Learning how interstellar dust turns into stars is a major challenge. But the AKARI satellite, an infrared observatory created by the Japan Aerospace Exploration Agency, could become a breakthrough tool in these studies. Launched in February of this year, AKARI is engaged in an infrared All Sky Survey, with spectacular early results from the Large Magellanic Cloud. The far-infrared image shown below shows that clouds of dust are found throughout this satellite galaxy of the Milky Way.
Image: This false-colour view of the Large Magellanic Cloud is a composite of images taken by AKARI at far-infrared wavelengths (60, 90 and 140 microns). The Large Magellanic Cloud is a neighbour galaxy to the Milky Way. Interstellar clouds in which new stars are forming are distributed over the entire galaxy. The bright region in the bottom-left is known as the ‘Tarantula Nebula’ and is a productive factory of stars. Credit: JAXA.
Infrared is ideal for these studies because stars tend to form within clouds of dusty interstellar gas, their light not visible at optical wavelengths. But as they grow, they heat the dust around them and that produces infrared radiation that AKARI can detect. Learning how dust is distributed in galaxies helps us understand how stars like our Sun came into being, and the Large Magellanic Cloud, being relatively close (160,000 light years), makes an ideal laboratory for these observations.
Centauri Dreams‘ note: Speaking of celestial views, as we have recently in these pages, I used to marvel at what an optical view of our Milky Way must be like from the LMC (although further study shows it might not be as spectacular as I had once imagined). Galaxies are, in fact, rather hard to see without resorting to tricks like CCD time exposures, as planet-hunter Greg Laughlin (UC-Santa Cruz) once pointed out:
Indeed, the great Andromeda Galaxy, M31, subtends an angle larger than the full Moon in the sky, and it is literally almost directly overhead right now (9:36 PM, Dec 3, latitude 36.97 deg N). The storms from earlier this week have blown through. The sky sparkles with brilliant clarity. Yet when I step outside and look up, I can’t see the Andromeda Galaxy at all. It’s too faint. In a 1:10,000,000,000,000 scale model of M31, the stars are like fine grains of sand separated by miles. Our Galaxy, the Andromeda Galaxy, and the Sombrero Galaxy are all essentially just empty space. To zeroth, to first, to second approximation, a galaxy is nothing at all.
On the other hand, if you’ve ever needed to re-awaken a sense of awe about the age and scale of the cosmos, the Large Magellanic Cloud serves quite well. Itself a galaxy of 10 billion stars, the LMC is thought to be the remnant of a barred spiral galaxy disrupted by the Milky Way. Ponder, too, that our galaxy may be headed for disruption and perhaps incorporation into the Andromeda galaxy, which itself was involved in a collision with dwarf galaxy Messier 32 (M32) more than 200 million years ago. Pondering galactic collisions works wonders with my own long-term perspective, even if they’re hard to see with the naked eye!
Astrophysics, abstract
astro-ph/0703196
From: Gurtina Besla [view email]
Date: Thu, 8 Mar 2007 22:16:54 GMT (630kb)
Are the Magellanic Clouds on their First Passage about the Milky Way?
Authors: Gurtina Besla (1), Nitya Kallivayalil (1), Lars Hernquist (1), Brant Robertson (2,3,4), T.J. Cox (1), Roeland P. van der Marel (5), Charles Alcock (1) ((1) Harvard-Smithsonian CfA, (2) KICP, (3) Enrico Fermi Institute, (4) Spitzer Fellow, (5) STScI)
Recent proper motion measurements of the Large and Small Magellanic Clouds (LMC and SMC, respectively) by Kallivayalil et al (2006a,b) suggest that the 3D velocities of the Clouds are substantially higher (~100 km/s) than previously estimated and now approach the escape velocity of the Milky Way (MW). Previous studies have also assumed that the Milky Way can be adequately modeled as an isothermal sphere to large distances. Here we re-examine the orbital history of the Clouds using the new velocities and a LCDM-motivated MW model with virial mass Mvir = 1e12 Msun (e.g. Klypin et al 2002). We conclude that the Clouds are either currently on their first passage about the MW or, if the MW can be accurately modeled by an isothermal sphere to distances >200 kpc (i.e., Mvir > 2e12 Msun), that their orbital period and apogalacticon distance must be a factor of two larger than previously estimated, increasing to 3 Gyr and 200 kpc, respectively. A first passage scenario is consistent with the fact that the LMC and SMC appear to be outliers when compared to other satellite galaxies of the MW: they are irregular in appearance and are moving faster. We discuss the implications of this orbital analysis for our understanding of the star formation history, the nature of the warp in the MW disk and the origin of the Magellanic Stream (MS), a band of HI gas trailing the LMC and SMC that extends roughly 100 degrees across the sky. Specifically, as a consequence of the new orbital history of the Clouds, the origin of the MS may not be explainable by current tidal and ram pressure stripping models.
http://arxiv.org/abs/astro-ph/0703196
Little Dwarf Galaxies survive within Bigger Dwarfs: Why Some Dwarfs Go Dark and Others Stay Luminous
Authors: George Lake, Elena D’Onghia ((1) University of Zurich)
(Submitted on 31 Jan 2008)
Abstract: The Large Magellanic Cloud (LMC) was the largest galaxy in a group of galaxies that entered the Milky Way (MW) system at late times. Seven of the 11 brightest dwarf galaxies of the MW may have been part of this system. The association of dwarfs with the plane of the LMCs orbit has been used to argue that they are formed from tidal debris from LMC and SMC (Kroupa et al 2005). Instead, we find that they owe to the tidal breakup of the Magellanic Group.
The Cold Dark Matter (CDM) paradigm suffers from the small-scale structure problem where 500 galaxies as massive as Draco and Ursa Minor are expected, but only 11 are seen. If seven of the 11 observed were part of the LMC group, the substructure in this group is close to CDM predictions. There are other likely nearby dwarf groups, including a second Milky Way group associated with Fornax (Lynden-Bell 1982), great circles of satellites in M31 (Koch & Grebel 2006) and groupings of otherwise isolated dwarfs (Tully et al. 2006). For the first time, we build he circular velocity distribution of the satellites in system as small as groups of dwarf galaxies, using the dwarf galaxies that likely entered with the LMC system and data on nearby dwarf systems. Our work points to natural mechanisms that lead to less suppression of satellites in dwarf groups providing an explanation for the missing satellite problem in the Local Group.
Comments: Submitted as ApJ Letters
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0802.0001v1 [astro-ph]
Submission history
From: Elena D’Onghia [view email]
[v1] Thu, 31 Jan 2008 21:00:04 GMT (400kb)
http://arxiv.org/abs/0802.0001
Breaking up the Magellanic Group into the Milky Way Halo: understanding the ultra-compact dwarf galaxy properties
Authors: Elena D’Onghia (University of Zurich)
(Submitted on 3 Feb 2008)
Abstract: We use a numerical simulation of a loose group containing a Milky Way halo to probe that in the hierarchical universe the Magellanic Clouds and some dSphs have been accreted into the Milky Way halo from a late infalling group of dwarfs. Our simulations show that the tidal breakup of the Magellanic group occurs before it enters the Milky Way halo. Only half of the satellites contributed from the group are predicted to be inside the Milky Way virial radius. Half of its subhalos survive outside the current virial radius in the form of satellites, whereas the remaining material contributes to the diffuse Milky Way halo. At z~0 the disrupted group contributes less than 10% to the Milky Way halo mass but 20% of the brightest dwarf galaxies of the Milky Way have been part of this group.
This scenario points out that some dSphs might have been form away from giant spirals and been accreted already as spheroids, by a late infall group in contrast with the classical picture of tidal stripping of dSph formation models. This would naturally explain several peculiarities of the local dSph: why Draco and the other luminous dSphs exist compared to other ultra-faint satellite galaxies, the location of Tucana and Cetus in the outskirts of the Local Group and the mismatch in metallicity between the stellar halo of the Milky Way and the dwarf galaxies that many have suspected dissolved to build it.
Comments: ApJ Letter submitted
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0802.0302v1 [astro-ph]
Submission history
From: Elena D’Onghia [view email]
[v1] Sun, 3 Feb 2008 21:00:06 GMT (153kb)
http://arxiv.org/abs/0802.0302
Cosmic Finger Taps Our Galaxy’s Shoulder
Gas streaming from the Magellanic Clouds has hooked
onto our galaxy’s starry disk.
http://www.space.com/scienceastronomy/080205-st-galaxy-finger.html
The orbital poles of Milky Way satellite galaxies: a rotationally supported disc-of-satellites
Authors: Manuel Metz, Pavel Kroupa, Noam I. Libeskind
(Submitted on 26 Feb 2008)
Abstract: Available proper motion measurements of Milky Way (MW) satellite galaxies are used to calculate their orbital poles and projected uncertainties. These are compared to a set of recent cold dark-matter (CDM) simulations, tailored specifically to solve the MW satellite problem. We show that the CDM satellite orbital poles are fully consistent with being drawn from a random distribution, while the MW satellite orbital poles indicate that the disc-of-satellites of the Milky Way is rotationally supported.
Furthermore, the bootstrapping analysis of the spatial distribution of theoretical CDM satellites also shows that they are consistent with being randomly drawn. The theoretical CDM satellite population thus shows a significantly different orbital and spatial distribution than the MW satellites, most probably indicating that the majority of the latter are of tidal origin rather than being DM dominated sub-structures. A statistic is presented that can be used to test a possible correlation of satellite galaxy orbits with their spatial distribution.
Comments: Accepted for publication in ApJ
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0802.3899v1 [astro-ph]
Submission history
From: Manuel Metz [view email]
[v1] Tue, 26 Feb 2008 21:00:24 GMT (293kb)
http://arxiv.org/abs/0802.3899