Dwarfs Gobbling Dwarfs: A Stellar Tidal Stream Around NGC 4449 and Hierarchical Galaxy Formation on Small Scales

Abstract

A candidate diffuse stellar substructure was previously reported in the halo of the nearby dwarf starburst galaxy NGC 4449 by Karachentsev et al. We map and analyze this feature using a unique combination of deep integrated-light images from the Black Bird 0.5-meter telescope, and high-resolution wide-field images from the 8-meter Subaru telescope, which resolve the nebulosity into a stream of red giant branch stars, and confirm its physical association with NGC 4449. The properties of the stream imply a massive dwarf spheroidal progenitor, which after complete disruption will deposit an amount of stellar mass that is comparable to the existing stellar halo of the main galaxy. The ratio between luminosity or stellar-mass between the two galaxies is ∼ 1:50, while the indirectly measured dynamical mass-ratio, when including dark matter, may be ∼ 1:10-1:5. This system may thus represent a "stealth" merger, where an infalling satellite galaxy is nearly undetectable by conventional means, yet has a substantial dynamical influence on its host galaxy. This singular discovery also suggests that satellite accretion can play a significant role in building up the stellar halos of low-mass galaxies, and possibly in triggering their starbursts.

The remnant of a merger between two dwarf galaxies in Andromeda II

“…At similar mass-scales, around 10 11 solar masses in stars, further observational evidence of merging activity is also ample 3,4,5 . Mergers of lower-mass galaxies are expected within the hierarchical process of galaxy formation 6 , but have hitherto not been seen for galaxies with less than about 10 9 solar masses in stars 7,8 . Here, we report the kinematic detection of a stellar stream in one of the satellite galaxies of Andromeda, the dwarf spheroidal galaxy Andromeda II, which has a mass of only 10 7 solar masses in stars 9 .…”

New Spectroscopic Technique Based on Coaddition of Surface Brightness Fluctuations: NGC 4449 and Its Stellar Tidal Stream

“…We apply this technique to the stellar stream of NGC 4449 (Martínez-Delgado et al 2012;Rich et al 2012). Due to the distance to NGC 4449 (3.82 Mpc; Annibali et al 2008), the apparent magnitudes of individual RGB stars are too faint to target them spectroscopically, and its extremely low surface brightness also prevents us from obtaining integrated light spectroscopy (the TRGB of NGC 4449ʼs stream is found at I = 24.06 and its surface brightness is m = 26.75 g mag arcsec −2 ; Martínez-Delgado et al 2012).…”

“…We use our Subaru/Suprime-Cam r − i CMD, which Martínez-Delgado et al (2012) used to confirm the distance of the stream, to select candidates in the stellar stream. Our primary targets are objects above the TRGB (  i 24.2) and in the color range -< -< r i 0.1 0.5.…”

“…The stream has an elongated morphology that is ∼8 kpc long by ∼1.5 kpc wide (~¢´¢ 6.4 1.4; Martínez-Delgado et al 2012). This size is smaller than the Keck/DEIMOS field of view ( ¢´¢ 16.7 5.0).…”

“…The stellar stream, on the other hand, has a much steeper luminosity function because it is mainly composed of old stars (Martínez-Delgado et al 2012), which means that the bulk of the stars have very similar luminosity. This makes the blending effects in the stellar stream very important.…”