Abstract
We report the discovery of Pegasus IV, an ultra-faint dwarf galaxy found in archival data from the Dark Energy Camera processed by the DECam Local Volume Exploration Survey. Pegasus IV is a ...compact, ultra-faint stellar system (
r
1
/
2
=
41
−
6
+
8
pc;
M
V
= −4.25 ± 0.2 mag) located at a heliocentric distance of
90
−
6
+
4
kpc
. Based on spectra of seven nonvariable member stars observed with Magellan/IMACS, we confidently resolve Pegasus IV’s velocity dispersion, measuring
σ
v
=
3.3
−
1.1
+
1.7
km s
−1
(after excluding three velocity outliers); this implies a mass-to-light ratio of
M
1
/
2
/
L
V
,
1
/
2
=
167
−
99
+
224
M
⊙
/
L
⊙
for the system. From the five stars with the highest signal-to-noise spectra, we also measure a systemic metallicity of Fe/H =
−
2.63
−
0.30
+
0.26
dex, making Pegasus IV one of the most metal-poor ultra-faint dwarfs. We tentatively resolve a nonzero metallicity dispersion for the system. These measurements provide strong evidence that Pegasus IV is a dark-matter-dominated dwarf galaxy, rather than a star cluster. We measure Pegasus IV’s proper motion using data from Gaia Early Data Release 3, finding (
μ
α
*
,
μ
δ
) = (0.33 ± 0.07, −0.21 ± 0.08) mas yr
−1
. When combined with our measured systemic velocity, this proper motion suggests that Pegasus IV is on an elliptical, retrograde orbit, and is currently near its orbital apocenter. Lastly, we identify three potential RR Lyrae variable stars within Pegasus IV, including one candidate member located more than 10 half-light radii away from the system’s centroid. The discovery of yet another ultra-faint dwarf galaxy strongly suggests that the census of Milky Way satellites is still incomplete, even within 100 kpc.
Abstract
We present a spectroscopic analysis of Eridanus IV (Eri IV) and Centaurus I (Cen I), two ultrafaint dwarf galaxies of the Milky Way. Using IMACS/Magellan spectroscopy, we identify 28 member ...stars of Eri IV and 34 member stars of Cen I. For Eri IV, we measure a systemic velocity of
v
sys
=
−
31.5
−
1.2
+
1.3
km
s
−
1
, and velocity dispersion
σ
v
=
6.1
−
0.9
+
1.2
km
s
−
1
. Additionally, we measure the metallicities of 16 member stars of Eri IV. We find a metallicity of
Fe
/
H
=
−
2.87
−
0.07
+
0.08
, and resolve a dispersion of
σ
Fe/H
=0.20 ± 0.09. The mean metallicity is marginally lower than all other known ultrafaint dwarf galaxies, making it one of the most metal-poor galaxies discovered thus far. Eri IV also has a somewhat unusual right-skewed metallicity distribution. For Cen I, we find a velocity
v
sys
= 44.9 ± 0.8 km s
−1
, and velocity dispersion
σ
v
=
4.2
−
0.5
+
0.6
km
s
−
1
. We measure the metallicities of 27 member stars of Cen I, and find a mean metallicity Fe/H = −2.57 ± 0.08, and metallicity dispersion
σ
Fe
/
H
=
0.38
−
0.05
+
0.07
. We calculate the systemic proper motion, orbit, and the astrophysical J-factor for each system, the latter of which indicates that Eri IV is a good target for indirect dark matter detection. We also find no strong evidence for tidal stripping of Cen I or Eri IV. Overall, our measurements confirm that Eri IV and Cen I are dark-matter-dominated galaxies with properties largely consistent with other known ultrafaint dwarf galaxies. The low metallicity, right-skewed metallicity distribution, and high J-factor make Eri IV an especially interesting candidate for further follow-up.
ABSTRACT
The current and next observation seasons will detect hundreds of gravitational waves (GWs) from compact binary systems coalescence at cosmological distances. When combined with independent ...electromagnetic measurements, the source redshift will be known, and we will be able to obtain precise measurements of the Hubble constant H0 via the distance–redshift relation. However, most observed mergers are not expected to have electromagnetic counterparts, which prevents a direct redshift measurement. In this scenario, one possibility is to use the dark sirens method that statistically marginalizes over all the potential host galaxies within the GW location volume to provide a probabilistic source redshift. Here we presented H0 measurements using two new dark sirens compared to previous analyses using DECam data: GW190924$\_$021846 and GW200202$\_$154313. The photometric redshifts of the possible host galaxies of these two events are acquired from the DECam Local Volume Exploration Survey (DELVE) carried out on the Blanco telescope at Cerro Tololo. The combination of the H0 posterior from GW190924$\_$021846 and GW200202$\_$154313 together with the bright siren GW170817 leads to $H_{0} = 68.84^{+15.51}_{-7.74}\, \rm {km\, s^{-1}\, Mpc^{-1}}$. Including these two dark sirens improves the 68 per cent confidence interval (CI) by 7 per cent over GW170817 alone. This demonstrates that the addition of well-localized dark sirens in such analysis improves the precision of cosmological measurements. Using a sample containing 10 well-localized dark sirens observed during the third LIGO/Virgo observation run, without the inclusion of GW170817, we determine a measurement of $H_{0} = 76.00^{+17.64}_{-13.45}\, \rm {km\, s^{-1}\, Mpc^{-1}}$.
Abstract
We report the discovery of six ultra-faint Milky Way satellites identified through matched-filter searches conducted using Dark Energy Camera (DECam) data processed as part of the second ...data release of the DECam Local Volume Exploration (DELVE) survey. Leveraging deep Gemini/GMOS-N imaging (for four candidates) as well as follow-up DECam imaging (for two candidates), we characterize the morphologies and stellar populations of these systems. We find that these candidates all share faint absolute magnitudes (
M
V
≥ −3.2 mag) and old, metal-poor stellar populations (
τ
> 10 Gyr, Fe/H < −1.4 dex). Three of these systems are more extended (
r
1/2
> 15 pc), while the other three are compact (
r
1/2
< 10 pc). From these properties, we infer that the former three systems (Boötes V, Leo Minor I, and Virgo II) are consistent with ultra-faint dwarf galaxy classifications, whereas the latter three (DELVE 3, DELVE 4, and DELVE 5) are likely ultra-faint star clusters. Using data from the Gaia satellite, we confidently measure the proper motion of Boötes V, Leo Minor I, and DELVE 4, and tentatively detect a proper-motion signal from DELVE 3 and DELVE 5; no signal is detected for Virgo II. We use these measurements to explore possible associations between the newly discovered systems and the Sagittarius dwarf spheroidal, the Magellanic Clouds, and the Vast Polar Structure, finding several plausible associations. Our results offer a preview of the numerous ultra-faint stellar systems that will soon be discovered by the Vera C. Rubin Observatory and highlight the challenges of classifying the faintest stellar systems.
Abstract
We present the discovery of a candidate ultra-faint Milky-Way satellite, Eridanus IV (DELVE J0505−0931), detected in photometric data from the DECam Local Volume Exploration survey (DELVE). ...Eridanus IV is a faint (
M
V
= − 4.7 ± 0.2), extended (
r
1
/
2
=
75
−
13
+
16
pc
), and elliptical (
ϵ
= 0.54 ± 0.1) system at a heliocentric distance of
76.7
−
6.1
+
4.0
kpc
, with a stellar population that is well described by an old, metal-poor isochrone (age of
τ
∼ 13.0 Gyr and metallicity of Fe/H ≲ − 2.1 dex). These properties are consistent with the known population of ultra-faint Milky-Way satellite galaxies. Eridanus IV is also prominently detected using proper-motion measurements from Gaia Early Data Release 3, with a systemic proper motion of
(
μ
α
cos
δ
,
μ
δ
)
=
(
+
0.25
±
0.06
,
−
0.10
±
0.05
)
mas yr
−1
measured from its horizontal branch and red-giant-branch member stars. We find that the spatial distribution of likely member stars hints at the possibility that the system is undergoing tidal disruption.
ABSTRACT
We use the SMASH survey to obtain unprecedented deep photometry reaching down to the oldest main-sequence turn-offs in the colour–magnitude diagrams (CMDs) of the Small Magellanic Cloud ...(SMC) and quantitatively derive its star formation history (SFH) using CMD fitting techniques. We identify five distinctive peaks of star formation in the last 3.5 Gyr, at ∼3, ∼2, ∼1.1, ∼0.45 Gyr ago, and one presently. We compare these to the SFH of the Large Magellanic Cloud (LMC), finding unequivocal synchronicity, with both galaxies displaying similar periods of enhanced star formation over the past ∼3.5 Gyr. The parallelism between their SFHs indicates that tidal interactions between the MCs have recurrently played an important role in their evolution for at least the last ∼3.5 Gyr, tidally truncating the SMC and shaping the LMC’s spiral arm. We show, for the first time, an SMC–LMC correlated SFH at recent times in which enhancements of star formation are localized in the northern spiral arm of the LMC, and globally across the SMC. These novel findings should be used to constrain not only the orbital history of the MCs but also how star formation should be treated in simulations.
Abstract
We report the detection of three RR Lyrae (RRL) stars (two RRc and one RRab) in the ultra-faint dwarf (UFD) galaxy Centaurus I (Cen I) and two Milky Way (MW)
δ
Scuti/SX Phoenicis stars based ...on multi-epoch
giz
DECam observations. The two RRc stars are located within two times the half-light radius (
r
h
) of Cen I, while the RRab star (CenI-V3) is at ∼6
r
h
. The presence of three distant RRL stars clustered this tightly in space represents a 4.7
σ
excess relative to the smooth distribution of RRL in the Galactic halo. Using the newly detected RRL stars, we obtain a distance modulus to Cen I of
μ
0
= 20.354 ± 0.002 mag (
σ
= 0.03 mag), a heliocentric distance of
D
⊙
= 117.7 ± 0.1 kpc (
σ
= 1.6 kpc), with systematic errors of 0.07 mag and 4 kpc. The location of the Cen I RRL stars in the Bailey diagram is in agreement with other UFD galaxies (mainly Oosterhoff II). Finally, we study the relative rate of RRc+RRd (RRcd) stars (
f
cd
) in UFD and classical dwarf galaxies. The full sample of MW dwarf galaxies gives a mean of
f
cd
= 0.28. While several UFD galaxies, such as Cen I, present higher RRcd ratios, if we combine the RRL populations of all UFD galaxies, the RRcd ratio is similar to the one obtained for the classical dwarfs (
f
cd
∼ 0.3). Therefore, there is no evidence for a different fraction of RRcd stars in UFD and classical dwarf galaxies.
Abstract
The DECam Local Volume Exploration survey (DELVE) is a 126-night survey program on the 4 m Blanco Telescope at the Cerro Tololo Inter-American Observatory in Chile. DELVE seeks to understand ...the characteristics of faint satellite galaxies and other resolved stellar substructures over a range of environments in the Local Volume. DELVE will combine new DECam observations with archival DECam data to cover ∼15,000 deg
2
of high Galactic latitude (∣
b
∣ > 10°) southern sky to a 5
σ
depth of
g
,
r
,
i
,
z
∼ 23.5 mag. In addition, DELVE will cover a region of ∼2200 deg
2
around the Magellanic Clouds to a depth of
g
,
r
,
i
∼ 24.5 mag and an area of ∼135 deg
2
around four Magellanic analogs to a depth of
g
,
i
∼ 25.5 mag. Here, we present an overview of the DELVE program and progress to date. We also summarize the first DELVE public data release (DELVE DR1), which provides point-source and automatic aperture photometry for ∼520 million astronomical sources covering ∼5000 deg
2
of the southern sky to a 5
σ
point-source depth of
g
= 24.3 mag,
r
= 23.9 mag,
i
= 23.3 mag, and
z
= 22.8 mag. DELVE DR1 is publicly available via the NOIRLab Astro Data Lab science platform.
Abstract
We present the discovery of DELVE 6, an ultra-faint stellar system identified in the second data release of the DECam Local Volume Exploration (DELVE) survey. Based on a maximum-likelihood ...fit to its structure and stellar population, we find that DELVE 6 is an old (
τ
> 9.8 Gyr at 95% confidence) and metal-poor (Fe/H < −1.17 dex at 95% confidence) stellar system with an absolute magnitude of
M
V
=
−
1.5
−
0.6
+
0.4
mag and an azimuthally averaged half-light radius of
r
1
/
2
=
10
−
3
+
4
pc. These properties are consistent with the population of ultra-faint star clusters uncovered by recent surveys. Interestingly, DELVE 6 is located at an angular separation of ∼10° from the center of the Small Magellanic Cloud (SMC), corresponding to a 3D physical separation of ∼20 kpc given the system’s observed distance (
D
⊙
= 80 kpc). This also places the system ∼35 kpc from the center of the Large Magellanic Cloud (LMC), lying within recent constraints on the size of the LMC’s dark matter halo. We tentatively measure the proper motion of DELVE 6 using data from Gaia, which we find supports a potential association between the system and the LMC/SMC. Although future kinematic measurements will be necessary to determine its origins, we highlight that DELVE 6 may represent only the second or third ancient (
τ
> 9 Gyr) star cluster associated with the SMC, or one of fewer than two dozen ancient clusters associated with the LMC. Nonetheless, we cannot currently rule out the possibility that the system is a distant Milky Way halo star cluster.
Abstract
We perform a detailed photometric and astrometric analysis of stars in the Jet stream using data from the first data release of the DECam Local Volume Exploration Survey DR1 and Gaia EDR3. ...We discover that the stream extends over ∼ 29° on the sky (increasing the known length by 18°), which is comparable to the kinematically cold Phoenix, ATLAS, and GD-1 streams. Using blue horizontal branch stars, we resolve a distance gradient along the Jet stream of 0.2 kpc deg
−1
, with distances ranging from
D
⊙
∼ 27–34 kpc. We use natural splines to simultaneously fit the stream track, width, and intensity to quantitatively characterize density variations in the Jet stream, including a large gap, and identify substructure off the main track of the stream. Furthermore, we report the first measurement of the proper motion of the Jet stream and find that it is well aligned with the stream track, suggesting the stream has likely not been significantly perturbed perpendicular to the line of sight. Finally, we fit the stream with a dynamical model and find that it is on a retrograde orbit, and is well fit by a gravitational potential including the Milky Way and Large Magellanic Cloud. These results indicate the Jet stream is an excellent candidate for future studies with deeper photometry, astrometry, and spectroscopy to study the potential of the Milky Way and probe perturbations from baryonic and dark matter substructure.
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