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Register a new userThe Saga of M81: Global View of a Massive Stellar Halo in Formation
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Recent work has shown that Milky Way-mass galaxies display an incredible range of stellar halo properties, yet the origin of this diversity is unclear. The nearby galaxy M81 $-$ currently interacting with M82 and NGC 3077 $-$ sheds unique light on this problem. We present a Subaru Hyper Suprime-Cam survey of the resolved stellar populations around M81, revealing M81s stellar halo in never-before-seen detail. We resolve the halo to unprecedented $V$-band equivalent surface brightnesses of 33 mag arcsec$^{-2}$, and produce the first-ever global stellar mass density map for a Milky Way-mass stellar halo outside of the Local Group. Using the minor axis, we confirm M81s halo as one of the lowest mass and metal-poorest known ($M_{star} simeq 1.16{times}10^9 M_{odot}$, [Fe/H] $simeq {-}1.2$) $-$ indicating a relatively quiet prior accretion history. Yet, our global halo census finds that tidally unbound material from M82 and NGC 3077 provides a substantial infusion of metal-rich material ($M_{star} simeq 5.4{times}10^8$ $M_{odot}$, [Fe/H] $simeq {-}$0.9). We further show that, following the accretion of its massive satellite M82 (and the LMC-like NGC 3077), M81 will host one of the most massive and metal-rich stellar halos in the nearby universe. Thus, the saga of M81: following a passive history, M81s merger with M82 will completely transform its halo from a low-mass, anemic halo rivaling the MW, to a metal-rich behemoth rivaled only by systems such as M31. This dramatic transformation indicates that the observed diversity in stellar halo properties is primarily driven by diversity in the largest mergers these galaxies have experienced.
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Using the MegaCam imager on the Canada-France-Hawaii Telescope, we have resolved individual stars in the outskirts of the nearby large spiral galaxy M81 (NGC 3031) well below the tip of the red giant branch of metal-poor stellar populations over 60 kpc * 58 kpc. In this paper, we report the discovery of new young stellar systems in the outskirts of M81. The most prominent feature is a chain of clumps of young stars distributed along the extended southern HI tidal arm connecting M 81 and NGC 3077. The colour-magnitude diagrams of these stellar systems show plumes of bright main sequence stars and red supergiant stars, indicating extended events of star formation. The main sequence turn-offs of the youngest stars in the systems are consistent with ages of ~40 Myr. The newly reported stellar systems show strong similarities with other known young stellar systems in the debris field around M81, with their properties best explained by these systems being of tidal origin.
We present the first results of a wide-field mapping survey of the M81 group conducted with Hyper Suprime-Cam on the Subaru Telescope. Our deep photometry reaches $sim2$ magnitudes below the tip of the red giant branch (RGB) and reveals the spatial distribution of both old and young stars over an area of $sim 100times115$ kpc at the distance of M81. The young stars ($sim30-160$ Myr old) closely follow the neutral hydrogen distribution and can be found in a stellar stream between M81 and NGC,3077 and in numerous outlying stellar associations, including the known concentrations of Arps Loop, Holmberg,IX, an arc in the halo of M82, BK3N, and the Garland. Many of these groupings do not have counterparts in the RGB maps, suggesting they may be genuinely young systems. Our survey also reveals for the first time the very extended ($geq 2times rm{R_{25}}$) halos of RGB stars around M81, M82 and NGC,3077, as well as faint tidal streams that link these systems. The halos of M82 and NGC,3077 exhibit highly disturbed morphologies, presumably a consequence of the recent gravitational encounter and their ongoing disruption. While the halos of M81, NGC,3077 and the inner halo of M82 have the similar $(g-i)_{0}$ colors, the outer halo of M82 is significantly bluer indicating it is more metal-poor. Remarkably, our deep panoramic view of the M81 group demonstrates that the complexity long-known to be present in HI is equally matched in the low surface brightness stellar component.
The Central Molecular Zone (CMZ), a $sim$200 pc sized region around the Galactic Centre, is peculiar in that it shows a star formation rate (SFR) that is suppressed with respect to the available dense gas. To study the SFR in the CMZ, young stellar objects (YSOs) can be investigated. Here we present radio observations of 334 2.2 $mu$m infrared sources that have been identified as YSO candidates. Our goal is to investigate the presence of centimetre wavelength radio continuum counterparts to this sample of YSO candidates which we use to constrain the current SFR in the CMZ. As part of the GLOSTAR survey, D-configuration VLA data was obtained for the Galactic Centre, covering -2$^{circ}<l<$2$^{circ}$ and -1$^{circ}<b<$1$^{circ}$, with a frequency coverage of 4-8 GHz. We matched YSOs with radio continuum sources based on selection criteria and classified these radio sources as potential HII regions and determined their physical properties. Of the 334 YSO candidates, we found 35 with radio continuum counterparts. We find that 94 YSOs are associated with dense dust condensations identified in the 870 $mu$m ATLASGAL survey, of which 14 have a GLOSTAR counterpart. Of the 35 YSOs with radio counterparts, 11 are confirmed as HII regions, based on their spectral indices and the literature. We estimated their Lyman continuum photon flux in order to estimate the mass of the ionising star. Combining these with known sources, the present-day SFR in the CMZ is calculated to be $sim$0.068 M$_{odot}$ yr$^{-1}$, which is $sim$6.8$%$ of the Galactic SFR. Candidate YSOs that lack radio counterparts may not have yet evolved to the stage of exhibiting an HII region or, conversely, are older and have dispersed their natal clouds. Since many lack dust emission, the latter is more likely. Our SFR estimate in the CMZ is in agreement with previous estimates in the literature.
In this paper, we present a comprehensive analysis of star-forming galaxies (SFGs) at intermediate redshifts (z~1). We combine the ultra-deep optical spectro-photometric data from the Survey for High-z Absorption Red and Dead Sources (SHARDS) with deep UV-to-FIR observations in the GOODS-N field. Exploiting two of the 25 SHARDS medium-band filters, F687W17 and F823W17, we select [OII] emission line galaxies at z~0.84 and z~1.23 and characterize their physical properties. Their rest-frame equivalent widths (EW$_{mathrm{rf}}$([OII])), line fluxes, luminosities, star formation rates (SFRs) and dust attenuation properties are investigated. The evolution of the EW$_{mathrm{rf}}$([OII]) closely follows the SFR density evolution of the universe, with a trend of EW$_{mathrm{rf}}$([OII])$propto$(1+z)$^3$ up to redshift z~1, followed by a possible flattening. The SF properties of the galaxies selected on the basis of their [OII] emission are compared with complementary samples of SFGs selected by their MIR and FIR emission, and also with a general mass-selected sample of galaxies at the same redshifts. We demonstrate observationally that the UVJ diagram (or, similarly, a cut in the specific SFR) is only partially able to distinguish the quiescent galaxies from the SFGs. The SFR-M$_*$ relation is investigated for the different samples, yelding a logarithmic slope ~1, in good agreement with previous results. The dust attenuations derived from different SFR indicators (UV(1600), UV(2800), [OII], IR) are compared and show clear trends with respect to both the stellar mass and total SFR, with more massive and highly star-forming galaxies being affected by stronger dust attenuation.
We have mapped cold atomic gas in 21cm line HI self-absorption (HISA) at arcminute resolution over more than 90% of the Milky Ways disk. To probe the formation of H2 clouds, we have compared our HISA distribution with CO J=1-0 line emission. Few HISA features in the outer Galaxy have CO at the same position and velocity, while most inner-Galaxy HISA has overlapping CO. But many apparent inner-Galaxy HISA-CO associations can be explained as chance superpositions, so most inner-Galaxy HISA may also be CO-free. Since standard equilibrium cloud models cannot explain the very cold HI in many HISA features without molecules being present, these clouds may instead have significant CO-dark H2.
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