Stellar Stream Around Messier 61

• The stellar stream around M61 is a narrow tidal feature extending ~50 kpc, formed from the accretion of a disrupted dwarf galaxy.
• It exhibits extremely faint surface brightness (~28 AB mag arcsec⁻²) and red colors, typical of an old, quenched stellar population.
• The interaction likely triggered dynamical changes in M61, demonstrating LSST’s potential to reveal detailed galactic accretion processes.

A stellar stream has been identified around the spiral galaxy Messier 61 (M61, NGC 4303) by the Vera C. Rubin Observatory in its First Look imaging campaign, representing the first stellar stream discovery with this instrument. The stream exhibits a narrow, radially-oriented morphology, extending approximately 50 kpc from the disk, and is characterized by exceptionally faint surface brightness ($\mu_g \sim 28$ AB mag arcsec$^{-2}$), red color ($(g - z)_0 \sim 1.0$), and high stellar mass ($M_\star \sim 2 \times 10^8~M_\odot$). These properties indicate that it is the tidal debris of an accreted dwarf galaxy, with significant implications for hierarchical galaxy formation and the broader census of galactic accretion features as anticipated with the Legacy Survey of Space and Time (LSST) (Romanowsky et al., 28 Oct 2025).

1. Discovery and Observational Context

The stream around M61 was first visually identified by Giuseppe Donatiello in 2020 via small-aperture telescopes; however, it was unambiguously revealed within the Rubin Observatory’s commissioning data. The observations encompassed $\sim 25$ deg$^2$ of the Virgo Cluster, utilizing LSSTCam in ugriz bands. The imaging depth reached levels comparable to a five-year LSST stack, with sensitivity to surface brightness features as faint as $\mu_g \sim 30$–$31$ mag arcsec$^{-2}$. The enhanced data quality and coverage enabled detailed morphological and photometric analysis, leveraging preservation of low-surface-brightness (LSB) features critical for detecting extended, faint structures.

2. Morphological and Structural Properties

The stellar stream manifests as a $\sim 50$ kpc feature projected nearly straight northward from M61’s outer disk (20 kpc from the center), initially $\sim 2$ kpc wide, broadening to $\sim 4$ kpc toward the northern extremity. The northern terminus culminates in a plume roughly $9 \times 4$ kpc, accompanied by a faint $\sim 5$ kpc extension. Lower-surface-brightness satellites and possible peripheral plumes are also noted but remain unconfirmed. Figure 1 in the cited work provides a color composite delineating the stream morphology and context within M61’s halo, illustrating both the core structure and intricate debris field.

Feature	Extent	Notable Description
Main stream	~50 kpc (length)	Radially-oriented, narrow structure
Width at disk edge	~2 kpc	Increases to ~4 kpc at far end
Plume	~$9 \times 4$ kpc	Complex structure at northern tip
Extension	~5 kpc	Faint, further extension

3. Photometric and Stellar Population Characteristics

The stream is defined by an extremely faint $g$-band surface brightness, with $\mu_g \sim 27.2$ mag arcsec$^{-2}$ near the disk, declining to $\mu_g \sim 28.6$ mag arcsec$^{-2}$ at its terminus; the mean value is $\mu_g = 27.9$ mag arcsec$^{-2}$. Integrated color measurements yield $(g - r)_0 = 0.70$ and $(g - z)_0 = 1.00 \pm 0.1$, consistent with an old, quenched dwarf galaxy stellar population. The total $g$-band luminosity is $L_g \sim 9 \times 10^7~L_{g,\odot}$, with a stellar mass estimate of $M_\star \sim 2 \times 10^8~M_\odot$ based on mass–color calibrations. These quantitative properties are analogous to those of the Sagittarius stream’s progenitor in the Milky Way.

Property	Value	Contextual Note
Surface brightness	$\mu_g \sim 28$ mag arcsec$^{-2}$	Extremely faint regime
Integrated color	$(g-z)_0 = 1.00 \pm 0.1$	Quenched dwarf population
Luminosity	$L_g \sim 9 \times 10^7~L_{g,\odot}$	Comparable to Sagittarius stream
Stellar mass	$M_\star \sim 2 \times 10^8~M_\odot$	Dwarf galaxy scale

4. Dynamical Origin and Impact on Messier 61

The tidal stream is interpreted as the debris of a disrupted dwarf galaxy accreted by M61. The infall halo mass of the progenitor is estimated at $\sim 8 \times 10^{10}~M_\odot$. This interaction plausibly triggered several dynamical phenomena within M61, including the formation of a central bar, a $\sim 10$ Myr old nuclear starburst, and potentially AGN activity. This scenario directly parallels the proposed astrophysical impacts observed in the Milky Way due to the Sagittarius infall. The stream’s debris exhibits limited phase mixing, with stellar material confined to a narrow range of orbital phase, suggesting an earlier stage of disruption compared to the more dispersed Sagittarius stream.

5. Methodologies and Data Reduction Techniques

Data acquisition relied on Rubin Observatory’s LSSTCam, collecting high-fidelity ugriz imaging and emphasizing low-surface-brightness feature retention during reduction. Initial surface photometry was performed using DECaLS DR10 data due to the provisional status of Rubin’s photometric calibration. Image processing employed Gnuastro for background subtraction, contamination masking, and aperture photometry, supplemented by CDS-provided RGB FITS cubes for visualization. These techniques were essential for quantifying the stream’s photometric parameters and morphological traits.

6. Implications for Galaxy Formation and LSST Prospects

The detection exemplifies Rubin Observatory’s capability to identify faint tidal features around nearby galaxies and signals the advent of comprehensive galactic accretion census via LSST. The anticipated ten-year LSST Legacy Survey will enable probing of galactic halos to depths of $\mu_g \sim 30$–$31$ mag arcsec$^{-2}$, fostering the discovery of a wide array of substructures. This will directly benefit hierarchical galaxy formation models, refine dark matter halo characterizations, and inform the evolutionary history of galactic environments.

A plausible implication is that establishing a census of accretion features like the M61 stream will constrain merger rates, starburst triggers, and the assembly history in disk galaxies with high statistical significance.

7. Connections to Galactic Archaeology and Theoretical Context

The morphology, photometric properties, and dynamical scenario of the M61 stream reinforce the prevailing hierarchical assembly paradigm, wherein galactic halos accrete and disrupt dwarf companions. The observed parallels with the Sagittarius stream emphasize the universality of such accretion events across disk galaxies. These empirical findings will inform both semi-analytic and numerical models, placing robust constraints on the timescales, structural evolution, and feedback mechanisms associated with dwarf galaxy accretion. The LSST-enabled expansion of stream detections will have profound consequences for tests of Lambda-CDM predictions and dark matter substructure phenomenology.