A New Tidal Stream Discovered in Gaia DR3

Thanks to the precise astrometric measurements of proper motions by the Gaia mission, a new tidal stellar stream has been discovered in the northern hemisphere. The distribution of star count shows that the stream is approximately $80$ degrees long and $1.70$ degrees wide. Observations of $21$ member stars, including 14 RR Lyrae stars, indicate that the stream has an eccentric and retrograde orbit with $e=0.58$. The low metallicity, high total energy, and large angular momentum suggest that it is associated with the merging event Sequoia. This discovery suggests the possibility of finding more substructures with high eccentricity orbits, even in the inner halo.


INTRODUCTION
As a relic of merging events, stellar streams keep the memory of their progenitors' chemical and dynamical information, even after a few giga years (Helmi 2020).Many cold streams have been discovered in photometric surveys using the matched-filter method (Rockosi et al. 2002;Grillmair & Dionatos 2006;Grillmair 2009Grillmair , 2006;;Grillmair & Johnson 2006;Grillmair 2014;Grillmair et al. 2013;Bernard et al. 2016).Most of those cold streams are formed from stripped globular clusters that have smaller velocity dispersion.After the second data release of the Gaia mission, many such cold streams were discovered using proper motions (Malhan & Ibata 2018;Grillmair 2022).Besides those cold streams, clearer views of a few broad or diffuse substructures are also shown, such as the Sagittarius Stream and the Virgo Overdensity.These substructures are believed to be formed through the stripped dwarf galaxies.All these substructures show us a strongly disturbed Milky Way halo.Moreover, the accurate astrometric data from Gaia Mission also reveals a lot of phase mixed debris in the phase space (Koppelman et al. 2018(Koppelman et al. , 2019)), especially the largest merging event in the Milky Way, Gaia-Enceladus-Sausage (Helmi et al. 2018;Belokurov et al. 2018), whose debris dominates the inner halo.
With different methods, there are more than a hundred streams, most of which are cold streams, discovered by now (Mateu et al. 2018).What should be noticed is that many of those streams have not been yet confirmed with spectroscopic observations.The newdiscovered broad or diffuse streams, on the other hand, are not increased, since they are associated with more intensive but less-frequently occurred mergers and often with lower surface number density.This means that they are much more difficult to discover from the field stars than the cold streams.
In this work, we use Gaia DR3 data to search substructures in the halo with clean removal of field star contamination.We introduce the data selection in Section 2. The density distribution of the discovered stream is shown in Section 3. The properties of the newly discovered stream are studied in Section 4. Finally, the conclusions are drawn in Section 5.
The constraints on the parallax ω are used to remove the majority of nearby stars (d < 10 kpc), most of which are disk populations.This helps us significantly reduce the foreground stars and clarify the signal of the substructures, especially the faint ones.The stars with larger uncertainties in the proper motions are also removed.The constraint on RUWE is used to remove those possible binary stars.All those selections leave the samples mainly brighter than G = 19.The contamination of galaxies in the faint magnitudes will not affect our results because the proper motions of the galaxies are around 0 with a dispersion of around 0.2 mas yr −1 .

SLICING THE PROPER MOTION AND DENSITY DISTRIBUTION
In principle, the member stars of a unique substructure in a small space volume will have similar positions and velocities.Then their proper motions should be also similar.In other words, the spatial overdensity of the stars with similar proper motions should be of high probability to be a substructure from the same progenitor.We therefore check the spatial density distributions with different proper motion ranges in the selected samples of Gaia DR3.The step of 0.5 mas yr −1 is adopted to split out the data in both of the proper motions µ * α and µ δ .The sky is divided into equal-area pieces by the package Healpy with nside= 64 (Górski et al. 2005;Zonca et al. 2019).The smoothing process is applied using a Gaussian kernel with σ ∼ 55 ′ .With these procedures, many known substructures are revealed in the density distributions.All those substructures, in form of overdensities in the spatial density distribution, are crossmatched with the latest list of known nearby streams collected by Mateu et al. (2018).
As seen in Figure 1, the Anticenter Stream is the most significant substructure in the spatial density distribution with galactic longitude between 120 • and 200 • (Laporte et al. 2020).This proves that our method works well to reveal the substructures.Meanwhile, the orbit of the Sagittarius Stream is indicated by the solid black line (Mateu et al. 2018;Antoja et al. 2020).

Discovery of the new stream
A new 80 • long stream with a high signal-to-noise ratio is discovered with −1.0 < µ δ < −0.5 mas yr −1 and 0.5 < µ * α < 2.0 mas yr −1 , which is shown in Figure 1 viewed from the Galactic North Pole.The new stream is significantly located at a high galactic latitude around 30 • to 60 • with galactic longitude from 270 • to 30 • .Different parts of the stream are revealed in different proper motion ranges because of the projection effect of the Solar motion and the velocity gradient of the stream itself.Although the surface density is very low, the stream is still significant compared to its neighborhood.On the right panel in Figure 1 with 1.5 < µ * α < 2.0 mas yr −1 , we can still find the extension of the stream following the elongation at a lower galactic latitude (b < 45 • and l ∼ 30 • ).
To avoid the projection effect in the equatorial frame, we convert the coordinate to a new frame, where the longitude ϕ 1 is generally along the stream so that the stream is located at the latitude ϕ 2 ∼ 0 • .The conversion is done with the function transform to from the package gala 1 (Price-Whelan 2017).Two points on the stream are used to define the great circle during the conversion, (α, δ) = (192.0• , −11.5 • ) and (249.0 • , 4.2 • ).The zero point is defined as the spherical midpoint of those two points by gala with default setting.Figure 2 shows the field of the streams in the new coordinates, including the orbits of the Sagittarius Stream (Antoja et al. 2020) and the Cocytos Stream (Grillmair 2009), which are represented by the solid and dashed lines, respectively.The scanning law of Gaia DR3 is represented in the background, which is the number distribution sampled with an interval of 10 seconds during its observations2 .That proves that the stream is not an artefact relative to the Gaia measurements.
Focusing on the new stream, we first select the stars with longitude ϕ 1 between −50 • and 30 • .The latitude distribution ϕ 2 of the stars is shown in Figure 3.A significant peak is around ϕ 2 ∼ 0 • .To figure out the significance, we use the Gaussian function to fit the distribution of ϕ 2 , i.e. y = A * Gaussian( φ2 , σ ϕ2 ) + H.The amplitude of the signal is constrained as A = 133.48stars, the mean value and dispersion of the Gaussian distribution are ( φ2 , σ ϕ2 ) = (0.19 • ± 0.16 • , 1.70 • ± 0.18 • ), and the background is H = 8.98 ± 1.1.The fluctuation of the background σ H is calculated by the dispersion of the bins outside 3σ ϕ2 , i.e. |ϕ 2 | > 5 • .Then the signal-to-noise ratio of the structure is estimated by SN R = A/σ H = 39.5.We take all the 227 stars within 3σ ϕ2 as the member stars of the stream, the stream has a length of ∼ 80 • and a width of 1.70 • (1σ ϕ2 ).We also estimate that the contamination of field stars within 10 bins from ϕ 2 = −5 • to 5 • is about 10 * H = 90 in total.
For further validation, we check the color-magnitude diagram (CMD) of the member candidates.Because of the large coverage on the sky, we divide the member   From the CMD of the member samples, we can find the significant signatures of the possible horizontal branch stars.To make sure if that is the horizontal branch, rather than the possible nearby turnoff stars, those fainter stars within the same proper motion ranges, but with larger uncertainties are also represented by the gray dots.From the distribution of all the dots, we prefer that the those black dots around G ∼ 18 should be the horizontal branch stars.By cross-matching with the RR Lyrae catalog identified by Clementini et al. (2023), there are 14 RR Lyrae stars (RRLs) in the member candidates.We find that those RRLs are located with consistent position of the horizontal branch stars, which indicates that all of those RRLs should be the members of the stream.All of those 14 RRLs are marked with red triangles in the corresponding panels.Then, the distances of those RRLs are calculated given a constant absolute magnitude in G-band, i.e. 0.63 mag (Muraveva et al. 2018).The resulting distances of these RRLs are listed in Table 1.
Except the RRLs, there are another 8 stars in the member candidates, which are included in LAMOST DR9 (Liu et al. 2020).The radial velocity (RV ) and the metallicity [M/H] have been estimated by SLAM (Zhang et al. 2020).Figure 5  We then adopt the isochrone with metallicity [M/H] = −1.3 and age τ = 12 Gyr to fit the stream member candidates, which are represented by the blue dots in Figure 4.The isochrone is shifted according to the distances of the RRLs in the corresponding panel.Note that we do not find RRLs in the control samples.The isochrone in each subsample is shifted with the same distance as the member sample with same ϕ 1 range.

PROPERTIES OF THE STREAM
With those 14 RRLs identified by Gaia and the 7 LAMOST member stars with radial velocities, we can study the geometric and kinematic features of the stream.

The spatial distribution
First, the distance distribution of the 14 RRLs is shown in the middle panel of Figure 6.We fit the distance as a function of ϕ 1 with a spline function with a large smoothing factor s = 5000, i.e.UnivariateSpline from the package scipy.The best-fit line is represented by the dashed line in the middle panel of Figure 6.
Then, the distances of the 7 LAMOST member stars are estimated by applying the above relation.Note that those 7 stars are located with ϕ 1 > −10 • because LAMOST can only observe stars with δ > −10 • , the whole stream cannot be traced completely with LAM-OST data.Figure 7 shows the distributions in the Cartesian X-Y plane of the 14 RRLs and those 7 LAM-OST member stars with black and red dots, respectively.The Galactocentric distance r of the RRLs varies from 12.61 to 33.57kpc, which provides a lower limit of the eccentricity which means that the stream has a radial orbit.

Orbit property
Currently, we do not have the radial velocities for all the member candidates of the stream.However, the RRLs can provide information about the orbit of the stream, because of their accurate distance estimation.The proper motions can be converted to the tangential velocities.We, therefore, remove the contribution of the Solar motion, i.e. (U, V, W ) = (11.1,12.24 + 232, 7.25) km s −1 from the observed proper motion.The proper motions with respect to the position of the Sun are then converted to the coordinates of the stream with gala again, i.e.  1 smoothly increases as a function of ϕ 1 and V ϕ2 is strictly along zero with small random dispersion.Since that the increasing direction of ϕ 1 is approaching the Galactic center and the distance to the Galactic center declines with increasing ϕ 1 , the increasing V ϕ * 1 implies that the member stars of the stream accelerate when they are approaching the pericenter point close to the Galactic center.Meanwhile, the essential zero value of V ϕ2 of the RRLs is evident that they move exactly along the stream.
The motions of the RRLs are also represented with arrows color-coded by their distances in Figure 2. Similar to what we have seen in the bottom panel of Figure 6, the stream is perfectly moving along the stream and approaching the Galactic center.
The distances of the 7 LAMOST member stars are estimated by using the d − ϕ 1 relation fitted by the spline (the dashed line in the top panel of Figure 6) from the RRLs, which are represented by the red symbols in Fig- ure 6.Then, their 3D velocities can be derived and are represented by the red arrows in Figure 7. Like the tangential velocities of the RRLs, it also shows that the 6 stars move along the stream.With the full 6D information of those LAMOST stars, we estimate their total energy and the angular momenta with AGAMA (Vasiliev 2019) with a potential including a Dehnen bulge of mass 2 × 10 7 M ⊙ and scale radius 1.0 kpc, a Miyamoto-Nagai disk of 5 × 10 10 M ⊙ , scale radius 3.0 kpc and scale height 0.3 kpc, and an NFW halo with mass of 5.5 × 10 11 M ⊙ and scale radius 15.0 kpc.As shown in Figure 8, the LAMOST member stars show a clear retrograde rotation.Meanwhile, the average eccentricity of those 6 member stars is around 0.58, which is consistent with the coarse estimate from the spatial distributions of the RRLs.As a comparison, the globular clusters are also represented as gray dots (Vasiliev & Baumgardt 2021).The large total energy E and the retrograde motion indicate that this stream is possibly associated with the merging event Sequoia.This discovery indicates that there should be more streams in the halo, which are not discovered yet because of the observational and methodological limits, especially for those streams with highly radial orbits and low surface luminosity.With all 14 RR Lyrae stars and 7 LAMOST member stars, we find that the stream has an eccentric orbit with e ∼ 0.58.Assuming the stream has at least 14 RRLs, its progenitor should be of luminosity brighter than M V = −6.9(Mateu et al. 2018).To further study the substructure, spectroscopic observations are necessary to constrain their chemical and full dynamical features, such as LAMOST (Liu et al. 2020), SDSS-V3 , DESI (Cooper et al. 2023)   Table 1.The member RR Lyrae stars of the new stream are listed.Columns from left to the right are the source ID from Gaia DR3, the coordinates (α, δ), the proper motions, the radial velocities and its uncertainty, the signal-to-noise ratio and the distance obtained from the interpolation with RRL stars.corrected.Table 2.The member RGB stars of the new stream are listed.Columns from left to the right are the source ID from Gaia DR3, the coordinates (α, δ), the distance d, the G−band magnitude, the proper motions and their uncertainties along α and δ, the new coordinate (ϕ1, ϕ2), and the proper motions along ϕ1 and ϕ2 with Solar's movement corrected.
Figure 1.The density distributions of three subsamples from Gaia DR3 with different proper motion ranges are shown with view from the north galactic pole.The newly discovered stream is located on the top right in each panel with 270 • < l < 30 • and 30 • < b < 60 • .The orbit of the Sagittarius Stream (Mateu et al. 2018; Antoja et al. 2020) is represented with solid black lines.

Figure 2 .
Figure 2. The stars with −1.0 < µ δ < −0.5 mas yr −1 and 0.5 < µ * α < 2.0 mas yr −1 are represented by the gray dots in the target region.The region of the new stream is marked with gray shadow.The orbits of the nearby Cocytos Stream and the Sagittarius Stream are represented by the dashed and dotted lines.The arrows represent the tangential velocities of the RR Lyrae stars relative to the position of the Sun, but with the Solar movement corrected.The background represents the scanning law of the Gaia DR3.
candidates into subsamples according to the longitude ϕ 1 with the step of 10 • .The distribution in the CMD of those candidates are represented by the black dots in the top panels of Figure 4.Meanwhile, all the stars with proper motion uncertainties smaller than 0.5 mas yr −1 are shown with gray dots.For comparison, control samples are selected with with 5 • < |ϕ 2 | < 10 • , −50 • < ϕ 1 < 30 • , and proper motion uncertainties smaller than 0.5 mas yr −1 .The CMD of those control samples are shown in the bottom panel of Figure 4.In total, we obtained 8 member subsamples and 8 control samples.

Figure 3 .
Figure 3.The distribution of the latitude ϕ2 for the stream is shown with −50 • < ϕ1 < 30 • .The thick dashed lines represent the constraints on the latitude for the member selection.The thin dashed line represents a Gaussian fitting results with dispersion of σ ϕ 2 = 1.70 • , amplitude of A = 133.48and a background of H = 8.98 ± 1.1.
shows the distribution in [M/H] vs. RV plane of the 8 stars with color-coded signal-to-noise ratio in i−band (SNR i ).With a Gaussian kernel involving the uncertainties of RV , we obtain the kernel-smoothed distribution of RV .There is a clear peak showing at RV ∼ −87.2 km s −1 in the RV distribution displayed in the subplot in the top left inset.This peak represents the characteristic radial velocity of the stream.It is noted that there are two stars whose uncertainties are not provided.A median uncertainty of 8 km s −1 is then adopted for them during the calculation of the kernel-smoothed distribution.Then all 7 of the 8 stars with RV > −150 km s −1 are confirmed as the members (hereafter LAMOST member stars), including one horizontal branch star and six red giant branch stars.The radial velocity distribution versus the longitude ϕ 1 of the 8 stars is shown in the top panel of Figure6.All those selected 7 stars are marked with red dots.The mean metallicity [M/H] = −1.3 of them may represent the metallicity of the stream.
Figure 4.The CMD distributions of the member candidate stars of the stream in each subsample are shown in the top panels.The black and the gray dots are the stars with uncertainties of proper motions smaller than 0.2 and 0.5 mas yr −1 , respectively.The isochrone with metallicity and age of −1.3 and 13 Gyr is represented by the blue dots with distance module derived from the RR Lyrae stars in the corresponding subsample, which are marked with red triangles.Similar distributions of the control samples with 5 • < ϕ2 < 10 • and uncertainties smaller than 0.5 mas yr −1 are shown in the bottom panels.

Figure 5 .
Figure 5. Eight candidate stars observed by LAMOST are showed in the space of the radial velocity RV versus metallicity [M/H], color coded by the signal-to-noise ratio in i−band.The subplot shows the probability distribution of the radial velocities evolving Gaussian kernel with width of the uncertainties of RV .The median uncertainty 8.4 km s −1 is adopted for the two stars whose uncertainties are not given.

Figure 6 .
Figure 6.The variances of the radial velocity RV , the distance d and tangential velocities of the RRLs (black dots) are shown as a function of longitude ϕ1 from the top to the bottom panels, respectively.7 of the LAMOST member stars are represented by the red dots.The dashed line in the middle panel represents the best-fit spline.In the bottom panel, the filled black triangles and dots show the two tangential velocity components along ϕ1 and ϕ2 of the RRLs, respectively.The dark and light gray regions represent the limits for the two tangential velocities, which are caused by the proper motion cut during sample collection.The red dots represent the tangential velocity distributions of the 7 LAMOST member stars.The horizontal dashed line in the bottom panel indicates the zero velocity along ϕ2.
measurements, we select a catalog of distant stars with accurate proper motions σ µ * α < 0.2 and σ µ δ < 0.2 mas yr −1 .From the density distributions with different proper motion ranges, we find a new stellar stream, which is ∼ 80 • long and ∼ 1.70 • wide (1σ ϕ2 assuming a Gaussian distribution).According to the spectra of 7 LAMOST member stars, the stream has a metallicity of [M/H] = −1.3,which is consistent with that of the merging event Sequoia(Feuillet et al. 2021).The location of the stream in phase space E versus L Z also indicates the same association.

Figure 7 .
Figure 7.The 14 RRLs and 7 LAMOST member stars of the stream are represented in the spaces with black and red dots, respectively.The absolute magnitude 0.63 (Muraveva et al. 2018) is adopted for distance calculation of the RRLs.The movements of the 7 LAMOST member stars are represented by the red arrows.The position of the Sun is marked as the gray pentagon.The dashed lines represent the position of the Galactic center.

Figure 8 .
Figure 8.The 7 LAMOST member stars and the globular clusters (Vasiliev & Baumgardt 2021) are represented in the phase space E versus LZ with red dots and gray dots, respectively.