A Herschel Survey of the [N II] 205 micron Line in Local Luminous Infrared Galaxies --- The [N II] 205 micron Emission as a Star Formation Rate Indicator

0.28 MB·English

by  Yinghe Zhao

Draftversion January31,2013
PreprinttypesetusingLATEXstyleemulateapjv.12/16/11
A Herschel SURVEY OF THE [Nii] 205 µm LINE IN LOCAL LUMINOUS INFRARED GALAXIES — THE [Nii]
⋆
205 µm EMISSION AS A STAR FORMATION RATE INDICATOR
Yinghe Zhao1,2,3, Nanyao Lu2, C. Kevin Xu2, Yu Gao1,3, S. Lord2, J. Howell2, K. G. Isaak4,
V. Charmandaris5,6, T. Diaz-Santos7, P. Appleton2, A. Evans8,9, K. Iwasawa10, J. Leech11, J. Mazzarella2,
A. O. Petric12, D. B. Sanders13, B. Schulz2, J. Surace7, P. P. van der Werf14
Draft version January 31, 2013
3 ABSTRACT
1
We present, for the first time, a statistical study of [Nii] 205 µm line emission for a large sample of
0
local luminous infrared galaxies using Herschel Spectral and Photometric Imaging Receiver Fourier
2
TransformSpectrometer(SPIREFTS)data. Foroursampleofgalaxies,weinvestigatethecorrelation

n betweenthe[Nii]luminosity(L )andthetotalinfraredluminosity(L ),aswellasthedependence
[NII] IR
a
ofL /L ratioonL ,farinfraredcolors(IRASf /f )andthe[Oiii]88µmto[Nii]luminosity
J [NII] IR IR 60 100
0 ratio. We find that L[NII] correlates almost linearly with LIR for non AGN galaxies (all having
3 LIR <1012L⊙)inoursample,whichimpliesthatL[NII]canserveasaSFRtracerwhichisparticularly
usefulforhighredshiftgalaxieswhichwillbeobservedwithforthcomingsubmmspectroscopicfacilities

] such as the Atacama Large Millimeter/submillimeter Array. Our analysis shows that the deviation
O from the mean L -L relation correlates with tracers of the ionization parameter, which suggests
[NII] IR
C thescatterinthisrelationismainlyduetothevariationsinthehardness,and/orionizationparameter,
of the ambient galactic UV field among the sources in our sample.
.
h
Keywords: galaxies: evolution — galaxies: ISM — galaxies: starburst — infrared: ISM
p
-
o
1. INTRODUCTION lengths (e.g. Calzetti et al. 2009 and references therein;
r
t Kennicutt & Evans 2012). However, few of these SFR
s Thestarformationrate(SFR)isoneofthefundamen-
indicatorscanbeobservedoveralargeredshiftrangeus-
a talphysicalparametersforcharacterizinggalaxies. Hav-
[ inganeffectivewaytoderiveSFRsforgalaxiesspanning ing current facilities. Using the capabilities of the ESA
a largerange of look-back times is crucial for our under- Herschel SpaceObservatory(hereafterHerschel;Pilbratt
1 etal. 2010),itispossibletodefinenewSFRindicatorsin
standingofgalaxyevolutionleadingbacktoinitialcondi-
v
the farinfrared(FIR) window,whichcanbe followedup
8 tions. Currently, SFRs have been inferred using contin-
by the Atacama Large Millimeter/submillimeter Array
1 uumorspectrallineemissionfromawiderangeofwave-
(ALMA) over a large redshift range.
3
The[Nii]205µmline,whicharisesfromthe3P →3P
7 1PurpleMountainObservatory,ChineseAcademyofSciences, transition (205.197 µm; hereafter [Nii](1→0))1of the0
. Nanjing210008, China;[email protected]
1 2InfraredProcessingandAnalysisCenter,CaliforniaInstitute ground state of the singly ionized nitrogen, is expected
0 ofTechnology, MS100-22,Pasadena, CA91125, USA to be an excellent indicator of SFR. This FIR line emis-
3 3Key Laboratory of Radio Astronomy, Chinese Academy of sion essentially traces all of the warm ionized interstel-
1 Sciences, Nanjing210008, China
v: 22040EASAGANsotorrodpwhyijskic,sTMheisNsieotnhserDlaivnidsison,ESTEC, PO Box 299, l(a1r4.m53edeiVu)mis(IoSnMly)∼. T0h.9eeioVnilzaargtieornthpaontetnhtaiatlooffhnyidtrrooggeenn
i 5Department ofPhysics andITCP, UniversityofCrete, GR- (13.6 eV), allowing the [Nii] luminosity to be used as
X 71003Heraklion,Greece anestimate of the flux of ionizing photons (e.g. Bennett
r 6IESL/FoundationforResearchandTechnology-Hellas,GR- et al. 1994). The critical density of [Nii](1→0) is only
a 7d1e1P1a0r,isH,eFra-7k5li0o1n4,,GPraereisc,eFarnadncCehercheur Associ´e, Observatoire 44 cm−3 at T = 8000 K (Oberst et al. 2006), and the
7Spitzer Science Center, California Institute of Technology, energy level that emits the line corresponds to only 70
MS8D22e0p-a6r,tmPaesnatdoenfaA,sCtrAon9o1m12y,5,UUnSivAersity of Virginia, 530 Mc- K, so that it can be easily collisionally excited. In addi-
tion, the [Nii] 205 µm emission is usually optically thin
CormickRoad,Charlottesville,VA22904, USA
9National Radio Astronomy Observatory, 520 Edgemont becauseofitssmallEinsteincoefficient,andsuffersmuch
Road,Charlottesville,VA22903, USA less dust extinction than optical and near infrared lines.
10ICREA and Institut de Ci`encies del Cosmos (ICC), Uni-
In particular, this line is accessible using ALMA for a
versitat de Barcelona (IEEC-UB), Mart´ı i Franqu`es 1, 08028,
Barcelona,Spain significant fraction of the 0.6 . z . 16 redshift range
11DepartmentofPhysics,UniversityofOxford,DenysWilkin- (bands 3-10)16. For more local galaxies, the [Nii](1→0)
sonBuilding,KebleRoad,Oxford,OX13RH,UK line can be observedin the two windows around 200 µm
12AstronomyDepartment,CaliforniaInstituteofTechnology, (1.25−1.4 THz and 1.45−1.6 THz; e.g. Paine et al.
Pasadena, CA91125, USA
13UniversityofHawaii,InstituteforAstronomy,2680Wood- 2000; Yang et al. 2010) from the very driest sites with
lawnDrive,Honolulu,HI96822, USA future facilities such as the Sub-mm/THz telescope at
14LeidenObservatory,LeidenUniversity,POBox9513,2300
RALeiden,TheNetherlands
⋆Herschel is an ESA space observatory with science instru- 17RedshiftintervalscorrespondingtodifferentALMAreceivers:
mentsprovidedbyEuropean-ledPrincipalInvestigatorconsortia 11.6.z.16.3(Band3); 3.0.z.10.6(Bands 4−7);2.0.z.
andwithimportantparticipationfromNASA. 2.7(Band8);1.0.z.1.4(Band9);0.6.z.0.8(Band10)
2 Zhao et al.
the Chinese Antarctic Observatory under development
(e.g. Cui 2010).
Early studies have shown that the [Nii] 205 µm line
is comparatively bright. In most normal star-forming
galaxies, it is the third to fifth brightest FIR/submm
line after [Cii] 158 µm, [Oiii] 88 µm, [Oi] 63 µm and
[Nii] 122 µm lines (e.g., Wright et al. 1991; Brauher et
al. 2008). The luminosity of [Nii] 205 µm line (L )
[NII]
may be up to ∼10−4 times the totalinfraredluminosity
(L ). For example, L is about 3 × 10−4L and
IR [NII] IR
5×10−5L in the Milky Way (Wright et al. 1991) and
IR
M82(Petuchowskietal. 1994),respectively. Withsucha
highluminosity,this line hasalreadybeendetected with
ALMA for luminous/ultraluminous infrared galaxies at
high redshift (e.g. Nagao et al 2012; z =4.76).
The [Nii] 205 µm line is generally inaccessible to
ground-based facilities, and its rest wavelength is longer
than the cutoff of the Infrared Space Observatory (ISO)
Long Wavelength Spectrometer (LWS). Therefore, this
line was observed in only a handful of extragalactic ob-
jects using satellite and airborne platforms (e.g. Wright
et al. 1991; Petuchowski et al. 1994; Lord et al. 1995)
prior to the advent of Herschel. Here we report our
firstresults onthe [Nii] 205 µm line emissionfor a large
sample of galaxies, observed with the Fourier-transform
spectrometer(FTS) of the SPIRE instrument(Griffin et
Figure 1. Typicalspectra(correctedtothesourcerestframe)of
al. 2010) on board Herschel. We present the correla- the [Nii] 205 µm line for galaxies with a range of f60/f100 colors
tion between L and L , to characterize the [Nii] (going redder from top to bottom). In each panel, the blue, red
[NII] IR
205 µm line as a SFR indicator. We also investigate and black lines show the observed spectrum, the fitted profile of
the [Nii] line plus continuum, and the residual, respectively. In
possible dependences of [Nii] 205 µm to IR luminosity
the left panels, the [Nii] line is fitted using a Sinc function with
ratio(L[NII]/LIR)onLIR,FIRcolor(f60/f100,wheref60 afreewidthparameter. Therightpanelsshowthesamedatabut
andf arethe IRAS60µmand100µmfluxes,respec- thelineisfittedusingafixedinstrumentalSincfunctionconvolved
100
tively), and the [Oiii] 88 µm to [Nii] 205 µm emission witha freewidthgaussian shape (SCG). It is clear that the SCG
functioncangiveabetter fittingresultforthisline.
ratio(L /L ),to revealthe causeofthe scatterin
[OIII] [NII]
this relation. Throughout the paper, we adopt a Hub-
ble constant of H = 70 km s−1 Mpc−1, Ω = 0.3, and 2013) gives a brief program description and some initial
0 M results on the observed CO line emission.
Ω =0.7.
Λ TheobservationswereconductedwiththeSPIREFTS
in staring and high spectral resolution mode between
2. SAMPLE,OBSERVATIONSANDDATAREDUCTION 194–672 µm, yielding a resolution of 0.04 cm−1 (=1.2
The sample discussed in this Letter is part of the Her- GHz). The data were reduced using a customized ver-
schel opentime projectHerschel Spectroscopic Survey of sion of the standard SPIRE reduction and calibration
Warm Molecular Gas in Local Luminous Infrared Galax- pipeline for point source mode included in HIPE version
ies (LIRGs) (PI: N. Lu), whichaims primarily atstudy- 7.3 (Ott 2010). We used an averaged telescope relative
ing the dense and warm molecular gas properties of 125 spectral response function (RSRF) in place of the daily
LIRGs (LIR ≡ L(8 − 1000µm) > 1011L⊙, where LIR RSRF,toperformthefluxcalibration. Duringthereduc-
was calculated by using the IRAS four-band fluxes and tion, we did not apply any apodization to the spectra.
the equation given in Sanders & Mirabel (1996)), which To extractthe finalspectrum, a “background”emission,
comprise a flux limited subset of the Great Observato- whichwasobtainedbyfittingthemedianspectrumfrom
ries All-Sky LIRGs Survey sample (GOALS; Armus et thespatiallysurroundingdetectors,wassubtractedfrom
al. 2009). The full program details as well as obser- the central detector.
vational data on individual galaxies will be given in a In most cases the [Nii] 205 µm line is the brightest
future paper upon the expected completion of the pro- line in the SPIRE FTS wavelength range, and almost
gram in early 2013. Here we present a subsample of 70 all sources show high signal-to-noise ratio (S/N) in the
galaxies,including63LIRGsand7ULIRGs(L >1012 [Nii]line. InFig. 1weplotseveraltypicalspectraofthe
IR
L⊙), all of which are point sources with respect to the [Nii] line for galaxies with a range of FIR colors (0.4 .
HerschelSPIRE/FTSbeam. Thedeterminationofthese f /f .1.4),andsuperimposeafittedcontinuumand
60 100
galaxiesasrelativepointsourceswasbasedbothontheir the line profile. From the figure, one can see that the
observed mid-infrared emission extent (for example, see fixed Sinc function convolvedwith a free width gaussian
D´ıaz-Santos et al. 2010, 2011), and on the consistency (SCG) profile gives a better fitting result for the [Nii]
betweentheobservedemissionstrengthseenintheover- line since the instrumentalresolutionof the SPIRE FTS
lappingFTSbandsbetween316and324µm(beamsizes in high resolution mode at ∼ 210 µm is only about 300
are ∼20′′ and ∼37′′ for SSW and SLW, respectively, in km s−1 and the line width has an influence on the line
this wavelength range). A companion letter (Lu et al. shape. Therefore, we adopted the SCG profiles for the
[Nii] 205 µm line as a SFR indicator 3
integrated [Nii] line flux measurements except for one of significance, which indicates a very strong correlation
galaxy (CGCG448-020) where a single Sinc profile was between L and L . Our fit shows that, on average,
[NII] IR
better due to its narrow line width (< 200 km s−1 for LIR scaleswith Lα[NII] with α between 0.8 and1.1 at the
low J CO lines; e.g. Baan et al. 2008; Leech et al. 3σ significance. This nearly linear relation suggests that
2010) and relatively low S/N (∼ 4) in the line. The the power source of the [Nii] 205 µm emission tightly
resulting full width at half maximum of the [Nii] line relates to the star-forming activities. Using the SFR-
(excluding CGCG448-020) is 140−610 km s−1, with a L calibration given in Kennicutt & Evans (2012), we
IR
median value of ∼ 300 km s−1. In most cases, the 1σ can also build a direct relation between SFR and L ,
[NII]
statisticaluncertaintiesintheintegratedlinefluxareless namely
than 10%, and the median value is 7%.
logSFR(M⊙yr−1)=(−5.31±0.32)+(0.95±0.05)logL[NII](L⊙)
3. RESULTSANDDISCUSSIONS (2)
Comparing to the IR luminosity, the [Nii] 205 µm
3.1. L -L Correlation
[NII] IR emission could be a more convenient SFR indicator in
Our sample galaxies are all (U)LIRGs and 23 out of the sense that it is more unambiguous, i.e. less affected
70 show AGN/LINER features (identified using optical, by emissions from older stars (e.g. in the Milkyway the
mid infrared, or X-ray data). In order to increase the [Nii]205µmfluxislowerthanthatpredictedbyalinear
size and dynamic range of the sample used to study the L -L relationwhenIRintensitygoinglower;seeFig.
[NII] IR
L[NII]-LIR relation, we also include 30 unresolved galax- 4inBennettetal. 1994),andinter-comparablesincethe
ies observed by ISO LWS and compiled by Brauher et techniques for obtaining L for low and high redshift
IR
al. (2008), in our analysis. For these objects, most of objects are so instrument-dependent. The majority of
which are normal star-forming galaxies, the luminosity massive galaxies up to z = 5−6 are already chemically
distances were derived with the same method as used evolved systems based on their CO and dust emissions
for our (U)LIRG sample (e.g. Armus et al. 2009), i.e. (Solomon & Vanden Bout 2005) or optical/IR line spec-
bycorrectingtheheliocentricvelocityforthe 3-attractor troscopy (Hamann & Ferland 1999). Therefore, Eq. (2)
flow model of Mould et al. (2000). Their [Nii] 205 µm canbereasonablyapplicabletohigh-z starburstgalaxies
fluxes are derivedfromthe observed[Nii] 122µm fluxes despite that the L -L relation possibly depends on
[NII] IR
giveninBrauheretal. (2008),usingthetheoretical[Nii] metallicity.
122µmto[Nii]205µmemissionratio(hereafterR )at
21
ne = 80 cm−3 (the median value for late type galaxies; 3.2. The scatter in the LIR-L[NII] relation
see e.g. Ho et al. 1997) of R21=2.6. Here we compute InFig. 2wecanalsoseeanindicationofasteeperrela-
tthateiotnhefoorrettihcealgrRo2u1ndb-ystaastseulmevinelgs eolfecNt+ro,nanimdpuascitngextchie- tTiohnisbisetmweoerne Lcl[eNaIrIl]yanddemLoInRstfroartLedIRin>F1i0g1.13La⊙, ignalwaxhiiecsh.
fittedresults(Draine2011)ofHudson&Bell(2005)with we plot L /L vs L for the same set of galaxies.
[NII] IR IR
T =8000Ktocalculatethecollisionstrengths(morede- ForULIRGs, themeanL /L (−4.69dex)is consid-
tails are provided in Zhao et al. 2013, in preparation). [NII] IR
erably lower than that (−4.14 dex) of the galaxies with
Regardingthe inclusionofBrauheret al. galaxiesin our
lower IR luminosities. Due to the small ULIRG sample
study, we note that (1) about 90% of the galaxies in Ho
size and the prevalence of AGN we can reach no firm
et al. (1997) have ne <300 cm−3, yielding a theoretical conclusion as to whether the L /L decreases with
sRe2e1a=no0b.9s4er−v5ed.3R(neo=f11.05−−340.30fcomr−th3e);n(i2n)eegmalpaixriiceasl(leyi,gwhet LIR>1012L⊙ for starburst-dom[NinIaI]tedIRgalaxies.
21 However, the scatter in the L -L relation is as
inoursampleplusM82)thathaveboth[Nii]122µmand IR [NII]
large as ∼ 0.3 dex, and seems to increase with L ,
[Nii] 205 µm data. Therefore, the uncertainty in L IR
[NII] specially when ULIRGs are taken into account. The
caused by extrapolating [Nii] 205 µm fluxes from [Nii]
scatter could have several different origins: metallicity
122 µm emissions is a factor of a few (∼ 0.3 dex), and
variations and most likely, differing ionizing conditions.
is significantly smaller than the two order-of-magnitude
These latter effects include: the stellarUV input via the
span of L found in our sample, thus yielding useful
IR initial mass function, the aging of starbursts, the Hii
results concerning the L -L relation.
[NII] IR region/ISM geometry or the presence of an AGN. Our
In Fig. 2, we plot L against L for both our
[NII] IR sample of galaxies contains few low metallicity objects,
(U)LIRGs (cirles) and Brauher et al.’s galaxies (dia- and therefore we only address ionization effects here.
monds), as well as the starburst galaxy M82 (star- We note that the ionization parameter (U) can be es-
symbol; Petuchowski et al. 1994). The solid symbols peciallyhighin(U)LIRGs(e.g. Farrahetal. 2007,Petric
indicateAGNs/LINERs. Generally,L[NII] increaseswith et al. 2011), where U is defined as the dimensionless ra-
LIR,albeitthescatterseemstoincreasewithLIR(wewill tio ofthe number density ofincident ionizingphotons to
return to this later). In fact, the scatter at the high LIR the number density of hydrogen nuclei. Clearly, U can
end will be much reduced if we only look at the non- be measured through ionization parameter-sensitive line
active galaxies. A nonweighted least-squares linear fit, ratios, such as [Oiii]/[Oii] and [Niii]/[Nii]. Moreover,
using a geometrical mean functional relationship (Isobe theoreticalmodelshavealsoshownthatU canberoughly
et al. 1990), to the non-active galaxies, gives indicated by the IR color, f /f , by the means that
60 100
logU increases with f /f (see e.g. Abel et al. 2009).
logL =(4.51±0.32)+(0.95±0.05)logL (1) 60 100
IR [NII] In Fig. 3b we plot the dependence of L /L on
[NII] IR
with a scatter of 0.26 dex in L . The Spearman rank f /f . We can see from the figure that there only ex-
IR 60 100
correlationcoefficientρofthetrendis0.78ata>5σlevel ists a very weak dependence of L /L on f /f
[NII] IR 60 100
4 Zhao et al.
Figure 2. [Nii] 205 µm plotted against total infraredluminosities for normal and luminous infraredgalaxies. Circles(red) are galaxies
havingHerschelobservations. Diamonds (blue) aregalaxies fromBrauher et al. (2008), forwhichanadditional uncertainty of ∼0.3dex
shouldbetaken intoaccount besides the plotted errorbar. The superposedlinerepresents a geometrical mean, least-squares linearfit to
alldatapointsexceptforthosesolidsymbols.
when log(f /f )<−0.2. However, L /L clearly both small L /L and L /L . Excluding Mrk
60 100 [NII] IR [NII] IR [OIII] [NII]
decreases as f /f increases when log(f /f ) & 231, and using an ordinary least-squares linear fit to
−0.2, except fo6r0 a1f0e0w galaxies with the wa6r0me1s0t0FIR the data, we obtain log(L[NII]/LIR) = (−3.63±0.06)+
colors in the sample. Moreover, the scatter in this cor- (−0.61 ± 0.06)log(L /L ). The Spearman rank
[OIII] [NII]
relation is less than that in the L /L vs. L plot correlation coefficient ρ of this correlation is −0.93 at a
[NII] IR IR
(see Fig. 3a) and depends little on f /f . But we >5σ level of significance.
60 100
note that the scatter in the L /L -f /f plot is Galaxies plotted in Fig. 3c (hereafter [Oiii] sample)
[NII] IR 60 100
still large, and we hypothesize that it is caused by the are also marked with red circles in Figs. 3a and 3b,
variations in the radiationfield, since N+ needs high en- which indicates that the hardness variation of the heat-
ergy (29.6eV) to formN++, and the population ofionic ingradiationfieldamonggalaxiescanlargelyaccountfor
N is determined mainly by the slope of the background the scatter shown in the LIR-L[NII] relation, at least for
UV spectrum, i.e. the hardness, at a given U. LIR <1012 L⊙ galaxies. Actually, this is also supported
To try to account for the effects of the hardness of bythe factthatthescatteroflessluminousgalaxies(0.2
ionizing radiation field on L /L , we use the ratio dex for Brauher et al. sample; thus probably less active
[NII] IR
of the [Oiii] 88 µm emission, rather than [Niii] 57 µm instarformationandlowerU)issmallerthanthatofour
emissionwherethere arefew detections,to the [Nii]205 luminous sample (0.28 dex). Keeping in mind that the
µm emission to trace the hardness of the ionizing pho- conversion from [Nii] 122 µm flux to [Nii] 205 µm flux
tons. It takes 35 eV to form O++, which is only ∼5 eV may contribute largely to the scatter of Brauher et al.
higher than that needed for the formation of N++, and sample, the real scatter in the L[NII]-LIR relation might
thus the [Oiii]/[Nii]line ratiosareevenstrongerindica- probably be even smaller for these galaxies.
torofthehardness. Furthermore,thisratioisinsensitive To obtain a direct view of how much the scatter
to the gas density (Rubin 1984). Except for M82 (Duffy can be reduced, we also display the hardness-corrected
et al. 1987) and Mrk 231 (Fischer et al. 2010), all of L[NII]/LIR, which was calculated according to the fitted
the [Oiii] 88 µm fluxes are adopted from Brauher et al. L /L -L /L relation,forthe[Oiii]samplein
[OIII] [NII] [NII] IR
(2008). Fig. 3d (arbitrarily scaled). Comparing to Fig. 3a, one
Fig. 3c shows the relation between L[OIII]/L[NII] and can see that the scatter is reduced significantly (from
L /L . Clearly, there exists a good correlation such 0.26dex to 0.11dex by excluding Mrk 231,whichis still
[NII] IR
as L /L decreasesas L /L increases. There an outlier due to that the hardness correction has little
[NII] IR [OIII] [NII]
is only one outlier, the ULIRG Mrk 231, which shows effect on it, and is outside the plotted range).
[Nii] 205 µm line as a SFR indicator 5
Figure 3. Ratioof[Nii]205µmtoIRluminosityisplottedagainst(a)and(d): IRluminosity,(b): IRAS60µm/100µmcolor,and(c):
[Oiii]88µm/[Nii]205µmfluxratio. SymbolsarethesameasFig. 2exceptthatherelargercirclesrepresentULIRGs. Symbolsenclosed
byanadditionalredcircleinpanels(a) and(b)arethesamesourcesasshowninpanels(c)and(d). Eq. (1)isshownbythedashedline
in panel (a). The ordinate of panel (d) is a nominal value after the correction for the hardness effect (using the equation shown in this
panel). Mrk231isoutsidetheplottedrangeofpanel(d).
All of the seven ULIRGs in our sample show some and ∼ −3.6 dex, and varies systematically with
deficit in L relative to L . Four of the seven, in- the hardness/ionizationparameterofthe radiation
[NII] IR
cluding Arp 220, and Mrk 231, were observed by ISO field. However,additionaleffects are needed to ex-
LWS, but no [Oiii] 88 µm emission was detected at the plainthescatterintheL -L relationandtheir
[NII] IR
3σ limit (e.g. Brauher et a. 2008), despite all showing relatively lower L /L ratio for more IR lumi-
[NII] IR
someAGNcharacteristics. Thesegalaxiesalsoshowvery nous galaxies in our sample.
warm FIR colors (see Fig. 3b). Therefore, very dusty
Hii regions with high U might be needed to explain the
deficit of [Nii] emission in these galaxies(e.g. Fischer et Wethankananonymousrefereeforhis/hercomments,
al. 1999; Abel et al. 2009; Draine 2011). which improved this manuscript. This work is based
in part on observations made with Herschel, a Euro-
4. SUMMARY
pean Space Agency Conerstone Mission with signifi-
In this Letter we have presented our initial results of cant participation by NASA. Support for this work was
the [Nii] 205 µm emission for a sample of 70 (U)LIRGs provided in part by NASA through an award issued
observed by the Herschel SPIRE FTS. Combining with by JPL/Caltech. Research for this project is partially
the ISO andIRAS data, we havestudied the correlation supported by the Natural Science Foundation of China
betweenL[NII] andLIR,andinvestigatedthedependence (grant10833006)andJiangsuProvince(SBK201120678).
of L /L on L , FIR color f /f and the UV Y.Z. thanks IPAC for the hospitality and the financial
[NII] IR IR 60 100
hardnessindicator[Oiii]/[Nii]. The mainconclusionsof support during his visit. This research has made use of
our work are: the NASA/IPAC Extragalactic Database (NED), which
is operatedby the Jet PropulsionLaboratory,California
1. For star-forming galaxies with LIR < 1012L⊙, InstituteofTechnology,undercontractwiththeNational
thereexistsastrongcorrelationbetweenL[NII] and Aeronautics and Space Administration.
L , namely logL = (4.51 ± 0.32) + (0.95 ±
IR IR
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