X-ray gaseous emission in the galaxy M82
Piero Ranalli, Andrea Comastri, Livia Origlia, Roberto Maiolino
aa r X i v : . [ a s t r o - ph . GA ] A p r X-ray gaseous emission in the galaxy M82
Piero Ranalli ∗ , Andrea Comastri † , Livia Origlia † and Roberto Maiolino ∗∗ ∗ Università di Bologna, via Ranzani 1, 40127 Bologna, Italy † INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy ∗∗ INAF – Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Roma, Italy
Abstract.
The main results from a deep X-ray observation of M82 are summarised: spatially-dependent chemical abundances, temperature structure of the gas, charge-exchange emission linesin the spectrum. We also present an update of the chemical abundances, based on a more refinedextraction of spectra.
Keywords: galaxies: individual: M82 – galaxies: abundances – X-rays: ISM – plasmas – atomicprocesses.
PACS:
INTRODUCTION
We performed a very deep (100 ks) observation of the starburst galaxy M82 with theEPIC and RGS instruments on-board XMM-
Newton . The analysis has been publishedin [1]; we refer for any detail to that paper. A brief summary of the main results ispresented in the following.M82 has a large and luminous outflow, extending for several kpc out of both sides ofthe galaxy plane. On the sky, it extends for about 10 arcmin. A good-quality spectrumwas obtained with the RGS instrument. However, the main difficulty in analysing X-ray grating spectra from extended sources is the dependence of the line-spread-functionfrom the spatial shape of the source. The rgsxsrc model which convolves the spectralresponse with the shape of the source was essential to this analysis. Also, the parts ofthe outflow which lie inside of the galactic disc are heavily absorbed: thus the outflowappears with a different shape at different energies. For this reason, we had to analyseseparately the long- (18–30 Å) and short-wavelength (6-18 Å) regions of the spectrum.The EPIC instrument allowed the spatially-resolved spectroscopy of the outflow,albeit at a lower resolution than RGS. The outflow was divided in eleven slices, eachone parallel to the galactic plane, and spectra were extracted and analysed for each slice,deriving the temperature structure, abundances, and physical parameters of the plasma.Finally, we publish here update figures and table: we refined the extraction regions,after discovering that the regions used for spectral extractions in [1] were actually largerthan intended. No significant change in any conclusion or derived value has occurred.
MAIN RESULTS
At least three spectral components are present in the broad-band spectrum: i) continuumemission from point sources; ii) thermal plasma emission from hot gas; iii) charge
ABLE 1.
Physical parameters of the plasma across the different regions of the outflow. The dependencies on the fillingfactor f have been explicited for simplicity only in the first column, but they apply to all columns. N5 N4 N3 N2 N1 centre S1 S2 S3 S4 S5
Normalisation (10 − ) 0.079 0.24 0.50 0.41 0.85 27 1.7 0.92 0.20 0.099 0.047Volume (kpc ) 0.61 1.2 1.1 0.43 0.41 1.4 0.24 0.35 0.26 0.21 0.22Density (10 − cm − ) 2 . f − − dine cm − ) 2 . f − M ⊙ ) 0 . f + erg) 0 . f + − erg cm − ) 4 . f + f +
500 380 360 300 100 150 250 400 380 490 e x c h a ng ee m i ss i on fr o m n e u t r a l m e t a l s ( M g a nd S i ) . T h ec h e m i ca l a b s o l u t ea bund a n ce s o f t h e t h e r m a l p l a s m a d e p e ndon t h e d i s t a n ce fr o m t h e g a l ac ti c p l a n e :t h e y a r e l a r g e r i n t h e ou t s k i r t s a nd s m a ll e r c l o s e t o t h e g a l a xy ce n t r e . T h ea bund a n ce r a ti o s a l s o s ho w s p a ti a l v a r i a ti on s ( s ee b e l o w ) . T h i s m i gh t b e du e t o t h e d e p e nd e n ce o f s up e r nov a ( S N ) y i e l d s on t h e p r og e n it o r m a ss , i f t h e m a tt e r e xp e ll e dby t h e fi r s t S N i n a s t a rf o r m a ti onbu r s ti s a l s o t h e fi r s t on e t o t r a v e l ou t o f t h e g a l ac ti c p l a n e . t may also be a clear example of metals being pushed in the inter-galactic medium.The X-ray derived Oxygen abundance is lower than that measured in the atmospheresof red supergiant stars, leading to the hypothesis that a significant fraction of Oxygenions has already cooled off and no longer emits at energies > ∼ . > . ∼ SPATIALLY-DEPENDENT CHEMICAL ABUNDANCES
We defined 11 regions in the outflow of M82, slicing it parallel to the galactic plane.In the figures, negative and positive heights above the galactic plane are assigned to thenorthern and southern regions, respectively. The spectral model was a multi-temperatureAPEC plasma, with temperatures ranging from 0.1 to 10 keV. Point sources wereexcluded from all spectral regions except the galaxy centre, where their contribution wasadded to the model in the form of an absorbed power-law. The best-fitting abundancesare shown in Fig. 1 (left panels) along with results from infrared observations for thecentral regions [5] which show the abundances of stars born before the start of the currentburst. Lighter a -elements are more concentrated in the outflow than in the centre. Thiseffect is larger for elements with lower atomic mass, becomes less evident for Si andreverses for S. The centre/outskirt abundance ratio in the centre is about ∼ /
10 for Oand Ne. Fe is also more concentrated in the outflow.The abundances ratios (Fig. 1 right panel) have smaller variations, and present differ-ent trends for light and massive elements: while the O/Fe and Ne/Fe ratios are lower inthe centre than in the outskirts, the opposite holds for Si/Fe and S/Fe, with Mg/Fe beingan intermediate case showing no variation. The scatter between values for centre andoutskirts is a factor < ∼ ∼
10 from the centre to the outskirts, while the cooling time increases.
FIGURE 1.
Left panels:
Variation of chemical abundances with increasing height on the galactic plane.Black: abundances from X-ray MOS and pn data. Dark grey: abundances from X-ray RGS data (due to thecharacteristics of the RGS spectrometer, they represent space-averaged values). Light grey: abundancesfrom infrared data (corresponding to red supergiant stars in the galaxy central region). Negative values ofdistance refer to the north direction, positive values to south. Right panel:
Abundance ratios (X/Fe) observed in EPIC spectra of the outflow. Negative values of heightrefer to the north direction, positive values to south.
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