C: - 0004-637X_778_2

The Astrophysical Journal

, 778:102 (12pp), 2013 December 1

doi:

10.1088/0004-637X/778/2/102

AN INTENSELY STAR-FORMING GALAXY AT

∼

7 WITH LOW DUST AND METAL

CONTENT REVEALED BY DEEP ALMA AND

HST

OBSERVATIONS

Masami Ouchi

, Richard Ellis

, Yoshiaki Ono

, Kouichiro Nakanishi

, Kotaro Kohno

Rieko Momose

, Yasutaka Kurono

,M.L.N.Ashby

, Kazuhiro Shimasaku

, S. P. Willner

G. G. Fazio

, Yoichi Tamura

, and Daisuke Iono

Institute for Cosmic Ray Research, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8582, Japan;

ouchims@icrr.u-tokyo.ac.jp

Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8583, Japan

Department of Astrophysics, California Institute of Technology, MS 249-17, Pasadena, CA 91125, USA

The Graduate University for Advanced Studies (SOKENDAI), 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan

Joint ALMA Observatory, Alonso de Cordova 3107, Vitacura, Santiago 763-0355, Chile

Institute of Astronomy, University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan

Research Center for the Early Universe (WPI), University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan

Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA

Department of Astronomy, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan

Received 2013 June 15; accepted 2013 August 23; published 2013 November 11

ABSTRACT

We report deep ALMA observations complemented by associated

Hubble Space Telescope

(

HST

) imaging for a

luminous (

25) galaxy, “Himiko,” at a redshift of

6.595. The galaxy is remarkable for its high star

formation rate, 100

−

, which has been securely estimated from our deep

HST

and

Spitzer

photometry, and

the absence of any evidence for strong active galactic nucleus activity or gravitational lensing magnification. Our

ALMA observations probe an order of magnitude deeper than previous IRAM observations, yet fail to detect a

1.2 mm dust continuum, indicating a flux of

Jy, which is comparable to or weaker than that of local dwarf

irregulars with much lower star formation rates. We likewise provide a strong upper limit for the flux of [C

]

158

]

, which is a diagnostic of the hot interstellar gas that is often described as a valuable

probe for early galaxies. In fact, our observations indicate that Himiko lies off the local

]

–star formation rate

scaling relation by a factor of more than 30. Both aspects of our ALMA observations suggest that Himiko is a

unique object with a very low dust content and perhaps nearly primordial interstellar gas. Our

HST

images provide

unique insight into the morphology of this remarkable source, highlighting an extremely blue core of activity and

two less extreme associated clumps. Himiko is undergoing a triple major merger event whose extensive ionized

nebula of Ly

emitting gas, discovered in our earlier work with Subaru, is powered by star formation and the dense

circumgalactic gas. We are likely witnessing an early massive galaxy during a key period of its mass assembly

close to the end of the reionization era.

Key words:

cosmology: observations – galaxies: formation – galaxies: high-redshift

1. INTRODUCTION

Much progress has been achieved in recent years in charting

the abundance and integrated properties of the earliest galaxies

beyond a redshift of

6 selected via optical and near-infrared

(NIR) photometry (e.g., Bouwens et al.

2010a

; McLure et al.

2010

2012

; Castellano et al.

2010

; Ouchi et al.

2010

; Ellis et al.

2013

; Schenker et al.

2012

). The emerging picture indicates

that the redshift period 6

10 was a formative one in the

assembly history of normal galaxies. Sources at

7–8 show

moderately blue ultraviolet continua, which may be consistent

with young, metal-poor stellar populations with a star-formation

rate(SFR)of1–10

−

(e.g., Bouwens et al.

2010b

;

Finkelstein et al.

2010

; Schaerer & de Barros

2010

; Dunlop

et al.

2012

). Their small physical sizes (

7 kpc; Oesch et al.

2010

; Ono et al.

2012a

) and modest stellar masses (10

–10

;

Labb

eetal.

2010

) suggest that they quickly merge into larger,

more luminous systems. The abundance of sub-luminous, small

galaxies at high redshift also indicates that significant merging

occurred at early times, given that the faint-end slope of the UV

luminosity function changes from a steep

−

9at

7–8

(Schenker et al.

2012

; McLure et al.

2012

)to

−

7at

2–3 (e.g., Reddy & Steidel

2009

In practice, it is hard to decipher the physical processes

that govern the early assembly of galaxies from integrated

properties alone. We therefore seek to complement statistical

measurements such as SFRs and stellar masses with detailed

evidence from well-studied individual examples. Likewise, our

understanding of early cosmic history may be incomplete

given that so much is currently deduced from optical and

NIR data alone (Robertson et al.

2013

). Although optical and

NIR-selected sources at high redshift suggest that they contain

little or no dust (Bouwens et al.

2012

; Dunlop et al.

2013

), this

may be a selection bias. Star formation obscured by dust cannot

be quantified without identifying cold dust emission. Further-

more, the gas phase metallicity remains a key measurement

for understanding early systems, most notably in locating the

highly prized pristine “first generation” systems unpolluted by

supernova enrichment. Neither optical nor NIR facilities can

currently address this important question since the diagnostic

metal lines used at lower redshift, such as

[Oii]

λλ

3726, 3729

and

[Oiii]

λλ

5007, 4959, cannot be measured beyond

until the launch of the

James Web Space Telescope

For this reason, state of the art sub-millimeter facilities

such as the Atacama Large Millimeter Array (ALMA) offer

enormous promise. First, they can quantify the possible bias in

our current “optical” view of early galaxy formation by detecting

the hidden cold dust in high redshift galaxies. Second, the

] 158

m features prominent in star-forming regions

in the local universe offer a valuable tracer of metallicity at

The Astrophysical Journal

, 778:102 (12pp), 2013 December 1

Ouchi et al.

early times. Thus far, neither the cold dust continuum nor these

low-ionization tracers of metallicity have been observed beyond

∼

6 (Vieira et al.

2013

; Capak et al.

2011

; Riechers et al.

2010

;

Coppin et al.

2010

). Although a few QSOs have been observed

at sub-millimeter wavelengths to

4–7

1 (Maiolino et al.

2005

; Iono et al.

2006

; Walter et al.

2009

; Venemans et al.

2012

;

Willott et al.

2013

; Wang et al.

2013

), the presence of a powerful

active galactic nucleus (AGN) undoubtedly complicates any

understanding of the physical conditions in their host galaxies.

Detecting these important diagnostic signals of dust and

metallicity from typical

7 galaxies is clearly a major

observational challenge. Only upper limits on [C

] and sub-

millimeter continuum fluxes have been presented so far for the

abundant population of Lyman break galaxies (LBGs) and Ly

emitters (LAEs) at

∼

7. These limits have come from deep

exposures with the sub-millimeter Common-User Bolometer

Array (SCUBA; Holland et al.

1999

) facility on the James

Clerk Maxwell telescope and Plateau de Bure interferometric

observations (e.g., Ouchi et al.

2009b

; Walter et al.

2012

;

Kanekar et al.

2013

). Very recently, one

34 source

was studied in this way following a comprehensive search

for red objects in the

Herschel

HerMES blank field survey at

50–500

m (Riechers et al.

2013

). This source, HFLS3, has very

strong far-infrared (FIR) continuum emission and prominent

molecular

low-ionization lines. Its SFR, inferred from its FIR

luminosity, is extremely high, 2900

−

. Clearly, we need

to understand the context of this remarkable object by observing

other sources at a similar redshift.

The present work is concerned with undertaking such a

study for an extraordinarily luminous star-forming galaxy which

will hopefully complement the study of HFLS3 by Riechers

et al. (

2013

). Ouchi et al. (

2009b

) reported the discovery

of a star-forming galaxy at

595, “Himiko,”

with a

Spitzer

IRAC counterpart. This source was identified from an

extensive 1 deg

optical survey for

6 galaxies in the

UKIDSS

UDS field conducted with the Subaru telescope. The

redshift was subsequently confirmed spectroscopically using

Keck

DEIMOS. The unique features of this remarkable source

are evident when compared to the total sample of 207 galaxies

6 found in the panoramic Subaru survey. Not only

is Himiko by far the most luminous example (

25;

(Ly

)

erg s

−

), but it is spatially extended in

emission, whose largest isophotal area is 5.22 arcsec

corresponding to a linear extent of over 17 kpc. The lower

limit, SFR

−

, is placed on the SFR of Himiko

by the spectral energy distribution (SED) fitting analysis with

the early photometric measurements and the stellar-synthesis

and nebular-emission models (Ouchi et al.

2009b

).Duetothe

large uncertainties of photometric measurements, Ouchi et al.

(

2009b

) cannot constrain

(

−

), and provide only the lower

limit of SFR with

(

−

)

The present paper is concerned with the analysis of uniquely

deep ALMA and

Hubble Space Telescope

(

HST

) observations

of this remarkable source. Given its intense luminosity and high

SFR, we presume that it is being observed at a special time in

its assembly history. We seek to use the cold dust continuum

and [C

] measures from ALMA to understand its dust content

and gas phase metallicity. Likewise, the matched resolution of

HST

will allow us to address its morphologic nature. Fortunately,

one of the

HST

intermediate band filters closely matches the

intense Ly

emission observed for this source with Subaru.

See Ouchi et al. (

2009b

) for the meaning of this name.

Ultimately, we seek to understand the physical source of the

energy that powers the extensive Ly

nebula.

The plan of the paper is as follows. We describe our

ALMA and

HST

observations in Section

, and present the

detailed properties such as dust-continuum and metal-line emis-

sion, morphology, and stellar population in Section

.We

discuss the nature of this object in Section

and summa-

rize our findings in Section

. Throughout this paper, magni-

tudes are in the AB system. We adopt (

,σ

)

8).

2. OBSERVATIONS AND MEASUREMENTS

2.1. ALMA

To understand whether obscured star-formation is an impor-

tant issue and to determine the metallicity of Himiko, a key

source at high redshift, we carried out deep ALMA Band 6

observations in 2012 July 15, 18, 28, and 31 with a 16 12 m

antenna array under the extended configuration of 36–400 m

baseline. The precipitable water vapor ranged from 0.7 to

1.6 mm during the observations. We targeted Himiko’s [C

]

rest-frame 1900.54 GHz (157.74

m), which is redshifted to

250.24 GHz (1.198 mm) at a redshift of

595. Since

we expect a brighter dust continuum at a higher frequency in

the 1.2 mm regime, we extended our upper sideband (USB)

to the high-frequency side. Thus, we targeted the [C

] line

with the lowest spectral window (among four spectral windows)

in the lower sideband (LSB) and set the central frequency of the

four spectral windows to be 250.24 and 252.11 GHz in LSB, and

265.90 and 267.78 GHz in USB with a bandwidth of 1875 MHz.

The two spectral windows and each sideband contiguously cover

the frequency ranges. The total on-source integration time was

3.17 hr. We used 3c454.3 and J0423

−

013 for bandpass cal-

ibrators and J0217+017 for a phase calibrator. The absolute

flux scale was established by observations of Neptune and Cal-

listo. Our data were reduced with the Common Astronomy Soft-

ware Applications package. We rebin our data to a resolution

of 166 MHz (200 km

−

). The FWHM beam size of the final

image is 0

58 with a position angle of 79

◦

5. The 1

noise of the continuum image is

cont

Jy beam

−

over

the total bandwidth of 19

417 GHz, whose 7

5 GHz is sampled.

The 1

noise of the [C

] line image is

line

Jy beam

−

at 250.239 GHz over a channel width of 200 km

−

Further details of the ALMA observations and sensitivities

are summarized in Table

We averaged fluxes over the two spectral windows of

LSB (249.30–253.05 GHz or 1.203–1.185 mm) and USB

(264.96–268.71 GHz or 1.131–1.116 mm) in the range of

frequency free from the [C

] line. Figure

presents the

resulting ALMA continuum data at the 259

01 GHz fre-

quency (or 1.167 mm in wavelength) with a 1

sensitivity of

Jy beam

−

. There is a

∼

flux peak in the beam size

at the position of Himiko. However, there are a series of nega-

tive pixels nearby that correspond to the 2

–3

level per beam.

We conclude therefore that Himiko remains undetected in the

1.2 mm continuum with a 3

upper limit of

Jy beam

−

We note that this sensitivity is two and one order(s) of

magnitudes better than those previously obtained by deep

SCUBA

SHADES and IRAM

PdBI observations (Ouchi et al.

2009b

; Walter et al.

2012

). This clearly indicates that Himiko

has very weak millimeter emission. Table

summarizes the flux

upper limits for the continuum and [C

] line derived from our

ALMA data.

The Astrophysical Journal

, 778:102 (12pp), 2013 December 1

Ouchi et al.

Table 1

ALMA Observations and Sensitivities

cont

line

cont

line

cont

line

FIR

]

(GHz)

(

Jy beam

−

)(

Jy beam

−

)(

Jy)

(

Jy)

(10

)(10

)

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

259.007

250.239

17.4

83.3

250

Notes.

Columns: (1) and (2) central frequencies of continuum and [C

] line observations that correspond to 1

and 1

20 mm, respectively. (3) and (4) 1

sensitivities for continuum and [C

] line in a unit of

Jy beam

−

.The

continuum sensitivity given in the total bandwidth for the continuum measurement is 19

417 GHz or 86

894

which is a sum of four spectral windows (see text). The line sensitivity is defined with a channel width of

200 km

−

. (5) and (6) 3

upper limits of continuum and [C

]lineinaunitof

Jy. (7) and (8) 3

upper limits

of far-infrared continuum luminosity (8–1000

m) and [C

] line luminosity in a unit of 10

and 10

solar

luminosities, respectively. We estimate 3

upper limits of far-infrared continuum luminosities at 40–500

and 42

5–122

mtobe

and

, respectively. These far-infrared luminosities are

estimated with the assumptions of the graybody,

5, and a dust temperature of

40 K.

Figure 1.

ALMA continuum data for Himiko at 259 GHz (1.16 mm). The

gray scale indicates the intensity at each position where darker regions imply

higher intensities. The black contours denote

−

,and

−

levels, while

yellow contours show +1

,+2

,and+3

significance levels where the 1

flux corresponds to 17

Jy beam

−

. The white cross indicates the position of

Himiko. The ellipse in the lower corner denotes the beam size.

Figure

shows the ALMA [C

] velocity channel maps for

Himiko. We searched for a signal of [C

] over 600 km

−

around

the frequency corresponding to

595. In Figure

, there

are noise peaks barely reaching the 3

level in 0 km s

−

and

−

200 km s

−

that are slightly north and south of Himiko’s

optical center position, respectively. However, neither of these

sources is a reliable counterpart of Himiko. Although the

ALMA data reach a 1

noise of 83

Jy beam

−

,no[C

]

line is detected. The corresponding 3

upper limit for [C

]is

250

Jy in a channel width of 200 km s

−

. The associated

luminosity limit is

2.2. HST

The primary goal of the associated

HST

observations of

Himiko is related to the morphological nature of this remark-

able source. We carried out deep

HST

WFC3-IR broad-band

(

125

and

160

)

and medium-band (

098

) observations for

Himiko. The two broad-band filters

125

and

160

measure the

rest-frame UV continuum fluxes and are free from contamina-

tion from Ly

emission, thus maximizing the amount of in-

formation on the stellar content for SED fitting (Section

3.3

The intermediate-band filter of

098

fortuitously includes

the spectroscopically confirmed Ly

line of Himiko at 9233 Å

(Ouchi et al.

2009b

) with a system throughput of 40%, which

is close to the peak throughput of this filter (

∼

45%). Thus,

the

098

image is ideal for mapping the distribution of Ly

emitting gas.

Our observations were conducted in 2010 September 9,

12, 15–16, 18, and 26 with an orientation of 275

◦

. Some

observations were partially lost because

HST

went into “safe

mode” on 2010 September 9, 22:30 during the execution of

one visit. The total integration times for usable imaging data

are 15670.5, 13245.5, 18064.6 s for

098

125

, and

160

respectively. The various WFC3 images were reduced with

the WFC3 and MULTIDRIZZLE packages on PyRAF. To

optimize our analyses, in the multidrizzle processing we chose

a final

pixfrac

5 and pixel scale of 0

05132. We degraded

images of

098

and

125

to match the point spread functions

(PSFs) of these images with that of H

160

, which has the largest

size among the

HST

images. We ensured that the final WFC3

images have a matched PSF size of 0

19 FWHM.

Figure

presents a color composite

HST

UV-continuum

image of Himiko as well as a large ionized Ly

cloud identified

by the Subaru observations (Ouchi et al.

2009b

). This image

reveals that the system comprises three bright clumps of starlight

surrounded by a vast Ly

nebula

17 kpc across. We denote

the three clumps as A, B, and C. Figure

shows the

HST

Subaru, and

Spitzer

images separately. The

098

image in

Figure

detects only marginal extended Ly

emission, because

of the shallower surface brightness limit of the 2.4 m

HST

compared to the 8 m Subaru telescope. Nevertheless, we have

found a possible bright extended component at position D in

Figure

. We perform 0

4 diameter aperture photometry for

clumps A–C and location D as well as 2

diameter aperture

photometry, which we adopt as the total magnitude of the

system. Tables

and

summarize the photometric properties.

It should be noted that Himiko is not only identified as an

LAE, but also would be regarded as an LBG or “dropout”

galaxy. Using the optical photometry of Ouchi et al. (

2009b

;

seealsoTable

), we find no blue continuum fluxes for filters

125

and

160

are referred to as

125

and

160

, respectively.

The Astrophysical Journal

, 778:102 (12pp), 2013 December 1

Ouchi et al.

Figure 2.

As Figure

,butfor[C

] velocity channel maps of Himiko whose 1

intensity is 83

Jy beam

−

. The six panels present maps of 200 km

−

width at

central velocities of

−

600,

−

400,

−

200, 0, +200, and +400 km s

−

from the top left to the bottom right. 0 km s

−

corresponds to [C

] emission at the redshift

595, i.e., 250.24 GHz (1.198 mm).

Figure 3.

Color composite image of Himiko. Blue and green represent

HST

WFC3 continua of

125

and

160

, respectively. Red indicates Ly

emission

resolved with sub-arcsec-seeing Subaru observations. The Ly

emission image

comprises the Subaru

921 narrowband data with a subtraction of the

continuum estimated from the seeing-matched

HST

WFC3 data. The three

continuum clumps are labeled A, B, and C.

through

up to the relevant detection limits of 28–29 mag.

The very red color of

−

1 meets typical dropout

selection criteria (e.g., Bouwens et al.

2011

). Because the

band photometry includes the Ly

emission line and an Ly

continuum break, we can also estimate the continuum break

color using our

HST

photometry of

125

and

160

and the optical

-band photometry. Assuming the continuum spectrum is flat

Figure 4.

HST

, Subaru, and

Spitzer

images of Himiko; north is up and east

is to the left. Each panel presents 5

images at

098

125

,and

160

bands from

HST

WFC3, 3

mand4

m bands from

Spitzer

SEDS. The

image is a Subaru

921 image continuum subtracted using

125

and

includes intensity contours. The Subaru image has a PSF size of 0

8. The solid

red circles indicate the positions of 0

4 diameter apertures for Clumps A, B,

C, and D photometry in the

HST

images (see Section

2.2

for details), while the

dashed red circles denote 2

diameter apertures used for the defining the total

magnitudes.

(

const.), we obtain a continuum break color

−

125

−

160

0, further supporting Himiko’s classification as

an LBG. Importantly, these classifications also apply to clumps

A–C, ruling out the possibility that some could be foreground

sources.

The Astrophysical Journal

, 778:102 (12pp), 2013 December 1

Ouchi et al.

Table 2

Properties of the Subcomponents

Component

(pix)

125

160

βL

(Ly

)EW

SFR(UV)

SFR(Ly

)

(mag)

(10

erg s

−

)(Å)(

−

)(

−

)

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

(11)

Total

2815.4

2790.4

−

578

−

235

(F098M)

···

(24

08)

···

(30

(61

+28

−

)

···

(28

14)

A(clump)

2803.0

2789.0

−

968

+14

−

B(clump)

2816.5

2790.5

−

C(clump)

2826.3

2791.9

−

+12

−

2821

2801

···

123

Notes.

Columns: (1) Name of the component. (2) and (3) Positions in pixels. (4)–(6) Aperture magnitudes in

125

,and

160

indicates a narrow

intermediate

band magnitude determined with

098

for all lines except in Total(NB921). The upper limit corresponds to a 3

limit. (7) UV-continuum slope. (8) Ly

luminosity.

(9) Rest-frame apparent equivalent width of Ly

emission line in Å. (10) Star-formation rate estimated from the UV magnitude. (11) Star-formation rate estimated

from the UV magnitude.

The

magnitude corresponds to

921. The quantities of Columns 8, 9, and 11 are estimated from

921 photometry.

Table 3

Total Magnitudes and Fluxes of Himiko

Band

Mag

Flux(Total)

921

098

125

160

(24

2 mm)

([C

])

250

Notes.

In units of AB magnitudes, if not specified. The

upper limits are 2

and 3

magnitudes in

BVRi

and

8–24

m bands, respectively.

Measurements obtained in Ouchi et al. (

2009b

). The

continuum magnitudes from

through

are defined

with a 2

diameter aperture photometry that matches

to the photometry of the total magnitudes of NIR

bands.

Total magnitudes from UKIDSS

UDS DR8 data

that are estimated with a 2

diameter aperture pho-

tometry and the aperture correction in the same man-

ner as Ono et al. (

2010b

Three sigma upper limits derived with our ALMA

data.

The UV continuum magnitudes of clumps A–C range from

26.4 to 27.0 mag in

125

and

160

. Each clump has a UV

luminosity corresponding to the characteristic luminosity

∗

of a

∼

7 galaxy,

8 mag (Ouchi et al.

2009a

; Bouwens

et al.

2011

). Moreover, the variation in luminosity across the

components is small; there is no single dominant point source

in this system, confirming earlier deductions that the system

does not contain an active nucleus.

The

098

image shows that Ly

emission is not uniformly

distributed across the three clumps. Clump A shows intense Ly

emission with a rest-frame equivalent width (EW

)of68

+14

−

Å,

placing it in the category of a LAE, whereas clumps B and

C have emission more typical of LBGs with a rest-frame Ly

equivalent width (EW

) less than 20 Å given the measurement

uncertainties.

In summary, the

HST

and Subaru data indicate that Himiko

is a triple

∗

galaxy system comprising one LAE and two

LBGs surrounded by an extensive 17 kpc diffuse Ly

halo.

Importantly, from the above morphological studies, we can

easily eliminate the possibility that Himiko is gravitationally

lensed by a foreground concentration. Ouchi et al. (

2009a

)have

already made a strong case against lensing given that Keck

spectroscopy revealed a velocity gradient of 60 km s

−

across

the system. We can further reject this supposition given that

there are clear asymmetries in the outermost images (one has

strong Ly

emission and the other does not).

2.3. Spitzer

Although

Spitzer

cannot match the resolution of the above

morphological data, we use the very deep

Spitzer

IRAC SEDS

data reaching 26 mag at the 3

level (Ashby et al.

2013

)to

investigate the counterpart of the overall Himiko system at the

m and 4

m bands. To improve the relative astrometric

accuracy, we have realigned the SEDS images to the

HST

images, referring to bright stellar objects commonly detected

in the

Spitzer

and

HST

images. The relative astrometric errors

are estimated to be

1 rms. We obtain total magnitudes for

the

Spitzer

IRAC images from a 3

diameter aperture and

use an aperture correction given in Yan et al. (

2005

). The

total magnitudes are 23

09 mag and 24

19 mag

at the 3

m and 4

m bands, respectively. Because the

Spitzer

IRAC 5

m and 8

m and

Spitzer

MIPS 24

band images are not available in the SEDS data set, we use

the relatively shallow

Spitzer

SpUDS (PI: J. Dunlop, 2007)

photometry measurements presented in Ouchi et al. (

2009b

Table

summarizes these total magnitudes and fluxes.

3. RESULTS

Ouchi et al. (

2009b

) found that Himiko has a high SFR

(

−

) and derived a moderately high stellar mass

5–5

) from the Subaru photometry and shallow

The Astrophysical Journal

, 778:102 (12pp), 2013 December 1

Ouchi et al.

Figure 5.

Optical to far-infrared SED of Himiko in the observed frame. The

filled square shows the upper limit from our deep ALMA Band 6 observations

and the filled circles represent photometry from

HST

WFC3

125

and

160

photometry and

Spitzer

SEDS 3

6and4

m. The filled pentagons indicate the

UKIDSS-UDS DR8

,and

photometry. The cross and plus symbols denote

HST

WFC3

098

and Suprime-Cam

921 photometry that includes Ly

emission and the Gunn–Peterson trough in their bandpasses. The open circles

and arrows are data points and the upper limits taken from Ouchi et al. (

2009b

The open diamond with an arrow shows the upper limit from the IRAM

observations (Walter et al.

2012

). The red, magenta, green, and blue lines

represent the SEDs of local galaxies, Arp220, M82, M51, and NGC 6946 (Silva

et al.

1998

), respectively, redshifted to

595. SEDs of local dwarf irregular

galaxies similarly redshifted are presented with cyan lines (Dale et al.

2007

All local galaxy SEDs are normalized in the rest-frame UV, where Himiko’s

SED is reliably determined.

Spitzer

SpUDS data. Here, we attempt to improve upon these

estimates and for the first time secure information on dust

content and inter-stellar medium (ISM) metallicity.

3.1. Far Infrared SED

We investigate obscured star-formation and dust properties

of Himiko from its FIR SED using the newly available ALMA

1.2 mm continuum data. The SED from the optical to millime-

ter wavelengths is shown in Figure

, together with that of

various local starburst templates. The figure demonstrates that

Himiko’s millimeter flux is significantly weaker than that of

dusty starbursts in the local universe such as Arp220 and M82,

as well as the spiral galaxy NGC 6946; it is more compara-

ble to those of dwarf galaxies of much lower mass. Similarly,

Himiko’s rest-frame optical flux derived from the

Spitzer

IRAC

6 and 4

m photometry is significantly weaker than that of

dusty starbursts and spiral galaxies. Given its intense rest-frame

UV luminosity and moderately high stellar mass, Himiko’s dust

emission and evolved stellar flux are remarkably weak. Both

properties imply a low extinction and a relatively young stellar

age (Section

3.3

). In this sense, Himiko may be similar to many

luminous

∼

3 LBGs whose cold-dust continuum emission is

also comparable to unreddened local starburst galaxies (Ouchi

et al.

1999

We can estimate a FIR luminosity of Himiko from our

1.2 mm continuum limit. Assuming an optically thin graybody

of modified blackbody radiation with a dust emissivity power-

law spectral index of

5 and a dust temperature of

40 K (Eales et al.

1989

; Klaas et al.

1997

), we obtain

upper limit of

FIR

integrated over

8–1000

m. We also estimate 3

upper limits of

and

at 40–500

m and 42

5–122

m, respectively.

Note that these upper limits depend upon the assumed dust

temperature and

.For

25 K and

60 K, the 3

upper limit luminosities in 8–1000

mare

and

, respectively. Similarly, for

0 and

the 3

upper limit luminosities in 8–1000

mare

and

, respectively.

The preceding upper luminosity limits do depend somewhat

on dust temperature and spectral index. Based on the

Herschel

measurements, Lee et al. (

2012

) find that the average dust

temperature is

∼

30 K under

5 for relatively high redshift

(

∼

4) LBGs with a luminosity of

∗

comparable to

Himiko. In the local universe, the median dust temperatures are

33 K, 30 K, and 36 K, for E

S0, Sb-Sbc, and infrared bright

galaxies, respectively (Sauvage & Thuan

1994

; Young et al.

1989

). Recent numerical simulations have claimed that LAEs

may have a relatively high dust temperature, due to the proximity

of dust to star-forming regions. However, even in this case, the

maximum temperature reaches only

40 K (Yajima et al.

2012b

). On the other hand, Himiko’s dust must be heated to

some lower limit by the cosmic microwave background (CMB),

whose blackbody temperature scales as

CMB

(1+

), where

CMB

is the temperature of the present-day CMB,

CMB

73 K.

Assuming local thermal equilibrium between the ISM of Himiko

and the CMB at

595 (da Cunha et al.

2013

) yields a

lower limit of

21 K. Thus, it is appropriate to consider

a range of

20–40 K with

5. Because the larger

assumed dust temperature

40 K with

5 provides a

weaker upper limit, we adopt a conservative 3

upper limit of

FIR

(8–1000

m). Tables

and

present the

luminosity upper limit.

3.2. ISM Metallicity from [C

] Emission

We now turn to estimating the metallicity of the ISM of

Himiko using [C

] emission as a valuable tracer in star-forming

regions. Despite our significant integration, no line is seen.

Figure

(and Table

) presents the upper limit to the [C

]

luminosity in the context of the correlation with the SFR

(de Looze et al.

2011

). In the case of Himiko, the SFR was

obtained by SED fitting of the rest-frame UV to optical data,

including a correction for dust extinction (Section

3.3

). Himiko

clearly departs significantly from the scaling relation; the deficit

amounts to a factor of

times 30. Given the SFRs of de

Looze et al. (

2011

) for local galaxies are derived in a similar

manner to that for Himiko, including contributions from dust-

free and dusty starbursts with

Galaxy Evolution Explorer

’s UV

and

Spitzer

’s infrared fluxes, respectively, it is difficult to believe

that this deficiency is due to some form of bias arising from

comparing different populations.

Graci

a-Carpio et al. (

2011

) and Diaz-Santos et al. (

2013

)

present

]

FIR

ratios for local starbursts that depend on

FIR

and the FIR and mid-IR surface brightnesses. As a result,

Diaz-Santos et al. (

2013

) argue that

]

may not represent a

particularly reliable indicator of SFR. However, FIR and mid-

IR luminosities only trace dusty starbursts and typically exclude

dust-free measures such as the UV luminosity. Because galaxies

with fainter FIR

mid-IR luminosities have a larger ratio of

]

FIR

in the datasets probed by Graci

a-Carpio et al. (

2011

)

and Diaz-Santos et al. (

2013

), more dust-free star-formation is

expected in such systems. In this sense, the analysis of de Looze

et al. (

2011

) is perhaps more relevant as a prediction of what to

expect for Himiko. Nonetheless, given the importance of using

The Astrophysical Journal

, 778:102 (12pp), 2013 December 1

Ouchi et al.

Figure 6.

] luminosity as a function of SFR. The filled square indicates the

upper limit luminosity of Himiko and the open diamond presents estimates

of HFLS3 (Riechers et al.

2013

). The solid line is the local scaling relation

determined with the data shown with bars (de Looze et al.

2011

). Note that the

bars are obtained by re-calculating the SFR values using the data of de Looze

et al. (

2011

), following the formula shown in de Looze et al. (

2011

). The shaded

region indicates the observed scatter.

]

as a possible tracer and the discussion that follows below,

independent studies of

]

as a function of UV luminosity

and

FIR

would be desirable. Figure

also shows that HFLS3 at

3 (Riechers et al.

2013

) follows the local scaling relation.

However, it should be noted that the SFR of HFLS3 is derived

from the FIR luminosity, and thus any contribution from dust-

free star-formation would be missing. In this sense, the SFR

may be a lower limit, in which case HFLS3 may also depart

somewhat from the local relation.

The absence of [C

] emission in Himiko is perhaps the most

surprising result from our ALMA campaign. The emission line

is often assumed to be the most robust far-IR tracer of star

formation in high redshift galaxies, such that it may replace

optical lines such as Ly

in securing spectroscopic redshifts in

the reionization era. Our failure to detect this line in one of the

most spectacular

7 galaxies has significant implications,

which we discuss in Section

3.3. Improved Physical Properties from the Near-infrared SED

Although we derived some constraints on the integrated prop-

erties of Himiko in our earlier work (Ouchi et al.

2009b

we did not obtain an

(

−

) estimate and only the lower

limit of SFR with

(

−

)

0 was obtained, due to the

large uncertainties of photometric measurements. We now re-

fine these estimates based on our significantly deeper

HST

and

Spitzer

data. Our near-IR SED is taken using total magni-

tudes from the

HST

images (Section

2.2

), the

Spitzer

IRAC

SEDS images (Section

2.3

), and

JHK

DR8 data from the

UKIDSS

UDS survey. We tabulate these total magnitudes in

Table

including ground-based optical data previously given in

Ouchi et al. (

2009b

We present the SED of Himiko in Figure

and undertake

the

fitting of a range of stellar synthesis models in the

same manner as Ono et al. (

2010b

), using the stellar synthesis

models of Bruzual & Charlot (

2003

) with the dust attenuation

formulation given by Calzetti et al. (

2000

). We adopt a Salpeter

Figure 7.

Optical to near-infrared SED of Himiko newly obtained by our deep

HST

and

Spitzer

observations, together with photometry from ground-based

observations. The red lines represent the best-fit SEDs of stellar synthesis models

with (left) and without (right) nebular lines (see Ono et al. 2010 for detailed

model descriptions). The filled squares denote

HST

and

Spitzer

SEDS fluxes

of Himiko defined by the total magnitudes. The open squares show

-band

fluxes that are not used for the SED fitting, due to the Ly

line contamination.

The large error bars at

mand

m are those obtained by Subaru and

Spitzer

SpUDS observations given by Ouchi et al. (

2009b

). The red crosses

represent the broadband fluxes expected from the best-fit SED models. For

various assumptions the fits indicate that Himiko has a SFR of 100

−

stellar mass of 2–3

, and a selective extinction of

(

−

)

(see the text for details).

initial mass function (IMF; Salpeter

1955

) with lower and upper

mass cutoffs of 0.1 and 100

, respectively. Applying models

of constant and exponentially decaying star-formation histories

with metallicities ranging from

02–1

, we search

for the best-fit model in a parameter space of

(

−

)

0–1

and age

1–810 Myr (where the latter upper limit corresponds

to the cosmic age at

595). Nebular continuum and line

emission, estimated from the ionizing photons from young stars,

are optionally included following the metallicity-dependent

prescriptions presented in Schaerer & de Barros (

2009

) and

Ono et al. (

2010b

For a constant SFR history with no nebular emission and a

fixed metallicity of

, we find that our best-fit model

has a stellar mass of

∗

−

, a stellar age of

−

yr, a SFR of 98

−

, and an extinction of

(

−

)

15 with a reduced

of 3

1. This is a significant

improvement over our much weaker earlier constraints which

did not have the benefit of the

HST

WFC3 or

Spitzer

SEDS

data(Ouchi et al.

2009b

). The new infrared data play a critical

role in determining the Balmer break, thereby resolving the

degeneracy between extinction and age. On the other hand, the

fit itself is not very satisfactory. The reduced

is large and

there is a significant discrepancy at 3

m. Since the 3

and 4

m bands sample the strong nebular lines of H

[Oiii]

and H

, respectively, at

595, this encourages us to include

nebular emission in our fitting procedure. In fact, in Figure

we note that the IRAC 4

m emission shows a positional

offset with respect to that at 3

m, suggesting the possibility

of contamination by nebular emission.

Adding nebular emission to the stellar SED models given

above, the best fit has a more satisfactory reduced

of 1

6, and

we derive a reduced stellar mass of

∗

−

and

a younger stellar age of 1

−

yr, but similar values for

the SFR of 100

−

and extinction of

(

−

)

15.

Table

summarizes the results of our SED fitting with the

pure stellar and stellar+nebular models. In the stellar+nebular

models, we assume that all ionizing photons lead to nebular

emission lines corresponding to an escape fraction

esc

0. If we allow

esc

to be a free parameter, then following

The Astrophysical Journal

, 778:102 (12pp), 2013 December 1

Ouchi et al.

Table 4

Stellar Population of Himiko

Model

∗

(

−

)

∗

Age

SFR

sSFR

dof

(

)

(mag)

(Myr)

(

−

)(yr

−

)

(1)

(2)

(3)

(4)

(5)

(6)

(7)

Stellar+nebular

−

182

+22

−

100

−

Pure stellar

−

363

+44

−

Notes.

Columns: (1) models with or without nebular emission. (2) Stellar mass. (3) Color excess of dust extinction for

stellar continua. (4) Stellar age. (5) Star-formation rate. (6) Specific star-formation rate. (7) Reduced

. The degree of

freedom (dof) is six.

The uncertainty of color excess is smaller than our model-parameter grid of

(

−

)

01.

Ono et al. (

2012b

) we find no change from the model above

(i.e.,

esc

0) and formally establish that

esc

Labb

eetal.(

2010

) and Finkelstein et al. (

2010

)have

suggested from their pure stellar models that

HST

7–8

dropout galaxies have modest stellar masses (10

–10

) and

are quite young (30–300 Myr), in contrast with Himiko’s stellar

mass (

∗

) and age (360 Myr) estimated

with our pure stellar models. Of course, Himiko is more

massive and energetic than typical LBGs seen in the small

area of the Hubble Ultra Deep Field. Its most notable feature

is its high SFR of

100

−

, which is more than an

order of magnitude larger than those of the

HST

LBGs at

similar redshifts (

1–10

−

; Labb

eetal.

2010

). Himiko’s

selective extinction,

(

−

)

15, is also larger than that

HST

dropouts, more than half of which are consistent with

no extinction (Finkelstein et al.

2010

). On the other hand, the

stellar mass of Himiko is only about 1

10th that of many sub-

millimeter galaxies (SMGs) at

∼

3 (Chapman et al.

2005

We estimate a specific star-formation rate (sSFR), defined by a

ratio of SFR to stellar mass, to be sSFR

−

and sSFR

−

, for the pure stellar

and stellar+nebular cases, respectively. Even though the stellar

masses are very different, Himiko, SMGs, and LBGs at

∼

share comparable sSFRs

∼

−

–10

−

(seeFigure12of

Ono et al.

2010a

3.4. UV Spectral Slopes on the Spatially Resolved Images

The new

HST

data give us the first reliable measurement of the

UV continuum slope for each of the morphological components

identified in Figure

. The UV spectral slope provides a valuable

indicator of the combination of dust extinction, metallicity,

the upper IMF, and stellar age. We estimated the UV slope,

, from the

125

and

160

photometry, which samples the

continua at the rest-frame wavelengths of

∼

1600 and

∼

2100 Å,

neither of which is contaminated by either Ly

emission or the

-continuum break.

We calculate

via

=−

125

−

160

5log

(

/λ

)

−

(1)

where

and

are the central wavelengths of the

125

and

160

filters, respectively. The estimates for each component are

summarized in Table

. We obtain

=−

57 for the

entire system of Himiko, which is comparable to the average

UV slope of

∗

LBGs,

=−

22 (Bouwens et al.

2012

, see also Dunlop et al.

2013

). Figure

shows the UV-to-

FIR luminosity ratio, log(

FIR

1600

), and the UV-continuum

slope,

, for the entire system of Himiko, and compares these

Figure 8.

UV to FIR luminosity ratio, log(

FIR

1600

), as a function of the

UV-continuum slope,

. The filled square presents the upper limit of

log(

FIR

1600

) and the measurement of

for the total luminosities of Himiko.

The solid line denotes the relation for local starbursts given by Meurer et al.

(

1999

estimates with the relation of local starbursts (Meurer et al.

1999

). Figure

indicates that Himiko has log(

FIR

1600

values comparable with or smaller than those of local dust-poor

starbursts. Since the Small Magellanic Cloud (SMC) extinction

has a smaller log(

FIR

1600

) value at a given

(see Figure 10

of Reddy et al.

2010

) due to SMC’s steeper extinction curve in

-1

/λ

than that for local starbursts, it may be more

appropriate for Himiko. Our result also suggests that Himiko

is not associated with additional FIR sources that are invisible

in the rest-frame UV. These implications are consistent with

the conclusions of the UV-FIR luminosity ratio discussed in

Figure

More interestingly, the UV slopes of the individual substruc-

tures provide valuable information on the nature of Himiko.

Clumps B and C have

=−

47 and

=−

28, respectively, which are comparable to the average UV

slope of

∗

LBGs. However, Clump A presents a very blue

UV slope,

=−

32. Because this component is de-

tected at the

∼

level in both

125

and

160

, the UV slope

is quite reliable. Bouwens et al. (

2012

) claim that selection and

photometric biases lead to an error of only

1for

the brightest of their sources with

∼

photometry (see also

Dunlop et al.

2013

). Even including such a possible bias, Clump

A remains significantly bluer than the average

∗

LBGs at the

level.

As presented in Section

2.2

, Clump A also shows Ly

emission. Together with the blue UV slope, this suggests a

The Astrophysical Journal

, 778:102 (12pp), 2013 December 1

Ouchi et al.

Figure 9.

UV-continuum slope,

, as a function of rest-frame Ly

equivalent

width. The filled circle, square, pentagon, and diamond denote measures for the

entire system, and Clumps A, B, and C, respectively. The blue, cyan, green, and

red solid lines represent predictions based on instantaneous starburst models of

PopIII,

−

,and0

with nebular and stellar continua

(Raiter et al.

2010

). The thin lines are the same, but for constant star-formation

models. The associated dotted lines show the effect of ignoring nebular emission.

The arrow indicates the effect of applying an extinction with

(

−

)

(see the text for details).

very young and

or metal poor component. However, the Ly

equivalent width is only EW

+14

−

Å. To understand the

significance of this, in Figure

, we compare

and EW

for the

entire Himiko system and the various clumps with the stellar

and nebular models of Raiter et al. (

2010

), where a Salpeter

IMF is assumed. In Figure

, the arrow size in

for the stellar

extinction of

(

−

)

1 is calculated using a combination

of the empirical relation,

1600

43 + 1

(Meurer et al.

1999

), and Calzetti extinction,

1600

(

−

)

, where

1600

is 10 (Ouchi et al.

2004

). Similarly, the arrow size in EW

for

(

−

)

1 is estimated from the relation given in

Ono et al. (

2010a

) under the assumption of a

flat continuum

and the standard SFR relations of UV and Ly

luminosities in

the case of B recombination. Figure

shows that the data points

of Himiko fall on the tracks of star-formation photoionization

models (Raiter et al.

2010

) within the measurement errors and

the dust-extinction correction uncertainties, and indicates that

the Ly

emission of Himiko can be explained by photoionization

by massive stars.

4. DISCUSSION

We now bring together our key results, both from the earlier

Subaru program (Ouchi et al.

2009b

) and the present

HST

and

ALMA campaigns, in order to understand the significance of

our upper limits on the [C

] and dust emission, and thereby the

nature of Himiko.

4.1. The Low Dust and Metal Content of Himiko

We have shown (Figure

) that Himiko’s sub-millimeter

emission is comparable with or weaker than that of local

dwarf irregulars with far lower SFRs, indicating intensive star-

formation in a dust-poor gaseous environment. In fact, assuming

the local starburst SFR–

(FIR) relation of Kennicutt (

1998

)

with Himiko’s FIR upper limit luminosity of

,we

obtain SFR(FIR)

−

, which is far smaller than not

only our best optical-NIR estimate SFR of

100

−

,but

also the UV-luminosity SFR of SFR(UV)

−

with no dust extinction correction. This is also true under the

assumption of the SFR–

(FIR) relation (Buat & Xu

1996

)

which is valid for local dust poor disk systems of Sb and later

galaxies, which provides SFR(FIR)

−

. In this way,

Himiko does not follow the SFR–

(FIR) relation of typical

local galaxies, indicating a dust-poor gaseous environment. This

seems similar to observations that find extended Ly

emission in

dust poor low-

galaxies (Hayes et al.

2013

) and a high-

QSO

(Willott et al.

2013

). Based on numerical simulations, Dayal

et al. (

2010

) find that

∼

6–7 LAEs are dust poor with a dust-

to-gas mass ratio smaller than the Milky Way by a factor of 20.

Dayal et al. (

2010

) predict a 1.4 mm continuum flux of

for sources with

(Ly

)

2–3

erg s

−

6, a

result comparable to our ALMA observations. Deeper ALMA

observations could further test the model of Dayal et al. (

2010

)

and place important constraints on the dust-to-gas mass ratio.

Similarly, our strong upper limit on the [C

] 158

mline

(Figure

) places it significantly below the scaling relation of

]

and SFR that is obeyed by lower redshift galaxies. This

discovery indicates the following four possibilities: Himiko

has (1) a hard ionizing spectrum from an AGN, (2) a very

high density of photo-dissociation regions (PDRs), (3) a low

metallicity, and (4) a large column density of dust. In case

(1), a hard ionizing spectrum from an AGN can produce

little [C

] luminosity relative to FIR luminosity, due to the

intense ionization field (Stacey et al.

2010

). As we discuss

in (2) of Section

4.2

, there are no AGN signatures; there

are no detections of X-ray and high-ionization lines, as well

as extended sources plus non-AGN-like Ly

profile+surface

brightness. We can rule out possibility (1). In case (2), a

very high density of PDRs provides more rapid collisional de-

excitations for the forbidden line of [C

], and quenches a [C

]

emission line. In case (3), the PDRs in Himiko are composed

of metal poor gas that may be quite typical of normal galaxies

observed at early epochs. De Looze (

2012

) has argued that

offsets from the [C

]–SFR relation can be explained in terms of

metal abundance and this would imply a gas-phase metallicity

. Indeed, for our young mean stellar age of

160–410 Myr, standard ionization-photon bounded

Hii

regions

with a local chemical abundance would yield [C

] emission

somewhat above the local scaling relation, due to the expected

large PDRs. Moreover, the recent numerical simulations predict

that a [C

] flux drops as metallicity decreases (Vallini et al.

2013

). Vallini et al. (

2013

) claim that Himiko’s gas-phase

metallicity is sub-solar on the basis of the comparison of their

models with the previous IRAM [C

] upper limit. Comparing

these numerical models with our strong ALMA upper limit

of [C

] would place further constraints on the metallicity of

Himiko. In case (4), the depth of C

zones in PDRs is determined

by dust extinction. Since the C

zones extend over the dust

extinction up to



4 (Malhotra et al.

2001

), heavy dust

extinction in the ISM does not allow the creation of a large

zones emitting [C

]. However, from the no detection of

a 1.2 mm dust continuum discussed above, the heavy dust

extinction narrowing the PDRs is unlikely. As dust extinction

and gas phase metallicity generally correlate closely (Storchi-

Bergmann et al.

1994

; see also Finlator et al.

2006

), the weak

dust emission also suggests a very low metallicity gas. Thus,

case (3) is probably true, which contributes the weak [C

]

emission. Case (2) could also help to weaken the [C

] emission.

To summarize, faint [C

] and weak dust emission can be

explained in a self-consistent manner with a very low metallicity

gas and little dust in a near-primordial system.