We report Hubble Space Telescope/Cosmic Origins Spectrograph observations of the Ly alpha emission and interstellar absorption lines in a sample of 10 star-forming galaxies at z ~ 0.2. Selected on ...the basis of high equivalent width optical emission lines, the sample, dubbed "Green Peas," make some of the best analogs for young galaxies in an early universe. We detect Ly alpha emission in all ten galaxies, and 9/10 show double-peaked line profiles suggestive of low H I column density. We measure Ly alpha /H alpha flux ratios of 0.5-5.6, implying that 5%-60% of Ly alpha photons escape the galaxies. These data confirm previous findings that low-ionization metal absorption (LIS) lines are weaker when Ly alpha escape fraction and equivalent width are higher. However, contrary to previously favored interpretations of this trend, increased Ly alpha output cannot be the result of a varying H I covering: the Lyman absorption lines (Ly beta and higher) show a covering fraction near unity for gas with N sub(H I) > ~ 10 super(16) cm super(-2). Moreover, we detect no correlation between Ly alpha escape and the outflow velocity of the LIS lines, suggesting that kinematic effects do not explain the range of Ly alpha /H alpha flux ratios in these galaxies. In contrast, we detect a strong anticorrelation between the Ly alpha escape fraction and the velocity separation of the Ly alpha emission peaks, driven primarily by the velocity of the blue peak. As this velocity separation is sensitive to H I column density, we conclude that Ly alpha escape in these Green Peas is likely regulated by the H I column density rather than outflow velocity or H I covering fraction.
How Gas Accretion Feeds Galactic Disks Ho, Stephanie H.; Martin, Crystal L.; Turner, Monica L.
The Astrophysical journal,
04/2019, Volume:
875, Issue:
1
Journal Article
Peer reviewed
Open access
Numerous observations indicate that galaxies need a continuous gas supply to fuel star formation and explain the star formation history. However, direct observational evidence of gas accretion ...remains rare. Using the EAGLE cosmological hydrodynamic simulation suite, we study cold gas accretion onto galaxies and the observational signatures of the cold gas kinematics. For EAGLE galaxies at z = 0.27, we find that cold gas accretes onto galaxies anisotropically with typical inflow speeds between 20 and 60 km s−1. Most of these galaxies have comparable mass inflow rates and star formation rates, implying that the cold inflowing gas plausibly accounts for sustaining the star-forming activities of the galaxies. As motivation for future work to compare the cold gas kinematics with measurements from quasar sightline observations, we select an EAGLE galaxy with an extended cold gas disk, and we probe the cold gas using mock quasar sightlines. We demonstrate that by viewing the disk edge on, sightlines at azimuthal angles below 10° and impact parameters out to 60 pkpc can detect cold gas that corotates with the galaxy disk. This example suggests that cold gas disks extending beyond the optical disks possibly explain the sightline observations that detect corotating cold gas near galaxy major axes.
We present observations of 50 pairs of redshift z 0.2 star-forming galaxies and background quasars. These sightlines probe the circumgalactic medium (CGM) out to half the virial radius, and we ...describe the circumgalactic gas kinematics relative to the reference frame defined by the galactic disks. We detect halo gas in Mg ii absorption, measure the equivalent-width-weighted Doppler shifts relative to each galaxy, and find that the CGM has a component of angular momentum that is aligned with the galactic disk. No net counter-rotation of the CGM is detected within 45° of the major axis at any impact parameter. The velocity offset of the circumgalactic gas correlates with the projected rotation speed in the disk plane out to disk radii of roughly 70 kpc. We confirm previous claims that the Mg ii absorption becomes stronger near the galactic minor axis, and we show that the equivalent width correlates with the velocity range of the absorption. We cannot directly measure the location of any absorber along the sightline, but we explore the hypothesis that individual velocity components can be associated with gas orbiting in the disk plane or flowing radially outward in a conical outflow. We conclude that centrifugal forces partially support the low-ionization gas and galactic outflows kinematically disturb the CGM producing excess absorption. Our results firmly rule out schema for the inner CGM that lack rotation and suggest that angular momentum as well as galactic winds should be included in any viable model for the low-redshift CGM.
Measurements of interstellar Na I lambda lambda 5890, 5896 absorption lines in 18 ultraluminous infrared galaxies (ULIGs) have been combined with published Na I data, to reassess the dependence of ...galactic outflow speeds on starburst luminosity and galactic mass. The Doppler shifts reveal outflows of relatively cool gas in 15 of 18 ULIGs with an average outflow speed at the line center of 330 plus or minus 100 km s super(-1). The relation between outflow speed and star formation rate (SFR), defined by the distribution's upper envelope over 4 orders of magnitude in SFR, demonstrates that winds from more luminous starbursts accelerate interstellar gas to higher speeds roughly as upsilon proportional to SFR super(0.35). This result is surprising since, in the traditional model for starburst-driven winds, these relatively cool gas clouds are accelerated by the ram pressure of a hot, supernova-heated wind that exhibits weak (if any) X-ray temperature variation with increasing galactic mass. The lack of evidence for much hotter winds is partly a sensitivity issue, but the Na I velocities in ultraluminous starbursts actually are consistent with acceleration by the tepid wind, indicating that a hotter component is unlikely to dominate the momentum flux. The Na I velocities in the dwarf starburst winds do not reach the terminal velocity of a hot wind at the measured temperature of kT similar to 0.73 keV, a result that could be interpreted simply as evidence that the hot superbubbles are too confined in dwarf starbursts to generate a free-flowing wind. A dynamically motivated scenario, however, is that the dwarf starburst winds simply lack enough momentum to accelerate the clouds to the velocity of the hot wind. Among the subsample of starbursts with well-constrained dynamical masses, the terminal outflow velocities are always found to approach the galactic escape velocity. Motivated by a similar scaling relation for stellar winds, the galactic Eddington luminosity for dusty starbursts is shown to be within the range measured for ULIGs. If radiation pressure on dust grains, coupled to the cool wind, is indeed important for galactic wind dynamics, then feedback is stronger in massive galaxies than previously thought, helping shape the high-mass end of the galaxy luminosity function. Regardless of the nature of the acceleration mechanism in ULIGs, the mass flux of cool gas estimated from these data demonstrates that starburst-driven winds transport significant gas during the assembly stage of field elliptical galaxies, a factor<< Measurements of interstellar Na I lambda lambda 5890, 5896 absorption lines in 18 ultraluminous infrared galaxies (ULIGs) have been combined with published Na I data, to reassess the dependence of galactic outflow speeds on starburst luminosity and galactic mass. The Doppler shifts reveal outflows of relatively cool gas in 15 of 18 ULIGs with an average outflow speed at the line center of 330 plus or minus 100 km s super(-1). The relation between outflow speed and star formation rate (SFR), defined by the distribution's upper envelope over 4 orders of magnitude in SFR, demonstrates that winds from more luminous starbursts accelerate interstellar gas to higher speeds roughly as upsilon proportional to SFR super(0.35). This result is surprising since, in the traditional model for starburst-driven winds, these relatively cool gas clouds are accelerated by the ram pressure of a hot, supernova-heated wind that exhibits weak (if any) X-ray temperature variation with increasing galactic mass. The lack of evidence for much hotter winds is partly a sensitivity issue, but the Na I velocities in ultraluminous starbursts actually are consistent with acceleration by the tepid wind, indicating that a hotter component is unlikely to dominate the momentum flux. The Na I velocities in the dwarf starburst winds do not reach the terminal velocity of a hot wind at the measured temperature of kT similar to 0.73 keV, a result that could be interpreted simply as evidence that the hot superbubbles are too confined in dwarf starbursts to generate a free-flowing wind. A dynamically motivated scenario, however, is that the dwarf starburst winds simply lack enough momentum to accelerate the clouds to the velocity of the hot wind. Among the subsample of starbursts with well-constrained dynamical masses, the terminal outflow velocities are always found to approach the galactic escape velocity. Motivated by a similar scaling relation for stellar winds, the galactic Eddington luminosity for dusty starbursts is shown to be within the range measured for ULIGs. If radiation pressure on dust grains, coupled to the cool wind, is indeed important for galactic wind dynamics, then feedback is stronger in massive galaxies than previously thought, helping shape the high-mass end of the galaxy luminosity function. Regardless of the nature of the acceleration mechanism in ULIGs, the mass flux of cool gas estimated from these data demonstrates that starburst-driven winds transport significant gas during the assembly stage of field elliptical galaxies, a factor/SUP>that helps explain the rapid decline in SFR in these systems inferred from elemental abundance ratios.
Abstract We present integral field spectroscopy toward and around J1044+0353, a rapidly growing, low-metallicity galaxy that produces extreme O iii line emission. A new map of the O32 flux ratio ...reveals a density-bounded ionization cone emerging from the starburst. The interaction of the hydrogen-ionizing radiation, produced by the very young starburst, with a cavity previously carved out by a galactic outflow, whose apex lies well outside the starburst region, determines the pathway for global Lyman continuum (LyC) escape. In the region within a few hundred parsecs of the young starburst, we demonstrate that superbubble breakthrough and blowout contribute distinct components to the O iii line profile: broad and very broad emission line wings, respectively. We draw attention to the large O iii luminosity of the broad component and argue that this emission comes from photoionized, superbubble shells rather than a galactic wind as is often assumed. The spatially resolved He ii λ 4686 nebula appears to be photoionized by young star clusters. Stellar wind emission from these stars is likely the source of line wings detected on the He ii line profile. This broader He ii component indicates slow stellar winds, consistent with an increase in stellar rotation (and a decrease in effective escape speed) at the metallicity of J1044+0353. At least in J1044+0353, the recent star formation history plays a critical role in generating a global pathway for LyC escape, and the anisotropic escape would likely be missed by direct observations of the LyC.
Low-ionization-state Mg ii gas has been extensively studied in quasar sightline observations to understand the cool, ∼104 K gas in the circumgalactic medium. Motivated by recent observations showing ...that the Mg ii gas around low-redshift galaxies has significant angular momentum, we use the high-resolution EAGLE cosmological simulation to analyze the morphological and rotation structures of the z 0.3 circumgalactic Mg ii gas and examine how they change with the host galaxy properties. Around star-forming galaxies, we find that the Mg ii gas has an axisymmetric instead of a spherical distribution, and the axis of symmetry aligns with that of the Mg ii gas rotation. A similar rotating structure is less commonly found in the small sample of simulated quiescent galaxies. We also examine how often Mg ii gas around galaxies selected using a line-of-sight velocity cut includes gas physically outside of the virial radius (rvir). For example, we show that at an impact parameter of 100 pkpc, a 500 km s−1 velocity cut around galaxies with stellar masses of 109-109.5 M (1010-1010.5 M ) selects Mg ii gas beyond the virial radius 80% (6%) of the time. Because observers typically select Mg ii gas around target galaxies using such a velocity cut, we discuss how this issue affects the study of circumgalactic Mg ii gas properties, including the detection of corotation. While the corotating Mg ii gas generally extends beyond 0.5rvir, the Mg ii gas outside of the virial radius contaminates the corotation signal and makes observers less likely to conclude that gas at large impact parameters (e.g., 0.25rvir) is corotating.
We analyze the physical conditions in the low-ionization component of starburst outflows (in contrast to the high-ionization wind fluid observed in X-rays), based on new Keck/LRIS spectroscopy of ...partially resolved absorption troughs in near-ultraviolet and optical spectra of Ultraluminous Infrared Galaxies. The large velocity width and blueshift present in seven atomic transitions indicate a macroscopic velocity gradient within the outflowing gas. The separation of the Mg II 2796, 2803 (and Fe II 2587, 2600) doublet lines in these data constrains the gas kinematics better than previous studies of the heavily blended Na I 5892, 5898 doublet. The identical shape of the Mg II 2796 absorption troughs to that of the normally weaker transition at 2803 A (after accounting for emission filling) requires both transitions be optically thick at all outflow velocities. The fraction of the galactic continuum covered by the outflow at each velocity therefore dictates the shape of these absorption troughs. We suggest that the velocity offset of the deepest part of the troughs, where the covering factor of low-ionization gas is near unity, reflects the speed of a shell of swept-up, interstellar gas at the time of blowout. In a spherical outflow, we show that the fragments of this shell, or any clouds that expand adiabatically in rough pressure equilibrium with the hot wind, expand slowly relative to the geometrical dilution, and the covering fraction of low-ionization gas decreases with increasing radius. Our measurement of a covering factor that decreases with increasing velocity can therefore be interpreted as evidence that the low-ionization outflow is accelerating, i.e., absorption at higher velocity comes from gas at larger radii. We also present measurements of Cf (v) in four species, place an upper limit of ne 3000 cm-3 on the density of the outflowing gas, and discuss lower limits on the mass outflow rate.
We constrain gas inflow speeds in star-forming galaxies with color gradients consistent with inside-out disk growth. Our method combines new measurements of disk orientation with previously described ...circumgalactic absorption in background quasar spectra. Two quantities, a position angle and an axis ratio, describe the projected shape of each galactic disk on the sky, leaving an ambiguity about which side of the minor axis is tipped toward the observer. This degeneracy regarding the 3D orientation of disks has compromised previous efforts to measure gas inflow speeds. We present Hubble Space Telescope and Keck/LGSAO imaging that resolves the spiral structure in five galaxies at redshift z 0.2. We determine the sign of the disk inclination for four galaxies, under the assumption that spiral arms trail the rotation. We project models for both radial infall in the disk plane and circular orbits onto each quasar sightline. We compare the resulting line-of-sight velocities to the observed velocity range of Mg ii absorption in spectra of background quasars, which intersect the disk plane at radii between 69 and 115 kpc. For two sightlines, we constrain the maximum radial inflow speeds as 30-40 km s−1. We also rule out a velocity component from radial inflow in one sightline, suggesting that the structures feeding gas to these growing disks do not have unity covering factor. We recommend appropriate selection criteria for building larger samples of galaxy-quasar pairs that produce orientations sensitive to constraining inflow properties.
We analyze Mg II lambdalambda2796, 2803 and Fe II lambdalambda2586, 2600 absorption profiles in individual spectra of 105 galaxies at 0.3 < z < 1.4. 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) < 30degrees (face-on), while the wind detection rate is ~45% in objects having i > 50 gamma (edge-on). We find that wind velocity is correlated with galaxy M sub(*) at 3.4sigma significance, while outflow equivalent width is correlated with SFR at 3.5sigma significance, suggesting hosts with higher SFR launch more material and/or generate a larger velocity spread for the absorbing clouds. However, the gas carries sufficient energy to reach distances gap50 kpc, and may therefore be a viable source of material for the massive, cool circumgalactic medium around bright galaxies at z ~ 0.
We report the first direct and robust measurement of the faint-end slope of the Ly alpha emitter (LAE) luminosity function (LF) at z = 5.7. Candidate LAEs from a low-spectral-resolution blind search ...with IMACS on Magellan-Baade were targeted at higher resolution to distinguish high-redshift LAEs from foreground galaxies. All but 2 of our 42 single-emission-line systems have flux F < 2.0 x 10 super(-17) ergs s super(-1) cm super(-2), making these the faintest emission-lines observed for a z = 5.7 sample with known completeness, an essential property for determining the faint end slope of the LAE LF. We find 13 LAEs as compared to 29 foreground galaxies, in very good agreement with the modeled foreground counts predicted in Dressler et al. that had been used to estimate a faint-end slope of alpha = -2.0 for the LAE LF. A 32% LAE fraction, LAE/(LAE+foreground) within the flux interval F = 2-20 x 10 super(-18) ergs s super(-1) cm super(-2) constrains the faint end slope of the LF to -2.35 < alpha < -1.95 (1sigma). We show how this steep LF should provide, to the limit of our observations, M sub(UV) ~ -16, more than 20% of the flux necessary to maintain ionization at z = 5.7, with a factor of 10 extrapolation in flux reaching more than 50%. This is in addition to the comparable contribution by brighter Lyman Break Galaxies M sub(UV) <, ~ -18. We suggest that this bodes well for a sufficient supply of Lyman continuum photons by similar, low-mass star-forming galaxies within the reionization epoch at z approx = 7, only 250 Myr earlier.
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