Sagittarius II, Draco II and Laevens 3: three new Milky Way satellites discovered in the Pan-STARRS 1 3pi Survey (1507.07564v2)

Abstract: We present the discovery of three new Milky Way satellites from our search for compact stellar overdensities in the photometric catalog of the Panoramic Survey Telescope and Rapid Response System 1 (Pan-STARRS 1, or PS1) 3pi survey. The first satellite, Laevens 3, is located at a heliocentric distance of d=67+/-3 kpc. With a total magnitude of Mv=-4.4+/-0.3 and a half-light radius rh=7+/-2 pc, its properties resemble those of outer halo globular clusters. The second system, Draco II/Laevens 4 (Dra II), is a closer and fainter satellite (d~20 kpc, Mv =-2.9+/-0.8), whose uncertain size (rh = 19 +8/-6 pc) renders its classification difficult without kinematic information; it could either be a faint and extended globular cluster or a faint and compact dwarf galaxy. The third satellite, Sagittarius II/Laevens 5 (Sgr II), has an ambiguous nature as it is either the most compact dwarf galaxy or the most extended globular cluster in its luminosity range (rh = 37 +9/-8 pc and Mv=-5.2+/-0.4). At a heliocentric distance of 67+/-5 kpc, this satellite lies intriguingly close to the expected location of the trailing arm of the Sagittarius stellar stream behind the Sagittarius dwarf spheroidal galaxy (Sgr dSph). If confirmed through spectroscopic follow up, this connection would locate this part of the trailing arm of the Sagittarius stellar stream that has so far gone undetected. It would further suggest that Sgr II was brought into the Milky Way halo as a satellite of the Sgr dSph.

Discovery of Three New Milky Way Satellites in the Pan-STARRS 1 3π Survey

Overview

The paper entitled "Sagittarius II, Draco II, and Laevens 3: three new Milky Way satellites discovered in the Pan-STARRS 1 3$\pi$ survey" outlines the significant finding of three new satellites in the Milky Way: Laevens 3, Draco II, and Sagittarius II. These discoveries were made using the comprehensive data from the Panoramic Survey Telescope and Rapid Response System 1 (Pan-STARRS 1, or PS1) 3$\pi$ survey, which serves as a crucial platform for detecting faint astronomical objects. The identification of these satellites contributes to the ongoing exploration of the outer regions of the Milky Way and enhances our understanding of its structure and satellite interactions.

Methodology

The research team employed a methodical approach to identify compact stellar overdensities using the photometric catalog from the PS1 survey. They applied convolution techniques to isolate potential satellite stars based on specific color-magnitude criteria and computed statistical significance maps to flag significant detections that did not correspond to previously known objects. This approach was pivotal in detecting the three new satellites with significance levels comfortably exceeding the established thresholds.

Satellite Descriptions

1. Laevens 3: With a heliocentric distance of $67 \pm 3$ kpc, Laevens 3 manifests properties akin to outer halo globular clusters. Its visual magnitude of $M_V = -4.4 \pm 0.3$ and a half-light radius $r_h = 7 \pm 2$ pc suggest a compact structure typical of such clusters.
2. Draco II (Dra II): This satellite, located at approximately 20 kpc, displays an ambiguous nature due to uncertain size parameters ($r_h = 19^{+8}_{-6}$ pc). Its total magnitude is estimated at $M_V = -2.9 \pm 0.8$, indicating it could either be a faint globular cluster or a compact dwarf galaxy.
3. Sagittarius II (Sgr II): This entity, at a heliocentric distance of $67 \pm 5$ kpc, potentially represents the most compact dwarf galaxy or the most extended globular cluster within its luminosity parameters ($r_h = 37^{+9}_{-8}$ pc, $M_V = -5.2 \pm 0.4$). Its proximity to the anticipated trailing arm of the Sagittarius stellar stream suggests it might have been accreted into the Milky Way halo as a former satellite of the Sagittarius dwarf spheroidal galaxy (Sgr dSph).

Implications and Speculations

The discoveries offer vital insights into the classification and distribution of objects in the Milky Way halo, focusing on the distinction between dwarf galaxies and globular clusters. The possible association of Sagittarius II with the Sagittarius stellar stream, if confirmed, could provide valuable constraints on modeling the stream's dynamics and the accretion history of the Milky Way. Furthermore, these satellites' properties challenge the traditional boundaries between globular clusters and dwarf galaxies, underscoring the necessity for nuanced classification criteria based on both photometric and spectroscopic analyses.

Future Directions

Continued observation and spectroscopic follow-up of these satellites are essential to affirm their nature and explore their kinematics and chemical compositions further. A deeper understanding of their properties will significantly advance knowledge about the Milky Way's formation and satellite accretion processes. Moreover, these findings prompt ongoing searches within wide-field surveys for additional faint satellites, potentially unearthing further minor galaxies or globular clusters that blend into the expansive darkness of the Milky Way's halo.