A Stellar Tidal Stream Around the Whale Galaxy, NGC 4631

Abstract

We report the discovery of a giant stellar tidal stream in the halo of NGC 4631, a nearby edge-on spiral galaxy interacting with the spiral NGC 4656, in deep images taken with a 40 cm aperture robotic telescope. The stream has two components: a bridge-like feature extending between NGC 4631 and NGC 4656 (stream SE ) and an overdensity with extended features on the opposite side of the NGC 4631 disk (stream NW ). Together, these features extend more than 85 kpc in projection. The orientation of stream SE relative to the orientations of NGC 4631 and NGC 4656 is not consistent with an origin from an interaction between these two spirals, and is more likely debris from a satellite encounter. The stellar tidal features can be qualitatively reproduced in an N-body model of the tidal disruption of a single, massive dwarf satellite on a moderately eccentric orbit (e = 0.6) around NGC 4631 over ∼3.5 Gyr. Both modeling and inferences from the morphology of the streams indicate these are not associated with the complex HI tidal features observed between both spirals, which likely originate from a more recent, gas-rich accretion event. The structure of stream NW suggests that it may contain the progenitor of the stream, in agreement with the N-body model. However, we cannot exclude other possibilities such as the satellite dwarf galaxy NGC 4627 being the progenitor based on these data. In addition, stream NW is roughly aligned with two very faint dwarf spheroidal candidates. The system of dwarf galaxies and the tidal stream around NGC 4631 can provide an additional interesting case for exploring the anisotropy distribution of satellite galaxies recently reported around Local Group spiral galaxies by means of future follow-up observations.

Resolved Stellar Streams around NGC 4631 from a Subaru/Hyper Suprime-Cam Survey

“…This is in good agreement with a total initial stellar mass (5.4 × 10 8 M ⊙ ) of the progenitor presumed in the N -body simulation of Martínez-Delgado et al (2015). However, red/metal-rich ends of these MDFs suffer from incompleteness even if we carefully construct them by considering completeness function derived in Section 2.3.…”

“…Furthermore, the lack of young stellar populations in Stream SE and NW is consistent with spatial consistent with lack of spatial correlation between these streams and the H I distribution discussed in Martínez-Delgado et al (2015). In the subsequent sections, we investigate the photometric properties of these two stellar streams based on the resolved stellar population.…”

“…The two red dashed-ellipses indicate Stream SE and NW, while the two red dashed-rectangles illustrate control fields for statistically estimating background and foreground contaminations in these stream fields. The four orange circles present the candidates of dwarf galaxies reported by the previous studies (Seth et al 2005a;Karachentsev et al 2014;Martínez-Delgado et al 2015;Javanmardi et al 2016). The eight blue ones show the candidates of dwarf galaxies newly detected through the visual inspection in this study.…”

Chemical Tagging in the SDSS-Iii/Apogee Survey: New Identifications of Halo Stars With Globular Cluster Origins

“…Stars from the earliest events in hierarchical assembly should be found throughout the Galaxy (e.g., Brook et al 2007;Tumlinson 2010), while stars from later accretion of lower-mass galaxies mainly orbit in the outer halo, where streams of debris are long-lived thanks to long dynamical timescales. Indeed, the spatial coherence of merger debris can be observed in deep imaging of many galaxies (e.g., Malin & Carter 1980;Martínez-Delgado et al 2015) and kinematic coherence has been identified through spectroscopic studies in the Milky Way (e.g., Schlaufman et al 2009) and M31 (e.g., Gilbert et al 2009). Minor mergers clearly play a crucial role in assembling the outer halo, with its high degree of substructure and small or negative net rotation (e.g., Font et al 2011;Deason et al 2012;Pillepich et al 2014).…”

Metallicity and Age of the Stellar Stream Around the Disk Galaxy NGC 5907

“…The Astronomical Journal, 152:72 (11pp), 2016 September doi:10.3847/0004-6256/152/3/72 in Martínez-Delgado et al 2015), Martínez-Delgado et al (2008) were not able to measure colors in the stream. Martínez-Delgado et al (2008) also produced a simple N-body model that mimicked the observed loopy stellar stream as a fossil of the tidal disruption of a single satellite in a merger event (total mass ratio of 1:4000), rejecting the hypothesis of multiple merger events in the halo of NGC 5907.…”