Far-Ultraviolet Observations of Outflows From Infrared-Luminous Galaxies

Abstract

We obtained medium-resolution ultraviolet (UV) spectra between 1150 and 1450 Å of the four UV-bright, infraredluminous starburst galaxies IRAS F08339+6517, NGC 3256, NGC 6090, and NGC 7552 using the Cosmic Origins Spectrograph on board the Hubble Space Telescope. The selected sightlines toward the starburst nuclei probe the properties of the recently formed massive stars and the physical conditions in the starburst-driven galactic superwinds. Despite being metal-rich and dusty, all four galaxies are strong Lyα emitters with equivalent widths ranging between 2 and 13 Å. The UV spectra show strong P Cygni-type high-ionization features indicative of stellar winds and blueshifted low-ionization lines formed in the interstellar and circumgalactic medium. We detect outflowing gas with bulk velocities of ∼400 km s −1 and maximum velocities of almost 900 km s −1 . These are among the highest values found in the local universe and comparable to outflow velocities found in luminous Lyman-break galaxies at intermediate and high redshift. The outflow velocities are unlikely to be high enough to cause escape of material from the galactic gravitational potential. However, the winds are significant for the evolution of the galaxies by transporting heavy elements from the starburst nuclei and enriching the galaxy halos. The derived mass outflow rates of ∼100 M yr −1 are comparable to or even higher than the star formation rates. The outflows can quench star formation and ultimately regulate the starburst as has been suggested for high-redshift galaxies.

Physical properties and evolutionary state of the Lyman alpha emitting starburst galaxy IRAS 08339+6517

“…If the Cardelli law is assumed, E(B − V) = 0.20 would be required, but the fits neither improve, nor reproduce the slope of the UV continuum correctly. Leitherer et al (2013) measured E(B − V) = 0.25 when fitting the spectrum of IRAS 0833 obtained with HST/COS (circular aperture with radius =1.25 ), which only includes the spectral range 1100−1450 Å. This fit is significantly worse than that achieved with the LMC law and E(B − V) = 0.15 when the whole UV range is considered.…”

“…Therefore, there is no evidence of additional stellar knots potentially hidden within dense dust clouds, which would be invisible in the UV, but would contribute to L(Hα) and L FIR , as found in other similar galaxies like Haro 2 (Otí-Floranes et al 2012). Leitherer et al (2013) considered that the HST/COS aperture was large enough (radius = 1.25 ) to contain short-lived massive bursts at different ages, mimicking a quasi-continuous star formation rate, and adopted a characteristic age of 20 Myr. The detection by Pellerin & Robert (1999) of the Ca II λλ8498, 8542, 8662 Å and Mg 2 λ5177 Å absorption lines in an IR-optical spectrum of the source, which are signatures of red (super)giants (RSGs) and late-type stars, respectively, supported this assumption in principle, since the presence of both O stars and RSGs requires starbursts extended over at least ∼10 Myr.…”

“…Moreover, when we tried to reproduce the different observables assuming synthesis models for extended bursts with the parameters considered by Leitherer et al (2013), the Si IV and C IV spectral profiles could not be simultaneously fitted, and the stellar mass value which would account for the UV emission from STB overestimated the predicted L(Hα) and L FIR by factors 7 and 3, respectively. As discussed above, the measured WR/(WR + O) ratio is also not consistent with an extended starburst.…”

“…They fitted its spectral profile using the radiation transfer code described by Schaerer et al (2011), which is an improved version of the models developed by Verhamme et al (2006). Assuming the simple geometry of an expanding spherical shell, with expansion velocity, Doppler parameter, column density, and dust optical depth as free parameters, Leitherer et al (2013) obtained a spectral profile which nicely reproduces the primary Lyα spectral component and predicted the presence of a secondary emission. Nevertheless, the model fails to properly reproduce the wavelength of the secondary emission peak, as well as the profile of the blue, P Cyg-like absorption wing.…”

POSSIBLE SIGNATURES OF A COLD-FLOW DISK FROM MUSE USING A z ∼ 1 GALAXY–QUASAR PAIR TOWARD SDSS J1422−0001*

“…For a biconical flow, this wind speed represents the flow speed along any radial trajectories, but if the absorption originates from regions close to the disk where the flow is more cylindrical, V out ought to be corrected for the galaxy inclination ( i cos ) and would be −160km s −1 since the galaxy inclination is~ 60 . Following Leitherer et al (2013), Wood et al (2015), and Heckman et al (2015), one can estimate the hydrogen column density using the gas-to-dust ratio and the relationship between N H and reddening (e.g., Bohlin et al 1978 Wood et al (2015), this value represents an upper limit on the column density, as the extinction traces the column density to the star cluster and thus likely includes contributions from gasin both the disk and the wind. Another lower limit comes from the Mg II rest-frame equivalent widthW 3.5 r 2796 Å (Table 3), which implies a column density >2 10 20 -cm 2 from the Ménard & Chelouche (2009) column density-Mg IIequivalent width correlation, i.e., at least about 40% of the column density is in the wind.…”