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Register a new userThe elusive stellar halo of the Triangulum Galaxy
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Added by
Geraint F. Lewis Prof
Publication date
2016
fields
Physics
and research's language is
English
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The stellar halos of large galaxies represent a vital probe of the processes of galaxy evolution. They are the remnants of the initial bouts of star formation during the collapse of the proto-galactic cloud, coupled with imprint of ancient and on-going accretion events. Previously, we have reported the tentative detection of a possible, faint, extended stellar halo in the Local Group spiral, the Triangulum Galaxy (M33). However, the presence of substructure surrounding M33 made interpretation of this feature difficult. Here, we employ the final data set from the Pan-Andromeda Archaeological Survey (PAndAS), combined with an improved calibration and a newly derived contamination model for the region to revisit this claim. With an array of new fitting algorithms, fully accounting for contamination and the substantial substructure beyond the prominent stellar disk in M33, we reanalyse the surrounds to separate the signal of the stellar halo and the outer halo substructure. Using more robust search algorithms, we do not detect a large scale smooth stellar halo and place a limit on the maximum surface brightness of such a feature of ${mu}_V$ = 35.5 mags per square arcsec, or a total halo luminosity of $L < 10^6L_{odot}$.
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As large-scale stellar surveys have become available over the past decade, the ability to detect and characterize substructures in the Galaxy has increased dramatically. These surveys have revealed the Triangulum-Andromeda (TriAnd) region to be rich with substructure in the distance range 20-30 kpc, and the relation of these features to each other -- if any -- remains unclear. This complex situation motivates this re-examination of the TriAnd region with a photometric and spectroscopic survey of M giants. An exploration using 2MASS photometry reveals not only the faint sequence in M giants detected by Rocha-Pinto et al. (2004) spanning the range $100^{circ}<l<160^{circ}$ and $-50^{circ}<b<-15^{circ}$ but, in addition, a second, brighter and more densely populated M giant sequence. These two sequences are likely associated with the two distinct main-sequences discovered (and labeled TriAnd1 and TriAnd2) by Martin et al. (2007) in an optical survey in the direction of M31, where TriAnd2 is the optical counterpart of the fainter RGB/AGB sequence of Rocha-Pinto et al. (2004). Here, the age, distance, and metallicity ranges for TriAnd1 and TriAnd2 are estimated by simultaneously fitting isochrones to the 2MASS RGB tracks and the optical MS/MSTO features. The two populations are clearly distinct in age and distance: the brighter sequence (TriAnd1) is younger (6-10 Gyr) and closer (distance of $sim$ 15-21 kpc), while the fainter sequence (TriAnd2) is older (10-12 Gyr) and is at an estimated distance of $sim$ 24-32 kpc. A comparison with simulations demonstrates that the differences and similarities between TriAnd1 and TriAnd2 can simultaneously be explained if they represent debris originating from the disruption of the same dwarf galaxy, but torn off during two distinct pericentric passages. [Abridged]
Rapid advance has been made recently in accurate distance measurements for nearby ($D < 11$ Mpc) galaxies based on the magnitude of the tip of red giant branch stars resolved with the Hubble Space Telescope. We use observational properties of galaxies presented in the last version of Updated Nearby Galaxy Catalog to derive a halo mass of luminous galaxies via orbital motion of their companions. Our sample contains 298 assumed satellites with known radial velocities around 25 Milky Way-like massive galaxies and 65 assumed satellites around 47 fainter dominant galaxies. The average total mass-to-$K$-band luminosity ratio is $31pm6 M_odot/L_odot$ for the luminous galaxies, increasing up to $sim200 M_odot/L_odot$ toward dwarfs. The bulge-dominated luminous galaxies are characterized with $langle{}M_T/L_Krangle = 73pm15 M_odot/L_odot$, while the disc-dominated spirals have $langle{}M_T/L_Krangle = 17.4pm2.8 M_odot/L_odot$. We draw attention to a particular subsample of luminous spiral galaxies with signs of declining rotation curve, which have a radial velocity dispersion of satellites less than 55 km/s and a poor dark matter halo with $langle{}M_T/L_Krangle = 5.5pm1.1 M_odot/L_odot$. We note that a fraction of quenched (dSph, dE) companions around Milky Way-like galaxies decreases with their linear projected separation as $0.75 exp(-R_p/350,mathrm{kpc})$.
We reveal the highly structured nature of the Milky Way stellar halo within the footprint of the PAndAS photometric survey from blue main sequence and main sequence turn-off stars. We map no fewer than five stellar structures within a heliocentric range of ~5 to 30 kpc. Some of these are known (the Monoceros Ring, the Pisces/Triangulum globular cluster stream), but we also uncover three well-defined stellar structures that could be, at least partly, responsible for the so-called Triangulum/Andromeda and Triangulum/Andromeda 2 features. In particular, we trace a new faint stellar stream located at a heliocentric distance of ~17 kpc. With a surface brightness of Sigma_V ~ 32-32.5 mag/arcsec^2, it follows an orbit that is almost parallel to the Galactic plane north of M31 and has so far eluded surveys of the Milky Way halo as these tend to steer away from regions dominated by the Galactic disk. Investigating our follow-up spectroscopic observations of PAndAS, we serendipitously uncover a radial velocity signature from stars that have colors and magnitudes compatible with the stream. From the velocity of eight likely member stars, we show that this stellar structure is dynamically cold, with an unresolved velocity dispersion that is lower than 7.1 km/s at the 90-percent confidence level. Along with the width of the stream (300-650 pc), its dynamics points to a dwarf-galaxy-accretion origin. The numerous stellar structures we can map in the Milky Way stellar halo between 5 and 30 kpc and their varying morphology is a testament to the complex nature of the stellar halo at these intermediate distances.
Minor accretion events with mass ratio M_sat : M_host ~ 1:10 are common in the context of LCDM cosmology. We use high-resolution simulations of Galaxy-analogue systems to show that these mergers can dynamically eject disk stars into a diffuse light component that resembles a stellar halo both spatially and kinematically. For a variety of orbital configurations, we find that ~3-5e8 M_sun of primary stellar disk material is ejected to a distance larger than 5 kpc above the galactic plane. This ejected contribution is similar to the mass contributed by the tidal disruption of the satellite galaxy itself, though it is less extended. If we restrict our analysis to the approximate solar neighborhood in the disk plane, we find that ~1% of the initial disk stars in that region would be classified kinematically as halo stars. Our results suggest that the inner parts of galactic stellar halos contain ancient disk stars and that these stars may have been liberated in the very same events that delivered material to the outer stellar halo.
We present an analysis of the large-scale structure of the halo of the Andromeda galaxy, based on the Pan-Andromeda Archeological Survey (PAndAS), currently the most complete map of resolved stellar populations in any galactic halo. Despite copious substructure, the global halo populations follow closely power law profiles that become steeper with increasing metallicity. We divide the sample into stream-like populations and a smooth halo component. Fitting a three-dimensional halo model reveals that the most metal-poor populations ([Fe/H]<-1.7) are distributed approximately spherically (slightly prolate with ellipticity c/a=1.09+/-0.03), with only a relatively small fraction (42%) residing in discernible stream-like structures. The sphericity of the ancient smooth component strongly hints that the dark matter halo is also approximately spherical. More metal-rich populations contain higher fractions of stars in streams (86% for [Fe/H]>-0.6). The space density of the smooth metal-poor component has a global power-law slope of -3.08+/-0.07, and a non-parametric fit shows that the slope remains nearly constant from 30kpc to 300kpc. The total stellar mass in the halo at distances beyond 2 degrees is 1.1x10^10 Solar masses, while that of the smooth component is 3x10^9 Solar masses. Extrapolating into the inner galaxy, the total stellar mass of the smooth halo is plausibly 8x10^9 Solar masses. We detect a substantial metallicity gradient, which declines from [Fe/H]=-0.7 at R=30kpc to [Fe/H]=-1.5 at R=150kpc for the full sample, with the smooth halo being 0.2dex more metal poor than the full sample at each radius. While qualitatively in-line with expectations from cosmological simulations, these observations are of great importance as they provide a prototype template that such simulations must now be able to reproduce in quantitative detail.
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