The elliptical galaxy NGC 5044 Stellar population and ionized gas

a r X i v :0711.4372v 1 [a s t r o -p h ] 27 N o v 2007Astronomy &Astrophysics manuscript no.(will be inserted by hand later)The Elliptical Galaxy NGC 5044:Stellar Population and IonizedGasM.G.Rickes 1,M.G.Pastoriza 1and Ch.Bonatto 1Universidade Federal do Rio Grande do Sul (UFRGS),Instituto de F ´ısica,Departamento de Astronomia e-mail:maurogr@if.ufrgs.br;mgp@if.ufrgs.br and charles@if.ufrgs.br Received –;accepted –Abstract.In this work we investigate the stellar population,metallicity distribution and ionized gas in the elliptical galaxy NGC 5044,using long-slit spectroscopy and a stellar population synthesis method.We found differences in the slope of metal-line profiles along the galaxy which suggests an enhancement of αelements,particularly towards the central region.The stellar population synthesis showed that the component with [Z/Z ⊙]∼0.0dominates the λ5870˚A flux in the central region of NGC 5044,contributing with ∼42%of the total flux,while in the external regions the contribution decreases to ∼8.0%.The component with [Z/Z ⊙]∼−0.4contributes with ∼32%in the central region,and ∼55%in the external regions.The component with [Z/Z ⊙]∼−1.1contributes with ∼26%in the central region,and ∼37%in the external regions.The three components have ∼1010years.The presence of a non-thermal ionization source,such as a low-luminosity AGN and/or shock ionization,is implied by the large values of the ratio [N ii ]Send offprint requests to :Ch.Bonatto -charles@if.ufrgs.brbeing larger in the southern part of the galaxy (Macchetto et al.1996;Goudfrooij 1991).The dust distribution has an irregular morphology,concentrated in the inner 10′′,where two central dark clouds can be seen (Ferrari et al.1999).NGC 5044has also been detected by IRAS at 60and 100µm and also at 6,12,and 18µm by ISO (Ferrari et al.2002).Its IRAS luminosity is 14×108L ⊙.The galaxy has a nuclear radio source and extended X-ray emission (Fabbiano et al.1992)and shows a very complex gas and stellar kinematics.The gas velocity profile is very irreg-ular,with many humps and dips;its radial velocity is systematically blueshifted with respect to the stellar sys-temic velocity by ∼60−100km s −1.Furthermore,the stellar velocity curves observed on a fairly large area near the galaxy center counter-rotate with respect to the outer regions.Very slow or no stellar rotation is detected along the galaxy’s minor axis.The velocity dispersion of the gas peaks at ∼200−230km s −1in the center,decreasing rapidly outwards (Caon et al.2000).The peculiar kine-matics and gas morphology described above may be sig-natures either of interactions of NGC 5044with the envi-ronment or a post-merger event.The goal of this paper is to investigate the stellar popu-lation,metallicity distribution and the presence of ionized gas in NGC 5044,which are fundamental parameters to understand the formation and evolution of this galaxy.2M.G.Rickes et al.:Stellar population and ionized gas in NGC5044Parameter NGC5044Table1.Data obtained at the NASA/IPAC Extragalactic Database(NED)which is operated by the Jet Propulsion Laboratory,California Institute of Technology,under contract with the National Aeronautics and Space Administration.This paper is organized as follows:Sect.2presents the observations and data reduction;Sect.3deals with the equivalent width analysis of the absorption lines and their radial dependence;Sect.4describes the stellar population synthesis,whose results are discussed in Sect.5;Sect.6 presents an analysis of the emission lines and the nature of the ionization source.Finally,in Sect.7a general dis-cussion of the results and conclusions will be drawn.2.Observations and spectra extractionNGC5044is a bright southern elliptical galaxy for which the general parameters are given in Table1.The star formation history,metallicity distribution and emission-gas properties of NGC5044are studied using long-slit spectroscopy.The observations were ob-tained with the ESO3.6m telescope at La Silla,Chile, equipped with EFOSC1.The spectra cover the range λλ5100−6800˚A with a3.6˚A/pixel resolution.The spa-tial scale of the observational configuration is0.6′′pixel−1; the slit length was3.1′while its width wasfixed at1.5′′corresponding approximately to the average seeing;two 2-dimensional spectra were obtained with the same ex-position time(approximately45minutes)and position angle P A=10◦.The slit positioning on the galaxy is shown in Fig.1(right panel),in which the dashes mark the spatial location of each extracted spectrum.Afinal one-dimensional spectrum was obtained combining(by weighted average)the two extractions for each position marked in Fig.1.Details on the extracted spectra are given in Table2. The spectra have been extracted within different areas in order to increase the signal to noise ratio(S/N),since more external regions have lowerflux.Each spectrum was cor-rected for radial velocity(V r=2704km s−1)and Galactic extinction(A V=0.07).Thefinal average spectra,nor-malized atλ5870˚A are shown in Fig.2,separately for the north and south directions on the galaxy.Table2.Spatial extractionsSpectra R/R ef f(kpc)(kpc)20.000.06bN,bS111.060.13dN,dS322.120.25fN,fS533.190.30hN,hS85M.G.Rickes et al.:Stellar population and ionized gas in NGC 504435000540058006200λ (A)0.511.522.533.5R e l a t i v e F λSouth5300570061006500λ (A)NorthCenter3.05"6.10"9.15"12.20"15.25"18.30"24.40"3.05"6.10" ]9.15"12.20"15.25"18.30"24.40"FeI5782Mg2FeI5270FeI5709FeI5406FeI5335Fig.2.Spatial extractions for the South (left panel)and North (right panel)directions normalized at λ5870˚A .The distance to the galaxy center is indicated on each spec-trum.Except for the bottom ones,the spectra have been shifted by an arbitrary constant,for clarity purposes portant absorption lines used in stellar population studies.However,in this spectral region the galaxy presents sev-eral neutral iron lines,e.g.Fe i λ5270,Fe i λ5335,Fe i λ5406,Fe i λ5709and Fe i λ5782,as well as the Mg 2band.These absorption features present strong dependence on the age and metallicity of the underlying stellar population (Bica &Alloin 1986).Previous to the measurements,all spectra have been normalized at λ5870˚A .The continuum tracing and spec-tral windows of the absorption lines used in the present paper are those defined by the Lick system (Faber et al.1985),which are reproduced in Table 3.Each equivalent width (EW)and the Mg 2index have been measured three times taking into account the uncertainties in the contin-uum level definition.This procedure allowed us to esti-mate the average value and corresponding standard devi-ation for each measurement.Although Na i λ5895is the strongest absorption feature present in our spectra,it was not used in the analysis be-cause it may be contaminated by dust concentrated in the central region of the galaxy (Ferrari et al.1999).Besides the EWs and Mg 2index,we measured as well contin-uum points at 5300˚A ,5313˚A ,5406˚A ,5800˚A ,5870˚A andTable 3.Lick indices definitionsIndex Line/Band∆λ(˚A )AC (˚A )F e λ5270Fe i5247.375—5287.3755234.875—5249.8755287.375—5319.875F e λ5406Fe i5390.250—5417.7505379.000—5390.2505417.750—5427.750F e λ5782Fe i5778.375—5798.3755767.125—5777.1255799.625—5813.375Table Notes.Column 3:Spectral window;Column 4:AdjacentContinuum.6630˚A ,as defined in Bica &Alloin (1986).These contin-uum points,normalized at λ5870˚A ,are listed in Table 5.In order to compare our results for NGC 5044with other elliptical galaxies which present similar radial gra-dients in the Mg 2band and in Fe i 5270and Fe i 5335lines (e.g.Davies et al.,1987;Carollo et al.1993),we first cor-rected these features for the line broadening due to the internal stellar velocity dispersion.The spectrum of the G giant star HR 5333was used to estimate the velocity dispersion correction.HR 5333was assumed to have zero intrinsic velocity and its spectrum was broadened with a series of gaussian filters with a ve-locity dispersion σvarying from 0−300km s −1,in steps of 10km s −1.For each absorption feature considered we calculate an empirical correction index C(σ),so that C (σ)Mg 2=Mg 2(0)W λ(σ).The velocity dispersion inNGC 5044varies from ∼260in the center to ∼230km s −1in the external regions (Caon et al.2000).We have calcu-lated the correction index C (σ)for each marked position in Fig.1using Caon et al.’s (2000)σvalues.Our correc-tion factors vary from 1.23to 1.27for the Mg 2index and from 1.20to 1.23for the Fe I lines,in the velocity range 230–260km s −1.Our correction factors for the Mg 2in-dex are somewhat larger than those given by Davies et al.(1993),while those for Fe i λ5270are similar.As can be seen in Fig.3,except for Fe i λ5406(panel(d)),the other absorption features present strong variations with distance to the center.In particular,the Mg 2band (panel (a))clearly presents a positive gradient,i.e.increas-ing towards the center of the galaxy,whereas the other Fe i lines present evidence of negative gradients.3.1.Metallicity gradientThe radial distributions of the Mg 2index and those of the EWs of Fe i 5270,Fe i 5335,Fe i 5406,Fe i 5709and Fe i 5782,corrected for velocity dispersion,are shown in Fig.??.The corrected Mg 2index (panel(a))presents an enhancement of the gradient which decreases from 0.42mag in the cen-ter to ∼0.32mag in the external regions of the galaxy.On the other hand,the corrected EW of Fe i 5270(panel(b))4M.G.Rickes et al.:Stellar population and ionized gas in NGC5044Table4.Mg2index and EWs of the Mg2and Fe i lines.R(′′)0.370±0.00813.33±0.382.67±0.111.17±0.250.96±0.321.35±0.030.97±0.06 6.10S0.339±0.01411.72±0.702.29±0.641.21±0.300.77±0.241.35±0.010.93±0.07 12.20S0.317±0.01411.69±0.702.87±0.131.31±0.300.45±0.241.45±0.150.74±0.23 18.30S0.320±0.00611.23±0.363.58±0.623.12±0.501.22±0.012.24±0.261.15±0.490.365±0.01713.29±0.381.99±0.112.00±0.250.82±0.161.08±0.130.93±0.13 6.10N0.335±0.00211.47±0.701.92±0.121.55±0.300.63±0.020.89±0.120.99±0.12 12.20N0.330±0.00511.54±0.172.21±0.131.16±0.300.66±0.200.98±0.150.64±0.15 18.30N0.320±0.01011.29±0.362.92±0.621.59±0.501.15±0.172.05±0.270.52±0.27Cλ/C5870λ5300λ5313λ5546λ5800λ5822λ66300.95±0.031.02±0.070.99±0.021.02±0.051.02±0.050.98±0.123.05S1.03±0.051.03±0.020.98±0.051.01±0.021.02±0.040.95±0.069.15S1.01±0.051.01±0.010.97±0.050.99±0.020.99±0.010.94±0.0615.25S1.01±0.041.02±0.020.98±0.041.02±0.031.01±0.030.96±0.0724.40S3.05N0.99±0.041.00±0.020.95±0.061.00±0.011.01±0.030.94±0.059.15N1.00±0.041.00±0.030.97±0.010.99±0.031.01±0.020.95±0.0615.25N1.00±0.061.00±0.020.97±0.021.00±0.011.00±0.020.94±0.0524.40NTable Notes.N and S correspond to north and south respectively.increases from2.30˚A in the center to∼3.57˚A in the ex-ing Poveda’s approximation,M tot=0.9σ2R eternal regions.This result is in reasonable agreement withCarollo et al.(1993)who found no correlation of the Mg2index with either Fe i5270and Fe i5335.In Fig.4we plotthe velocity dispersion corrected values of the Mg2indexvs.the EWs of Fe i5270and Fe i5335,in which we see nocorrelation between these features.The different slope ofthe Mg2gradient relative to those of Fe i5270and Fe i5335can be accounted for by an enhancement ofα-elements ingeneral,Mg in particular(Faber et al.1992and Wortheyet al.1994).In NGC5044this effect is more intense in thecenter than in the outer regions.The dependence of metallicity gradients on fundamen-tal parameters was investigated by Carollo et al.(1993)who found that the Mg2gradient shows a bimodal trend.For objects with masses lower than1011M⊙,the Mg2gradient slope correlates with galaxy mass,whereas forobjects more massive than1011M⊙,no such correlationappears.In order to compare our results with the data of Carolloet al.(1993),first we estimate the mass of NGC5044us-M.G.Rickes et al.:Stellar population and ionized gas in NGC 50445R (arcsec)123E W (A )1234E W (A )0.30.350.40.45m a g n i t u d eR (arcsec)0.511.5E W (A )123E W (A )0.511.5E W (A )Fig.3.Spatial variation of absorption features in NGC 5044.which are based on models of galactic globular clus-ters with 18Gyrs of age.From Figure 1of Worthey et al.(1992)we found that for our corrected Mg 2index,[Fe/H]=0.5and 0.17in the central and externalregions,respectively.In the same way,using Fe i 5270we found [Fe/H]=−0.5,0.2;and for Fe i 5335,we found [Fe/H]=−1.0,−0.5in the central and external regions,respec-tively.Therefore,we found an [Fe/H]abundance above solar using as metallicity indicator the Mg 2index,while [Fe/H]is lower than solar for Fe i 5270and Fe i 5335lines;in addition Mg is above solar in NGC 5044.4.Stellar population synthesisFor a good understanding of some properties of NGC 5044,such as the presence of central gas emission,a precise determination of the star formation process is necessary.Therefore,the age of the constituent stars in NGC 5044is an important parameter to be determined.The integrated spectrum of a given galaxy contains sig-nificant information on its stellar content and chemical en-richment (Bica &Alloin,1985).In principle,this informa-tion together with a stellar population synthesis method,can be used to determine the star formation history (Bica &Alloin,1986).In this work we employ the stellar pop-ulation synthesis method developed by Bica (1988)whichMg2 (mag.)0.00.51.01.52.02.53.03.5E W (A )1.01.52.02.53.03.54.04.5E W (A )Fig.4.Mg 2index vs.Fe i 5270and Fe i 5335line strength.is based on integrated spectra of star clusters and H ii regions,characterized by different ages and metallicities.In the present case,due to the small number of observa-tional constraints (EWs and continuum points),we use three old components with different metallicities in order to minimize the degeneracy between age and metallicity of the different population components.These metallici-ties components (templates)are:G 1→[Z/Z ⊙]∼0.0,G 2→[Z/Z ⊙]∼−0.4and G 3→[Z/Z ⊙]∼−1.1,the three with age 1010years.Alternatively,we have also performed a stellar popu-lation synthesis with a set of three components with sim-ilar (solar)metallicities and different ages:G1(1010yr),Y3(108yr)and RHII (106yr).We found that the spectra presented in Fig.2are better fitted by a range in metal-licities than a range in ages.The base elements that we use are taken from the star cluster population templates described in Bica &Alloin (1986),and are presented in Fig.5.The corresponding EWs and continuum points for the base elements have been measured similarly as those for NGC 5044and are given in Tables 6and 7,respec-tively.We remind that,for the stellar population analysis we use the EW of Mg 2,which is given in column (3)of Table 4.Basically the algorithm uses the selected EWs and pivot points for the continuum measured in a given spec-trum and compares them to those of a model computed from a base of simple stellar population elements.The6M.G.Rickes et al.:Stellar population and ionized gas in NGC 50445100535056005850610063506600λ (A)0.650.750.850.951.05R e l a t i v e F λG1G2G3Fig.5.Population templates (age components)used in the synthesis,normalized at λ5870˚A .algorithm is not a minimization procedure,instead it gen-erates all combinations of the base elements according to a given flux contribution step.The code also successively dereddens galaxy input continuum points and compares them against a given base model.Every combination is subsequently tested against a set of windows of maximum allowed difference between the observed and the resulting features obtained.Finally,the solutions satisfying all fea-ture windows are averaged out,and this average is adopted as the final synthesis solution.Initially we used a 10%step for testing flux contribu-tion at λ5870˚A generating about ∼5200combinations for each assumed E (B −V )i .Reddenings were tested in the range 0.00≤E(B-V)i ≤0.50with a step of 0.01.Thus,in total,∼260000combinations are tested for each ex-traction,and the number of possible solutions amounts to less than 1%.After probing as above the space of combi-nations,we calculate the solution with finer steps of 5%,2%and finally 1%.We tested the Galactic (Seaton 1979),LMC (Fitzpatrick 1986)and SMC (Pr´e vot et al.1984)reddening laws for the synthesis of each extraction,and concluded that the Galactic law applies in all cases.5.Synthesis resultsThe results of our synthesis are given in Table 8,where we present the percentage contribution of each base elementTable 6.Equivalent widths for the baseMg 2band Fe i 5270Fe i 5335Fe i 5406Fe i 5709Fe i 5782∼0.0∼−0.4∼−1.1Table 7.Continuum points for the baseG 10.970.980.991.011.000.94G 3to the flux at λ5870˚A.The values of E(B −V)i are also given in the table.The G1component ([Z/Z ⊙]∼0.0)dominates the λ5870˚A flux in the central extractions of NGC 5044.In the nucleus,the G1component contributes with ∼42%of the total flux,while in the external regions the contri-bution decreases to ∼8.0%.The G2component ([Z/Z ⊙]∼−0.4)contributes with ∼32%in the central region,and ∼55%in the external re-gions (24.40′′).The G3component ([Z/Z ⊙]∼−1.1)con-tributes with ∼26%in the central region,and ∼37%in the external regions,as can be seen in Table 8.Flux frac-tions and the internal reddening as a function of distance to the center are shown in Fig.7.Notice that the spa-tial distribution of the internal reddening E(B −V)i along the north and south directions (Table 8and panel (d)in Fig.7)does not characterize a gradient.The spectra representing the stellar population of NGC 5044have been constructed using the star cluster templates (Sect.4)combined according to the propor-tions given by the synthesis.We illustrate this procedure in Fig.6in which we show the synthesis for the central extraction (top panel).The resulting pure emission spec-trum,which is obtained after the subtraction of the pop-ulation template,is shown in the bottom panel.The goodness of the above stellar population synthesis method can be assessed by the residuals both in the EWs and continuum points.As can be seen in Table 9,most of the absorption features are reasonably well reproduced by the synthesis method.The residuals are almost equally distributed between positive and negative ones,probably indicating that NGC 5044has the same metallicity than that of the templates.The continuum fit is quite good as well,as can be seen by the residuals in Table 10.5.1.Integrated Colour IndexIn order to put NGC 5044in the context of other elliptical galaxies rich in interestellar medium,we build the colour-colour diagram (J-K)vs.(V-K),using the J,K and V photometric data for a sample of elliptical galaxies from Rembold et al.(2002)and NED.The results are presentedM.G.Rickes et al.:Stellar population and ionized gas in NGC 504475100560061006600λ (A)R e l a t i v e F λ1(obs)Σ = (b) + (c) + (d)(a) (b) (c)Obs. − Synth.Central extraction [(E(B−V) = 0.15]Fig.6.Stellar population synthesis of the central extrac-tion.Top panel:(obs)-observed spectrum corrected for reddening;(a)-G1population template;(b)-G2popu-lation template;(c)-G3population template and ()-synthesized spectrum.Bottom panel:pure emission spec-trum.Spectrum (obs)has been shifted by a constant.Table 8.Synthesis results in terms of flux fractionsFlux fraction at λ5870˚A G1G2G3E (B −V )i0.0023.43±2.0442.29±2.4034.29±0.880.006.10S 23.79±1.7632.46±2.2743.75±1.20.0112.20S 11.13±2.7944.95±3.4043.92±1.450.0218.30S 9.57±2.7558.16±3.1432.27±1.010.0119.09±3.1745.17±3.2535.74±0.740.016.10N 22.69±2.5337.93±3.8939.38±2.130.0112.20N 14.33±1.4984.00±2.581.67±1.490.0018.30N7.83±1.8849.64±2.6942.53±1.200.00Mg 2band Fe i 5270Fe i 5335Fe i 5406Fe i 5709Fe i 57820.48−0.21−0.34−0.73−0.020.023.05S0.850.690.19−0.290.410.389.15S 0.570.47−0.16−0.490.370.0415.25S 0.801.161.19−0.441.670.6624.40S 3.05N 0.67−0.200.30−0.370.150.089.15N 0.79−0.40−0.12−0.41−0.220.0115.25N0.18−0.010.070.030.080.6224.40NTable Notes.Residuals correspond to the observed −synthe-sized values.index,NGC 5044also presents mid-IR (6.7to 15µm )dust emission(Ferrari et al.2002).6.Ionized gasEmission gas has been detected in a large number of elliptical galaxies.However,the origin of this gas and the ionization source are not yet conclusively established.NGC 5044presents conspicuous emission lines throughout8M.G.Rickes et al.:Stellar population and ionized gas in NGC5044 Table10.Continuum residualsR(′′)0.000.040.05−0.020.010.000.01 6.10S0.060.07−0.020.000.00−0.01 12.20S0.080.080.02−0.010.00−0.02 18.30S0.060.070.020.020.010.000.040.04−0.030.000.010.01 6.10N0.050.05−0.010.01−0.010.00 12.20N0.060.060.000.020.000.01 18.30N0.040.04−0.010.010.000.00lines FλFλ(10−16erg s−1cm−2)(˚A)Hα6.80±0.231.0013.62 [NII]λ65847.33±0.251.07±0.0521.27[SII]λ67315.83±0.110.85±0.0323.64Hα4.29±0.110.63±0.0311.82 [NII]λ65842.44±0.060.35±0.0111.21[SII]λ67311.96±0.060.28±0.0111.10Hα3.27±0.140.48±0.0311.78 [NII]λ65841.97±0.080.28±0.0112.24[SII]λ67311.42±0.070.21±0.0110.73Hα2.91±0.110.43±0.0215.08 [NII]λ65841.45±0.050.21±0.0112.19[SII]λ67311.24±0.070.18±0.0112.12Hα2.49±0.090.37±0.0218.32 [NII]λ65840.78±0.030.11±0.0012.59[SII]λ67310.57±0.040.08±0.0011.12Hα3.39±0.200.49±0.0332.46 [NII]λ65840.80±0.050.11±0.0110.30[SII]λ67311.15±0.050.17±0.0112.84Hα3.43±0.130.50±0.0212.55 [NII]λ65841.82±0.070.26±0.0111.88[SII]λ67311.15±0.170.17±0.0210.26Hα4.06±0.230.60±0.0417.73 [NII]λ65841.62±0.090.24±0.0113.16[SII]λ67311.09±0.140.16±0.0212.98[NII]λ65487.84±0.621.15±0.1011.853.05N[SII]λ67172.32±0.450.34±0.0610.73[NII]λ65484.56±0.230.67±0.0412.116.10N[SII]λ67170.96±0.070.14±0.0110.61[NII]λ65483.51±0.190.51±0.0312.809.15N[SII]λ67170.72±0.060.10±0.0111.35[NII]λ65482.65±0.210.39±0.0315.8512.20N[SII]λ67170.50±0.020.07±0.0011.00[NII]λ65482.41±0.120.35±0.0210.3115.25N[SII]λ67170.40±0.090.06±0.0110.89[NII]λ65481.93±0.130.28±0.0211.8818.30N[SII]λ67170.54±0.140.08±0.028.76[NII]λ65484.82±0.270.71±0.0510.7124.40N[SII]λ67171.60±0.130.23±0.0211.87 Table Notes.Column4:Flux normalized to the central region F Hα;Column5:FWHM according to a gaussianfit.region,and the FWHM for the above emission lines.The uncertainty attributed to each measuredflux is based on a gaussian least-squaresfit.Assuming that the emission lines are formed only by recombination,the number of ionizing photons(Q(H))can be calculated asQ(H)=L HααHα(H0,T),(1)where the Hαluminosity is L Hα=4πD2F Hα,αB(H0,T) is the recombination coefficient summed over all energy levels,andαHα(H0,T)is the recombination coefficient at Hα.With a Hubble constant of H0=75km s−1Mpc−1,M.G.Rickes et al.:Stellar population and ionized gas in NGC50449 Table12.Luminosity and ionizing photonsR(′′)L HαQ(H)(×1037erg s−1)(×1050photons s−1)3.05N7.33±0.091.81±0.036.10N5.79±0.041.42±0.019.15N3.22±0.030.78±0.0112.20N1.93±0.010.46±0.0015.25N5.54±0.041.33±0.0118.30N2.04±0.010.51±0.0024.40N6.16±0.041.52±0.01Hα(panel (a)),[S ii]λλ6717,31M⊙Number N Lyα(×1048photons s−1)Table Notes.Column3:Number of stars of each spectral type.In the central region,NGC5044presents[N ii]Hα>1.Notice that the ratio[N ii]Hα>2.However,ac-cording to the ionization models for old stars in elliptical galaxies(Binette et al.1994)the ratio N[ii]Hα.A high ratio N[ii]Hα∼2can be accounted for by post-AGB stars.7.ConclusionsThe stellar population,metallicity distribution and ion-ized gas in NGC5044have been investigated in this pa-per by means of long-slit spectroscopy and stellar popula-tion synthesis.With respect to the spatial distributionof10M.G.Rickes et al.:Stellar population and ionized gas in NGC 5044Diameter (kpc)−50050015002500350045005500F l u xFig.10.Spatial profiles of H αand [NII].Notice that [NII]/H α>1along the galaxy.metal-strength indices (corrected for velocity dispersion),the Mg 2profile increases towards the central region,con-trary to what is observed for the profile of Fe i 5270.This difference in slopes may be accounted for by an enhance-ment of α-elements in general,Mg in particular.The Mg 2gradient slope compared with the mass of NGC 5044in-dicates a monolitic collapse.However,the measure [Fe/H]gradient slope is not characteristic of one monolitic col-lapse.The stellar population synthesis gives that most metal-lic component ([Z/Z ⊙]∼0.0)dominates the λ5870˚A flux in the central region of NGC 5044.In the nucleus,this component contributes with ∼42%of the total flux,while in the external regions the contribution decreases to ∼8.0%.The component with [Z/Z ⊙]∼−0.4contributes with ∼32%in the central region,and ∼55%in the exter-nal regions.The metal-poor component ([Z/Z ⊙]∼−1.1)contributes with ∼26%in the central region,and ∼37%in the external regions.The spatial distribution of the in-ternal reddening E(B −V)i along the north and south di-rections does not characterize a gradient.The large values of the ratio [N ii ]−30−20−100102030R (arcsec)0123E W (A )1234E W (A )0.30.350.40.45m a g n i t u d e−30−20−100102030R (arcsec)0.511.5E W (A )123E W (A )0.511.5E W (A)。