Volume 574, February 2015
|Number of page(s)||21|
|Published online||16 January 2015|
IRAM PdBI maps of the FIR continuum showing the marginal detection of an additional source in the field, north-west of SDSS J1148+5251, at a projected distance of 10.5 arcsec, originally discovered by Herschel. Left: the map on the left panel has been obtained by collapsing the line-free channels of the dataset at 256 GHz as explained in Sect. 3.1. The synthesised beam is 1.3″ × 1.2′′. Negative and positive contours correspond to − 3σ, 2σ, 4σ to 18σ in steps of 2σ (1σ rms noise is 0.159 mJy beam-1). Right: the map on the right panel has been obtained by merging the two ancillary datasets at 262 GHz and 259.4 GHz (average observed frequency of 260.7 GHz). This is the same map as Fig. 11, which is employed for studying the spatial extent of the FIR continuum. The synthesised beam is 1.1′′×1.0′′. Negative and positive contours correspond to − 3σ, 3σ, 5σ to 40σ in steps of 5σ (1σ rms noise is 0.082 mJy beam-1).
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Herschel observations revealed the presence of an additional FIR continuum source, north-west of SDSS J1148+5251, at a distance of ~10′′ (Leipski et al. 2010, 2013, 2014). This north-western source is detected at the ~5σ level in our PdBI observations. More specifically, Fig. A.1 shows that a FIR continuum source at a distance of 10.5 ′′ from SDSS J1148+5251 is independently detected in both our FIR continuum maps at 256 GHz (4.5σ) and at 261 GHz (5.6σ).
In Sect. 3.1 we have evidenced a 2σ inconsistency between our new FIR continuum flux density estimate at 256 GHz of 3.3 ± 0.7 mJy and the flux density of 4.8 ± 0.6 mJy expected at the same observed frequency from the MAMBO-II bolometric observations by Bertoldi et al. (2003). Such small discrepancy can be fully explained by the presence of the north-western source contaminating the MAMBO-II flux measurement. We further test this hypothesis by performing a fitting of the FIR SED of SDSS J1148+5251, in which we select only the continuum measurements available in the literature that are not contaminated by the north-western source or that have been corrected for this effect (Fig. A.2). In particular, we select the 100 μm and 160 μm Herschel/PACS observations by Leipski et al. (2013) (corrected for the contaminating source), the 1.1 mm PdBI observations by Gallerani et al. (2014) (observed frequency of 262 GHz), and the 1.3 mm and 2.8 mm PdBI observations by Riechers et al. (2009) (observed frequencies of 225 GHz and 109 GHz, respectively). We exclude from the SED fitting our PdBI data point at 256 GHz (shown as a red star in Fig. A.2), as the purpose of the fit is showing the consistency of our continuum measurement with the previous observations, once corrected for the contaminating north-western source. The FIR SED in Fig. A.2 is fit with a single temperature modified blackbody combined with a mid infrared (MIR) power law, following Casey (2012). By fixing the emissivity β = 1.6, we obtain a dust temperature of 64 K. We note that our PdBI continuum observation at 256 GHz sits perfectly on the SED model in Fig. A.2.
We then use the FIR SED fit model for estimating the expected MAMBO-II flux density, by taking into account the spectral response of the bolometer (transmission curve). We obtain an expected MAMBO-II flux density of 3.8 mJy, i.e. significantly lower than the value reported by Bertoldi et al. (2003), which would in turn predict a flux density at 256 GHz of 3.6 ± 0.4 mJy, consistent with our PdBI measurement.
FIR SED of SDSS J1148+5251, where only the continuum measurements corrected for the contaminating north-west source are shown, i.e. the Herschel/PACS observations by Leipski et al. (2013), the PdBI observations by Riechers et al. (2009) and Gallerani et al. (2014), and our new PdBI observations at 256 GHz (Sect. 3.1). Different symbols have been used for different datasets (see legend at the bottom-left corner of the plot). The FIR SED has been fit with a single dust temperature modified blackbody with fixed emissivity β = 1.6, combined with a MIR power law, following Casey (2012). Our new PdBI data point at 256 GHz has been excluded from the fit, as explained in the text.
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In Table B.1 we report the results of the spectral fitting to the bright features A-F of the extended [C ii] 158 μm source, shown in Fig. 3. It is immediately evident from Table B.1 that the features A-F are characterised, on average, by high velocity dispersions, which can be as high as σv ~ 800 km s-1, suggesting that they are mostly associated with the outflow discovered by Maiolino et al. (2012). This is confirmed by the analysis of the maps of the broad wings (Sect. 3.4). However, we note that, in correspondence of some positions (e.g. B, C, F), there is also a significant contribution from “narrow” emission at the systemic velocity, hinting at the presence of a very extended “quiescent” (i.e. non outflowing) [C ii] component, which is investigated in Sect. 3.5.
IRAM PdBI continuum-subtracted maps of the blue- (top panels) and red- (bottom panels) shifted [C ii] 158 μm emission of SDSS J1148+5251, obtained at different velocities. The corresponding velocity integration ranges are indicated on each map. Negative and positive contours are in steps of 1.5σ. In all panels the big central cross indicates the pointing and phase centre, corresponding to the optical position of the quasar. The small crosses mark the position of the 48 blobs that we identified as belonging to the outflow. For each of these clumps in outflow we estimate the dynamical time scale τdyn = R/v, where R is the distance from the quasar position, and v is the average velocity of each map.
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In this section we explain the method we employ for estimating the outflow mass-loss rate using the resolved information provided by our IRAM PdBI observations. We produce channel maps of the [C ii] 158 μm blue- and red-shifted emission at velocities v ≤ −200 km s-1and v ≥ 200 km s-1, respectively. The maps are shown in Fig. C.1. The channel widths vary with velocities, because we aim to obtain an approximately constant signal-to-noise for the extended emission in each map. For this reason, at lower velocities, i.e. closer to the [C ii] emission peak, we use narrow channels with Δv = 100 km s-1, while at higher velocities, tracing the fainter high-velocity component of the outflow, we integrate over wider channels of Δv ~ 200−500 km s-1. The centroid positions of the 48 clumps that we ascribe to the outflow are indicated in Fig. C.1. These are selected to be spatially included within the region corresponding to the total [C ii] source shown in Fig. 2 and to have in the maps a signal-to-noise ratio SNR ≥ 3. We note that we have conservatively excluded from the outflow computation the central components of maps B1 (⟨ v ⟩ = −250 km s-1) and R1 (⟨ v ⟩ = 250 km s-1), because, due to low velocities probed by these two maps, it is difficult to assess to which extent the central extended emission belongs to
the outflow. For similar reasons we have not taken into account the central core emission within ± 200 km s-1, although there may be a significative outflow contribution of ~20% even at these low velocities, as suggested by the Gaussian fits to the line profile (Fig. 1). However, we have included in the outflow the central blobs visible in maps B2 and R2 (mean velocities of ± 350 km s-1), because they are offset with respect to the quasar position and appear to be co-spatial with the higher velocity emission in maps B3 and R3, which is undoubtedly tracing the outflow.
For each blob (belonging to the outflow) we calculate from the maps its projected distance from the central quasar and, accordingly, obtain a measure of its dynamical time scale. The dynamical time scale is defined as τdyn = R/v, where R is the distance travelled by the clump (assumed equal to the projected distance between the centroid of each clump and the optical position of the quasar) and v is the velocity of the gas. The error on R is estimated differently for resolved and unresolved clumps; in particular, for unresolved clumps, σR is the FWHM of the synthesised beam divided by the signal-to-noise ratio. For resolved clumps, instead, we set , where rmax is the maximum (projected) radius of the blob. We conservatively adopt for v the average (projected) velocity of each map. We note that the uncertainty on v, defined as σv=Δv/2, is obviously higher in the higher velocity maps, where we have integrated the emission over larger velocity channels (Δv). The distribution of τdyn that we measure within the outflow is shown in Fig. 6.
To estimate the mass of (atomic) gas in outflow, we first measure the [C ii] flux associated with each outflowing clump, by using apertures centred on the positions of the crosses (indicated in Fig. C.1). We then convert the [C ii] fluxes (integrated over their corresponding velocity ranges Δv) into [C ii] 158 μm luminosities, and the [C ii] luminosities into (lower limits on the) atomic
gas mass, following Hailey-Dunsheath et al. (2010) as explained in Sect. 3.4. By adding up the mass-loss rate contribution, i.e. , from all the 48 clumps in outflow, we obtain a total integrated outflow rate of 1400 ± 300 M⊙ yr-1. The error is simply , where is the uncertainty on Ṁout for a given clump in outflow, obtained by propagating errors on τdyn and on the velocity-integrated flux associated with the clump. We note, however, that the uncertainty on the conversion from [C ii] luminosity to atomic gas mass is not taken into account in our error estimate.
We show in Fig. D.1 the first and second moment maps of the [C ii] emission within v ∈(−200, 200) km s-1, obtained by applying a flux threshold of 2.8 mJy. The moments maps suggest that the bulk of [C ii] does not trace gas in a regularly rotating disk. Our IRAM PdBI observations are dominated by the extended [C ii] component which, in this velocity range, contributes to ~70% of the total flux (Sect. 3.5). It is however possible that the compact source, unresolved by our observations, is rotationally-supported, as suggested by Walter et al. (2009). Moreover, the first moment map shows a north-south velocity gradient south of the quasar position, with velocities ranging from 20 km s-1 to 100 km s-1, whose origin is not clear. In conclusion, no constrains can be put on the total dynamical mass of the system from our [C ii] observations.
First (top) and second (bottom) moment maps obtained within v ∈(−200, 200) km s-1 by applying a flux threshold of 2.8 mJy. Velocity contours in both maps are in steps of 20 km s-1.
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© ESO, 2015
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