Absorption‐Line Probes of Gas and Dust in Galactic Superwinds (original) (raw)
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On the enrichment of the intergalactic medium by galactic winds
Monthly Notices of the Royal Astronomical Society, 1997
Observations of metal lines in Lyα absorption systems of small H I column density and their ubiquitous nature suggest that the intergalactic medium (IGM) was enriched to about Z ∼ 0.01 Z ⊙ by a redshift z ∼ 3. We investigate the role of winds from small star-forming galaxies at high z in enriching the IGM. The existence of large numbers of small galaxies at high z follows naturally from hierarchical clustering theories (e.g. CDM). For analytical simplicity we assume that the galactic winds escape the galaxies at a single characteristic redshift z in , and we model the galactic winds as spherical shock waves propagating through the IGM. We then calculate the probability distribution of the metallicity of the IGM, as a function of time (for different values of z in), adopting plausible galaxy mass functions (from Press-Schechter formalism), cooling physics, star-formation efficiencies, gas ejection dynamics, and nucleosynthesis yields. We compare this expected distribution with the observed distribution of metallicities in the Lyα forest at z = 3, the metal poor stars in the halo of our Galaxy, and with other observational constraints on such a scenario. We find that galactic winds at high z could have enriched the IGM to a mean metallicity of Z ∼ 0.01Z ⊙ at z ∼ 3, with a standard deviation of the same order, if z in < ∼ 5, and that this satisfies all the observational constraints.
2016
We present an analysis of the Mg II λλ2796, 2803 and Fe II λλ2586, 2600 absorption line profiles in individual spectra of 105 galaxies at 0.3 < z < 1.4. The galaxies, drawn from redshift surveys of the GOODS fields and the Extended Groth Strip, fully sample the range in star formation rates (SFRs) occupied by the star-forming sequence with stellar masses log M * /M ⊙ 9.5 at 0.3 < z < 0.7. Using the Doppler shifts of the Mg II and Fe II absorption lines as tracers of cool gas kinematics, we detect large-scale winds in 66 ± 5% of the galaxies. High-resolution Hubble Space Telescope/Advanced Camera for Surveys imaging and our spectral analysis indicate that the outflow detection rate depends primarily on galaxy orientation: winds are detected in ∼ 89% of galaxies having inclinations (i) < 30 • (face-on), while the wind detection rate is only ∼ 45% in objects having i > 50 • (edge-on). Combined with the comparatively weak dependence of the wind detection rate on intrinsic galaxy properties (including SFR surface density), this suggests that biconical outflows are ubiquitous in normal, starforming galaxies at z ∼ 0.5, with over half of the sample having full wind cone opening angles of ∼ 100 •. We find that the wind velocity is correlated with host galaxy M * at 3.4σ significance, while the equivalent width (EW) of the flow is correlated with host galaxy SFR at 3.5σ significance, suggesting that hosts with higher SFR may launch more material into outflows and/or generate a larger velocity spread for the absorbing clouds. The large (> 1Å) Mg II outflow EWs typical of this sample are rare in the context of Mg II absorption studies along QSO sightlines probing the extended halos of foreground galaxies, implying that this wind material is not often detected at impact parameters > 10 kpc. Assuming that the gas is launched into dark matter halos with simple, isothermal density profiles, the wind velocities measured for the bulk of the cool material (∼ 200 − 400 km s −1) are sufficient to enable escape from the halo potentials only for the lowest-M * systems in the sample. However, the highest-velocity gas in the outflows typically carries sufficient energy to reach distances of 50 kpc, and may therefore be a viable source of cool material for the massive circumgalactic medium observed around bright galaxies at z ∼ 0.
Galactic outflows and evolution of the interstellar medium
Monthly Notices of the Royal Astronomical Society, 2012
We present a model to self-consistently describe the joint evolution of starburst galaxies and the galactic wind resulting from this evolution. This model will eventually be used to provide a subgrid treatment of galactic outflows in cosmological simulations of galaxy formation and the evolution of the intergalactic medium (IGM). We combine the population synthesis code Starburst99 with a semi-analytical model of galactic outflows and a model for the distribution and abundances of chemical elements inside the outflows. Starting with a galaxy mass, formation redshift, and adopting a particular form for the star formation rate, we describe the evolution of the stellar populations in the galaxy, the evolution of the metallicity and chemical composition of the interstellar medium (ISM), the propagation of the galactic wind, and the metal-enrichment of the intergalactic medium. The model takes into account the full energetics of the supernovae and stellar winds and their impact on the propagation of the galactic wind, the depletion of the ISM by the galactic wind and its impact on the subsequent evolution of the galaxy, as well as the evolving distributions and abundances of metals in the galactic wind. In this paper, we study the properties of the model, by varying the mass of the galaxy, the star formation rate, and the efficiency of star formation. Our main results are the following: (1) For a given star formation efficiency f * , a more extended period of active star formation tends to produce a galactic wind that reaches a larger extent. If f * is sufficiently large, the energy deposited by the stars completely expels the ISM. Eventually, the ISM is being replenished by mass loss from supernovae and stellar winds. (2) For galaxies with masses above 10 11 M ⊙ , the material ejected in the IGM always falls back onto the galaxy. Hence lower-mass galaxies are the ones responsible for enriching the IGM. (3) Stellar winds play a minor role in the dynamical evolution of the galactic wind, because their energy input is small compared to supernovae. However, they contribute significantly to the chemical composition of the galactic wind. We conclude that the history of the ISM enrichment plays a determinant role in the chemical composition and extent of the galactic wind, and therefore its ability to enrich the IGM.
The Astronomical Journal, 2000
Arcsecond-resolution X-ray imaging of the nucleus of the nearby starburst galaxy NGC 253 with Chandra reveals a well-collimated, strongly limb-brightened, kiloparsec-scale conical outflow from the central starburst region. The outflow is very similar in morphology to the known Hα outflow cone, on scales down to 20 pc. This provides, for the first time, robust evidence that both X-ray and Hα emission come from low volume filling factor regions of interaction between the fast energetic wind of SN-ejecta and the denser ambient interstellar medium (ISM), and not from the wind fluid itself. We provide estimates of the (observationally and theoretically important) filling factor of the X-ray emitting gas, of between ∼ 4 and 40 per cent, consistent with an upper limit of ∼ 40 per cent based directly on the observed limb-brightened morphology of the outflow. Only 20 per cent of the observed X-ray emission can come from the volume-filling, metal-enriched, wind fluid itself. Spatially-resolved spectroscopy of the soft diffuse thermal X-ray emission reveals that the predominant source of spectral variation along the outflow cones is due to strong variation in the absorption, on scales of ∼ 60 pc, there being little change in the characteristic temperature of the emission. We show that these observations are easily explained by, and fully consistent with, the standard model of a superwind driven by a starburst of NGC 253's observed power. If these results are typical of all starburst-driven winds, then we do not directly see all the energy and gas (in particular the hot metal-enriched gas) transported out of galaxies by superwinds, even in X-ray emission.
The Astrophysical Journal, 2014
We present the first observation of a galaxy (z = 0.2) that exhibits metal-line absorption back-illuminated by the galaxy ("down-the-barrel") and transversely by a background quasar at a projected distance of 58 kpc. Both absorption systems, traced by Mg II, are blueshifted relative to the galaxy systemic velocity. The quasar sight-line, which resides almost directly along the projected minor axis of the galaxy, probes Mg I and Mg II absorption obtained from Keck/LRIS and Ly α, Si II and Si III absorption obtained from HST/COS. For the first time, we combine two independent models used to quantify the outflow properties for down-the-barrel and transverse absorption. We find that the modeled down-the-barrel deprojected outflow velocities range between V dtb = 45 − 255 km s −1 . The transverse bi-conical outflow model, assuming constant-velocity flows perpendicular to the disk, requires wind velocities V out f low = 40 − 80 km s −1 to reproduce the transverse Mg II absorption kinematics, which is consistent with the range of V dtb . The galaxy has a metallicity, derived from Hα and N II, of [O/H]=−0.21±0.08, whereas the transverse absorption has [X/H] = −1.12 ± 0.02. The galaxy star-formation rate is constrained between 4.6-15 M ⊙ yr −1 while the estimated outflow rate ranges between 1.6-4.2 M ⊙ yr −1 and yields a wind loading factor ranging between 0.1 − 0.9. The galaxy and gas metallicities, the galaxy-quasar sight-line geometry, and the down-the-barrel and transverse modeled outflow velocities collectively suggest that the transverse gas originates from ongoing outflowing material from the galaxy. The ∼1 dex decrease in metallicity from the base of the outflow to the outer halo suggests metal dilution of the gas by the time it reached 58 kpc.
The Astronomical …, 2010
We analyze star forming galaxies drawn from SDSS DR7 to show how the interstellar medium (ISM) Na I λλ5890, 5896 (Na D) absorption lines depend on galaxy physical properties, and to look for evidence of galactic winds. We combine the spectra of galaxies with similar geometry/physical parameters to create composite spectra with signal-to-noise ∼ 300. The stellar continuum is modeled using stellar population synthesis models, and the continuum-normalized spectrum is fit with two Na I absorption components. We find that: (1) ISM Na D absorption lines with equivalent widths EW > 0.8 Å are only prevalent in disk galaxies with specific properties -large extinction (A V ), high star formation rates (SFR), high star formation rate per unit area (Σ SFR ), or high stellar mass (M * ).
Starburst-driven Superwinds in Quasar Host Galaxies
The Astrophysical Journal
During the past five decades astronomers have been puzzled by the presence of strong absorption features including metal lines, observed in the optical and ultraviolet spectra of quasars, signaling inflowing and outflowing gas winds with relative velocities up to several thousands of km s −1. In particular, the location of these windsclose to the quasar, further out in its host galaxy, or in its direct environment-and the possible impact on their surroundings have been issues of intense discussion and uncertainty. Using our Herschel Space Observatory data, we report a tendency for this so-called associated metal absorption to occur along with prodigious star formation in the quasar host galaxy, indicating that the two phenomena are likely to be interrelated, that the gas winds likely occur on the kiloparsec scale and would then have a strong impact on the interstellar medium of the galaxy. This correlation moreover would imply that the unusually high cold dust luminosities in these quasars are connected with ongoing star formation. Given that we find no correlation with the AGN strength, the wind feedback that we establish in these radio-loud objects is most likely associated with their host star formation rather than with their black hole accretion.
Enrichment of the High-Redshift IGM by Galactic Winds
2001
This paper discusses a semi-numerical method of investigating the enrichment of the intergalactic medium by galactic winds. We find that most galaxies at z ∼ > 3 should be driving winds, and that (if these winds are similar to those at low z) these winds should escape to large distances. Our calculations -which permit exploration of a large region of model parameter space -indicate that the wind velocity, the mass of the wind-driving galaxies, the fraction of ambient material entrained, and the available time (between wind launch and the observed redshift) all affect wind propagation significantly; other physical effects can be important but are sub-dominant. We find that under reasonable assumptions, the enrichment by 3 ∼ < z ∼ < 6 galaxies could account for the quantity of metals seen in the Lyα forest, though it is presently unclear whether this enrichment is compatible with the intergalactic medium's detailed metal distribution or relative quiescence.
The Astrophysical Journal, 2014
We present an analysis of the Mg II λλ2796, 2803 and Fe II λλ2586, 2600 absorption line profiles in individual spectra of 105 galaxies at 0.3 < z < 1.4. The galaxies, drawn from redshift surveys of the GOODS fields and the Extended Groth Strip, fully sample the range in star formation rates (SFRs) occupied by the star-forming sequence with stellar masses log M * /M ⊙ 9.5 at 0.3 < z < 0.7. Using the Doppler shifts of the Mg II and Fe II absorption lines as tracers of cool gas kinematics, we detect large-scale winds in 66 ± 5% of the galaxies. High-resolution Hubble Space Telescope/Advanced Camera for Surveys imaging and our spectral analysis indicate that the outflow detection rate depends primarily on galaxy orientation: winds are detected in ∼ 89% of galaxies having inclinations (i) < 30 • (face-on), while the wind detection rate is only ∼ 45% in objects having i > 50 • (edge-on). Combined with the comparatively weak dependence of the wind detection rate on intrinsic galaxy properties (including SFR surface density), this suggests that biconical outflows are ubiquitous in normal, starforming galaxies at z ∼ 0.5, with over half of the sample having full wind cone opening angles of ∼ 100 • . We find that the wind velocity is correlated with host galaxy M * at 3.4σ significance, while the equivalent width (EW) of the flow is correlated with host galaxy SFR at 3.5σ significance, suggesting that hosts with higher SFR may launch more material into outflows and/or generate a larger velocity spread for the absorbing clouds. The large (> 1Å) Mg II outflow EWs typical of this sample are rare in the context of Mg II absorption studies along QSO sightlines probing the extended halos of foreground galaxies, implying that this wind material is not often detected at impact parameters > 10 kpc. Assuming that the gas is launched into dark matter halos with simple, isothermal density profiles, the wind velocities measured for the bulk of the cool material (∼ 200 − 400 km s −1 ) are sufficient to enable escape from the halo potentials only for the lowest-M * systems in the sample. However, the highest-velocity gas in the outflows typically carries sufficient energy to reach distances of 50 kpc, and may therefore be a viable source of cool material for the massive circumgalactic medium observed around bright galaxies at z ∼ 0.