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1. Background
Gas outflows are thought to be ubiquitous as a way of expelling metals from the interstellar medium and enriching the intergalactic and circumgalactic medium (Tumlinson et al. 2017). These gas outflows are predominantly detected in the foreground or blueshifted relative to the galaxy as expelled clouds move toward us, the observer, along the line of sight; clouds expelled on the far side of the galaxy are not directly observable. The speed at which this foreground material is moving gives a measurement of galaxy scale feedback, which is of key interest to cosmological simulations hoping to formulate realistic models for the causes, strength, and effects of this feedback on galaxy evolution and star formation (Hopkins et al. 2014). Most outflow measurements of high redshift galaxies are either of relatively unobscured galaxies (e.g., Martin et al. 2012) or lensed galaxies (e.g., Spilker et al. 2018). These galaxies do not usually have a high star formation rate (SFR). Here we present the measurement of a gas outflow for an unlensed, typical dusty star-forming galaxy, SMM J100031.9+022013.0 (also known as 450.06) that has a M⋆ = 1.1 × 1011 M⊙ and SFR = 720 M⊙ yr−1 (Casey et al. 2017) at z ∼ 2.
2. Data
The data for this galaxy, 450.06, located at 10:00:31.91+02:20:13.00, were obtained using the MOSFIRE and DEIMOS spectrographs at the W. M. Keck Observatory on 2013 December 31 and 2012 December 10–11 respectively. The seeing on 2013 December 31 was 0
6 and on 2012 December 10–11 was 13. The spectra (H, K, and optical bands) were extracted using IRAF APALL. Each of the spectra were smoothed with a Gaussian kernel, with convolved instrumental resolution and seeing. Emission lines from the night sky were masked.
3. Measurements
The systemic redshift was calculated using the Hα emission line in combination with the two [N ii] lines detected in the MOSFIRE spectrum of this galaxy, shown in Figure 1 (upper left). We constrain the galaxy's systemic redshift to be z = 2.085 ± 0.001. From this redshift, the expected values of Iron absorption lines were calculated and used as reference points in the DEIMOS rest-frame UV spectrum. After fitting the off-line continuum as a constant in Fλ, the absorption features were then fit with Gaussians to find the widths and central wavelengths and to determine if there is a systemic blueshift indicative of a wind. The rest-frame UV absorption features from the DEIMOS spectrum are shown in Figure 1 (upper right). The three detected metal absorption features were stacked together using the systemic redshift to generate a reference velocity frame; a weighted mean by source SNR was used to derive the aggregate signature of gas outflow. The line of sight velocity was measured relative the systemic redshift and found to be −190 ± 50 kms−1.
Figure 1. Upper left: Hα and [N ii] emission features from K band MOSFIRE spectrum from which we derive the systemic redshift of 2.085 ± 0.001. Upper right: Fe ii absorption features in the DEIMOS spectrum. The vertical lines show where the absorption features were expected based on the redshift from Hα. The centroid of the gaussians appear at shorter wavelengths than expected, which traces galaxy scale winds. Lower left: stacked absorption features show speed of gas outflows. The gray lines are the slices of the spectrum and the blue is an interpolated curve from all three features. The zero velocity point is based on the systemic redshift. Lower right: outflow velocity for 450.06 plotted against data from Martin (2005) and Heckman et al. (2015) for outflow velocity as a function of star formation rate showing that though highly obscured, the outflow velocity is consistent with previous measurements from other types of galaxies. The data used to create this figure are available.
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4. Conclusions
In this Research Note we examined spectra at near-infrared and optical wavelengths in order to find Fe ii absorption features from gas outflows. We used the systemic redshift from Hα to find absorption features, co-added them, and calculated from their blueshift a velocity of −190 ± 50 km s−1. This value agrees with outflow speeds in Martin (2005) and Heckman et al. (2015) for galaxies with similar SFRs as shown in Figure 1. This suggests that the geometry of highly obscured galaxies is consistent with typical galaxies at similar redshifts for which gas outflows have been characterized.
The authors wish to thank the John W. Cox Endowment for the Advanced Studies in Astronomy for supporting this research. Additional thanks to Richard Seifert, Patrick Drew, and Justin Spilker for helpful conversations in completing the analysis of this project. The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.