We report the discovery of Specter, a disrupted ultrafaint dwarf galaxy
revealed by the H3 Spectroscopic Survey. We detected this structure via a pair
of comoving metal-poor stars at a distance of ...12.5 kpc, and further
characterized it with Gaia astrometry and follow-up spectroscopy. Specter is a
$25^\circ \times 1^\circ$ stream of stars that is entirely invisible until
strict kinematic cuts are applied to remove the Galactic foreground. The
spectroscopic members suggest a stellar age $\tau \gtrsim 12$ Gyr and a mean
metallicity $\langle\text{Fe/H}\rangle = -1.84_{-0.18}^{+0.16}$, with a
significant intrinsic metallicity dispersion $\sigma_{ \text{Fe/H}} =
0.37_{-0.13}^{+0.21}$. We therefore argue that Specter is the disrupted remnant
of an ancient dwarf galaxy. With an integrated luminosity $M_{\text{V}} \approx
-2.6$, Specter is by far the least-luminous dwarf galaxy stream known. We
estimate that dozens of similar streams are lurking below the detection
threshold of current search techniques, and conclude that spectroscopic surveys
offer a novel means to identify extremely low surface brightness structures.
The Milky Way's satellite galaxies ("surviving dwarfs") have been studied for
decades as unique probes of chemical evolution in the low-mass regime. Here we
extend such studies to the "disrupted ...dwarfs", whose debris constitutes the
stellar halo. We present abundances (Fe/H, $\alpha$/Fe) and stellar masses
for nine disrupted dwarfs with $M_{\star}\approx10^{6}-10^{9}M_{\odot}$ from
the H3 Survey (Sagittarius, $Gaia$-Sausage-Enceladus, Helmi Streams, Sequoia,
Wukong/LMS-1, Cetus, Thamnos, I'itoi, Orphan/Chenab). The surviving and
disrupted dwarfs are chemically distinct: at fixed mass, the disrupted dwarfs
are systematically metal-poor and $\alpha$-enhanced. The disrupted dwarfs
define a mass-metallicity relation (MZR) with a similar slope as the $z=0$ MZR
followed by the surviving dwarfs, but offset to lower metallicities by
$\Delta$Fe/H$\approx0.3-0.4$ dex. Dwarfs with larger offsets from the $z=0$
MZR are more $\alpha$-enhanced. In simulations as well as observations,
galaxies with higher $\Delta$Fe/H formed at higher redshifts -- exploiting
this, we infer the disrupted dwarfs have typical star-formation truncation
redshifts of $z_{\rm{trunc}}{\sim}1-2$. We compare the chemically inferred
$z_{\rm{trunc}}$ with dynamically inferred accretion redshifts and find almost
all dwarfs are quenched only after accretion. The differences between disrupted
and surviving dwarfs are likely because the disrupted dwarfs assembled their
mass rapidly, at higher redshifts, and within denser dark matter halos that
formed closer to the Galaxy. Our results place novel archaeological constraints
on low-mass galaxies inaccessible to direct high-$z$ studies: (i) the redshift
evolution of the MZR along parallel tracks but offset to lower metallicities
extends to $M_{\star}\approx10^{6}-10^{9}M_{\odot}$; (ii) galaxies at
$z\approx2-3$ are $\alpha$-enhanced with $\alpha$/Fe$\approx0.4$.
We report the discovery of Specter, a disrupted ultrafaint dwarf galaxy revealed by the H3 Spectroscopic Survey. We detected this structure via a pair of comoving metal-poor stars at a distance of ...12.5 kpc, and further characterized it with Gaia astrometry and follow-up spectroscopy. Specter is a \(25^\circ \times 1^\circ\) stream of stars that is entirely invisible until strict kinematic cuts are applied to remove the Galactic foreground. The spectroscopic members suggest a stellar age \(\tau \gtrsim 12\) Gyr and a mean metallicity \(\langle\text{Fe/H}\rangle = -1.84_{-0.18}^{+0.16}\), with a significant intrinsic metallicity dispersion \(\sigma_{ \text{Fe/H}} = 0.37_{-0.13}^{+0.21}\). We therefore argue that Specter is the disrupted remnant of an ancient dwarf galaxy. With an integrated luminosity \(M_{\text{V}} \approx -2.6\), Specter is by far the least-luminous dwarf galaxy stream known. We estimate that dozens of similar streams are lurking below the detection threshold of current search techniques, and conclude that spectroscopic surveys offer a novel means to identify extremely low surface brightness structures.
The Milky Way's satellite galaxies ("surviving dwarfs") have been studied for decades as unique probes of chemical evolution in the low-mass regime. Here we extend such studies to the "disrupted ...dwarfs", whose debris constitutes the stellar halo. We present abundances (Fe/H, \(\alpha\)/Fe) and stellar masses for nine disrupted dwarfs with \(M_{\star}\approx10^{6}-10^{9}M_{\odot}\) from the H3 Survey (Sagittarius, \(Gaia\)-Sausage-Enceladus, Helmi Streams, Sequoia, Wukong/LMS-1, Cetus, Thamnos, I'itoi, Orphan/Chenab). The surviving and disrupted dwarfs are chemically distinct: at fixed mass, the disrupted dwarfs are systematically metal-poor and \(\alpha\)-enhanced. The disrupted dwarfs define a mass-metallicity relation (MZR) with a similar slope as the \(z=0\) MZR followed by the surviving dwarfs, but offset to lower metallicities by \(\Delta\)Fe/H\(\approx0.3-0.4\) dex. Dwarfs with larger offsets from the \(z=0\) MZR are more \(\alpha\)-enhanced. In simulations as well as observations, galaxies with higher \(\Delta\)Fe/H formed at higher redshifts -- exploiting this, we infer the disrupted dwarfs have typical star-formation truncation redshifts of \(z_{\rm{trunc}}{\sim}1-2\). We compare the chemically inferred \(z_{\rm{trunc}}\) with dynamically inferred accretion redshifts and find almost all dwarfs are quenched only after accretion. The differences between disrupted and surviving dwarfs are likely because the disrupted dwarfs assembled their mass rapidly, at higher redshifts, and within denser dark matter halos that formed closer to the Galaxy. Our results place novel archaeological constraints on low-mass galaxies inaccessible to direct high-\(z\) studies: (i) the redshift evolution of the MZR along parallel tracks but offset to lower metallicities extends to \(M_{\star}\approx10^{6}-10^{9}M_{\odot}\); (ii) galaxies at \(z\approx2-3\) are \(\alpha\)-enhanced with \(\alpha\)/Fe\(\approx0.4\).
Ecological Models and Satellite Imagery Turner, Woody; Melton, Forrest
Bulletin of the Ecological Society of America,
10/2005, Letnik:
86, Številka:
4
Journal Article
PDF
