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

Abstract

We present a new spectroscopic technique based in part on targeting the upward fluctuations of the surface brightness for studying the internal stellar kinematics and metallicities of galaxies of low surface brightness effects both to galaxies and streams beyond the Local Group. The distance to these systems makes them unsuitable for targeting individual red giant branch (RGB) stars (tip of RGB at  I 24 mag) and their surface brightness is too low (  m 25 r mag arcsec −2 ) for integrated light spectroscopic measurements. This technique overcomes these two problems by targeting individual objects that are brighter than the tip of the RGB. We apply this technique to the star-forming dwarf galaxy NGC 4449 and its stellar stream. We use Keck/DEIMOS data to measure the line-ofsight radial velocity out to ∼7 kpc in the east side of the galaxy and ∼8 kpc along the stream. We find that the two systems are likely gravitationally bound to each other and have heliocentric radial velocities of 227.3 ± 10.7 km s −1 and 225.8 ± 16.0 km s −1 , respectively. Neither the stream nor the near half of the galaxy shows a significant velocity gradient. We estimate the stellar metallicity of the stream based on the equivalent width of its calcium triplet lines and find [Fe/H] = - 1.37 0.41, which is consistent with the metallicity-luminosity relation for dwarf galaxies in the Local Group. Whether the streamʼs progenitor was moderately or severely stripped cannot be constrained with this uncertainty in metallicity. We demonstrate that this new technique can be used to measure the kinematics and (possibly) the metallicity of the numerous faint satellites and stellar streams in the halos of nearby (∼4 Mpc) galaxies.

Star Formation Histories of the LEGUS Dwarf Galaxies. I. Recent History of NGC 1705, NGC 4449, and Holmberg II*

“…On the other hand, Martínez-Delgado et al (2012) and Rich et al (2012) detected and analyzed a stellar tidal stream in the halo of NGC 4449, arguing that this object is the result of the ongoing disruption of a dSph galaxy, while Annibali et al (2012) found a globular cluster of NGC 4449 which appears associated with two tails of blue stars, possibly the nucleus of a former gas-rich satellite galaxy undergoing tidal disruption by NGC 4449. Finally, using metallicity measurements of the stream, Toloba et al (2016) suggested a progenitor as massive as Fornax or Sagittarius. Interestingly, an interaction like that could have had a substantial impact on NGC 4449.…”

Simulating surveys for ELT-MOSAIC: status of the MOSAIC science case after phase A

“…MOSAIC will deliver several advances in the field by measuring the stellar kinematics in z~1 galaxies or in sub-structures of local systems. In particular, measuring the stellar kinematics of the tidal debris expected to lie in the outskirts of most Milky Way-like systems will allow us to test theoretical predictions and learn about the processes by which these galaxies formed (Toloba et al 2016), and provide unique views on their dark matter halos (Errani et al 2015). At earlier epochs (2<z<4), it will be important to measure spatially-resolved chemodynamical information in galaxies across a wide range of stellar masses (dwarfs to giants) and environments (field to clusters).…”

SPECTROSCOPIC CONFIRMATION OF THE DWARF SPHEROIDAL GALAXY d0994+71 AS A MEMBER OF THE M81 GROUP OF GALAXIES

“…To improve the reliability of v we coadd all 27 stars together (see Figure 2). Our spectral coaddition process is the same one as described by Toloba et al (2016a): (1) correct for possible offsets across the slit using the atmospheric A band seen in the continuum of all objects, this correction is < 10 km s −1 ; (2) rebin the spectra and their uncertainties to a common wavelength range; (3) renormalize the fluxes and their associated uncertainties; and (4) add, pixel by pixel, the fluxes of the renormalized rebinned spectra by performing a sigma clipping where those pixels that deviate more than 3σ from the median are rejected.…”

“…We estimate the stellar metallicity of d0944+71 following the same procedure as Ho et al (2015) and Toloba et al (2016a). This method transforms the equivalent width (EW) of the two strongest absorption Upper panels: flux (dark green) and error (light green) spectra that result after coadding all 27 targets used to measure the radial velocity of d0944+71.…”

“…We have devised a new method to obtain critical spectroscopic information via resolved stellar populations out to distances of ∼ 4 Mpc, utilizing powerful groundbased multi-object spectrographs (as recently described in Toloba et al 2016a). The method involves co-adding spectra from a single dwarf galaxy or stellar stream to obtain a final spectrum to measure a radial velocity or mean metallicity.…”