Deep VLT spectroscopy of the z=2.49 Radio Galaxy MRC 2104-242: Evidence for a metallicity gradient in its extended emission line region

5 Pages·0.19 MB·English

by  R. Overzier

A&A manuscript no. ASTRONOMY
(will be inserted by hand later) AND
Your thesaurus codes are: ASTROPHYSICS
missing; you have not inserted them
z .
Deep VLT spectroscopy of the = 2 49 Radio Galaxy
⋆
MRC 2104–242
Evidence for a metallicity gradient in its extended emission line region
R.A. Overzier, H.J.A. Ro¨ttgering, J.D. Kurk, and C. De Breuck
1
0 Sterrewacht Leiden,P.O. Box 9513, 2300 RA,Leiden, The Netherlands([email protected])
0
2 Received / Accepted

n
a
J Abstract. We present spectroscopic observations of the One of the most important questions in studying

0 rest-frame UV line emission around radio galaxy MRC HzRGs concernsthe ionizationofthe halo.Thereis strik-
1 2104–242 at z = 2.49, obtained with FORS1 on VLT ing evidence that the mechanism for ionization is either

Antu. The morphology of the halo is dominated by two photo-ionization by the active nucleus (Villar-Mart´ın et
1
spatiallyresolvedregions.Lyα is extendedby >12′′ along al. 1997) or shock ionization by jets interacting with the
v
2 the radio axis, C IV and He II are extended by ∼8′′. The gaseous medium (Best et al. 2000). Another important
5 overall spectrum is typical for that of high redshift radio question involves the origin of the emission line gas.If its
1 galaxies.ThemoststrikingspatialvariationisthatN Vis origin is external to the radio galaxy, it can be gas as-
1
present in the spectrum of the region associated with the sociated with galaxy merging (Heckman et al. 1986) or
0
center of the galaxy hosting the radio source, the north- the result of a massive cooling flow from the intraclus-
1
0 ernregion,while absentinthe southernregion.Assuming ter medium (Crawford& Fabian1996).Alternatively,the
/ that the gas is photoionized by a hidden quasar, the dif- gas could have been driven out by a starburst-wind or
h
ference in N V emission can be explained by a metallicity by shocks associated with the radio source. Studying the
p
- gradientwithin the halo, with the northernregionhaving propertiesofthe gasindetailmayhelptomakeadistinc-
o a metallicity ofZ ≈ 1.5Z⊙ and Z ≤ 0.4Z⊙ forthe south- tion between these scenarios.
r
t ernregion.This is consistentwith a scenarioinwhichthe In this letter we present spectroscopic observations of
s
a gas is associatedwith a massive coolingflow ororiginates the extended emission line halo around MRC 2104–242.
: from the debris of the merging of two or more galaxies. This radio source is identified with a galaxy at z = 2.49
v
and is one of the brightest known HzRGs in Lyα (Mc-
i
X Key words: cosmology:early Universe – galaxies: active Carthyetal.1990).NarrowbandLyαimagesshowatotal
r –galaxies:evolution–galaxies:individual:MRC2104–242 extent of > 12′′ (i.e. 136 kpc1) distributed in two distinct
a
– galaxies: kinematics and dynamics regions separated by ∼6′′. Spectroscopy shows that both
regions have large FWHM (∼1000−1500 km s−1), large
rest-frame equivalent widths (330 and 560 ˚A) and a ve-
locity difference of ∼500 km s−1 (McCarthy et al. 1996;
Koekemoer et al. 1996; Villar-Mart´ın et al. 1999a). The
1. Introduction
tworegionsalsoemitotherlinesandfaintcontinuum.HST
High redshift (i.e. z & 1) radio galaxies (HzRGs) are be- imaging(WFPC2andNICMOS)hasshownthatthehost
lieved to be the progenitors of massive elliptical galaxies galaxyisveryclumpy,suggestiveofamergingsystem(see
(e.g.Bestetal.1998).Therefore,thesegalaxiesareanim- Pentericciet al.1999foran optical-radiooverlay).One of
portanttoolforstudyingtheepochofgalaxyformationin the bright components hosted by the northern region is
the early universe. HzRGs are often surrounded by giant prominentinnear-infraredemissionandthereforeitisas-
halosofionizedgas,whichradiateluminousemissionlines sumed to be the center of the galaxy hosting the radio
in the UV/optical part of the spectrum. The continuum source.This nucleus and the other components in this re-
and the line emission,which canbe spatially extended by gion are not resolved in our spectra, so we will refer to
as much as 100 kpc, are often elongated along the direc- the whole as the northern region. The southern region is
tion of the radio axis (Chambers et al. 1987; for a review associated with a narrow filamentary component of ∼2′′
see McCarthy 1993). oriented in the direction of the radio axis.
Send offprint requests to: R.A. Overzier 1 We adopt H0 = 50 km s−1 Mpc−1, q0 = 0.1. At z = 2.49
⋆ Based on observations at theESO VLT Antutelescope this implies a linear size scale of 11.3 kpcarcsec−1.
2 R.A.Overzier et al.: Deep VLTspectroscopy of the z=2.49 Radio Galaxy MRC 2104–242
Theoutlineofthisletterisasfollows.In§2,wedescribe width comparable to that of N V in the northern region.
our VLT observations and we present the basic results 2.Fig.3showsthetwo-dimensionalemissionlinestructure
in §3. In §4, we show evidence for a metallicity gradient of Lyα,C IV andHe II. The peak of the C IV emissionin
betweenthetworegionsandin§5wewilldiscusshowthis the northern region is redshifted with respect to that of
relates to the origin of the halo. Lyα by ∼100 km s−1, while that of He II is blueshifted
Throughout this letter we shall abbreviate the emis- by ∼150 km s−1. In the southern region, Lyα shows two
sion lines as follows: N V for N V λ 1240, C IV separate peaks shifted bluewardfrom the northernregion
for C IV λλ 1549, He II for He II λ 1640, Si IV for by ∼1000 and ∼500 km s−1. This two-peak distribution
Si IV λλ 1400 and O III] for O III] λ 1665. isalsoseeninC IV andHe II,albeitatlowS/N.The fact
thatitisobservedinHe IIcouldindicatekinematicalsub-
structure in the halo and that the dip in the Lyα profile
2. VLT observations
is not due to H I absorption.
The observations were carriedout in service mode on UT 3. The northern region shows similar FWHM (∼700 km
1999September 2−5withFORS1 onthe 8.2mVLT Antu s−1) for Lyα, C IV and He II. In the southern, Lyα and
telescope(ESO-Chile).Weusedgrism600Bwitha1′′wide C IV have high FWHM (>1000 km s−1), while that of
slit. A 2×2 readout binning was used in order to increase He II is a factor 1.5 lower.
thesignal-to-noiseratio(S/N).Theresultantspectralres- 4. Within the errors, the emission line ratios of the two
olutionwas∼6˚A(FWHM).Theslitwaspositionedalong regions are the same, only those involving N V are dis-
the brightest components and the filamentary structure crepant. The N V/C IV and N V/He II line ratios are at
at a position angle of 2◦ North through East. The expo- least4and3timeshigherinthenorthernregioncompared
sure time was 3×3600s. The seeing during the observa- to the southern.
tions was ∼1′′ and conditions were photometric. Data re-
ductionfollowedthestandardproceduresusingtheNOAO 4. Difference in N V emission from the two
IRAFlong-slitpackage.Webias-subtractedtheindividual
regions: evidence for a metallicity gradient?
frames and divided them by a normalized dome flat-field
frame. Cosmic rays were removed from the background WehavedetectedN Vinthenorthernregion,whichissel-
subtracted images. We subtracted sky lines and shifted dom present at a significant level in HzRGs (R¨ottgering
the images into registration using stars on the CCD. For etal.1997;De Breucketal.2000).Inthe southernregion
wavelength calibration we used comparison spectra of a wedetectednoN V,whiletheotherline-ratiosaresimilar
He and a HgCd lamp. For flux calibration we observed in both regions. Vernet et al. (1999) found that HzRGs
the spectrophotometric standard star LTT7987. The re- follow a sequence in N V/C IV vs. N V/He II, parallel to
sulting photometric scale is believed to be accurate at the relation defined by the broad line regions (BLR) of
a level of ∼15 %. We corrected the spectra for atmo- quasars found by Hamann & Ferland (1993). These au-
spheric extinction and applied a galactic extinction cor- thors showed that this sequence can be explained by a
rectionofE(B−V)=0.057determinedfromthedustmaps variationofthemetallicityoftheBLR,causedbyarapidly
of Schlegel, Finkbeiner & Davis (1998). evolving starburst in the massive galactic core. Villar-
The extraction apertures centered on the two regions Mart´ın et al. (1999) showed that both shock ionization
were resp. 4′′ and 3′′, chosen to include most of the emis- andphoto-ionizationcouldnotexplaineithertheN Vcor-
sionwhilekeepinghighS/Nfortheweakerlines.Wemea- relation, or the strong N V emission in some HzRGs (e.g.
sured wavelengths, fluxes, FWHM and equivalent widths van Ojik et al. 1994). They found that a model of photo-
(EW) by fitting Gaussianprofilesto the lines (R¨ottgering ionizationandvariationofmetallicitybestexplainedboth
et al. 1997). The measured FWHM was deconvolved for properties. Therefore, we conclude that the difference in
the instrumental profile assuming Gaussian distributions. N V emission can be explained only by a metallicity gra-
dientwithinthe halo.Usingthe metallicitysequencewith
quadratic nitrogen enhancement (N ∝ Z2) from Vernet
3. Results
et al. (2000) we find a metallicity of Z ≈ 1.5 Z⊙ for the
The mainobservationalresults canbe summarizedas fol- northern region and an upper limit of Z ≤ 0.4 Z⊙ for the
lows: southern (Fig. 4).
1. Figs. 1 and 2 show the spectra of the northern and
southern regions.Both regions show Lyα, C IV and He II 5. Discussion
and weak Si IV, O III]. N V is detected in the northern
region,but it is absent in the southern. The emission line The supersolar metallicity of the gas associated with the
properties are listed in Table 1. For the undetected N V centralpartofthegalaxyimpliesthat2104–242hasexpe-
in the southern region we have calculated an upper limit rienced a period of intense star formation.Assuming that
assuming a Gaussian line shape with a peak 3 times the the difference in N V emission found within the halo is
rms of the continuum at the expected wavelength and a due to a metallicity gradient,it is likely that the emitting
R.A.Overzier et al.: Deep VLTspectroscopy of the z=2.49 Radio Galaxy MRC 2104–242 3
Fig.1. VLT/Antu spectrum of the northern region of Fig.2. VLT/Antu spectrum of the southern region of
emission of 2104–242.The extraction aperture was 4′′. emission of 2104–242.The extraction aperture was 3′′.
Fig.3. The two-dimensional emission line structures of Lyα, C IV and He II. Offset zero was chosen in between the
northern and southern regions. Velocity zero corresponds to the peak Lyα emission in the northern region.
Table 1. Wavelength, flux, FWHM and (rest-frame) EW for the emission lines in the northernand southernregions.
northern region southern region
Line Peak (˚A) Fluxa FWHM (km s−1) EW (˚A) Peak (˚A) Fluxa FWHM(km s−1) EW (˚A)
Lyα 4247±1 40±4 610±140 360±50 4238±1 42±4 1490±140 845±232
N V 4330±2 1.0±0.1 1100±300 13±2 ≤0.25
SiIV 4892±8 1.2±0.1 2200±1000 18±4 4887±5 1.2±0.2 2300±700 32±5
C IV 5411±1 2.4±0.3 850±150 42±6 5399±2 2.5±0.3 1200±300 57±17
HeII 5726±1 3.0±0.3 700±100 55±7 5717±2 2.0±0.2 800±200 58±22
O III] 5812±1 0.3±0.1 250±200 4±1 5803±7 0.3±0.1 750±700 6±2
a Flux is given in unitsof 10−16 erg s−1 cm−2
4 R.A.Overzier et al.: Deep VLTspectroscopy of the z=2.49 Radio Galaxy MRC 2104–242
that jet-cloud interactions occur and for some sources
1.0 12 there is evidence of shock ionization (Best et al. 2000).
9 However, large radio sources like 2104–242 are less likely
Quasars
6 to be ’shock-dominated’, because the shockfronts have
0) 0.5 4 passed well beyond the emission line halo.
4
16 HzRGs Also, the gas could have been expelled by a super-
λe II 0.0 Z=2 2 3 wind following an enormous starburst. We have shown
H
0 / evidence for intense star formation in 2104–242. Binette
λ 124 -0.5 1 NROEGRITOHNERN et al. (2000) showed that radio galaxy 0943–242 is sur-
V Z=0.4 roundedbyavestigegasshellofverylowmetallicity.They
N
g(10 conclude that this gas has been expelled from the parent
lo -1.0 galaxy during the initial starburst at the onset of its for-
SOUTHERN mation.
REGION
-1.5 We conclude that the emission line ratios are well ex-
plainedbyacombinationofphoto-ionizationandametal-
-1.5 -1.0 -0.5 0.0 0.5 licity gradient. This is consistent with scenarios in which
log (N V λ 1240 / C IV λ λ 1549)
10 the halo is formed by gas falling onto the radio galaxy
Fig.4. N V/He II vs. N V/C IV. The dotted line repre- located at the center of a forming cluster or by gas asso-
sentsthemetallicitysequencedefinedbyquasars(Hamann ciated with intense galaxy merging.
&Ferland1993),withthenumbersalongtheline indicat-
ing the metallicityinsolarunits.The solidline represents Acknowledgements. We acknowledge productive discussions
a metallicity sequence with N∝ Z2 (ionizationparameter with Wil van Breugel and Laura Pentericci.
U=0.035,powerlawspectralindex α=−1.0)fromVernet
et al. (2000). The two regions of 2104–242 are indicated.
References
Smallopencirclesindicateradiogalaxiesfromthesample
of De Breuck et al. (2000). Best P., Longair M., Ro¨ttgering H., 1998, MNRAS 295,
549
Best P., Ro¨ttgering H., Longair M., 2000, MNRAS 311,
gasnearthecenterandthegasfurtheroutareindifferent 23
stages of evolution. Binette L., Kurk J., Villar-Mart´ın M., Ro¨ttgering H.,
Theinfallofgasbymassivecoolingflowsisbelievedto 2000, A&A 356, 23
beanimportantprocessingalaxyformation(Crawford& Chambers K., Miley G., van Breugel W., 1987, Nat 329,
Fabian1996).Inthisscenario,gascoolsfromaprimordial 604
halo surrounding the radio sourceand provides the mate- De Breuck C., Ro¨ttgering H., Miley G., van Breugel W.,
rialfromwhichthegalaxyismade.Thiscouldalsobethe Best P., 2000, A&A 362, 519
case for 2104–242.Fardal et al. (2000) recently examined Fardal M. et al., 2000, in press, astro-ph/0007205
cooling radiation from forming galaxies, focusing on Lyα Crawford C., Fabian A., 1996, MNRAS 282, 1483
lineluminositiesofhighredshiftsystems.Theyfindthata Hamann F., Ferland G., 1993,ApJ 418, 11
significantamountoftheextendedLyαemissioncanarise Heckman T. et al., 1986, ApJ 311, 526
from cooling radiation. Kauffmann G., 1995, MNRAS 274, 161
However, because the two regions of 2104–242 both Koekemoer A., van Breugel W. & Bland-Hawthorn J.,
show emission lines from other elements than H and He, 1996, In: M. Bremer et al. (eds.) “Cold Gas at High
at least some of the gas must already have been pro- Redshifts”, Kluwer, p. 385
cessedin starsinthe past.The HST imagesrevealedthat McCarthy P., Kapahi V., van Breugel W., Subrahmanya
this galaxy actually consists of a number of larger and C., 1990, AJ 100, 1014
smaller components (Pentericci et al. 1999), which sup- McCarthy P., 1993, ARA&A 31, 639
ports the general idea that galaxies are formed by a pro- McCarthy P., Baum S., Spinrad H., 1996,ApJS 106, 281
cess of hierarchical buildup (Kauffmann 1995). We have Pentericci L. et al., 1999, A&A 341, 329
also found evidence for kinematical substructure within Ro¨ttgering H. et al., 1997, A&A 326, 505
the halo.Therefore, the emission line halo may be the re- Schlegel D., Finkbeiner D., Davis M., 1998, ApJ 500,525
sult of gas associatedwith suchintensive galaxymerging. van Ojik R. et al., 1994, A&A 289, 54
Alternatively, the gas in the halo may be the result of Vernet J. et al., 2000,in press, astro-ph/0010640
other mechanisms.Itmay havebeendrivenoutby strong Villar-Mart´ınM.,TadhunterC.,ClarkN.,1997,A&A323,
jet-cloud interactions. Line widths as large as 1500 km 21
s−1 are common in HzRGs, which indicate extreme, non- Villar-Mart´ınM.,BinetteL.,FosburyR.,1999,A&A346,
gravitational motions. The alignment effect also suggests 7
R.A.Overzier et al.: Deep VLTspectroscopy of the z=2.49 Radio Galaxy MRC 2104–242 5
Villar-Mart´ın M., Fosbury R., Binette L., Tadhunter C.,
Rocca-Volmerange B., 1999, A&A 351, 47