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Register a new userMilky Way vs Andromeda: a tale of two disks
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J. Yin
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We study the chemical evolution of the disks of the Milky Way (MW) and of Andromeda (M31), in order to reveal common points and differences between the two major galaxies of the Local group. We use a large set of observational data for M31, including recent observations of the Star Formation Rate (SFR) and gas profiles, as well as stellar metallicity distributions along its disk. We show that, when expressed in terms of the corresponding disk scale lengths, the observed radial profiles of MW and M31 exhibit interesting similarities, suggesting the possibility of a description within a common framework. We find that the profiles of stars, gas fraction and metallicity of the two galaxies, as well as most of their global properties, are well described by our model, provided the star formation efficiency in M31 disk is twice as large as in the MW. We show that the star formation rate profile of M31 cannot be fitted with any form of the Kennicutt-Schmidt law (KS Law) for star formation. We attribute those discrepancies to the fact that M31 has undergone a more active star formation history, even in the recent past, as suggested by observations of a head-on collision with the neighboring M32 galaxy about 200 Myr ago. The MW has most probably undergone a quiescent secular evolution, making possible a fairly successful description with a simple model. If M31 is more typical of spiral galaxies, as recently suggested by Hammer et al. (2007), more complex models, involving galaxy interactions, will be required for the description of spirals.
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We present the first comparison between the lifetime star formation histories (SFHs) of M31 and Milky Way (MW) satellites. Using the Advanced Camera for Surveys aboard the Hubble Space Telescope, we obtained deep optical imaging of Andromeda II (M$_{V} = -$12.0; log(M$_{star}$/M$_{odot}$) $sim$ 6.7) and Andromeda XVI (M$_{V} = -$7.5; log(M$_{star}$/M$_{odot}$) $sim$ 4.9) yielding color-magnitude diagrams that extend at least 1 magnitude below the oldest main sequence turnoff, and are similar in quality to those available for the MW companions. And II and And XVI show strikingly similar SFHs: both formed 50-70% of their total stellar mass between 12.5 and 5 Gyr ago (z$sim$5-0.5) and both were abruptly quenched $sim$ 5 Gyr ago (z$sim$0.5). The predominance of intermediate age populations in And XVI makes it qualitatively different from faint companions of the MW and clearly not a pre-reionization fossil. Neither And II nor And XVI appears to have a clear analog among MW companions, and the degree of similarity in the SFHs of And II and And XVI is not seen among comparably faint-luminous pairs of MW satellites. These findings provide hints that satellite galaxy evolution may vary substantially among hosts of similar stellar mass. Although comparably deep observations of more M31 satellites are needed to further explore this hypothesis, our results underline the need for caution when interpreting satellite galaxies of an individual system in a broader cosmological context.
Our Galaxy and the nearby Andromeda galaxy (M31) are the most massive members of the Local Group, and they seem to be a bound pair, despite the uncertainties on the relative motion of the two galaxies. A number of studies have shown that the two galaxies will likely undergo a close approach in the next 4$-$5 Gyr. We used direct $N$-body simulations to model this interaction to shed light on the future of the Milky Way - Andromeda system and for the first time explore the fate of the two supermassive black holes (SMBHs) that are located at their centers. We investigated how the uncertainties on the relative motion of the two galaxies, linked with the initial velocities and the density of the diffuse environment in which they move, affect the estimate of the time they need to merge and form ``Milkomeda. After the galaxy merger, we follow the evolution of their two SMBHs up to their close pairing and fusion. Upon the fiducial set of parameters, we find that Milky Way and Andromeda will have their closest approach in the next 4.3 Gyr and merge over a span of 10 Gyr. Although the time of the first encounter is consistent with other predictions, we find that the merger occurs later than previously estimated. We also show that the two SMBHs will spiral in the inner region of Milkomeda and coalesce in less than 16.6 Myr after the merger of the two galaxies. Finally, we evaluate the gravitational-wave emission caused by the inspiral of the SMBHs, and we discuss the detectability of similar SMBH mergers in the nearby Universe ($zleq 2$) through next-generation gravitational-wave detectors.
We study the Extended Chandra Deep Field South (E-CDFS) Very Large Array sample, which reaches a flux density limit at 1.4 GHz of 32.5 microJy at the field centre and redshift ~ 4, and covers ~ 0.3 deg^2. Number counts are presented for the whole sample while the evolutionary properties and luminosity functions are derived for active galactic nuclei (AGN). The faint radio sky contains two totally distinct AGN populations, characterised by very different evolutions, luminosity functions, and Eddington ratios: radio-quiet (RQ)/radiative-mode, and radio-loud/jet-mode AGN. The radio power of RQ AGN evolves ~ (1+z)^2.5, similarly to star-forming galaxies, while the number density of radio-loud ones has a peak at ~ 0.5 and then declines at higher redshifts. The number density of radio-selected RQ AGN is consistent with that of X-ray selected AGN, which shows that we are sampling the same population. The unbiased fraction of radiative-mode RL AGN, derived from our own and previously published data, is a strong function of radio power, decreasing from ~ 0.5 at P_1.4GHz ~ 10^24 W/Hz to ~ 0.04$ at P_1.4GHz ~ 10^22 W/Hz. Thanks to our enlarged sample, which now includes ~ 700 radio sources, we also confirm and strengthen our previous results on the source population of the faint radio sky: star-forming galaxies start to dominate the radio sky only below ~ 0.1 mJy, which is also where radio-quiet AGN overtake radio-loud ones.
Studying our Galaxy, the Milky Way (MW), gives us a close-up view of the interplay between cosmology, dark matter, and galaxy formation. In the next decade our understanding of the MWs dynamics, stellar populations, and structure will undergo a revolution thanks to planned and proposed astrometric, spectroscopic and photometric surveys, building on recent advances by the Gaia astrometric survey. Together, these new efforts will measure three-dimensional positions and velocities and numerous chemical abundances for stars to the MWs edge and well into the Local Group, leading to a complete multidimensional view of our Galaxy. Studies of the multidimensional Milky Way beyond the Gaia frontier---from the edge of the Galactic disk to the edge of our Galaxys dark matter halo---will unlock new scientific advances across astrophysics, from constraints on dark matter to insights into galaxy formation.
We study the two main constituent galaxies of a constrained simulation of the Local Group as candidates for the Milky Way (MW) and Andromeda (M31). We focus on the formation of the stellar discs and its relation to the formation of the group as a rich system with two massive galaxies, and investigate the effects of mergers and accretion as drivers of morphological transformations. We use a state-of-the-art hydrodynamical code which includes star formation, feedback and chemical enrichment to carry out our study. We run two simulations, where we include or neglect the effects of radiation pressure from stars, to investigate the impact of this process on the morphologies and star formation rates of the simulated galaxies. We find that the simulated M31 and MW have different formation histories, even though both inhabit, at z=0, the same environment. These differences directly translate into and explain variations in their star formation rates, in-situ fractions and final morphologies. The M31 candidate has an active merger history, as a result of which its stellar disc is unable to survive unaffected until the present time. In contrast, the MW candidate has a smoother history with no major mergers at late times, and forms a disc that grows steadily; at z=0 the simulated MW has an extended, rotationally-supported disc which is dominant over the bulge. Our two feedback implementations predict similar evolution of the galaxies and their discs, although some variations are detected, the most important of which is the formation time of the discs: in the model with weaker/stronger feedback the discs form earlier/later. In summary, by comparing the formation histories of the two galaxies, we conclude that the particular merger/accretion history of a galaxy rather than its environment at the LG-scales is the main driver of the formation and subsequent growth or destruction of galaxy discs.
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