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The Galaxy Counterparts of the two high-metallicity DLAs at z=2.412 and z=2.583 towards Q0918+1636

The quasar Q0918+1636 (z=3.07) has two intervening high-metallicity Damped Lyman-alpha Absorbers (DLAs) along the line of sight, at redshifts of z=2.412 and 2.583. The z=2.583 DLA is located at a large impact parameter of 16.2 kpc, and despite this large impact parameter it has a very high metallicity (consistent with solar), a substantial fraction of H_2 molecules, and it is dusty as inferred from the reddened spectrum of the background QSO. The z=2.412 DLA has a metallicity of [M/H]=-0.6 (based on ZnII and SiII). In this paper we present new observations of this interesting sightline. HST/WFC3 imaging was obtained in the F606W, F105W and F160W bands. This is complemented by ground-based imaging in the u-, g-bands as well as K_s observations in the near-infrared (NIR). In addition, we present further spectroscopy with the ESO/VLT X-Shooter spectrograph. Based on these observations we obtain the following results: By fitting stellar population synthesis models to the photometric SED we constrain the physical properties of the z=2.583 DLA galaxy, and we infer its morphology by fitting a Sersic model to its surface brightness profile. We find it to be a relatively massive (M_star 10^10 M_sun), strongly star-forming (SFR~30 M_sun / yr, dusty (E_(B-V)=0.4) galaxy with a disk-like morphology. We detect most of the strong emission lines from the z=2.583 DLA [OIII],3727, [OIII],4960, [OIII],5007, Hbeta, and Halpha, albeit at low signal-to-noise (SN) ratio except for the [OIII],5007 line. We also detect [OIII],5007 emission from the galaxy counterpart of the z=2.412 DLA at a small impact parameter (<2 kpc). Overall our findings are consistent with the emerging picture that high-metallicity DLAs are associated with relatively (compared to typical DLAs) luminous and massive galaxy counterparts.


INTRODUCTION
For a long time the only available method for studying galaxies at redshifts z > 1 (barring QSO host galaxies) was to look at them in absorption against the light of background QSOs (e.g., Weymann et al. 1981;Wolfe et al. 2005). Then, from the second half of the 1990ies, the study of high-z galaxies in emission went through a breakthrough that is still unfolding (e.g., Giavalisco 2002;Shapley 2011). However, combining the information from absorption and emission lines is still a poorly developed field. Although more than 10 000 of the so-called Damped Lyman-α Absorbers (DLAs) have been found so far (Noterdaeme et al. 2012b), and despite some progress (e.g., Møller et al. 2002) in finding their galaxy counterparts, we still have less than a dozen examples of such absorption selected galaxies (Krogager et al. 2012, see also Rauch et al. 2008and Schulze et al. 2012. Expanding the sample is of great interest as we in this manner obtain unique information about the kinematics and chemical composition of gas surrounding the central ∼1 kpc which are typically studied in emission. This information is vital for probing current ideas about the role of processes like inflow of pristine gas and outflow of enriched gas in galaxy formation and evolution (e.g., Dekel et al. 2009;Bouché et al. 2010;Fumagalli et al. 2011;Dekel & Krumholz 2013;Crighton et al. 2013, and references therein).
The z = 3.07 quasar SDSSJ 091826.16+163609.0 was selected in the survey for high-metallicity DLAs described in Fynbo et al. (2010) and Fynbo et al. (2011). It was selected due to the presence of a DLA at z = 2.412 with strong FeII lines. After obtaining deep X-Shooter spectroscopy of the QSO, Fynbo et al. (2011) serendipitously discovered a second DLA at z = 2.583 along the line of sight with even stronger metal lines. Fynbo et al. (2011) detected the forbidden [OII] and [OIII] emission lines of the galaxy counterpart of this second DLA. The galaxy is located at an impact parameter of 2 ′′ , corresponding to ∼ 16 kpc at z = 2.583. The z = 2.412 DLA was not detected in emission in that study.
In this paper, we present new results based on new observations of this sightline obtained with the Hubble Space Telescope (HST), ESO Very Large Telescope (VLT) and Nordic Optical Telescope (NOT). In Sect. 2 we give an overview of the observations and data reduction, and describe the data analysis and the results in Sect. 3 and Sect. 4. Finally, Sect. 5 contains a discussion of our findings and their implications for the field.
Throughout this paper, we use a flat Λ CDM cosmology with ΩΛ = 0.728, Ωm = 0.272 and a Hubble constant of H0 = 70.4 km s −1 Mpc −1 (Komatsu et al. 2011). All magnitudes are given in the AB system.

HST imaging
The field of Q 0918+1636 was observed with the Wide Field Camera 3 (WFC3) on the HST on two epochs in November 2011 (with the NIR detector in the F105W and F160W filters) and on April 18 2012 (with the UVIS detector in the F606W filter). The roll-angle of the telescope was set such that the z = 2.583 DLA galaxy falls between the diffraction spikes of the Point Spread Function (PSF) of the QSO. The two observations with the NIR detector were taken using the WFC3-IR-DITHER-BOX-MIN pattern providing an optimal 4-point sampling of the PSF. The UVIS observation was taken using the WFC3-UVIS-DITHER-BOX pattern. We have reduced and combined the images using the software package multidrizzle provided by the STScI. By shifting and combining the images taken with sub-pixel offsets one achieves a better sampling of the PSF, which in the case of the IR observations is crucial as the PSF is poorly sampled in the native 0. ′′ 13 px −1 images. For this work we have set the parameter pixfrac to 0.7 in all reductions and used a final pixel scale of 0. ′′ 06 px −1 for IR and 0. ′′ 024 px −1 for UVIS. For a detailed description of the parameters in the software we refer to the multidrizzle user manual.

NOT imaging
On the nights of Jan 25-26 2012 and March 3 2012 Q 0918+1636 was observed with the Andalucia Faint Object Spectrograph and Camera (ALFOSC) and with the Nordic Optical Telescope nearinfrared Camera and spectrograph (NOTCam) at the NOT. A total of 8400 s, 11500 s and 7830 s of exposure time was obtained in the g-band, u-band, and Ks-band, respectively (see Table 1). Observing conditions were clear, with an average seeing FWHM of ∼ 1 ′′ in the January 2012 nights, and clear and sub-arcsec seeing on March 3rd 2012.The optical images were reduced using IRAF 1 standard procedures. The NOTCam images were reduced with custom IDL scripts, using a running-median for sky-subtraction, and an object mask for 2nd pass sky-subtraction. The NOTCam distortion correction was applied using the IRAF/geotran task.

VLT/X-Shooter spectroscopy
Q 0918+1636 was observed with VLT/X-Shooter on April 15 2012. Both a stare observation at a position angle of 162 o East of North and a nodding observation at a position angle of −66 o East of North were obtained. The observation at position angle −66 o East of North was a mistake: it should have been at 66 o East of North with the purpose of covering the z = 2.583 DLA galaxy and the QSO. In a stare observation the target is kept at a fixed position on the slit throughout the observation, whereas in a nodding observation an observing block consists of four exposures between which the target is moved in an ABBA pattern along the slit. A deeper observation was obtained on March 15-16 2013 at the correct position angle of 66 o East of North for covering the z = 2.583 DLA galaxy and the QSO using the nodding template with a nod-throw of 4 ′′ . In Fig. 1 we show the orientation of the slits in all the X-Shooter observations, both the new observations reported here and the previous observations of Fynbo et al. (2011). The purpose of the stare observation at position angle of 162 o East of North was to determine the redshift of the galaxy seen in the bottom of Fig. 1 in order to establish if this could be the galaxy counterpart of the z = 2.412 DLA. For the full log of observations we refer to Table 1.
The spectra from March 2013 were reduced with the ESO X-Shooter pipeline 2.0 (Modigliani et al. 2010;Goldoni 2011). We use the default parameters for the first five recipes which perform the basic calibrations (master darks, order prediction, flat fields, and the 2D maps for later rectification of the spectra). For the reduction of the object frames we use the corresponding pipeline recipes for the stare and nodding modes, with parameters optimized to provide the best possible sky-subtraction. The flux standard star LTT3218 was observed in both nights. Those spectra were reduced with the same calibration data as the spectra of the QSO/DLA system, and sampled onto the same spatial and wavelength grid. The extracted 1-dimensional (1D) standard star spectra were divided by the known tabulated spectrum of the standard star (which is first interpolated to the same wavelength grid), and the result smoothed with a kernel of 30 pixels to obtain a clean response curve. Each individual 2D spectrum was first normalized to an integration time of 1s, and then divided by the also normalized response curve from the corresponding night. The resulting flux-calibrated spectra from the 3 individual observing blocks (each comprising one hour of integration time) were collapsed along the wavelength axis to determine the peak positions of their spectral PSFs (SPSFs), and subsequently aligned on the spatial axis. Thereafter, they were stacked by means of a median combination. Galactic extinction corrections were taken from Schlafly & Finkbeiner (2011) 2 and implemented with the f m unred code in IDL. The final SPSF was determined by collapsing the error-weighted 2D stack along the wavelength axis in the H-band wavelength range and the 1D spectrum of the quasar itself was extracted by applying the corresponding normalized weights, similar to the optimal extraction procedure described in Horne (1986). To check the flux level we integrated the 1D spectrum over the transmission curves of the NOTCam Jand H-band filters, and compared with the photometry from Fynbo et al. (2013). The measurements agree within the errorbars, thus no correction of the flux level was necessary. With a gaussian fit to the SPSF we determined the seeing of the combined spectrum to be ∼ 0. ′′ 8 in the H-band.

HST imaging
Based on the high resolution HST imaging we can now improve the relative astrometry over that presented in Fynbo et al. (2011). We find that the DLA galaxy is located at an impact parameter of 1.98 ± 0.02 arcsec from the QSO at a position angle of −115 o East 2 provided by http://ned.ipac.caltech.edu/   of North, consistent with the earlier measurements. This impact parameter corresponds to a proper distance of 16.2 kpc at z = 2.583.
We used the GALFIT tool (Peng et al. 2002;Peng 2010) to fit 2D Sersic models, convolved with the PSF, to the HST images of the DLA galaxy. The Point Spread Functions (PSFs) for the HST images were simulated using the software TINYTIM. We chose to simulate the PSFs instead of using an empirical PSF as the model PSF has higher S/N ratio in the outer parts, where the PSFs from the data have high noise due to the background. We did the PSF simulation by first creating models using TINYTIM for each position of the target in the four-point dither pattern, assuming a QSO spectrum for the wavelength dependent PSF modelling, and taking into account the aberrations of the telescope as specified in the auxiliary data files. The models were sub-sampled by a factor of 5 compared to the native pixel scale of the detectors in order to position the model PSF more accurately. We then re-sampled the model PSF images to the native sampling and convolved them with the appropriate filter-specific Charge Diffusion Kernel. The four "raw" PSF images were then combined by the pipeline task multidrizzle in IRAF using the same parameters as for the data reduction. This allows us to mimic the effects of the reduction procedures. The results from GALFIT are the best-fit values for the effective half-light radius r eff , the Sersic index n, and the axis ratio b a ), which quantify the structure of the galaxy. The circularized radius is calculated as a . GALFIT also delivered photometry in all three bands, summarized in Table 2. Galactic extinction corrections are taken from the Schlafly & Finkbeiner (2011) maps.
The z = 2.583 DLA galaxy has a disk-like morphology with a Sersic index consistent with 1. The galaxy is compact with a circularized radius of only 0.11 ′′ corresponding to 0.9 kpc.

F606w
F105w F160w u−band g−band Ks−band HST/F160W  Table 2. Results from the GALFIT fits and NOT photometry. b/a is the ratio of the minor and major axis radii and n is the Sersic index. The magnitudes in the HST filters were computed by GALFIT, whereas aperture photometry was done on the NOT images. Band

NOT/Alfosc imaging
We use magauto in SExtractor (Bertin & Arnouts 1996) to measure the total fluxes of SDSS stars in the u-and g-band images of the field of Q 0918+1636, which we use to derive the zeropoints. In order to do photometry of the DLA galaxy counterpart we first did PSF subtraction using the same procedure as in similar previous studies (e.g., Møller & Warren 1993;Fynbo et al. 1999Fynbo et al. , 2000. Magnitudes were measured in circular apertures. Again, Galactic extinction corrections are taken from the Schlafly & Finkbeiner (2011) maps. In the g-band we measure an AB magnitude of 25.9 ± 0.3 and in the u-band we do not detect the DLA galaxy down to a 3σ detection limit of 26.5 (in a 2 arcsec diameter aperture).

NOT/NOTCam imaging
For the NOTCam/Ks-band we determined the zeropoint with stars from the 2MASS catalog (Skrutskie et al. 2006). On the combined image in the Ks-band we again subtracted the PSF of the QSO using a PSF determined from stars in the field. The residual image does not contain significant emission at the position of the z = 2.583 DLA galaxy. There is residual flux at the expected position at the 2σ significance level, but we conservatively report a 3σ detection limit of 23.3 (on the AB system) measured in a 2 arcsec diameter aperture.

SED fitting
We fit stellar population synthesis models to the six broad-band photometric points from the HST and NOT imaging listed in Table 2, to derive the stellar mass, age and star formation rate with the same procedure as in Krogager et al. (2013). In summary the fitting code uses the stellar population templates from Bruzual & Charlot (2003) convolved with a large Monte Carlo library of star formation histories (exponential plus random bursts) assuming a Chabrier (2003) IMF. Dust is added following the two-component model of Charlot & Fall (2000), with the parameters being the total optical depth, τv, and the fraction of dust 3 contributed by the ISM, µ.
The metallicity is restricted to solar as inferred from the absorption analysis, but we find consistent results when using the full range of the models between 20% and 2.5 times solar. We then adopt a Bayesian approach by comparing the observed magnitudes to the ones predicted by all the models in the library, and we construct 3 For details on the prior distribution of the SFH and dust parameters see Salim et al. (2005) 1 Averaged over 1 Gyr, but averaged over a shorter timescale of 10 Myr we get a similar value within the errors.  the probability density functions of stellar mass, mean luminosityweighted stellar age, and star formation rate. Fortunately, none of the filters contains any of the strong emission line and hence we do not include emission lines in the fits. The results of the SED-fits are provided in Table 3.

VLT/X-Shooter spectroscopy
The galaxy which is responsible for the z = 2.583 DLA is located at a projected distance of 1. ′′ 98 from the QSO, which corresponds to 10 pixels on the spatial axis of the 2D spectrum. Given the good seeing of 0. ′′ 8 the two objects are well separated. Thus, we do not need to subtract the continuum of the QSO. At the spatial position where the DLA galaxy is located, there is very little continuum flux, but the [OIII] λ5007 emission line of the z = 2.583 DLA galaxy is clearly visible. The [OIII] λ5007 line is typically the line detected at highest S/N ratio for galaxies at similar redshifts (e.g., Fynbo et al. 2011). We then extract a 1D spectrum of the z = 2.583 DLA galaxy similarly as for the QSO spectrum, but this time using a gaussian SPSF with a FWHM of 0. ′′ 8, as there is not enough continuum signal to determine the SPSF. The [OIII] λ5007 emission line is clearly located in wavelength regions without sky-line residuals. For this line the flux is determined by summing up the flux in the 1D spectrum. In Fig 4 we show the [OIII] λ5007 line in 1 and 2 dimensions. The redshift determined from the [OIII] λ5007 line is z = 2.58277 ± 0.00010, which is 36 ± 20 km s −1 (where the uncertainty also includes the uncertainty on the absorption redshift) blueshifted compared to the centre of the low-ionisation absorption lines (Fynbo et al. 2011).
To search for emission at lower impact parameter we subtract the QSO continuum following the procedure described in Fynbo et al. (2010). In Fig. 5 we show a wider region around the [OIII] λ5007 line from the z = 2.583 DLA galaxy after subtraction of the SPSF of the QSO. There is no evidence for emission at smaller impact parameters.
The Hα, Hβ, [OIII] λ4960, and [OII] λ3727 lines are visible but detected at lower S/N ratio. We derive fluxes for these lines by fixing the redshift and width from the [OIII] λ5007 line. The resulting line fluxes are provided in Table 5. In particular Hα is very uncertain as it is located far in the red end of the K-band where the sky-background is very high. We use the emission-line ratio R23 (originally defined by Pagel et al. (1979)) to derive the oxygen abundance for the system. The index is defined as the ratio of [OII] λ3727 and [OIII] λλ 4959,5007 to Hβ. The R23 metallicity indicator is double-valued. Moreover, the calibration of the line ratio depends on the ionization parameter, which also depends on metallicity. We therefore solve the problem iteratively by use of the line ratio O32 as an indicator of the ionization parameter. By using the calibration of Kobulnicky & Kewley (2004) to infer the metallicity, we obtain the following two values: the upper branch solution is 12 + log(O/H) = 8.8 ± 0.2, and the lower branch solution is 12 + log(O/H) = 8.2 ± 0.2. We consider the upper branch solution most likely in this case given the other properties of the system (absorption metallicity, luminosity, mass), but we cannot establish this on the basis of the emission lines alone.
For the Hα emission line the observed line flux corresponds to a luminosity of LHα = 1.5 ± 0.5 × 10 42 erg s −1 . Converting the luminosity into SFR using Kennicutt (1998) gives SFRHα = 13 ± 5 M⊙ yr −1 . Converting to the assumed Chabrier IMF (Treyer et al. 2007) we find SFRHα = 8 ± 3 M⊙ yr −1 . Correcting for the extinction inferred from the SED fitting this corresponds to 22 ± 7 M⊙ yr −1 for the Chabrier IMF, consistent with the SFR derived from the SED fitting in Table 3.
Due to the increased S/N ratio of the spectrum of Q0918+1636 we are also able to detect much weaker absorption lines than in Fynbo et al. (2011). An example is TiII λ1910 for which we measure an observed equivalent width of 0.102±0.014Å corresponding to a metallicity of -0.98±0.05 implying that Titanium is depleted by close to 1 dex. This is consistent with observations of Titanium in the local group where Titanium is found to be highly depleted onto dust grains (e.g., Welty & Crowther 2010). It is also consistent with the large depletion of Fe, Mn and Cr (Fynbo et al. 2011).

Absorption line analysis
The original reason for targeting this QSO was the presence of a metal-strong DLA at z = 2.412. To characterize the absorption line properties of the z = 2.412 DLA we performed Voigt-profile fitting of the HI and metal absorption lines. For the DLA we derive an HI column density of log N/cm −2 = 21.26 ± 0.06 (Fig. 6).
Voigt-profiles were fitted to several metal lines using the VPfit software 4 , assuming turbulence-dominated internal motion in the system. The redshift measured from the low-ionization (SiII, FeII, ZnII, CrII, MnII) absorption lines is z = 2.4121 ± 0.0002. Redshifts and velocity dispersions were tied for each of the individual components of the low-ionisation lines. High-ionisation lines were fitted independently. The low-ionization absorption was found to be best fitted by six distinct components. A plot of the fit results is shown in Fig. 7, and inferred column densities are shown in Table 4. Integrated metallicities [M/H] based on low-ionization absorption are −0.6 ± 0.2, −0.6 ± 0.2, −1.2 ± 0.2, −1.2 ± 0.2 and −1.3 ± 0.2 for ZnII, SiII, CrII, FeII and MnII, respectively. In Fig. 7 we also show the intermediate and high-ionisation lines from AlIII and CIV. These lines are fitted independently and in this case only 5 sub-components suffice. It is striking that the v < 0 km s −1 absorption is strongest for AlIII and CIV whereas the strongest low-ionization absorption is at v > 0 km s −1 .
The resolution of X-Shooter is, as dicussed in several earlier works, not ideal for robust Voigt-profile fitting (e.g., Fynbo et al. 2010;Noterdaeme et al. 2012a;Krühler et al. 2013), but for our purposes of establishing that the system is metal-rich and for inferring the velocity width of the absorption, the data are sufficient.
We also determine the velocity width of the low-ionisation absorption following the prescription of Ledoux et al. (2006). Here we find ∆v = 349 km s −1 and 352 km s −1 for FeII,λ2260 and SiII,λ1808, respectively.
In conclusion, the metallicity of the system is well above our target selection criterion of 0.1 Z ⊙ and there is evidence for substantial depletion of refractory elements on dust grains.
Further details on the analysis of this system can be found in Thorsen (2011).

The galaxy counterpart
In Fynbo et al. (2011) no emission was found from the galaxy counterpart of this absorber. From the spectrum taken with the slit at the position angle of 162 o East of North we find that the galaxy seen at the bottom of Fig. 1 is at a lower redshift of z = 0.987 based on the detection of the [OII] λ3727 doublet and the [OIII] λ5007 line. In Fig. 2 there is no obvious other source at smaller impact parameters except the counterpart of the z = 2.583 DLA. One possibility is that the source is a faint galaxy at a small impact parameter. In the lower left sub-panel of the upper panel in Fig. 2 there is a hint of a source at a position angle of about 130 o East of North (marked with an arrow). Opposite to this is a ring-like residual consistent with what one would expect if the centroid of the PSF has been shifted slightly to the lower left by the presence of the foreground galaxy. We have attempted to include such an additional source in the GALFIT modelling, but without success. We have done one test to gauge the reality of this potential source. Using GALFIT with rotated PSFs (45 o , 90 o , 135 o , etc.), we find that the dimples surrounding the PSF core are either well subtracted or poorly subtracted (or, in general, partially subtracted) depending on the angle of rotation. Obviously, the main effect is that the diffraction spikes change such that with a rotation one sees the original spikes as a positive residual and a negative residual oriented along the diffraction spikes of the rotated PSF. However, the residual to the south-east, labeled in the image, remains fixed with roughly the same shape and brightness. In the end we are convinced that the residual is due to a real source and not an artifact from poor PSF subtraction. We note that the residual could also be related to the host galaxy of the QSO. The tail extending below the QSO is a result of Charge Transfer Inefficiency.
In the case of a very small impact parameter we expect potential emission lines from the galaxy to be included in all the slits shown in Fig. 1. We therefore co-added all the 2-dimensional spectra obtained up to 2012 and performed a SPSF-subtraction as described in Fynbo et al. (2010). In the upper panel of Fig. 8 we show the region around the position in the spectrum where the [OIII] λ5007 emission line is expected to fall. We tentatively, at about 3.5σ significance, detect an emission line at the expected position. The spectrum we obtained in March 2013 is substantially better due to better observing conditions and the use of the nodding observing template. In the lower panel of Fig. 8 we show the [OIII] λ5007 emission line from the z = 2.412 DLA now at a higher S/N ratio. The line is very narrow with a FWHM of 67±12 km s −1 , which is only slightly larger than the resolution of 45 km s −1 . Corrected for the resolution the FWHM is 50 km s −1 . The redshift determined from the line is z = 2.4128 ± 0.0002, which is redshifted 38 ± 25 km s −1 relative to the mean absorption redshift, but of course well within the full ∼350 km s −1 velocity extent of the low-ionisation absorption.
We do not detect other lines, but [OIII] λ5007 is the line expected to be detected at highest significance at these redshifts (e.g., Fynbo et al. 2010;Krühler et al. 2012Krühler et al. , 2013 and the non-detection of the other lines is expected on S/N grounds. Table 4. Ionic column densities in the 6 individual line components of the DLA system at z abs = 2.412.

Ion
Transition The impact parameter is consistent with 0 and a conservative upper limit is 0.25 arcsec corresponding to 2.0 kpc.

DISCUSSION
In this paper we have presented new observations of the two DLAs towards Q 0918+1636 and their galaxy counterparts. The galaxy counterpart of the z = 2.583 DLA was discovered previously (Fynbo et al. 2011), whereas the discovery of the counterpart of the z = 2.412 DLA is first reported here. (1) Transition rest frame wavelength inÅ.
(3) Line width at FWHM in units of km s −1 corrected for the instrumental resolution of 45 km s −1 .

The z = 2.583 DLA Galaxy
For the z = 2.583 DLA Galaxy we have the largest amount of information: detection of severeal strong emission lines and a clear detection of the galaxy in the HST images. We can use our information about the size of the galaxy and the kinematics, as probed by the [OIII] λ5007 emission line of the z = 2.583 DLA galaxy to get an estimate of the dynamical mass of the system. As in Krogager et al. (2013) we follow the method described in Rhoads et al. (2013) to estimate the dynamical mass given the measured size and velocity dispersion: where i denotes the inclination of the system with i = 90 o being edge on and G is the gravitational constant. In order to estimate the velocity dispersion of the system we use the FWHM of the emission lines as a probe of the integrated gas-kinematics of the system. We then use the width of the [OIII] λ5007 line to estimate the ve-locity dispersion to be σ = 107 ± 10 km s −1 , and we adopt the size from the GALFIT analysis: r eff = 1.4 kpc (see Table 2). From the GALFIT analysis we infer a (projected) axis ratio of the galaxy of b/a = 0.43. The system may be described as disclike, given the elongated shape, and the fact that we see a value of Sérsic n close to 1. We thus adopt a value of sin(i) = 0.5 and use the fitted half-light semi-major axis for our estimate of the dynamical mass of the system: M dyn ≈ 6.0 ± 1.3 × 10 10 M⊙. This estimate should only be considered a rough approximation given the assumptions.
From our SED fit to the broad band imaging data, we obtain a stellar mass of M⋆ 10 10 M⊙. We can use this measurement to test the mass-metallicity relation for DLA systems (Ledoux et al. 2006;Møller et al. 2013). Using the relation in Møller et al. (2013, their eq. 6) using as input solar metallicity (i.e., assuming that C [M/H] = 0) we predict a stellar mass of M⋆ = 3 × 10 10 M⊙. Given the substantial (∼ 0.38 dex) scatter in their relation, the agreement between the prediction and our best fit stellar mass from the SED fit is good.

The nature of DLA galaxies
The two DLA systems studied here are, as other systems in our survey (Fynbo et al. 2010(Fynbo et al. , 2011Krogager et al. 2012), drawn from the extreme high-metallicity end of the distribution and hence should not be considered typical examples of DLA galaxies. In Table 6 we compare the two systems and include also DLA galaxies from the literature for comparison (Weatherley et al. 2005;Fynbo et al. 2010Fynbo et al. , 2011Bouché et al. 2012;Noterdaeme et al. 2012a;Krogager et al. 2013). For consistency we re-calculate the velocity shifts for N-14-1C and N-14-2C using the centroids of the low-ionisation lines corresponding to z abs = 1.9205. The galaxy counterpart of the z = 2.412 DLA is the system with the highest velocity extent of the low-ionisation absorption. However, the FWHM of its [OIII] emission is the lowest in the sample. This indicates that additional influences than mass must be important in determining the velocity width of the low-ionisation absorption. One such possible influence is of course outflows. Another important reason for this may be the low impact parameter, which implies that a larger fraction of the gravitational potential is probed by the lineof-sight. We also observe here, as in Krogager et al. (2012), that the systems with the highest HI column densities have the smallest impact parameters. It would be interesting to carry out detailed comparisons of the quantities in Table 6 with simulations, e.g., similar to the works of Pontzen et al. (2008) and Rakic et al. (2013).
For the galaxy counterpart of the z = 2.583 we can establish further properties: It is a compact (r eff = 1 kpc), strongly star-forming galaxy with a centroid 16.2 kpc away from the lineof-sight to the background QSO. The galaxy photometry is well fitted by galaxy templates with ages up to several 100 Myr. The ratio between the HI gas scale length of this DLA galaxy, as measured by its impact parameter, and the light scale length, as measured by its half-light radii, is of order 10 as seen in previous cases of DLA galaxy counterparts (Møller et al. 2002;Krogager et al. 2013). This is very different from the situation in local galaxies, where the gas only extends up to a few times the extension of the light (Bosma 1981). Our data are deep, but the (1 + z) 4 dimming of surface brightness with redshift is a very strong effect. Hence, an important question is whether the measured compact morphology is only due to central high surface brightness regions embedded in lower surface brightness regions with extension more similar to the HI gas, but below the surface brightness detection limit of our data. Such HI central high surface brightness regions are also seen in local spiral galaxies (e.g., Carollo et al. 1997). The issue of morphology of starforming galaxies at these redshifts as inferred from WFC3/IR data has been studied intensively by Law et al. (2012a,b) who find that these systems are not rotationally supported disk galaxies. Rather, they appear to be predominantly unstable, dispersion-dominated, systems fueled by rapid gas accretion which presumably later form extended rotationally supported disks. They also argue that all these galaxies drive strong outflows with more massive galaxies driving less highly ionized outflows. Compared to their sample the z = 2.583 DLA galaxy is in the upper third of the mass distribution. For the z = 2.583 galaxy the distances and ages are also consistent with a wind scenario: for an age of 233 Myr a mean speed of ∼ 70 km s −1 is required to reach 16 kpc. Such winds speeds are well below what is seen in nearby (more modest) winds (e.g., Melioli et al. 2013). Bouché et al. (2013) argue for a similar system of a galaxy counterpart to a DLA at an even larger impact parameter (DLA2243-60 in Table 6) that the gas causing the DLA absorption is in a cold inflow. In their case the DLA metallicity is −0.72, which is too high to be pristine gas. Hence, also in this case, an outflow must have been important for determining the properties of the system. Rafelski et al. (2011) use statistical arguments to show that most DLAs must probe atomic gas with very low star-formation efficiencies. This would be consistent with a picture where metals in this gas originates from a wind rather than having been formed in situ.
The large impact parameter of the z = 2.583 galaxy could also be related to other processes like tidal stripping similar to what is seen in the Magellanic stream (Misawa et al. 2009). As DLAs are HI cross-section selected such systems will have a higher probability of being selected (see also Rauch et al. , 2013. However, we note that the correlation between metallicity and impact parameter found by Krogager et al. (2012) would not obviously result from such a scenario and we do not see evidence for a nearby galaxy that could have caused tidal stripping.
We note that none of the two galaxies have Lyα in emission. This may help explain the many non-detections resulting from searches for DLA galaxies in the previous few decades (Lowenthal et al. 1995;Møller et al. 2004;Fynbo et al. 2010, and references therein).
A coherent picture of DLAs and their relation to emission selected galaxies could be the following: DLAs originate from the outskirts of galaxies with properties (i.e., sizes, luminosities, stellar masses, metallicities) within the range of starforming Lyman-break galaxies at similar redshift, but due to their cross-section selection they are more likely to be drawn from the fainter end of the luminosity function than emission selected galaxies (Fynbo et al. 1999;Møller et al. 2002;Fynbo et al. 2008;Rauch et al. 2008;. There is evidence that DLA galaxies fulfill a metallicity-luminosity relation (Møller et al. 2004;Ledoux et al. 2006;Fynbo et al. 2008;Møller et al. 2013) and therefore high-metallicity DLAs are expected to have galaxy counterparts more similar to typical emission-selected galaxies ( i.e., Lyman-break galaxies seen in ground-based surveys) than DLAs in general which probably have extreme galaxy counterparts (Fynbo et al. 1999;Haehnelt et al. 2000;Rauch et al. 2008). The galaxy counterparts of the two DLAs towards Q0918+1636 are consistent with this picture.

Outlook
Thanks to new sensitive near-IR spectrographs the study of galaxy counterparts of z > 2 DLAs has now opened (Weatherley et al. 2005;Fynbo et al. 2010Fynbo et al. , 2011Bouché et al. 2012;Noterdaeme et al. 2012a;Krogager et al. 2013). The identification of intervening DLAs towards transient sources like Gamma-Ray Burst afterglows have also led to the detection of a galaxy counterpart and this approach hence also appears promising for the future (Schulze et al. 2012). At the moment observations like these are limited to the bright counterparts of the highest metallicity DLAs. With the advent of extremely large telescopes equipped with advanced adaptive optics in the next decade, however, such studies can be extended to the galaxy counterparts of more typical DLAs and hence a more complete unification of absorption and emission studies of high-z galaxies is within reach.