Unveiling the Boxy Bulge and Bar of the Andromeda Spiral Galaxy

Beaton, Rachael L.; Majewski, Steven R.; Guhathakurta, Puragra; Skrutskie, Michael F.; Cutri, R. M.; Good, J. C.; Patterson, Richard J.; Athanassoula, E.; Bureau, Martin

doi:10.1086/514333

Cited by 93 publications

(131 citation statements)

References 32 publications

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“…This is because after the pioneer work of Lindblad (1956) several other papers have pointed out in the inner region of M 31 the presence of morphological structures, such as a bar and a bulge, associated with streaming motion and non-circular orbits (e.g. Stark & Binney 1994;Athanassoula & Beaton 2006;Gordon et al 2006;Beaton et al 2007). In particular Berman (2001); Berman & Loinard (2002) have shown that the anomalous velocities observed in the inner region of M 31 can be explained as the response of the gas to the potential of a triaxial rotating bulge.…”

Section: Why Exclude the Innermost Region From Fittingmentioning

confidence: 99%

A wide-field H I mosaic of Messier 31

Corbelli

Lorenzoni

Walterbos³

et al. 2010

A&A

164

229

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Aims. We test cosmological models of structure formation using the rotation curve of the nearest spiral galaxy, M 31, determined using a recent deep, full-disk 21-cm imaging survey smoothed to 466 pc resolution. Methods. We fit a tilted ring model to the HI data from 8 to 37 kpc and establish conclusively the presence of a dark halo and its density distribution via dynamical analysis of the rotation curve. Results. The disk of M 31 warps from 25 kpc outwards and becomes more inclined with respect to our line of sight. Newtonian dynamics without a dark matter halo provide a very poor fit to the rotation curve. In the framework of modified Newtonian dynamic (MOND) however the 21-cm rotation curve is well fitted by the gravitational potential traced by the baryonic matter density alone. The inclusion of a dark matter halo with a density profile as predicted by hierarchical clustering and structure formation in a ΛCDM cosmology makes the mass model in newtonian dynamic compatible with the rotation curve data. The dark halo concentration parameter for the best fit is C = 12 and its total mass is 1.2 × 10 12 M . If a dark halo model with a constant-density core is considered, the core radius has to be larger than 20 kpc in order for the model to provide a good fit to the data. We extrapolate the best-fit ΛCDM and constant-density core mass models to very large galactocentric radii, comparable to the size of the dark matter halo. A comparison of the predicted mass with the M 31 mass determined at such large radii using other dynamical tracers, confirms the validity of our results. In particular the ΛCDM dark halo model which best fits the 21-cm data well reproduces the mass of M 31 traced out to 560 kpc. Our best estimate for the total mass of M 31 is 1.3 × 10 12 M , with 12% baryonic fraction and only 6% of the baryons in the neutral gas phase.

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Section: Why Exclude the Innermost Region From Fittingmentioning

confidence: 99%

A wide-field H I mosaic of Messier 31

Corbelli

Lorenzoni

Walterbos³

et al. 2010

A&A

164

229

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“…Bars are frequent in spiral galaxies (60 to 75% are barred according to the bar strength; e.g., Verley et al 2007), and it is likely that M 31 is barred or has been barred, because of its triaxial boxy bulge (Beaton et al 2007). However, up to now, the bar is not visible in the gas in either the morphological or kinematical data.…”

Section: Scenario 2: a Possible Barmentioning

confidence: 99%

“…the angle between the bar and the major axis of the galaxy is between 20 and 30 • . The size of the boxy-bulge is 1.4 kpc in radius, and the total length of the bar would be 4 kpc in radius, because a thin bar is identified beyond the boxy bulge (Beaton et al 2007). …”

Section: Previous Bar Propositions In M 31mentioning

confidence: 99%

Molecular gas in the inner 0.7 kpc-radius ring of M 31

Melchior

Combes

2011

A&A

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The study of the gas kinematic in the central 1.5 kpc × 1.5 kpc region of M 31 has revealed several surprises. The starting point of this investigation was the detection at the IRAM-30 m telescope of molecular gas with very large line splittings up to 260 km s −1 within the beam (∼40 pc). In this region, which is known for its low gas content, we also detect an ionised gas outflow in the circumnuclear region (within 75 pc from the centre) extending to the whole area in X-ray. Relying on atomic, ionised, and molecular gas, we account for most observables with a scenario that assumes that a few hundreds Myr ago, M 31 underwent a frontal collision with M 32, which triggered some star-formation activity in the centre, and this collision explains the special configuration of M 31 with two rings observed at 0.7 kpc and 10 kpc. The inner disc (whose rotation is detected in HI and ionised gas ([NII])) has thus been tilted (inclination: 43 deg, PA: 70 deg) with respect to the main disc (inclination: 77 deg, PA: 35 deg). One of the CO velocity components is compatible with this inner disc, while the second one comes from a tilted ring-like material with 40 deg inclination and PA = −35 deg. The relic star formation estimated by previous works to have occurred more than 100 Myr ago could have been triggered by the collision and could be linked to the outflow detected in the ionised gas. Last, we demonstrate that the amplitude of the line splittings detected in CO centred on the systemic velocity with a relatively high spatial resolution (40 pc) cannot be accounted for by a possible weak bar that is roughly aligned along the minor axis. Although M 31 has a triaxial bulge, there are no bar indicators in the gas component (photometry, no strong skewness of the isovelocities, etc.).

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“…M 31 is no exception; although Hubble classified it as normal type Sb, it is likely to possess a stellar bar. The presence of a bar in M 31 is suggested by its boxy bulge, which is conspicuous in infrared imaging 5,20 and is aligned almost parallel to the galaxys major axis. The boxy bulge might be an inflated bar, or it may hide a thinner parallel bar (at about 10 degrees from the major axis).…”

mentioning

confidence: 95%

An almost head-on collision as the origin of two off-centre rings in the Andromeda galaxy

Block

Bournaud

Combes

et al. 2006

Nature

117

161

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Abstract. The unusual morphology of the Andromeda Spiral (Messier 31, the closest spiral galaxy to the Milky Way) has long been an enigma. Although regarded for decades as showing little evidence of a violent history, M 31 has a well-known outer ring 1−7 of star formation at a radius of 10 kpc whose center is offset from the galaxy nucleus. In addition, the outer galaxy disk is warped as seen at both optical 8 and radio 9 wavelengths. The halo contains numerous loops and ripples. Here we report the discovery, based on analysis of previously-obtained data 10 , of a second, inner dust ring with projected dimensions 1.5 by 1 kpc and offset by ∼ 0.5 kpc from the center of the galaxy. The two rings appear to bedensity waves propagating in the disk.Numerical simulations offer a completely new interpretation for the morphology of M 31: both rings result from a companion galaxyplunging head-on through the center of the disk of M 31. The most likely interloper is M 32. Head-on collisions between galaxies are rare, but it appears nonetheless that one took place 210 million years ago in our Local Group of galaxies.Newly-acquired images 10 of M 31 secured by the Infrared Array Camera 11 (IRAC) onboard the Spitzer Space Telescope span the wavelength regime of 3.6 to 8.0 microns. These images offer unique probes of the morphologies of the stellar distribution and interstellar medium with no interference from extinction. Figure 1 shows the emission map of the interstellar medium at 8 microns, generated by subtracting a scaled 3.6 micron image (dominated by starlight) from the 8 micron image. The subtraction removes the contribution from stellar photospheres and leaves only the emission from dust grains 10 , which trace the interstellar medium of M 31. What is most striking in Figure 1 (and in the enlarged inset) is the presence of a complete though asymmetric inner ring of dust 6.9 by 4.4 arcmin in extent, translating to linear dimensions of about 1.5 by 1 kpc (assuming a distance 12 of 780 kpc). The inner ring lies between the two well known Baade spiral dust arms 13 , both of which are clearly seen in emission. The inner ring is elongated in a direction close to the minor axis and belongs to the central gas disk, which appears to be more face-on 14 . It is therefore not possible to know the inner rings precise ellipticity, but it is unlikely to be circular. IRAC imaging thus reveals two rings. The outer ring is offset by approximately 10 percent of its radius, while the inner ring is offset by about 40 percent or 0.5 kpc. The inner elliptical ring has been alluded to in earlier studies 15,16 , but all investigators have hitherto believed it to be a mini-spiral, related to a bar. Published Spitzer 24 micron images5 of M 31 show centrally-concentrated dust emission; the ring morphology is therefore disguised at these longer wavelengths. The IRAC images beautifully show the inner ring at high spatial resolution and furthermore confirm that this feature is a complete and continuous ring, even though offset and asymmetrical. Ther...

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Unveiling the Boxy Bulge and Bar of the Andromeda Spiral Galaxy

Cited by 93 publications

References 32 publications

A wide-field H I mosaic of Messier 31

A wide-field H I mosaic of Messier 31

Molecular gas in the inner 0.7 kpc-radius ring of M 31

An almost head-on collision as the origin of two off-centre rings in the Andromeda galaxy

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