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Register a new userGalactic Outflows in Absorption and Emission: Near-UV Spectroscopy of Galaxies at 1<z<2
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Dawn K. Erb
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We study large-scale outflows in a sample of 96 star-forming galaxies at 1<z<2, using near-UV spectroscopy of FeII and MgII absorption and emission. The average blueshift of the FeII interstellar absorption lines with respect to the systemic velocity is -85+/-10 km/s at z~1.5, with standard deviation 87 km/s; this is a decrease of a factor of two from the average blueshift measured for far-UV interstellar absorption lines in similarly selected galaxies at z~2. The profiles of the MgII 2796, 2803 lines show much more variety than the FeII profiles, which are always seen in absorption; MgII ranges from strong emission to pure absorption, with emission more common in galaxies with blue UV slopes and at lower stellar masses. Outflow velocities, as traced by the centroids and maximum extent of the absorption lines, increase with increasing stellar mass with 2-3sigma significance, in agreement with previous results. We study fine structure emission from FeII*, finding several lines of evidence in support of the model in which this emission is generated by the re-emission of continuum photons absorbed in the FeII resonance transitions in outflowing gas. In contrast, photoionization models indicate that MgII emission arises from the resonant scattering of photons produced in HII regions, accounting for the differing profiles of the MgII and FeII lines. A comparison of the strengths of the FeII absorption and FeII* emission lines indicates that massive galaxies have more extended outflows and/or greater extinction, while two-dimensional composite spectra indicate that emission from the outflow is stronger at a radius of ~10 kpc in high mass galaxies than in low mass galaxies.
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We use rest-frame UV spectroscopy to investigate the properties related to large-scale gas outflows, and to the dust extinction and star-formation rates of a sample of z ~ 2 star-forming galaxies from the Galaxy Mass Assembly ultradeep Spectroscopic Survey (GMASS). Dust extinction is estimated from the rest-frame UV continuum slope and used to obtain dust-corrected star-formation rates for the galaxies of the sample. For the entire sample, a mean value of the continuum slope <beta> = -1.11 pm 0.44 (r.m.s.) was derived, while the average SFR was found to be <SFR> = 52 pm 48 M_sun/yr (r.m.s.). A positive correlation between SFR and stellar mass was observed, in agreement with other works, the logarithmic slope of the relation being 1.10 pm 0.10. Low-ionization absorption lines, associated with the interstellar medium, were found to be blueshifted, with respect to the rest frame of the system, which indicates that there is outflowing gas with typical velocities of the order of ~ 100 km/s. Finally, investigating correlations between galaxy UV spectral characteristics and galaxy general properties, we report a possible correlation between the equivalent width of the interstellar absorption lines and SFR, stellar mass, and colour excess similar to that seen to hold at different redshifts.
We present an analysis of cool outflowing gas around galaxies, traced by MgII absorption lines in the co-added spectra of a sample of 486 zCOSMOS galaxies at 1 < z < 1.5. These galaxies span a range of stellar masses (9.45< log[M*/Msun]<10.7) and star formation rates (0.14 < log [SFR/Msun/yr] < 2.35). We identify the cool outflowing component in the MgII absorption and find that the equivalent width of the outflowing component increases with stellar mass. The outflow equivalent width also increases steadily with the increasing star formation rate of the galaxies. At similar stellar masses the blue galaxies exhibit a significantly higher outflow equivalent width as compared to red galaxies. The outflow equivalent width shows strong effect with star formation surface density ({Sigma}SFR) of the sample. For the disk galaxies, the outflow equivalent width is higher for the face-on systems as compared to the edge-on ones, indicating that for the disk galaxies, the outflowing gas is primarily bipolar in geometry. Galaxies typically exhibit outflow velocities ranging from -200 km/s to -300 km/s and on average the face-on galaxies exhibit higher outflow velocity as compared to the edge-on ones. Galaxies with irregular morphologies exhibit outflow equivalent width as well as outflow velocities comparable to face on disk galaxies. These galaxies exhibit minimum mass outflow rates > 5-7 Msun/yr and a mass loading factor ({eta} = dMout/dt /SFR) comparable to the star formation rates of the galaxies.
We use stellar population synthesis modeling to analyze the host galaxy properties of a sample of 33 UV-selected, narrow-lined active galactic nuclei (AGNs) at z ~ 2 - 3. In order to quantify the contribution of AGN emission to host galaxy broadband spectral energy distributions (SEDs), we use the subsample of 11 AGNs with photometric coverage spanning from rest-frame UV through near-IR wavelengths. Modeling the SEDs of these objects with a linear combination of stellar population and AGN templates, we infer the effect of the AGN on derived stellar population parameters. We also estimate the typical bias in derived stellar populations for AGNs lacking rest-frame near-IR wavelength coverage, and develop a method for inferring the true host galaxy properties. We compare AGN host galaxy properties to those of a sample of UV-selected, star-forming non-AGNs in the same redshift range, including a subsample carefully matched in stellar mass. Although the AGNs have higher masses and SFRs than the full non-active sample, their stellar population properties are consistent with those of the mass-selected sample, suggesting that the presence of an AGN is not connected with the cessation of star-formation activity in star-forming galaxies at z ~ 2 - 3. We suggest that a correlation between M_BH and galaxy stellar mass is already in place at this epoch. Assuming a roughly constant Eddington ratio for AGNs at all stellar masses, we are unable to detect the AGNs in low-mass galaxies because they are simply too faint.
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We use high-resolution cosmological zoom simulations with ~200 pc resolution at z = 2 and various prescriptions for galactic outflows in order to explore the impact of winds on the morphological, dynamical, and structural properties of eight individual galaxies with halo masses ~ 10^11--2x10^12 Msun at z = 2. We present a detailed comparison to spatially and spectrally resolved H{alpha} and other observations of z ~ 2 galaxies. We find that simulations without winds produce massive, compact galaxies with low gas fractions, super-solar metallicities, high bulge fractions, and much of the star formation concentrated within the inner kpc. Strong winds are required to maintain high gas fractions, redistribute star-forming gas over larger scales, and increase the velocity dispersion of simulated galaxies, more in agreement with the large, extended, turbulent disks typical of high-redshift star-forming galaxies. Winds also suppress early star formation to produce high-redshift cosmic star formation efficiencies in better agreement with observations. Sizes, rotation velocities, and velocity dispersions all scale with stellar mass in accord with observations. Our simulations produce a diversity of morphological characteristics - among our three most massive galaxies, we find a quiescent grand-design spiral, a very compact star-forming galaxy, and a clumpy disk undergoing a minor merger; the clumps are evident in H{alpha} but not in the stars. Rotation curves are generally slowly rising, particularly when calculated using azimuthal velocities rather than enclosed mass. Our results are broadly resolution-converged. These results show that cosmological simulations including outflows can produce disk galaxies similar to those observed during the peak epoch of cosmic galaxy growth.
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