a r X i v :0803.0679v 1 [a s t r o -p h ] 5 M a r 2008
Mon.Not.R.Astron.Soc.000,1–10(2008)Printed 5March 2008
(MN L A T E
X style ?le v2.2)Medium-resolution spectroscopy of FORJ0332-3557:Probing the
interstellar medium and stellar populations of a lensed Lyman-break galaxy at z=3.77?
R′e mi A.Cabanac 1,David Valls-Gabaud 2and Chris Lidman 3?
1LATT,
Universit′e de Toulouse,CNRS,57Avenue d’Azereix,65000Tarbes,France
2GEPI,
CNRS UMR 8111,Observatoire de Paris,5Place Jules Janssen,92195Meudon Cedex,France
3ESO,Vitacura,Alonso de Cordova,3107,Casilla 19001,Santiago,Chile
Accepted ...Received ...;in original form ...
ABSTRACT
We recently reported the discovery of FORJ0332-3557,a lensed Lyman-break galaxy at z =3.77in a remarkable example of strong galaxy-galaxy gravitational lensing.We present here a medium-resolution rest-frame UV spectrum of the source,which appears to be similar to the well-known Lyman-break galaxy MS1512-cB58at z =2.73.The spectral energy distribution is consistent with a stellar population of less than 30Ma,with an extinction of A V =0.5
mag and an extinction-corrected star formation rate SFR UV of 200–300h ?170M ⊙a
?1
.The Lyman-αline exhibits a damped pro?le in absorption produced by a column density of about N HI =(2.5±1.0)×1021cm ?2,superimposed on an emission line shifted both spatially (0.′′5with respect to the UV continuum source)and in velocity space (+830km s ?1with respect to the low-ionisation absorption lines from its interstellar medium),a clear signature of out?ows with an expansion velocity of about 270km s ?1.A strong emission line from He II λ164.04nm indicates the presence of Wolf-Rayet stars and reinforces the interpretation of a very young starburst.The metallic lines indicate sub-solar abundances of elements Si,Al,and C in the ionised gas phase.
Key words:galaxies:high-redshift –galaxies:starburst –galaxies:ISM –galaxies:abun-dances –galaxies:evolution –gravitational lensing –galaxies:stellar content
1INTRODUCTION
Very few high-redshift galaxies (z >3)have been observed spec-troscopically at medium and high resolution until now because they are generally too faint for 8/10-m class telescopes equipped with medium resolution spectrographs.Over the past years the most successful technique to probe these high-redshift galaxies con-sisted in building composite spectra of low signal-to-noise sam-ples (Shapley et al.2001,2003;Noll et al.2004)and redshift-selected sub-samples (Steidel et al.2001;Ando et al.2004).Very recently,high-redshift gamma-ray bursts have also led to a wealth of low-resolution spectral data on the interstellar medium of young star-forming regions in high-redshift galaxies (Berger et al.2006;Fynbo et al.2006;Vreeswijk et al.2004,2006).From these com-posite samples a picture emerges of very young and massive stellar populations of 1/10th-to 1/4th-solar metallicities,dominated by
?
Based on observations made at the ESO VLT under programs 74.A-0536and 78.A-0240?E-mail:remi.cabanac@ast.obs-mip.fr (RAC);david.valls-gabaud@obspm.fr (DVG);clidman@https://www.doczj.com/doc/db9528548.html, (CL)
on-going star formation,with strong out?ows,and a dusty compo-nent within abundant neutral gas.
As of today,only a handful of galaxies are bright enough to be studied spectroscopically at medium and high resolution,and among them the most well-known example is MS 1512-cB58(cB58)(Pettini et al.2000,2002;Savaglio et al.2002,z =2.73;look-back time=11.4Ga).A more recent example is the 6L ?z =5.5starburst (Dow-Hygelund et al.2005)discovered in the ?eld of a z =1.24galaxy cluster.The lensed galaxy FORJ0332-3557(Cabanac et al.2005,z =3.773,look-back time=12.1Ga),which is the object of the present study,is the most recent addition to this select few 1.Because the galaxy is magni?ed 13times by a fore-ground lens at z ~1,it is bright enough to be observed at medium to high spectral resolution.We present here a 9h-deep medium-resolution spectrum of the lensed galaxy taken with VLT/FORS2.The observations and data reduction are presented in Section 2while Section 3outlines the properties of the emission and absorp-
1
The advent of systematic surveys on wide ?elds is yielding many more
potential candidates,see e.g.Bolton et al.(2006);Cabanac et al.(2007).
2R.A.Cabanac,D.Valls-Gabaud and C.Lidman
tion lines and in Section4we analyse the spectral energy distribu-tion and the underlying stellar populations.
Throughout this paper we assume a?at FRW metric with
?Λ=0.7,?M=0.3and a Hubble constant normalised at
h70=70km s?1Mpc?1.Following the recommendations adopted by the IAU,the letter a is used for the non-SI unit of year.
2OBSERV ATIONS AND DATA REDUCTION
The observations were carried out with FORS2on the European
Southern Observatory VLT,under ESO programs74.A-0536and 78.A-0240.A total exposure time of8.71hours,split into6×1495
s(1′′slit)and16×1400s(0.′′8slit),was obtained(see Table1). We used the holographic grism600RI(0.163nm pixel?1)together with the GG435order sorting?lter,which results in resolving powers of1200and1000for the0.′′8the1′′slits,respectively. Since the0.′′8the1′′slits are slightly shifted with respect to each other in the focal plane,the resulting spectral coverage,529.7-862.8nm for the1′′slit and501.1-832.5nm for the0.′′8slit,differ slightly.The detector was the upgraded MIT2×4096×2048mo-saic,which with the standard resolution collimator yields a physical
scale of0.′′126pixel?1.We oriented the slits North-South and cen-tered them on the brightest part of the main arc(see Figure1).We used the standard2×2binned mode,which results in a?nal phys-ical scale of0.′′252pixel?1and a sampling of4and3.2pixels for the1′′and0.′′8slits,respectively.We placed the source on CCD1 (the upper northern chip),and used a20′′nod-on-slit strategy that allowed us to remove most sky artefacts.
The science data were taken during seven nights(2nights for 1′′slit,5nights for0.′′8).The ESO standard calibration scheme de-livers a set of bias,?at?eld and wavelength calibration(from an He-Ar arc)frames for each night.We,then,subtracted master bi-ases and divided by the2-D normalised master?at?elds.We eval-uated the fringing contamination to be ca.1%peak-to-valley in the most sensitive part of the frames.
We performed two independent reductions on the1′′slit dataset.Firstly,we reduced separately each science exposure and recombined the resulting one-dimensional spectra after all calibra-tions.Secondly,we combined all science exposures using the FITS header World Coordinate info(WCS keywords)and removed the instrumental and sky artefacts on the combined2-D image before extracting and calibrating the source spectrum.
In the?rst method we used standard NOAO/IRAF routines in order to extract the trace with a4th-degree Legendre polynomial
in the dispersion direction,weighted by the spectrum variance per-pendicular to the dispersion direction.We removed the sky emis-sion from1-D modelling of the sky emission as measured on both sides of the trace.We then calibrated each extracted trace with the wavelength calibration frames(1st order cubic spline)respecting the source spectrum slit position.FORS2?exures are very small and the?nal rms of±0.02nm derived for the wavelength frame was assumed to apply to the science exposures as well.All wave-lengths were shifted to the heliocentric reference frame.Finally the combination of the resulting61-D spectra(1′′slit data;Table1) was done using a median algorithm rejecting the minimum value and two maxima.
In the second method the sky substraction was done using an optimal?tting while combining the2-D spectra.The wavelength calibration frames proved to be stable between the two nights(1′′slit data;Table1).Figure2shows the resulting2-D frame after sky substraction.
The ISM and stellar populations of a LBG at z=3.83
Figure2.Continuum-normalised rest-frame ultraviolet spectrum of FORJ0332-3557(black solid line),superimposed on its error spectrum(blue solid line, offset for clarity)and the spectrum of the z=2.73lensed galaxy cB58(red solid line).The2-D spectrum of the source,part of a compact lensing con?guration, is also shown on the top.
4R.A.Cabanac,D.Valls-Gabaud and C.Lidman Table1.Description of spectroscopic data with VLT/FORS2.
2004-11-1601:02:191495 1.4460.56 1.0
2004-11-1601:28:011495 1.3260.55 1.0
2004-11-1601:54:291495 1.2310.65 1.0
2004-11-1602:20:091495 1.1610.76 1.0
2005-01-3001:20:451495 1.1030.50 1.0
2005-01-3001:46:261495 1.1540.54 1.0
2006-10-1606:02:291400 1.0280.660.8
2006-10-1606:26:571400 1.0200.530.8
2006-10-1606:51:331400 1.0210.610.8
2006-10-1607:15:521400 1.0310.500.8
2006-10-1607:55:521400 1.0680.730.8
2006-10-1608:20:311400 1.1040.610.8
2006-10-2107:37:591400 1.0700.480.8
2006-10-2108:02:091400 1.1060.610.8
2006-11-2107:03:261400 1.2660.850.8
2006-11-2204:33:111400 1.0230.850.8
2006-11-2204:57:201400 1.0360.890.8
2006-11-2804:24:071400 1.0300.540.8
2006-11-2804:48:361400 1.0480.630.8
2006-11-2805:20:591400 1.0870.500.8
2006-11-2805:45:141400 1.1300.570.8
2007-01-2504:30:281400 1.9160.670.8
The ISM and stellar populations of a LBG at z=3.8
5
Figure 4.The best-?t V oigt pro?le corresponding to a column density of (2.5±1)×1021cm ?2(solid and red dotted lines)is shown on the rest-frame spectrum (top frame).The bottom frames show the same ?t in ve-locity space ?v .On the panel at the bottom right the model is subtracted out of the spectrum,showing a conspicuous emission feature peaking at ~830km/s.This feature is also offset spatially by 2pixels (ca.0.′′5)
from
the
main
UV-continuum
emitting
source
(see
Fig.
3).
3.3
The
Helium
II
λ164.04nm
emission line:the
signature
of
very
massive stars A broad emission feature at 164.04nm is seen in Fig.2at a red-shift of z =3.7738consistent with other faint photospheric fea-tures.We associate this feature with the well-known He II emis-sion produced by Wolf-Rayet stars.Although not present in cB58,this feature has already been detected in absorption in young LBGs (Shapley et al.2001,;cf.Fig.15and 16),and in emission in the composite LGB spectrum of Shapley et al.(2003)but the origin of the feature cannot be asserted by the authors because the compos-ite spectrum mixes a wide range of stellar populations and ages.If FORJ0332-3557is similar to other LBGs,it belongs to the strongly absorbed Ly αquartile,and the case for a very young stellar popula-tion (~5Ma)of massive stars is strong.The He II feature of Fig.2shows the typical asymmetric pro?le produced by strong stellar out?ows of the evolved descendants of O stars more massive than M >20-30M ⊙.The equivalent width W 0(He II )yields the ratio of WR to O stars following Schaerer &Vacca (1998),log
WR
λ2f
,(2)
where f is the line oscillator strength.The equivalent width,W λ,and the wavelength,λ,are in nm.A tentative curve of growth indicates that the ion abundances could be 2-3dex larger for a Doppler parameter of b =50km s ?1.In this context,the most constraining line,besides Si II ?λ153.3which may be blended,is Fe II λ160.8,which appears unsaturated and whose small equiva-lent width is similar to the one measured in cB58and would yield b ~60km s ?1,similar to the b ~70km s ?1reported in cB58(Pettini et al.2002).
Compared to cB58,FORJ0332-3557W 0are lower by factors of 2-3(C IV λλ155.08155.08,Al II λ167.08,O I λ130.22)to a fac-tor of 1-1.2(Si IV λλ139.38140.28,Fe II λ160.84).More detailed analyses on elemental abundances and depletion in the interstellar medium of FORJ0332-3557will be presented elsewhere.
6R.A.Cabanac,D.Valls-Gabaud and C.Lidman
Table2.Interstellar absorption lines
0.0290--
H I Lyγ97.254--1.50±0.30+0.20
?0.10
0.07912--
H I Lyβ102.57--2.10±0.80+0.80
?0.60
H I Lyα1215.7---0.416421.4±0.2-
---WR feature
He II164.04783.09 3.7738?0.22±0.01+0.02
?0.02
C II133.45637.01 3.77320.21±0.03+0.03
0.127815.019±0.10>?2.8blended with C II*λ133.57
?0.02
0.190814.40±0.21>?3.6
C IV154.82738.74 3.77160.10±0.03+0.06
?0.03
0.0952214.97±0.25>?3.0sky contamination
C IV155.08740.00 3.77180.19±0.05+0.02
?0.02
N I?~120572.70 3.77180.36±0.18+0.08
0.0402315.84±0.25?1.6?triplet N Iλ119.95120.02120.07
?0.05
N III?132.43632.24 3.7741----blended with C IIλ132.39
0.0488715.30±0.17>?3.0blended with Si IIλ130.44
O I130.22621.31 3.77140.15±0.03+0.05
?0.03
---photospheric
O IV134.34641.27 3.77350.14±0.01+0.05
?0.03
1.83313.62±0.22>?
2.5sky contamination
Al II167.08797.41 3.77260.19±0.02+0.07
?0.03
---
S I/i?138.16659.29 3.77190.11±0.01+0.03
?0.02
S II/i?125.38598.57 3.77400.12±0.02+0.04
0.0108815.90±0.15?1.1?
?0.05
0.01624--blended with Si IIλ126.04
S II/i?125.95601.31 3.77420.19±0.04+0.03
?0.02
0.00545--photospheric
S V?150.18716.39 3.77410.3±0.03+0.04
?0.2
S II/i?151.12721.43 3.77400.07±0.01+0.02
---blended with Si IIλ151.21
?0.02
1.00714.13±0.11>?
2.8blended with S IIλ125.95
Si II126.04601.84 3.77490.19±0.04+0.03
?0.02
0.09415.05±0.37?1.9?blended with O Iλ130.22
Si II130.47622.49 3.77100.16±0.01+0.06
?0.03
0.13014.96±0.11>?2.0
Si II152.67728.39 3.77100.24±0.03+0.02
?0.02
0.13214.20±0.10?2.8?
Si II*?153.34731.40 3.76970.04±0.01+0.01
?0.01
0.514014.33±0.10>?2.6
Si IV139.38665.09 3.77180.19±0.03+0.03
?0.02
0.255314.66±0.12>?2.3blended with S Iλ140.15?
Si IV140.28669.38 3.77180.21±0.03+0.04
?0.02
0.05814.95±0.11?2.0
Fe II160.84767.68 3.77280.12±0.02+0.01
?0.02
The ISM and stellar populations of a LBG at z=3.87
argues for interstellar origin for the highly ionised lines.Finally, the single burst scenario seems to produce slightly deeper absorp-tion lines,whereas the constant SFR scenario better?ts the P-Cygni pro?les of C IVλλ155.08155.08nm and the red side of Lyα.We also tested different stellar wind models without any signi?cant dif-ferences.
4.2Spectra predicted by SED@