Gas, Dust, and Young Stars in the Outer Disk of M31

Cuillandre, Jean-Charles; Lequeux, J.; Allen, R. J.; Mellier, Y.; Bertin, E.

doi:10.1086/321358

Cited by 70 publications

(75 citation statements)

References 29 publications

(37 reference statements)

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“…Dust in the outskirt of Andromeda had been already indirectly detected, through extinction of both stars and background galaxies, by Cuillandre et al (2001), who exploited deep optical photometry in a ∼28 × 28 field roughly centred on our "E" region. This result, recently confirmed on a smaller scale by Bernard et al (2012, their "Outer Disc field"), implies an atomic gas-todust ratio about three times higher with respect to the average Galactic value, and is considered as an upper limit since it only takes into account the dust in front of the observed stars.…”

Section: Discussionmentioning

confidence: 99%

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TheHerschelExploitation of Local Galaxy Andromeda (HELGA)

Fritz

Gentile

Smith

et al. 2012

A&A

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Context. We have obtained Herschel images at five wavelengths from 100 to 500 μm of a ∼5.5 × 2.5 degree area centred on the local galaxy M 31 (Andromeda), our nearest neighbour spiral galaxy, as part of the Herschel guaranteed time project "HELGA". The main goals of HELGA are to study the characteristics of the extended dust emission, focusing on larger scales than studied in previous observations of Andromeda at an increased spatial resolution, and the obscured star formation. Aims. In this paper we present data reduction and Herschel maps, and provide a description of the far-infrared morphology, comparing it with features seen at other wavelengths. Methods. We used high-resolution maps of the atomic hydrogen, fully covering our fields, to identify dust emission features that can be associated to M 31 with confidence, distinguishing them from emission coming from the foreground Galactic cirrus. Results. Thanks to the very large extension of our maps we detect, for the first time at far-infrared wavelengths, three arc-like structures extending out to ∼21, ∼26 and ∼31 kpc respectively, in the south-western part of M 31. The presence of these features, hosting ∼2.2 × 10 6 M of dust, is safely confirmed by their detection in HI maps. Overall, we estimate a total dust mass of ∼5.8 × 10 7 M , about 78% of which is contained in the two main ring-like structures at 10 and 15 kpc, at an average temperature of 16.5 K. We find that the gas-to-dust ratio declines exponentially as a function of the galacto-centric distance, in agreement with the known metallicity gradient, with values ranging from 66 in the nucleus to ∼275 in the outermost region. Conclusions. Dust in M 31 extends significantly beyond its optical radius (∼21 kpc) and what was previously mapped in the farinfrared. An annular-like segment, located approximately at R 25 , is clearly detected on both sides of the galaxy, and two other similar annular structures are undoubtedly detected on the south-west side even further out.

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Section: Discussionmentioning

confidence: 99%

“…Cuillandre et al 2001;Bernard et al 2012), it had never been possible to study its properties and quantify its presence before.…”

Section: Introduction An Overall View Of the Interstellar Medium Of Tmentioning

confidence: 99%

TheHerschelExploitation of Local Galaxy Andromeda (HELGA)

Fritz

Gentile

Smith

et al. 2012

A&A

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“…The presence of dark baryons in the disk of galaxies is reinforced by the numerous signs of recent star formation in the far outer disk of galaxies, correlated with HI gas in NGC 6946 (Ferguson et al 1998), NGC 628 (Lelièvre & Roy 2000), M 101 Smith et al (2000), M 31 Cuillandre et al (2001), NGC 6822 (de Blok & Walter 2003), M 83 (Thilker et al 2005). Correlations between young stars and HI far from the center reveal that molecular gas, at the origin of a weak but existing star formation rate, must be present in abundance there, despite the lack of CO detection.…”

Section: Article Published By Edp Sciencesmentioning

confidence: 99%

Simulations of galactic disks including a dark baryonic component

Revaz¹,

Pfenniger²,

Combes³

et al. 2009

A&A

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The near proportionality between HI and dark matter in outer galactic disks prompted us to run N-body simulations of galactic disks in which the observed gas content is supplemented by a dark gas component representing between zero and five times the visible gas content. While adding baryons in the disk of galaxies may solve some issues, it poses the problem of disk stability. We show that the global stability is ensured if the ISM is multiphased, composed of two partially coupled phases, a visible warm gas phase and a weakly collisionless cold dark phase corresponding to a fraction of the unseen baryons. The phases are subject to stellar and UV background heating and gas cooling, and their transformation into each other is studied as a function of the coupling strength. This new model, which still possesses a dark matter halo, fits the rotation curves as well as the classical CDM halos, but is the only one to explain the existence of an open and contrasting spiral structure, as observed in the outer HI disks

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“…Our previously published observations of NGC 4414, covering almost the whole optical disk, show a clear decline in the CO(2 2 1)/CO(1 2 0) line ratio (independent of beam size) which we interpret 7 as a decrease in excitation temperature, meaning that even if substantial CO is present, the lines could be quite weak. Dust, which is necessary for H 2 formation, is clearly seen in absorption at the edge of the spiral disks of galaxies and even well beyond through the reddening of stars and background galaxies 8 . Observations of optical line ratios in H II regions at large radii also show that metals and star formation are present 9 .…”

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confidence: 99%

Molecular hydrogen beyond the optical edge of an isolated spiral galaxy

Braine

Herpin

2004

Nature

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Knowledge about the outermost portions of galaxies is limited owing to the small amount of light coming from them. It is known that in many cases atomic hydrogen (H I) extends well beyond the optical radius 1 . In the centres of galaxies, however, molecular hydrogen (H 2 ) usually dominates by a large factor [2][3][4] , raising the question of whether H 2 is also abundant in the outer regions. Here we report the detection of emission from carbon monoxide (CO), the most abundant tracer of H 2 , beyond the optical radius of the nearby galaxy NGC 4414. The host molecular clouds probably formed in the regions of relatively high H I column density and in the absence of spiral density waves. The relative strength of the lines from the two lowest rotational levels indicates that both the temperature and density of the H 2 are quite low compared to conditions closer to the centre. The inferred surface density of the molecular material continues the monotonic decrease from the inner regions. We conclude that although molecular clouds can form in the outer region of this galaxy, there is little mass associated with them.The CO spectra shown in Fig. 1 reveal the presence of cool, but not very cold, molecular gas out to 1.5 times the optical radius (R 25 ) of the isolated spiral NGC 4414. The CO observations were carried out in good conditions with the 30-m antenna on Pico Veleta (Spain) operated by the Institut de Radioastronomie Millimétrique, in May and November 2003, and March 2004. Cool H 2 is not directly observable because it has no permitted rotational transitions so CO, the most abundant heteronuclear molecular, is used as a proxy. The galaxy chosen is expected to be representative of a large class of spirals because it is of late type, quite axisymmetric 5 , and has no immediate neighbours with which it could interact. The atomic gas is extended, also indicating a lack of major interactions. This lack of interactions is important: although every interaction is different owing to the large number of parameters involved, isolated galaxies can reasonably be hoped to be representative. Furthermore, NGC 4414 is near the North Galactic Pole and suffers little from foreground (Galactic) confusion or extinction.Almost no constraints are as yet available on the H 2 content of the outer disks of spirals. To our knowledge, these are the first published detections of CO beyond the optical radius of an external spiral, reaching 1.5R 25 . A. Ferguson also reports detection (personal communication) of CO in NGC 6946 beyond R 25 . Digel et al. 6 detected isolated clouds at large radii in the Milky Way, although the mass implied was quite low. Our previously published observations of NGC 4414, covering almost the whole optical disk, show a clear decline in the CO(2 2 1)/CO(1 2 0) line ratio (independent of beam size) which we interpret 7 as a decrease in excitation temperature, meaning that even if substantial CO is present, the lines could be quite weak. Dust, which is necessary for H 2 formation, is clearly seen in absorption at...

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Gas, Dust, and Young Stars in the Outer Disk of M31

Cited by 70 publications

References 29 publications

TheHerschelExploitation of Local Galaxy Andromeda (HELGA)

TheHerschelExploitation of Local Galaxy Andromeda (HELGA)

Simulations of galactic disks including a dark baryonic component

Molecular hydrogen beyond the optical edge of an isolated spiral galaxy

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