The impact of galactic winds from LBGs on the Intergalactic Medium (original) (raw)
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The impact of galactic winds from Lyman-break galaxies on the intergalactic medium
Monthly Notices of the Royal Astronomical Society: Letters, 2006
An excess of sight-lines close to Lyman-break galaxies (LBGs) with little or no absorption in QSO absorption spectra has been reported and has been interpreted as the effect of galactic winds on the Intergalactic Medium. We use here numerical simulations to investigate the flux probability function close to plausible sites of LBGs. We show that the flux distribution near our LBGs in the simulation depends strongly on redshift, and is very sensitive to the averaging procedure. We show that a model without galactic winds and a model with a wind bubble size of 0.5 h −1 Mpc (comoving) are equally consistent with the new determination of the conditional flux distribution by . Models with the larger bubble sizes ( > ∼ 1 h −1 Mpc) suggested by the previous observations of Adelberger et al. based on a much smaller sample at higher redshift are not consistent with the new data. We, therefore, argue that the volume filling factor of galactic winds driven by LBGs may be much smaller than previously thought and that most of the metals responsible for the metal absorption associated with the low column density Lyα forest are unlikely to have been ejected by LBGs.
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.
Galactic winds and the Lyα forest
Monthly Notices of the Royal Astronomical Society, 2004
We study the effect of galactic outflows on the statistical properties of the Lyα forest and its correlation with galaxies. The winds are modelled as fully ionised spherical bubbles centered around the haloes in an N-body simulation of a ΛCDM model. The observed flux probability distribution and flux power spectrum limit the volume filling factor of bubbles to be less than 10%. We have compared the mean flux as a function of distance from haloes with the Adelberger et al. (ASSP) measurement. For a model of bubbles of constant size surrounding the most massive haloes, bubble radii of > ∼ 1.5 h −1 Mpc are necessary to match the high transmissivity at separations ≤ 0.5 h −1 Mpc but the increase of the transmissivity at small scales is more gradual than observed. The cosmic variance error due to the finite number of galaxies in the sample increases rapidly with decreasing separation. At separations ≤ 0.5 h −1 Mpc our estimate of the cosmic variance error is ∆F ∼ 0.3, 30% higher than that of ASSP. The difficulty in matching the rise in the transmissivity at separations smaller than the size of the fully ionised bubbles surrounding the haloes is caused by residual absorption of neutral hydrogen lying physically outside the bubbles but having a redshift position similar to the haloes. The flux level is thus sensitive to the amplitude of the coherent velocity shear near halos and to a smaller extent to the amplitude of thermal motions. We find that the velocity shear increases with halo mass in the simulation. A model where LBGs are starbursts in small mass haloes matches the observations with smaller bubble radii than a model where massive haloes host the LBGs. If we account for the uncertainty in the redshift position of haloes, a starburst model with a bubble radius of 1 h −1 Mpc and a volume filling factor of 2% is consistent with the ASSP measurements at the 1-1.5σ level. If this model is correct the sharp rise of the transmissivity at separations ≤ 0.5 h −1 Mpc in the ASSP sample is due to cosmic variance and is expected to become more moderate for a larger sample.
The Low-redshift Intergalactic Medium
Publications of the Astronomical Society of Australia, 1999
The low-redshift Lyα forest of absorption lines provides a probe of large-scale baryonic structures in the intergalactic medium, some of which may be remnants of physical conditions set up during the epoch of galaxy formation. We discuss our recent Hubble Space Telescope (HST) observations and interpretation of low-z Lyα clouds toward nearby Seyferts and QSOs, including their frequency, space density, estimated mass, association with galaxies, and contribution to Ωb. Our HST/GHRS detections of ∼ 70 Lyα absorbers with N hi ≥ 1012·6 cm−2 along 11 sightlines covering pathlength Δ(cz) = 114,000 km s−1 show f (>N hi ) α N hi −0·63±0·04 and a line frequency dN/dz = 200 ± 40 for N hi > 1012·6 cm−2 (one every 1500 km s−1 of redshift). A group of strong absorbers toward PKS 2155–304 may be associated with gas (400–800) kpc from four large galaxies, with low metallicity (≤0·003 solar) and D/H ≤ 2 × 10−4. At low-z, we derive a metagalactic ionising radiation field from AGN of J 0 = × 10−...
Cosmological Simulations of the Intergalactic Medium Evolution. II. Galaxy Model and Feedback
The Astrophysical Journal, 2015
We investigate models of self-consistent chemical enrichment of the intergalactic medium (IGM) from z = 6.0 → 1.5, based on hydrodynamic simulations of structure formation that explicitly incorporate outflows from star-forming galaxies. Our main result is that outflow parametrizations derived from observations of local starburst galaxies, in particular momentum-driven wind scenarios, provide the best agreement with observations of C IV absorption at z ∼ 2-5. Such models sufficiently enrich the high-z IGM to produce a global mass density of C IV absorbers that is relatively invariant from z = 5.5 → 1.5, in agreement with observations. This occurs despite continual IGM enrichment causing an increase in volumeaveraged metallicity by ∼ × 5-10 over this redshift range, because energy input accompanying the enriching outflows causes a drop in the global ionization fraction of C IV. Comparisons to observed C IV column density and linewidth distributions and C IV-based pixel optical depth ratios provide significant constraints on wind models. Our best-fitting outflow models show mean IGM temperatures only slightly above our no-outflow case, metal filling factors of just a few per cent with volume-weighted metallicities around 10 −3 at z ∼ 3, significant amounts of collisionally ionized C IV absorption and a metallicity-density relationship that rises rapidly at low overdensities and flattens at higher ones. In general, we find that outflow speeds must be high enough to enrich the low-density IGM at early times but low enough not to overheat it, and concurrently must significantly suppress early star formation while still producing enough early metals. It is therefore non-trivial that locally calibrated momentum-driven wind scenarios naturally yield the desired strength and evolution of outflows, and suggest that such models represent a significant step towards understanding the impact of galactic outflows on galaxies and the IGM across cosmic time.
Galactic Winds in Cosmological Simulations of the Circumgalactic Medium
2012
We explore new observationally-constrained sub-resolution models of galactic outflows and investigate their impact on the circumgalactic medium (CGM) in the redshift range z = 2 − 4. We perform cosmological hydrodynamic simulations, including star formation, chemical enrichment, and four cases of SNe-driven outflows: no wind (NW), an energy-driven constant velocity wind (CW), a radially varying wind (RVWa) where the outflow velocity has a positive correlation with galactocentric distance (r), and a RVW with additional dependence on halo mass (RVWb). Overall, we find that the outflows expel metal-enriched gas away from galaxies, significantly quench the star formation, reduce the central galactic metallicity and enrich the CGM. At z = 2, the radial profiles of gas properties around galaxy centers are most sensitive to the choice of the wind model for halo masses in the range (10 9 − 10 11)M ⊙. We infer that outflows in the RVWb model are least effective, with results similar to the NW case, except that the CGM is enriched more. Moreover, we find that the models CW and RVWa are similar, both showing the impact of effective winds, with the following notable differences. RVWa causes a greater suppression of star formation rate at z 5, and has a higher fraction of low-density (δ < 10), warm-hot (10 4 − 10 6 K) gas than in CW. Outflows in CW produce a higher and earlier enrichment of some IGM phases than in RVWa. By visual inspection, we note that the RVWa model shows galactic disks more pronounced than all the other wind models. We predict that some observational diagnostics are more promising to distinguish between different outflow driving mechanisms in galaxies: Z C of the CGM gas at r ∼ (30 − 300)h −1 kpc comoving, and CIV fraction of the inner gas at r < (4 − 5)h −1 kpc comoving.
The intergalactic medium over the last 10 billion years - I. Lyα absorption and physical conditions
Monthly Notices of the Royal Astronomical Society, 2010
The intergalactic medium (IGM) is the dominant reservoir of baryons at all cosmic epochs. In this paper, we investigate the evolution of the IGM from z = 2 → 0 in (48 h −1 Mpc) 3 , 110million particle cosmological hydrodynamic simulations using three prescriptions for galactic outflows. We focus on the evolution of IGM physical properties, and how such properties are traced by Lyα absorption as detectable using Hubble's Cosmic Origins Spectrograph (COS). Our results broadly confirm the canonical picture that most Lyα absorbers arise from highly ionized gas tracing filamentary large-scale structure. Growth of structure causes gas to move from the diffuse photoionized IGM into other cosmic phases, namely stars, cold and hot gas within galaxy haloes, and the unbound and shock-heated warm-hot intergalactic medium (WHIM). By today, baryons are comparably divided between bound phases (35 per cent in our favoured outflow model), the diffuse IGM (41 per cent) and the WHIM (24 per cent). Here we (re)define the WHIM as gas with overdensities lower than that in haloes (ρ/ρ 100 today) and temperatures T > 10 5 K, to more closely align it with the 'missing baryons' that are not easily detectable in emission or Lyα absorption. Strong galactic outflows can have a noticeable impact on the temperature of the IGM, though with our favoured momentum-driven wind scalings they do not. When we (mildly) tune our assumed photoionizing background to match the observed evolution of the Lyα mean flux decrement, we obtain line count evolution statistics that broadly agree with available (pre-COS) observations. We predict a column density distribution slope of f (N H I) ∝ N −1.70 H I for our favoured wind model, in agreement with recent observational estimates, and it becomes shallower with redshift. Winds have a mostly minimal impact, but they do result in a shallower column density slope and more strong lines. With improved statistics, the frequency of strong lines can be a valuable diagnostic of outflows, and the momentum-driven wind model matches existing data significantly better than the two alternatives we consider. The relationship between column density and physical density broadens mildly from z = 2 → 0, and evolves as ρ ∝ N 0.74 H I 10 −0.37z for diffuse absorbers, consistent with previous studies. Linewidth distributions are quite sensitive to spectral resolution; COS should yield significantly broader lines than higher resolution data. Thermal contributions to linewidths are typically subdominant, so linewidths only loosely reflect the temperature of the absorbing gas. This will hamper attempts to quantify the WHIM using broad Lyα absorbers, though it may still be possible to do so statistically. Together, COS data and simulations such as these will provide key insights into the physical conditions of the dominant reservoir of baryons over the majority of cosmic time.
Cosmological simulations of intergalactic medium enrichment from galactic outflows
Monthly Notices of the Royal Astronomical Society, 2006
We investigate models of self-consistent chemical enrichment of the intergalactic medium (IGM) from z = 6.0 → 1.5, based on hydrodynamic simulations of structure formation that explicitly incorporate outflows from star forming galaxies. Our main result is that outflow parameterizations derived from observations of local starburst galaxies, in particular momentum-driven wind scenarios, provide the best agreement with observations of C iv absorption at z ∼ 2 − 5. Such models sufficiently enrich the high-z IGM to produce a global mass density of C iv absorbers that is relatively invariant from z = 5.5 → 1.5, in agreement with observations. This occurs despite continual IGM enrichment causing an increase in volume-averaged metallicity by ∼ ×5 − 10 over this redshift range, because energy input accompanying the enriching outflows causes a drop in the global ionization fraction of C iv. Comparisons to observed C iv column density and linewidth distributions and C iv-based pixel optical depth ratios provide significant constraints on wind models. Our best-fitting outflow models show mean IGM temperatures only slightly above our no-outflow case, metal filling factors of just a few percent with volume-weighted metallicities around 10 −3 at z ∼ 3, significant amounts of collisionally-ionized C iv absorption, and a metallicity-density relationship that rises rapidly at low overdensities and flattens at higher ones. In general, we find that outflow speeds must be high enough to enrich the low-density IGM at early times but low enough not to overheat it, and concurrently must significantly suppress early star formation while still producing enough early metals. It is therefore non-trivial that locally-calibrated momentum-driven wind scenarios naturally yield the desired strength and evolution of outflows, and suggest that such models represent a significant step towards understanding the impact of galactic outflows on galaxies and the IGM across cosmic time.
The Astrophysical Journal, 2007
We explore the onset of star formation in the early Universe, exploiting the observations of high-redshift Lyman-break galaxies (LBGs) and Lyα emitters (LAEs), in the framework of the galaxy formation scenario elaborated by already successfully tested against the wealth of data on later evolutionary stages. Complementing the model with a simple, physically plausible, recipe for the evolution of dust attenuation in metal poor galaxies we reproduce the luminosity functions (LFs) of LBGs and of LAEs at different redshifts. This recipe yields a much faster increase with galactic age of attenuation in more massive galaxies, endowed with higher star formation rates. These objects have therefore shorter lifetimes in the LAE and LBG phases, and are more easily detected in the dusty submillimeter bright (SMB) phase. The short UV bright lifetimes of massive objects strongly mitigate the effect of the fast increase of the massive halo density with decreasing redshift, thus accounting for the weaker evolution of the LBG LF, compared to that of the halo mass function, and the even weaker evolution between z ≈ 6 and z ≈ 3 of the LAE LF. The much lower fraction of LBGs hosting detectable nuclear activity, compared to SMB galaxies, comes out naturally from the evolutionary sequence yielded by the model, which features the coevolution of galaxies and active nuclei. In this framework LAEs are on the average expected to be younger, with lower stellar masses, more compact, and associated to less massive halos than LBGs. Finally, we show that the intergalactic medium can be completely reionized at redshift z ≈ 6 − 7 by massive stars shining in protogalactic spheroids with halo masses from a few 10 10 M ⊙ to a few 10 11 M ⊙ , showing up as faint LBGs with magnitude in the range −17 M 1350 −20, without resorting to any special stellar initial mass function.