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chemical evolution of the universe has been one of the major issues in the modern astronomy .
heavy elements , or metals , have been generated and accumulated in the universe since its birth mainly through supernovae . an important clue to understand the chemical evolution of the universe
will no doubt be brought about by systematic study of a well - defined sample of supernova remnants . in x - ray observations , we are able to extract information of plasma temperature , metal abundance , explosion energy , ionization age , and so on from optically thin thermal plasma emission from supernova remnants ( snrs )
. these pieces of information will lead to understand galactic chemical evolution and star formation history .
a systematic study of snrs in the large magellanic cloud ( lmc ) suits well for this purpose , because of its well - known distance ( 50kpc ; @xcite ) and small interstellar absorption to lmc .
the fact that lmc is face - on to us is another advantage . because of these characteristics
, systematic x - ray studies of the thermal snrs in lmc have been carried out with some major x - ray observatories ( e.g. @xcite ) .
these observations are , however , limited in terms of energy resolution in the low energy band below @xmath01  kev , which is important to study the nature of the snr plasma in detail , because the temperature of the plasma in most snrs is less than @xmath02  kev .
in particular , hydrogenic and he - like oxygen lines that appear in 0.50.7  kev dominate a spectrum of the plasma with a temperature of @xmath61  kev , and hence , high resolution spectroscopy in this energy band is of great importance in evaluating the parameters of the low temperature plasma , such as the temperature , the density , the ionization parameter and the abundances of the metal .
we therefore have decided to perform a systematic study of the snrs in lmc with the suzaku xis ( @xcite , @xcite ) , which has the best energy resolution in the oxygen 0.50.7  kev energy band among the ccds currently in orbit . in this paper
, we study the snr n23 , as a beginning of our systematic study project .    n23 is identified as a snr with radio observations at 5 and 14.7  ghz for the first time ( @xcite ) .
its x - ray emission extends @xmath7 in the sky ( @xmath8pc at a distance of 50  kpc ) , showing a semi - circular morphology with only the south - eastern hemisphere being bright @xcite .
based on the observation , @xcite classified n23 as a young snr with an age of @xmath9 from the spectrum of the entire remnant .
@xcite carried out spatially resolved spectroscopy of n23 with , and estimated the age of north - western rim to be @xmath04600  yr .
these observations result in consistent estimation of the temperature and the ionization parameter ( @xmath10 ) in the bright south - eastern shell , which is @xmath00.50.7  kev and @xmath11@xmath12 , respectively .
they are , however , not be able to resolve hydrogenic and he - like lines from oxygen clearly because of limited energy resolution below @xmath01  kev .
in addition , @xcite have revealed that there is a point source around the center of n23 . from its 0.510  kev flux ( @xmath13 ) and
power - law spectrum with a photon index of 2.2 , the source is probably a rotation - powered pulsar and/or a pulsar wind nebula .
if so , the progenitor of n23 is a core - collapsed massive star ( @xcite ) .
this paper is organized as follows . after describing how the observation and data reduction are carried out in   [ sec : obs ] , we report the results of spectral analysis of n23 in   [ sec : spec_ana ] .
owing to high spectral resolution of the xis in the oxygen band , we have clearly resolved hydrogenic and he - like k@xmath14 emission lines from oxygen for the first time , which leads to identification of a new low temperature component .
based on these results , we discuss the plasma parameters and evolutionary phase of n23 in   [ sec : dis ] . finally , the summary is given in 
[ sec : sum ] .
n23 was observed with @xcite on 2005 august 1617 .
the observation log is summarized in table  [ tab : suzakuobslog ] .    .observation log . [ cols= " <
, < , < " , ]     using eq .
( [ eq : sedovage ] ) with @xmath15  pc , @xmath16 estimated from the low and high temperature components becomes @xmath08000 and @xmath05000  yr , respectively , which are consistent with the lower limit of @xmath17 .
this is a direct consequence of eq .
( [ eq : sedovage ] ) , implying @xmath16 being proportional to @xmath18 .
the high temperature component is younger than the low temperature component . in the sedov scheme , this can be interpreted as the high temperature component being heated recently . on the other hands , @xmath19 of the low and high temperature components are 9300@xmath20 and 12000@xmath20 , respectively , using eq .
( [ eq : coolage ] ) with @xmath21 given in table  [ tab : n23phi ] (  [ sec : n23_phys ] ) . since @xmath22 for both components , n23 is likely to be at a late stage of the sedov phase , and if so , its age is @xmath08000  yrs old from the newly discovered low temperature component .
note , however , that this discussion depends on the estimation of the volume filling factor @xmath23 .
accounting for the highly anisotropic image ( fig .
[ fig : n23image ] ) , we have normalized @xmath23 by 0.1 .
however , if the shock wave of n23 propagates in a higher density ism , like the edge of the open cluster hs  114 @xcite , the total volume should be smaller for the given emission measure , resulting in a smaller @xmath23 .
if , for instance , @xmath24 , @xmath25 of the low and high temperature components are @xmath05000 and @xmath07000  yr , respectively . since @xmath26 , the low temperature component should be regarded as in the radiative phase .
in addition , we would also like to remark that , in   [ sec : n23_phys ] , we have calculated the plasma volume , assuming that the shell thickness is 1/12 of the shell radius , which is expected for a spherical strong shock .
this holds only if n23 is still in the sedov ( adiabatic ) phase .
if n23 has been in the radiative phase , then the shell thickness should be smaller due to radiative cooling , and hence , the plasma density would be larger than the current estimation . as a result , the cooling time scale estimated from eq .
( [ eq : coolage ] ) should be smaller , and @xmath27 .
consequently , we can not deny possibility that n23 has been in the radiative phase .    in summary
, the age of n23 is estimated to be @xmath08000 yr from the newly discovered low temperature component as long as it still stays in sedov phase .
this condition is , however , uncertain , because the discussion depends on the volume filling factor of the plasma .
if , for instance , the shock propagate in the higher density ism , n23 may have already entered into the radiative cooling phase . in this case , the remnant age estimated from the cooling timescale is smaller .
finally , we can make an independent age estimation from the central source detected by @xcite .
its luminosity is @xmath28  erg in the 0.510.0  kev @xcite .
if the central source is a pulsar ( although the pulsation has not been detected ) , the characteristic age is @xmath29@xmath30  years , which is consistent with our age estimation update .
the main component of x - ray emission originates from the swept - up ism in n23 .
hence the elemental abundances of n23 reflect those of its environment .
figure  [ fig : ab ] shows the abundances obtained from the spectral analysis ( table  [ tab : n23bestfitpar ] ) .
( 80mm,80 mm ) ./figure
/ figure4.eps    the abundances of n , o , ne and fe agree roughly with the lmc average @xcite .
the si and mg abundances are smaller than those obtained ( @xcite ) , although their errors are relatively large .
the mg and si abundances obtained by suzaku are consistent with those from asca ( @xcite ) , respectively , which seem slightly less than the lmc average .
we observed n23 in the large magellanic cloud with suzaku , and discovered the new soft emission component with a temperature of @xmath31  kev , in addition to the @xmath320.7  kev component so far known .
this alters the estimate of the ionization parameter ( @xmath33 ) significantly from @xmath34 to @xmath35 . with the aid of imaging capability , we are able to calculate the density of the plasma , and have confirmed that the supernova explosion of n23 occurred in a high density region with an ism density of @xmath048  @xmath3 .
the relatively high ambient density may be related to the open cluster hs 114 @xcite near the remnant @xcite .
the parameters of the plasma from our analysis indicate that n23 is either at a late stage of the sedov phase or in the radiative cooling phase . assuming it is still in the sedov phase , we have estimated the age of the remnant to be @xmath08000  yr from the newly discovered soft component , which is twice as old as the estimation by @xcite .
given the uncertainty of the volume filling factor , however , we can not deny the possibility that n23 has already entered into the radiative phase .
the abundances of n23 is roughly consistent with the lmc average ( @xcite ) .
the si and mg abundances obtained by spectral fitting are smaller than those obtained by @xcite , although their errors on si and mg abundances are relatively large .
the mg and si abundances obtained by are consistent with those from ( @xcite ) .
we thank dr .
y. maeda for his useful discussion and comments .
we would like to express our gratitude to all members of the suzaku team for their contributions to the instrument preparation , spacecraft operation , software development , and in - orbit instrumental calibration .
we also thank an anonymous referee for useful comments .
x - ray emission from the supernova remnant n23 in the large magellanic cloud ( lmc ) is studied using the x - ray imaging spectrometer ( xis ) onboard . 
 thanks to superior energy resolution of the xis in the soft x - ray band , we resolved h - like and he - like oxygen emission lines from n23 with unprecedentedly high quality , and as a result , identified a new optically thin thermal emission component with a temperature @xmath00.2  kev , as well as that with a temperature of @xmath00.50.7  kev previously known . 
 this alters the estimate of the ionization timescale @xmath1 from @xmath0@xmath2 @xmath3  s to @xmath4@xmath5 @xmath3  s. under the assumption that n23 is still in the sedov phase , its age evaluated from the newly discovered low temperature component is @xmath08000  yr , although it is possible that n23 has already moved into the radiative phase . 
 the abundances of the heavy elements are found to be roughly consistent with those of the lmc average , which indicates that the origin of the x - ray emission of n23 is swept - up ambient material , as expected from its ionization timescale .