The VVVX quest for satellites around the Circinus galaxy

TL;DR

This study targets the Circinus galaxy halo to search for dwarf satellites using near-infrared data in the Milky Way's plane. It merges VVV/VVVX NIR photometry with ML-based photometric redshifts (ANNz) trained on a spectroscopic subset to filter foreground/background galaxies, and evaluates satellite candidates via half-light radii. Despite identifying 20 size-appropriate candidates, none have redshifts consistent with Circinus ($z\sim0.0015$), and only 8 lie at $z_{phot}<0.04$, far from the host; thus no satellites are confirmed. The work demonstrates the feasibility and limitations of NIR+ML approaches to study satellites at low Galactic latitudes and sets the stage for targeted spectroscopic follow-up.

Abstract

The Circinus galaxy is the nearest type-2 Seyfert galaxy, which is at a distance of 4.2 Mpc. Its environment is challenging to explore because it is located at low Galactic latitudes, behind the Galactic disc. The long-term goal is to characterise the Circinus galaxy halo and determine the presence of dwarf satellites using near-infrared data. We selected 1,542 galaxies from the VVV NIRGC within a 2-degree radius around Circinus, representing 2/3 of the virial radius. Structural parameters such as half-light radii and colours were used, and correlations were examined. A neural network was trained with 486 galaxies with known spectroscopic redshifts to estimate photometric redshifts for all galaxies. Potential satellites were defined based on half-light radii compatible with the typical sizes of dwarf satellites, and combined with photometric redshifts. The galaxy properties are reliably characterised down to $K_{s}$ $\sim$ 15.5 mag, which represents about 90% completeness of detections. At the distance of Circinus, this limiting magnitude corresponds to $K_{s}$ absolute magnitude of $-12.6$ mag, which allows us to find dwarf galaxies. There are 20 galaxies with half-light radii larger than 2.45 arcsec, only 8 have photometric redshifts below 0.04. None of these galaxies is close to Circinus, which has a redshift of 0.0015. The ANNz model exhibited a high degree of accuracy in the range $0.001 < z_{phot} < 0.023$. The presence of dwarf satellites could not be confirmed with the available data in the studied region. The apparent lack of satellites may be genuine, possibly related to AGN feedback effects. This work demonstrates the effectiveness of combining near-infrared data and machine learning techniques to estimate photometric redshifts at low Galactic latitudes, providing useful information for future spectroscopic follow-up campaigns.

Figures (10)

• Figure 1: The distribution of the VVV NIRGC galaxies in Galactic coordinates around the Circinus galaxy. The grey dashed circle represents the studied region, and the black ellipse shows the size and orientation of the Circinus galaxy. The galaxies are shown with black circles, and the three galaxies with available spectroscopy are indicated by blue triangles. The globular cluster candidates from Obasi2023Obasi2024 are represented with red circles.
• Figure 2: Colour-colour diagrams of the VVV NIRGC galaxies around the Circinus galaxy using the VVV and VVVX photometry (left panel) and DECaPS2 survey (right panel). The auxiliary bar represents the A$_V$ interstellar extinctions.
• Figure 3: Half-light radius distribution of the galaxies around the Circinus galaxy in logarithmic scale. The black dashed line shows the lower limit in half-light radius for dwarf satellites at the distance of the Circinus galaxy.
• Figure 4: Calibration results for the ANNz code. The solid line indicates the one-to-one relation between spectroscopic and photometric redshifts. The red points represent the mean photometric redshift within each spectroscopic redshift bin, with error bars indicating the standard deviation. The pink shaded region shows the calculated uncertainties in the distribution.
• Figure 5: Photometric redshift distribution of the galaxies around the Circinus galaxy in logarithmic scale. The blue line represents the median z$_{phot}$ of the distribution and the dashed red line shows the Circinus redshift.