Research Highlight

ALMA imprint of intergalactic dark structures in an Einstein ring

Our research team detected a gravitational imprint of intergalactic dark structures in the lensed submillimeter galaxy SDP.81 observed in 2014 October using Atacama Large Millimeter/submillimeter Array  (ALMA). The mass scale of the detected object is 10^9 solar mass, which  is the same as that of dwarf galaxies.  Firstly, we fitted a smooth lens model to the observed dust continuum image and obtained a residual image after fitted to the data.  Then, we found that the lensed 'B' image with a positive parity is systematically demagnified than the model prediction. In order to explain this feature, we need to consider a number of clustering halos or a locally underdense region in the line of sight. Although, we cannot distinguish these two possibilities observationally, the possibility of the former is considered to be smaller than the latter, because the tidal force in the lensing galaxy halo would strongly perturb subhalos falling into the parent halo.   On the other hand,  it is natural to have contributions from negative density objects in the latter case, because of spatial correlations between small halos that would eventually make voids in the intergalactic space.   Theoretical prediction of the strength of magnification perturbation of a source image due to intergalactic structures is consistent with the observation.  We also detected possible astrometric shifts of the order of 0.01 arcsec in the source image in a CO(8-7) line due to massive structures.  A compensated homogeneous spherical clump can explain the observed anomaly in the fluxes and astrometric shifts simultaneously. For detail, see arXiv:1510.00150. The result  is published in Monthly Notices of Royal Astronomical Society as "ALMA imprint of intergalactic dark structures in the gravitatioal lens SDP.81" MNRAS (2016) 457,2936. 

 

      Dust continuum image of the lensed submillimeter galaxy SDP.81 observed by ALMA. Aq1,

      Bq1, Cq1, Dq1 are quadruple image of a source q1 and Ad1, Dd1 and Ad2, Dd2 are double

       images of sources d1 and d2.

             Model lensed image of a best-fit smooth lens without any clumps.

                    Reconstructed source  image of the best-fit smooth lens model. q1 is splitted

                    into quadruple images, Aq1,Bq1,Cq1, Dq1 whereas d1 and d2 are splitted into

                    double images Ad1, Dd1 and Ad2, Dd2.

           Source images inverted from images A, B, C, and D for a best-fit smooth lens.  Inverted

           images of B and C  are distorted so that the fluxes are reduced by 20 %.  The contraction of

           B(C) image suggests a perturbation due to  a  locally underdense(overdense) object if projected

           onto the image plane .

                  Model lensed image of a best-fit smooth lens with a clump.  The full and dashed

                  curves corresponds to  the boundaries of a spherical clump. In between the two

                  curves, the density is negative (negative mass region).

           Source images inverted from images A, B, C, and D for a best-fit smooth lens with a clump

           with a negative mass wall.

 

 

         Observed (left) and model (right) images of emission line of CO(8-7) in SDP. 81. Because of 

        the gravitational effect of a possible clump, a new  bright image (inside a red circle) appears.  The corresponding

        other three images are thin arcs in the right hand side panel.