLUTION IN Z ≈ α GALAXIES
Atomic or molecular gas in star forming regions spreadthroughout galaxies is ideal for measuring the systemicredshift of the galaxy. At high redshift such measure-ments have been used to determine the ionized bubblesizes surrounding luminous quasars (Carilli et al. 2010).In this Section we use this information to determine thevelocity shifts of the observed Ly α emission from CLM 1and WMH 5. Ly α at z > ii ] and Ly α spectraon the same velocity scale after correcting the datasetsto the local standard of rest. For both galaxies the Ly α line is asymmetric with a broader red wing than blue andthe line center is shifted to the red from the systemic Willott et al.redshift defined by the [C ii ] observations. This highlyasymmetric line shape is characteristic of Ly α at high-redshift (Shimasaku et al. 2006) due to neutral hydrogenabsorption of the blue wing. For CLM 1 the measuredoffset is +430 ±
69 km s − and the Ly α flux drops to zerojust before the [C ii ] line center. For WMH 5 there is un-certainty in the systemic redshift due to the two compo-nents. If ‘A’, the component with larger [C ii ] linewidthand therefore likely higher mass, is to be identified as theLy α systemic redshift then the offset is +265 ±
52 km s − .However, the astrometry suggests that ‘B’ is the originof the rest-frame UV and therefore Ly α emission and inthis case the shift is +504 ±
52 km s − . For the remainderof this section we consider both cases as possible.The emergence of Ly α from galaxies is a complicatedprocess that involves resonant scattering off neutral hy-drogen and absorption by dust. The observed Ly α pro-files depend upon factors such as geometry, gas coveringfactor, dust and outflow velocity. Irrespective of the ion-ization state of the IGM, galaxies tend to show Ly α pro-files offset to the red due to resonant scattering and selec-tive absorption within galactic-scale outflows (see reviewin Dijkstra 2014). At z > α decreases the overall Ly α flux and shiftsthe line center further to the red (Laursen et al. 2011).The very sharp decrease in Ly α line strength of LBGsbetween z = 6 and z = 7 has been interpreted as evi-dence for a rapid change in IGM neutral fraction at thisepoch, in tension with reionization models that predict asmoother change in neutral fraction over this short cos-mic time (Pentericci et al. 2011; Schenker et al. 2012;Treu et al. 2013).The first measurement of Ly α velocity shifts at red-shifts relevant to the reionization epoch have recentlybeen made by Stark et al. (2014a). These authors usedthe UV nebular C iii ] λ α emitters. The gravitationally-lensed z = 6 .
027 galaxy A383-5.2 has a high Ly α equiv-alent width with strong asymmetry. Stark et al. (2014a)quote the Ly α peak velocity offset as 120 km s − . Forcomparison with our velocity offsets and previous mea-surements at lower redshift we determine from their Fig-ure 5 an approximate centroid-based velocity offset forthis galaxy of 150 ±
30 km s − . The z = 7 .
213 galaxy GN-108036 has a more moderate Ly α equivalent width and amuch lower significance detection of C iii ] was obtainedby Stark et al. (2014a) with a very low and uncertainvelocity offset of − ±
113 km s − .Stark et al. (2014a) noted that these offsets are smallcompared to those of LBGs at lower redshift and thismay be due to different physical conditions at high red-shift. In particular there is known to be a negative cor-relation between velocity shift and Ly α equivalent width(Shibuya et al. 2014; Erb et al. 2014) and Ly α strengthsincrease with redshift up to z = 6 (Stark et al. 2011).Since Ly α more easily escapes from galaxies via the wingsthan the line center, a decrease in velocity shift at high-redshift could be partially responsible for the sudden de-crease in Ly α emitters at this epoch. Choudhury et al.(2014) showed that a negative evolution of velocity shiftwith redshift can match the observed Ly α equivalent
200 0 200 400 600 800 1000 ∆ v Ly α (km/s)10 E W L y α () BA WMH 5CLM 1A383-5.2GN-108036
Fig. 9.—
Rest-frame equivalent width of the Ly α emission lineversus the velocity offset between Ly α and the systemic redshift.The two z > ii ] components, ‘A’ and‘B’, provide two estimates of the velocity offset connected by adotted line. Magenta circles show the only other two galaxies atsuch high redshifts with measured Ly α velocity offsets (Stark et al.2014a) based on the offset to the C iii ] λ α emission is given by‘B’, then the two z > width distribution evolution to smooth reionization his-tory models (see also Bolton & Haehnelt 2013).In Figure 9 we compare the Ly α velocity shifts mea-sured for CLM 1 and WMH 5 to those of A383-5.2 andGN-108036. We choose to plot the shifts as a functionof observed Ly α rest-frame equivalent width because ofthe correlation between velocity shift and Ly α equiv-alent width. In addition to these z > < z < .
1) as LAEs and LBGs (Erb et al. 2014, Stromet al. in prep.). For the lower redshift sample, spectro-scopic equivalent widths are plotted when available, oth-erwise photometric equivalent widths are used. Severalthings are evident from looking at Figure 9. The shiftsfor CLM 1 and WMH 5 are larger than for A383-5.2and GN-108036. None of the high-redshift galaxies haveshifts at odds with those observed at 2 < z < . α equivalent width.If we adopt ‘B’ as the systemic redshift for WMH 5,then both galaxies in this paper have Ly α velocity shiftsat the high end of the distribution, consistent with ad-ditional IGM absorption at higher redshift as predictedby the models of Laursen et al. (2011). Our analysisshows no observational evidence for velocity shifts thatdecrease as a function of redshift, at a given Ly α equiva-lent width. Obviously a much larger sample of observedhigh-redshift galaxies compared to realistic simulationsincorporating radiative transfer would be required to usethe Ly α velocity shift distribution to constrain the de-tails of reionization. CONCLUSIONS tar formation and the interstellar medium in z > ii ] line and dust contin-uum detections of two UV-luminous LBGs at redshift z >
6. These detections were made in relatively shortintegrations in
Early Science operations providing hopethat other galaxies in the reionization epoch with lowerSFR will be detectable with the full ALMA array. Ourresults, in accord with other recent ALMA observations(Capak et al. 2015), confirm that, despite unexpectedlylow L FIR , the increase in the [C ii ]/FIR ratio at high-redshift leads to [C ii ] lines that are bright and very use-ful tracers of the ISM, as predicted by Walter & Carilli(2008). The resolution of the merger of WMH 5 witha fairly compact ALMA array illustrates the power of[C ii ] line observations to understand the details of starformation and galaxy assembly at this epoch.CLM 1 and WMH 5 were selected for study becausetheir UV luminosity, stellar mass and redshift are compa-rable to the galaxy Himiko which was unexpectedly unde-tected by ALMA (Ouchi et al. 2013; Ota et al. 2014). Themain difference between our target galaxies and
Himiko (and most other high- z non-detections) are our relativelylow Ly α equivalent widths. Since the Ly α line strengthdecreases with increasing dust (and hence metallicity),this could explain the difference between these results.We note that the recent success with ALMA at 5 < z < α (Capak et al. 2015).However, the UV slopes of our galaxies are fairly blue,indicating little dust absorption of the UV photons andwe do observe a deficit of FIR photons compared to ex-pectations, as shown by the SEDs that are similar tonearby dwarf irregulars, not with nearby galaxies withsimilar SFRs of tens of solar masses per year. Futureobservations at shorter wavelength are critical to con-strain the full IR SED of high- z galaxies and constrainthe dust temperature. Are they very cool like low metal- licity dwarfs, despite the higher UV photon density andCMB temperature? Determining this is important as thefull IR SED is required to derive more accurate values ofSFR FIR to determine whether this is a more or less ac-curate SFR indicator at high-z than SFR [CII] or SFR UV .We have for the first time used millimeter lines to de-termine Ly α line velocity offsets from systemic for galax-ies in the reionization era. We find that the shifts in ourtwo galaxies are comparable to the high end of the dis-tribution at lower redshift, consistent with predictionsfrom Ly α line asymmetry due to neutral hydrogen ab-sorption (Laursen et al. 2011). More detailed studies inthe future of spatially-resolved kinematics of [C ii ] andLy α in such galaxies will allow stronger constraints to beplaced on the escape of Ly α emission from galaxies andits subsequent absorption by the IGM.Thanks to staff at the North America ALMA Re-gional Center for processing the ALMA data, Jean-Gabriel Cuby for providing the optical spectrum ofCLM 1, Dawn Erb for providing unpublished data onintermediate redshift Lyman Break Galaxies, and Pe-ter Capak and Caitlin Casey for interesting discussions.This paper makes use of the following ALMA data:ADS/JAO.ALMA Facility:
ALMA.
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