A novel 3D technique to study the kinematics of lensed galaxies

This schematic view shows lensed images in the top row and the source plane in the bottom row. Lensed data are shown for three representative velocity channels of the data cube; the respective grid on the image plane is regular. For each velocity channel, the position of a pixel in the image plane corresponds to a position on the source plane (lower panel), determined by the lens equation. The points form the vertices of a triangular adaptive grid on the source plane. The source grid automatically adapts with the lensing magnification, so that there is a high pixel density in the high-magnification regions close to the caustics. © MPA

Gravitational lensing offers the possibility to study faint, far-away galaxies. MPA researchers have now developed the first three dimensional lens modelling method, which allows not only the reconstruction of the mass distribution of the foreground galaxy but also the kinematics of the background galaxy. Consequently, the matter content can now be studied also in young galaxies.

In the standard model of cosmology, galaxies form as the baryonic gas cools at the centre of dark matter halos. They subsequently grow through accretion and mergers, leading to the hierarchical build-up of galaxy mass. While this general picture is well known, there are numerous physical mechanisms determining the relative contribution of baryons and dark matter within a galaxy and several open questions remain: What are the most important physical mechanisms that lead to the variety of galaxies we observe today? How do these mechanisms influence the matter content within galaxies? The answer to these questions is one of the significant challenges of modern astrophysics.

The study of galaxy kinematics has played a key role in this context. For example, in the local universe, the flatness of observed rotation curves is a well-established fact. The outer parts of the observed rotation curves cannot be explained by the mass predicted from the observed stellar and gas distribution and this discrepancy has been interpreted as evidence for the presence of a "dark matter" halo. Within high redshift galaxies, however, the relative content of baryons and dark matter is poorly known and also its evolution with cosmic time is not well understood. Neither current numerical simulations nor observational studies were able to produce consistent results on the fraction of dark matter within young galaxies.

Data and modelling for one simulated 3D dataset. The rows show three representative channel maps, corresponding to three velocities. Column 1 shows the input source, a rotating disc with its approaching (first row) and receding side (third row); the middle row shows the component which is at rest relative to the observer. Each row is then lensed forward to obtain the mock lensed data in Column 2. The model obtained with the 3D-lens modelling method is shown in column 3 and the residuals (difference of the data and the model) in column 4. From this model, both the source (column 5) and its kinematics (column 6) can be reconstructed. © MPA

The diverging results on the kinematics of high-redshift galaxies - and in consequence on their matter content - can be ascribed to the different methods used to overcome the observational limitations. The study of kinematics is mainly hampered by two factors: low spatial resolution and low signal-to-noise ratio.

These observational limitations can be successfully overcome by targeting galaxies for which the line of sight lies very close to a foreground galaxy. The gravitational field of the foreground galaxy then deflects the light from the distant background galaxy, producing distorted, magnified, and even multiple images of the background object. This effect is known as strong gravitational lensing and it offers the opportunity to study the background galaxies at high physical resolution and with good signal-to-noise. Furthermore, the magnifying power of gravitational lensing opens the possibility to study faint galaxies with low stellar masses, which are not easily accessible by surveys targeting unlensed galaxies.

The gravitational lensing group at MPA developed the first three dimensional lens modelling method (see Figure 1). This can be applied to 3D (IFU or radio) data, characterized by two spatial dimensions and one spectral dimension (velocity, frequency or wavelength), to simultaneously reconstruct both the mass distribution of the foreground galaxy and the kinematics of the background galaxy (see Figure 2).

For different mock background galaxies, these plots show the velocity fields (upper panels) and rotation curves (bottom panels). The velocity field is colour coded (see bar on the side) with red areas moving away from the observer and blue areas moving towards the observer. The original rotation curves are shown in blue and the best fit kinematic model is shown in red. The orange band shows the possible errors from uncertainties of the parameters that defined the rotation curves.

The mock data M1-M3 have input rotation curves described by functional forms, while for M4-M6 the rotation curves were taken from real galaxies. The rotation curves of M1 and M4 are typical of dwarf galaxies, the rotation curves of M2 and M5 are prototypes of spirals, while those of M3 and M6 are typical of massive spirals with a prominent bulge. © MPA 

Our method represents a significant improvement over those used until now, since it does not require the use of high-resolution imaging data for the derivation of the lens parameters, as these are derived from the same 3D data used for the kinematics of the background galaxy. Moreover, the latter is not obtained by fitting on the source plane, but directly the lensed data. This is achieved in a hierarchical Bayesian fashion, where the kinematics on the source plane is essentially a hyper-parameter of the model (i.e. a parameter defining the prior). We are thus able to study the possible degeneracies between the lens and kinematic parameters and estimate the uncertainties consistently.

With our technique we are able to recover both the lens and the kinematics parameters with great accuracy under different observational conditions. Furthermore, we have successfully tested the capability of this new method in recovering a variety of rotation curves with shapes which are prototypes of different morphological galaxy types, from dwarf to massive spiral galaxies (see Figure 3).

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Authors

Rizzo, Francesca

PhD student
Phone: 2019
Email: frizzo@mpa-garching.mpg.de
Room: 107

Vegetti, Simona
Scientific Staff
Phone: 2285
Email: svsvegetti@mpa-garching.mpg.de
Room: 10

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Original Publication

1. Rizzo F., Vegetti S., Fraternali F., Di Teodoro E.

A novel 3D technique to study the kinematics of lensed galaxies

Submitted to MNRAS in June 2018

Source: Max Planck Institute for Astrophysics

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Gemini Characterizes Cheshire Cat

Figure 1. Image of Cheshire Cat gravitationally lensed galaxies produced by combining Chandra X-ray data with Hubble Space Telescope optical data. Gemini spectroscopy provided critical spectroscopic data for characterizing the foreground lensing cluster and understanding its history and likely fate as a "fossil group." Image Credit: Hubble Space Telescope and Chandra X-ray Observatory

Figure 2. Color–magnitude diagram of all galaxies detected in the GMOS images with r' brighter than 24.5 mag (595 galaxies, black dots). Blue circles and red squares are the spectroscopically confirmed members of the G1 and G2 groups respectively, with filled and open symbols representing which of these galaxies lie inside and outside of 0.5 r₂₀₀ of the group. Red short dash and blue long dash vertical lines represent a two magnitude gap from the E and W eye galaxies, respectively. The black triangles represent spectroscopically confirmed background and foreground galaxies, while black dots represent objects without spectroscopic redshifts. The two central galaxies of the groups, SDSS J103843.58+484917.7 and SDSS J103842.68+484920.2 (the "eyes" of the Cheshire Cat) are represented by a letter (E and W) in the CMD. 

Gemini observations provide a key scientific context for a striking new image of the gravitationally lensed galaxy group popularly known as the Cheshire Cat. The image, released by NASA’s Chandra X-ray Observatory, combines X-ray data with optical images from the Hubble Space Telescope to paint a remarkable likeness to the famously devious smiling cat in Lewis Carrol’s Alice in Wonderland.

Behind the scenes, Gemini played a critical role in the image’s scientific story by taking the spectral fingerprints of many of the galaxies that make up the foreground cluster which bends the distant galactic light. For more details on the image, and how gravitational lenses work, see the Chandra press release here.

Gemini South astronomer Rodrigo Carrasco, a co-author on the paper which spawned the image, adds that Gemini provided the core spectroscopic data that allowed the team to fully characterize the cluster of foreground galaxies which serves as the lens. "With the Gemini data we were able to study the individual members of the cluster which we expect will eventually become fossil groups," says Carrasco. Fossil groups consist of a giant elliptical central galaxy which is the end-product of interactions with other galaxies over billions of years, and surrounded by smaller, less bright galaxies. "Our results indicate that merging of fossil groups – in this case, the Cheshire Cat – could be one of the ways that a large fraction of fossil groups in the nearby Universe formed." 

The paper, published in The Astrophysical Journal, is available at: http://xxx.lanl.gov/abs/1505.05501. 

Paper Abstract:

Abstract: The Cheshire Cat is a relatively poor group of galaxies dominated by two luminous elliptical galaxies surrounded by at least four arcs from gravitationally lensed background galaxies that give the system a humorous appearance. Our combined optical/X-ray study of this system reveals that it is experiencing a line of sight merger between two groups with a roughly equal mass ratio with a relative velocity of ∼1350 km s^-1. One group was most likely a low-mass fossil group, while the other group would have almost fit the classical definition of a fossil group. The collision manifests itself in a bimodal galaxy velocity distribution, an elevated central X-ray temperature and luminosity indicative of a shock, and gravitational arc centers that do not coincide with either large elliptical galaxy. One of the luminous elliptical galaxies has a double nucleus embedded off-center in the stellar halo. The luminous ellipticals should merge in less than a Gyr, after which observers will see a massive 1.2 − 1.5 x 10¹⁴ M_⊙ fossil group with an M_r = −24.0 brightest group galaxy at its center. Thus, the Cheshire Cat offers us the first opportunity to study a fossil group progenitor. We discuss the limitations of the classical definition of a fossil group in terms of magnitude gaps between the member galaxies. We also suggest that if the merging of fossil (or near-fossil) groups is a common avenue for creating present-day fossil groups, the time lag between the final galactic merging of the system and the onset of cooling in the shock-heated core could account for the observed lack of well-developed cool cores in some fossil groups.

Source: Gemini Observatory