The Metal Content of Dwarf Starburst Winds: Results from Chandra Observations of NGC 1569

By Crystal L. Martin, H. Kobulnicky, T. Heckman. 2002

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We present deep Chandra spectral imaging of the dwarf starburst galaxy NGC 1569. The unprecedented spatial resolution allows us to spatially identify the components of the integrated X-ray spectrum. Fitted spectral models require an intrinsic absorption component and higher metal abundances than previous studies indicated. Our results provide the first direct evidence for metal-enriched winds from dwarf starburst galaxies. We identify 14 X-ray point sources in NGC 1569. Most have properties consistent with those of high-mass X-ray binaries, but one is a steep-spectrum radio source that is probably a supernova remnant. The X-ray luminosity of NGC 1569 is dominated by diffuse, thermal emission from the disk (0.7 keV) and bipolar halo (0.3 keV). Photoelectric absorption from the inclined H I disk hardens the X-ray spectrum on the northern side of the disk relative to the southern side. Requiring the fitted absorption column to match the H I column measured at 21 cm implies that the metallicity of the H I disk is significantly less than solar but greater than 0.1 Z☉. Hence, much of the H I is enriched to levels comparable to the metallicity of the H II regions [O/H = 0.2(O/H)☉]. The X-ray color variations in the halo are inconsistent with a free-streaming wind and probably reveal the location of shocks created by the interaction of the wind with a gaseous halo. The X-ray spectrum of the diffuse gas presents strong emission lines from α-process elements. Fitted models require α-element abundances greater than 0.25 Zα, ☉ and ratios of α-elements to iron 2-4 times higher than the solar ratio. The best fit to the spectrum is obtained with solar mass fractions for the α-elements, 1.0 Zα, ☉, but a degeneracy between the metallicity and the spectral normalization prevents us from deriving an upper limit on the wind metallicity from the X-ray spectrum alone. We argue, however, that abundances larger than 2.0 Zα, ☉ pose awkward implications for the dynamical evolution of the wind based on our knowledge of the starburst properties. For consistency with our best-fitting abundances, the mass of interstellar gas entrained in the wind must be about 9 times the mass of stellar ejecta in the wind. Most of the oxygen carried by the wind comes from the stellar ejecta rather than entrained interstellar gas. The estimated mass of oxygen in the hot wind, 34,000 M☉, is similar to the oxygen yield of the current starburst. Apparently the wind carries nearly all the metals ejected by the starburst. These metals appear destined to contribute to the enrichment of the intergalactic medium. Much of the nucleosynthesis in NGC 1569 must have occurred during less violent periods of star formation, however, because our measurements imply that the neutral gas disk holds at least 5 times more oxygen than wind.

Subfields: Astronomy and Astrophysics

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Citations in the corpus

• DEMOGRAPHICS AND PHYSICAL PROPERTIES OF GAS OUTFLOWS/INFLOWS AT 0.4 < z < 1.4 (2012, background): “The escape of metals from low-mass galaxies in the local universe has been established empirically (Martin 1999; Martin et al. 2002; Tremonti et al. 2007).”
• Three-Dimensional Simulations of a Starburst-driven Galactic Wind (2008, result): “One of the most striking results of these observations is the close spatial relationship with the H -emitting gas (e.g., Strickland et al. 2000, 2002, 2004a; Cecil et al. 2002; Martin et al. 2002; Grimes et al. 2005; Ott et al. 2005), suggesting a close physical connection.”
• On Extending the Mass-Metallicity Relation of Galaxies by 2.5 Decades in Stellar Mass (2006, background): “Martin et al. (2002) showed in their Chandra observations that most of the oxygen produced in the starburst is in the enriched outflow.”
• Metallicity gradients in local field star-forming galaxies: insights on inflows, outflows, and the coevolution of gas, stars and metals (2015, background): “Evidences of the hot, wind fluid being more enriched than the ISM have been reported at least in one nearby dwarf galaxy NGC 1569 (Martin et al. 2002), perhaps indicating that the hot materials can survive the gravitational potential better than the cold entrained gas.”
• The Distribution of Metallicity in the Intergalactic Medium at z ~ 2.5: O VI and C IV Absorption in the Spectra of Seven QSOs (2003, background): “For example, the nearby dwarf NGC 1569 is driving a galactic wind that contains ∼ 34, 000M⊙ of oxygen at Z ∼ Z⊙ (Martin et al. 2002).”
• Galactic winds driven by cosmic ray streaming (2012, background): “The biconical structure of the hot gas cavities that formed in the 1010 h−1 M halo has some similarities with Hα-structures, e.g. in the well-known starburst galaxy M82 (Bland & Tully 1988) or with the dwarf starburst galaxy NGC 1569 (Martin et al. 2002).”
• The role of dwarf galaxy interactions in shaping the Magellanic System and implications for Magellanic Irregulars (2012, background): “Following Martin et al. (2002), the initial mass function (IMF) averaged metallicity of SNe ejecta is ZO,SN ∼ 8 times solar for oxygen.”
• ISM properties in low-metallicity environments II. The dust spectral energy distribution of NGC 1569 (2003, background): “Recent Chandra observations find large inhomogeneities in the metal abundances with ranges from 0.1 Z to 1 Z (Martin et al. 2002), evidence that the ISM has been affected by numerous supernovae explosions since the last burst.”
• THREE-DIMENSIONAL INTEGRAL FIELD OBSERVATIONS OF 10 GALACTIC WINDS. I. EXTENDED PHASE (≳10 Myr) OF MASS/ENERGY INJECTION BEFORE THE WIND BLOWS (2010, result): “Such an effect may have already been seen in dwarf starburst galaxies (Martin et al. (2002), Ott, Walter & Brinks (2005)), but we are unaware of similar studies in Seyfert outflow sources.”
• THE EFFECT OF STARBURST METALLICITY ON BRIGHT X-RAY BINARY FORMATION PATHWAYS (2010, methodology): “Using the Chandra X-Ray telescope, Martin et al. (2002) found 14 X-Ray point sources in NGC 1569, 12 of which are probable X-ray binaries.”
• A High Spatial Resolution X-Ray and Hα Study of Hot Gas in the Halos of Star-forming Disk Galaxies. I. Spatial and Spectral Properties of the Diffuse X-Ray Emission (2003, background): “Dust destruction by sputtering has a timescale comparable to the 107–108 yr timescale appropriate for winds and fountains (Popescu et al. 2000), so it is plausible that some fraction of the refractory elements are retained on dust grains in even hot halo gas.”
• Mapping Large-Scale Gaseous Outflows in Ultraluminous Galaxies with Keck II ESI Spectra: Variations in Outflow Velocity with Galactic Mass (2004, background): “Metallicity measurements from Xray spectra indicate the hot wind carries the heavy elements produced by the starburst population (Martin et al. 2002).”
• The Mass-Metallicity Relation at z≳2 (2006, background): “Metal-enhanced hot winds have been observed in X-rays in local starbursts; in dwarf galaxies, such winds may carry away nearly all the oxygen produced by the burst (Martin et al. 2002).”