Local Group Galaxies M31 Research Articles

Detection Prospects of Fast-merging Gravitational Wave Sources in M31

It is widely accepted that quite a number of double compact objects (DCOs) in the Milky Way can be identified by future space-based gravitational wave (GW) detectors, while systematic investigations on the detection of the GW sources in nearby galaxies are still lacking. In this paper, we present calculations of potential populations of GW sources for all types of DCOs in the Local Group galaxy M31. For M31, we use an age-dependent model for the evolution of the metallicity and the star formation rate. By varying assumptions of common-envelope ejection efficiencies and supernova-explosion mechanisms during binary evolution, we make predictions on the properties of DCOs that can be detected by the Laser Interferometer Space Antenna (LISA). Our calculations indicate that a few (a dozen) DCOs are likely to be observed by LISA during its 4 (10) yr mission. We expect that the sources with black hole components are more likely to be first identified during a 4 yr mission since these binaries have relatively large chirp masses, while the systems with white-dwarf components dominate the overall population of detectable GW sources during a 10 yr mission. LISA can only detect very tight fast-merging systems in M31, corresponding to the peak of orbital period distribution from ∼2 minutes for double white dwarfs to ∼20 minutes for double black holes.

The Red Supergiant Binary Fraction as a Function of Metallicity in M31 and M33

Abstract Recent work measuring the binary fraction of evolved red supergiants (RSGs) in the Magellanic Clouds points to a value between 15% and 30%, with the majority of the companions being unevolved B-type stars as dictated by stellar evolution. Here I extend this research to the Local Group galaxies M31 and M33 and investigate the RSG binary fraction as a function of metallicity. Recent near-IR photometric surveys of M31 and M33 have led to the identification of a complete sample of RSGs down to a limiting . To determine the binary fraction of these M31 and M33 RSGs, I used a combination of newly obtained spectroscopy to identify single RSGs and RSG+OB binaries, as well as archival UV, visible, and near-IR photometry to probabilistically classify RSGs as either single or binary based on their colors. I then adjusted the observed RSG+OB binary fraction to account for observational biases. The resulting RSG binary fraction in M33 shows a strong dependence on galactocentric distance, with the inner regions having a much higher binary fraction ( ) than the outer regions ( ). Such a trend is not seen in M31; instead, the binary fraction in lightly reddened regions remains constant at . I conclude that the changing RSG binary fraction in M33 is due to a metallicity dependence, with higher-metallicity environments having higher RSG binary fractions. This dependence most likely stems not from changes in the physical properties of RSGs due to metallicity but from changes in the parent distribution of OB binaries.

The WISE Extended Source Catalog (WXSC). I. The 100 Largest Galaxies

Abstract We present mid-infrared photometry and measured global properties of the 100 largest galaxies in the sky, including the well-studied Magellanic Clouds, Local Group galaxies M31 and M33, the Fornax and Virgo galaxy cluster giants, and many of the most spectacular Messier objects (e.g., M51 and M83). This is the first release of a larger catalog of extended sources as imaged in the mid-infrared, called the Wide-field Infrared Survey Explorer (WISE) Extended Source Catalog (WXSC). In this study, we measure their global attributes, including integrated flux, surface brightness, and radial distribution. The largest of the large are the LMC, SMC, and Andromeda galaxy, which are also the brightest mid-infrared galaxies in the sky. We interrogate the large galaxies using WISE colors, which serve as proxies for four general types of galaxies: bulge-dominated spheroidals, intermediate semi-quiescent disks, star-forming (SF) spirals, and AGN-dominated. The colors reveal a tight “sequence” that spans 5 mag in W2–W3 color, ranging from early to late types and low to high SF activity; we fit the functional form given by . Departures from this sequence may reveal nuclear, starburst, and merging events. Physical properties and luminosity attributes are computed, notably the diameter, aggregate stellar mass, and dust-obscured star formation activity. To effectively study and compare these galaxy characteristics, we introduce the “pinwheel” diagram, which depicts physical properties with respect to the median value observed for WISE galaxies in the local universe. Utilized with the WXSC, this diagram will delineate between different kinds of galaxies, identifying those with similar star formation and structural properties. Finally, we present the mid-infrared photometry of the 25 brightest globular clusters in the sky, of which many are also the largest and brightest objects orbiting the Milky Way, including Omega Centauri, 47 Tucanae, and a number of famed night-sky targets (e.g., M13).

A unified model for the maximum mass scales of molecular clouds, stellar clusters and high-redshift clumps

We present a simple, self-consistent model to predict the maximum masses of giant molecular clouds (GMCs), stellar clusters and high-redshift clumps as a function of the galactic environment. Recent works have proposed that these maximum masses are set by shearing motions and centrifugal forces, but we show that this idea is inconsistent with the low masses observed across an important range of local-Universe environments, such as low-surface density galaxies and galaxy outskirts. Instead, we propose that feedback from young stars can disrupt clouds before the global collapse of the shear-limited area is completed. We develop a shear–feedback hybrid model that depends on three observable quantities: the gas surface density, the epicylic frequency and the Toomre parameter. The model is tested in four galactic environments: the Milky Way, the Local Group galaxy M31, the spiral galaxy M83 and the high-redshift galaxy zC406690. We demonstrate that our model simultaneously reproduces the observed maximum masses of GMCs, clumps and clusters in each of these environments. We find that clouds and clusters in M31 and in the Milky Way are feedback-limited beyond radii of 8.4 and 4 kpc, respectively, whereas the masses in M83 and zC406690 are shear-limited at all radii. In zC406690, the maximum cluster masses decrease further due to their inspiral by dynamical friction. These results illustrate that the maximum masses change from being shear-limited to being feedback-limited as galaxies become less gas rich and evolve towards low shear. This explains why high-redshift clumps are more massive than GMCs in the local Universe.

Observations of M31 and M33 with the Fermi Large Area Telescope: A Galactic Center Excess in Andromeda?

Abstract The Fermi Large Area Telescope (LAT) has opened the way for comparative studies of cosmic rays (CRs) and high-energy objects in the Milky Way (MW) and in other, external, star-forming galaxies. Using 2 yr of observations with the Fermi LAT, Local Group galaxy M31 was detected as a marginally extended gamma-ray source, while only an upper limit has been derived for the other nearby galaxy M33. We revisited the gamma-ray emission in the direction of M31 and M33 using more than 7 yr of LAT Pass 8 data in the energy range 0.1 – 100 GeV , presenting detailed morphological and spectral analyses. M33 remains undetected, and we computed an upper limit of 2.0 × 10 − 12 erg cm − 2 s − 1 on the 0.1 – 100 GeV energy flux (95% confidence level). This revised upper limit remains consistent with the observed correlation between gamma-ray luminosity and star formation rate tracers and implies an average CR density in M33 that is at most half of that of the MW. M31 is detected with a significance of nearly 10 σ . Its spectrum is consistent with a power law with photon index Γ = 2.4 ± 0.1 stat + syst and a 0.1 – 100 GeV energy flux of ( 5.6 ± 0.6 stat + syst ) × 10 − 12 erg cm − 2 s − 1 . M31 is detected to be extended with a 4 σ significance. The spatial distribution of the emission is consistent with a uniform-brightness disk with a radius of 0.°4 and no offset from the center of the galaxy, but nonuniform intensity distributions cannot be excluded. The flux from M31 appears confined to the inner regions of the galaxy and does not fill the disk of the galaxy or extend far from it. The gamma-ray signal is not correlated with regions rich in gas or star formation activity, which suggests that the emission is not interstellar in origin, unless the energetic particles radiating in gamma rays do not originate in recent star formation. Alternative and nonexclusive interpretations are that the emission results from a population of millisecond pulsars dispersed in the bulge and disk of M31 by disrupted globular clusters or from the decay or annihilation of dark matter particles, similar to what has been proposed to account for the so-called Galactic center excess found in Fermi-LAT observations of the MW.

SENSITIVE 21 cm OBSERVATIONS OF NEUTRAL HYDROGEN IN THE LOCAL GROUP NEAR M31

ABSTRACT Very sensitive 21 cm H i measurements have been made at several locations around the Local Group galaxy M31 using the Green Bank Telescope at an angular resolution of 9.′1, with a 5σ detection level of N H i = 3.9 × 1017 cm−2 for a 30 km s−1 line. Most of the H i in a 12 square-degree area almost equidistant between M31 and M33 is contained in nine discrete clouds that have a typical size of a few kpc and a H i mass of 105 M ⊙. Their velocities in the Local Group Standard of Rest lie between −100 and +40 km s−1, comparable to the systemic velocities of M31 and M33. The clouds appear to be isolated kinematically and spatially from each other. The total H i mass of all nine clouds is 1.4 × 106 M ⊙ for an adopted distance of 800 kpc, with perhaps another 0.2 × 106 M ⊙ in smaller clouds or more diffuse emission. The H i mass of each cloud is typically three orders of magnitude less than the dynamical (virial) mass needed to bind the cloud gravitationally. Although they have the size and H i mass of dwarf galaxies, the clouds are unlikely to be part of the satellite system of the Local Group, as they lack stars. To the north of M31, sensitive H i measurements on a coarse grid find emission that may be associated with an extension of the M31 high-velocity cloud (HVC) population to projected distances of ∼100 kpc. An extension of the M31 HVC population at a similar distance to the southeast, toward M33, is not observed.

Multi-scale radio-infrared correlations in M 31 and M 33: The role of magnetic fields and star formation

Interstellar magnetic fields and the propagation of cosmic ray electrons have an important impact on the radio-infrared (IR) correlation in galaxies. This becomes evident when studying different spatial scales within galaxies. We investigate the correlation between the infrared (IR) and free-free/synchrotron radio continuum emission at 20 cm from the two local group galaxies M31 and M33 on spatial scales between 0.4 and 10 kpc. The multi-scale radio-IR correlations have been carried out using a wavelet analysis. The free-free and IR emission are correlated on all scales, but on some scales the synchrotron emission is only marginally correlated with the IR emission. The synchrotron-IR correlation is stronger in M33 than in M31 on small scales (<1 kpc), but it is weaker than in M31 on larger scales. Taking the smallest scale on which the synchrotron-IR correlation exists as the propagation length of cosmic ray electrons, we show that the difference on small scales can be explained by the smaller propagation length in M33 than in M31. On large scales, the difference is due to the thick disk/halo in M33, which is absent in M31. A comparison of our data with data on NGC6946, the LMC and M51 suggests that the propagation length is determined by the ratio of ordered-to-turbulent magnetic field strength, which is consistent with diffusion of CR electrons in the ISM. As the diffusion length of CR electrons influences the radio-IR correlation, this dependence is a direct observational evidence of the importance of magnetic fields for the radio-IR correlation within galaxies. The star formation rate per surface area only indirectly influences the diffusion length as it increases the strength of the turbulent magnetic field.

KINEMATICS OF OUTER HALO GLOBULAR CLUSTERS IN M31

We present the first kinematic analysis of the far outer halo globular cluster (GC) population in the Local Group galaxy M31. Our sample contains 53 objects with projected radii of ~20-130 kpc, of which 44 have no previous spectroscopic information. GCs with projected radii >30 kpc are found to exhibit net rotation around the minor axis of M31, in the same sense as the inner GCs, albeit with a smaller amplitude of 79 +/-19 km/s. The rotation-corrected velocity dispersion of the full halo GC sample is 106 +/-12 km/s, which we observe to decrease with increasing projected radius. We find compelling evidence for kinematic-coherence amongst GCs which project on top of halo substructure, including a clear signature of infall for GCs lying along the North-West stream. Using the tracer mass estimator, we estimate the dynamical mass of M31 within 200 kpc to be M_M31 = (1.2-1.5) +/- 0.2 x 10^12 M_sun. This value is highly dependent on the chosen model and assumptions within.

Water masers in the Local Group of galaxies

Received; accepted Abstract. We compare the number of detected 22 GHz H2O masers in the Local Group galaxies M31, M33, NGC 6822, IC 10, IC 1613, DDO 187, GR8, NGC 185, and the Magellanic Clouds with the water maser population of the Milky Way. To accomplish this we searched for water maser emission in the two Local Group galaxies M33 and NGC 6822 using the Very Large Array (VLA) and incorporated results from previous studies. We observed 62 Hii regions in M33 and 36 regions with Hemission in NGC 6822. Detection limits are 0.0015 and 0.0008 L⊙ for M33 and NGC 6822, respectively (corresponding to 47 and 50 mJy in three channels with 0.7 km s −1 width). M33 hosts three water masers above our detection limit, while in NGC 6822 no maser source was detected. We find that the water maser detection rates in the Local Group galaxies M31, M33, NGC 6822, IC 1613, DDO 187, GR8, NGC 185, and the Magellan ic Clouds are consistent with expectations from the Galactic water masers if one considers the different star formation rates of the galaxies. However, the gal axy IC 10 exhibits an overabundance of masers, which may result from a compact central starburst.

Deep ALTAIR + NIRI Imaging of the Disk and Bulge of M31

Deep J, H, and K' images, recorded with the ALTAIR adaptive optics system and NIRI imager on Gemini North, are used to probe the stellar content of the disk and bulge of the Local Group galaxy M31. With FWHM near 0.08 arcsec in K, these are the highest angular resolution near-infrared images yet obtained of this galaxy. Four fields that sample M31 at galactocentric radii of 62, 9, 4, and 2 arcmin were observed. The RGB-tip occurs between K = 17.0 and 17.2, and the color of the RGB in the field closest to the center of M31 is consistent with that of NGC 6528. After accounting for random photometric errors, the upper RGB in each field has a width on the (K, J-K) CMD that is consistent with a +/- 0.5 dex dispersion in [Fe/H], in rough agreement with what is seen in other disk and spheroid fields in M31. A population of very bright red stars, which we identify as C stars, are seen in the three fields that are closest to the center of M31. The spatial distribution of these objects suggests that they are well mixed throughout this part of M31, and so likely did not form in a compact region near the galactic nucleus, but more probably formed in the inner disk. We speculate that these C stars may be the most luminous members of the intermediate age population that has been detected previously in studies of the integrated spectrum of the central regions of M31.

Radial velocities, metallicities, and distances of Cepheids in M31 and M33

AbstractCepheid variable stars are fundamental to the calibration of the extragalactic distance scale. We present radial velocities, preliminary metallicities, and preliminary surface brightness distances of Cepheids in Local Group galaxies M31 and M33 using data obtained with the High Resolution Spectrograph on the Hobby-Eberly Telescope at McDonald Observatory. These data are the first step toward surface brightness distance measurements and metallicity calibrations of these extragalactic Cepheids.

ITAL]HUBBLE SPACE TELESCOPE[/ITAL][ITAL]Hubble Space Telescope[/ITAL] NICMOS Imaging of the Cores of M31 and M32

We present 1.1, 1.6, and 2.2 μm images of the cores of the Local Group galaxies M31 and M32 obtained with the Near Infrared Camera and Multi-Object Spectrometer of the Hubble Space Telescope (HST). These images are also compared with optical images of the galaxies obtained with the HST Wide Field Planetary Camera 2 (WFPC2). In M31, the nucleus remains distinctly asymmetric in all of the infrared bands, with no evidence of concentrated dust, although the distinction between the two nuclei, P1 and P2, is not as strong as in the WFPC2 and earlier WF/PC-1 images. This result is nonetheless consistent with the model of the nucleus as an eccentric stellar disk produced by the dynamical influence of a central supermassive black hole. Several individual M giant stars are detected within ~3'' of the nuclei and may represent stars that have escaped from the disk and phase-mixed around the nuclear center of mass. In M32 we also do not find strong evidence of dust, and the brightness profiles within the central ~1'' of the infrared images can be fitted by a power law of the same form as that fitted to the optical images. The infrared color profiles of the central few arcseconds of M32 show no strong gradients and indicate a population dominated by K giant stars. This is consistent with recent ground-based spectroscopy and photometry of the core region, which similarly show no strong gradients in age or metallicity within its stellar population.

Wolf-Rayet stars in the Local Group galaxies M31 and NGC 6822

view Abstract Citations (54) References (51) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Wolf-Rayet stars in the Local Group galaxies M 31 and NGC 6822. Moffat, A. F. J. ; Shara, M. M. Abstract While 21 W-R star candidates have been found in about two-thirds of the giant Sb galaxy M31 by a search to continuum magnitude of B value approximately 21.5, no candidates have been found in the irregular galaxy NGC 6822. Subsequent spectrophotometry has revealed that 17 of the 21 candidates are W-R. While the number of detected W-R stars is low, it is compatible with the total number of luminous and massive stars in each galaxy. In M31, most of the W-R stars lie in the ring of prominent star formation 7-12 kpc from the center. The variation of W-R subclasses with galactocentric distance implies a metallicity gradient similar to that in the Galaxy. Publication: The Astrophysical Journal Pub Date: October 1983 DOI: 10.1086/161391 Bibcode: 1983ApJ...273..544M Keywords: Andromeda Galaxy; Galactic Clusters; Local Group (Astronomy); Star Distribution; Stellar Spectrophotometry; Wolf-Rayet Stars; Abundance; Mass To Light Ratios; Milky Way Galaxy; Stellar Evolution; Astronomy full text sources ADS | data products SIMBAD (27) NED (24)