Bulge Of M31 Research Articles

A Chandra search for periodic X-ray sources in the bulge of M31

ABSTRACT We present a systematic search for periodic X-ray sources in the bulge of M31, using ∼2 Ms of archival Chandra observations spanning a temporal baseline of 16 yr. Utilizing the Gregory–Loredo algorithm that is designed for photon-counting, phase-folded light curves, we detect seven periodic X-ray sources, among which four are newly discovered. Three of these sources are novae, the identified periods of which range between 1.3 and 2.0 h and are most likely the orbital period. The other four sources are low-mass X-ray binaries, the identified periods of which range between 0.13 and 19.3 h and are also likely orbital due to a clear eclipsing/dipping behaviour in the light curve. We address implications on the X-ray binaries population of the M31 bulge. Our study demonstrates the potential of using archival X-ray observations to systematically identify periodic X-ray sources in external galaxies, which would provide valuable information about the underlying exotic stellar populations.

Radial gradients of CO absorptions and abundance ratios in the bulge of M31

We present new H- and K-band spectroscopy for the bulge of M31, taken with the LUCI spectrograph at the Large Binocular Telescope (LBT). We studied radial trends of CO absorption features (namely and ) in the bulge of M31, out to a galactocentric distance of $ 380$ pc). We find that most COs do not exhibit a strong radial gradient, despite the strong metallicity gradient inferred from the optical spectral range, except for showing a steep increase in the center. We compared the observed line strengths to predictions of different state-of-the-art stellar population models, including an updated version of EMILES models, which also uses the extended IRTF spectral library. The observed COs are close to models' predictions, but in some models they turn out to be underestimated. We find that the lack of radial gradients is due to the combination of increasing CO strength with metallicity and C abundance, and decreasing CO strength with IMF slope and O abundance. We speculate that the steep gradient of might be due to Na overabundance. Remarkably, we were able to fit, at the same time, optical indices and all the NIR COs (except for CO1.68) leaving abundance ratios (i.e. and ) as free-fitting parameters, imposing age and metallicity constraints from the optical spectral range with no significant contribution from intermediate-age populations ($ 1$ Gyr-old). For the majority of the bulge, we find 0.15$ dex larger than (by $ 0.1$ dex), and C abundance consistent with that of Mg. In the central (few arcsec) region, we still find an enhancement of O and Mg, but significantly lower . We find that the COs' line strengths of the bulge are significantly lower than those of massive galaxies, possibly because of a difference in carbon abundance, as well as, to some extent, total metallicity.

Bar-driven Gas Dynamics of M31

The large-scale gaseous shocks in the bulge of M31 can be naturally explained by a rotating stellar bar. We use gas dynamical models to provide an independent measurement of the bar pattern speed in M31. The gravitational potentials of our simulations are from a set of made-to-measure models constrained by stellar photometry and kinematics. If the inclination of the gas disk is fixed at i = 77°, we find that a low pattern speed of 16–20 km s−1 kpc−1 is needed to match the observed position and amplitude of the shock features, as shock positions are too close to the bar major axis in high Ω b models. The pattern speed can increase to 20–30 km s−1 kpc−1 if the inner gas disk has a slightly smaller inclination angle compared with the outer one. Including subgrid physics such as star formation and stellar feedback has minor effects on the shock amplitude, and does not change the shock position significantly. If the inner gas disk is allowed to follow a varying inclination similar to the H i and ionized gas observations, the gas models with a pattern speed of 38 km s−1 kpc−1, which is consistent with stellar-dynamical models, can match both the shock features and the central gas features.

The rotation curve and mass distribution of M31

ABSTRACT To gain a better understanding of the Andromeda galaxy M31 and its role in the Local Group, measuring its mass precisely is essential. In this work, we have constructed the rotation curve of M31 out to ∼125 kpc using 13 679 M31 objects obtained from various sources, including the LAMOST data release 9 (LAMOST DR9), the DESI survey, and relevant literature. We divide all objects in our sample into bulge, disc, and halo components. For the sources in the M31 disc, we have measured their circular velocities by a kinematic model with asymmetric drift corrections. For the bulge and halo objects, we calculate their velocity dispersions and use the spherical and projected Jeans equation to obtain the circular velocities. Our findings indicate a nearly isotropic nature for the M31 bulge, while the halo exhibits tangential anisotropy. The results show that the rotation curve remains constant at ∼220 km s−1 up to radius ∼25 kpc and gradually decreases to ∼170 km s−1 further out. Based on the newly determined rotation curve, we have constructed a mass distribution model for M31. Our measurement of the M31 virial mass is $M_{\rm vir} = 1.14^{+0.51}_{-0.35} \times 10^{12} M_\odot$ within rvir = 220 ± 25 kpc.

Prospects for annihilating dark matter from M31 and M33 observations with the Cherenkov Telescope Array

M31 and M33 are the closest spiral galaxies and the largest members (together with the Milky Way) of the Local group, which makes them interesting targets for indirect dark matter searches. In this paper we present studies of the expected sensitivity of the Cherenkov Telescope Array (CTA) to an annihilation signal from weakly interacting massive particles from M31 and M33. We show that a 100 h long observation campaign will allow CTA to probe annihilation cross-sections up to 〈συ〉 ≈ 5·10-25 cm3 s-1 for the τ + τ - annihilation channel (for M31, at a DM mass of 0.3 TeV), improving the current limits derived by HAWC by up to an order of magnitude. We present an estimate of the expected CTA sensitivity, by also taking into account the contributions of the astrophysical background and other possible sources of systematic uncertainty. We also show that CTA might be able to detect the extended emission from the bulge of M31, detected at lower energies by the Fermi/LAT.

The Chemodynamics of the Stellar Populations in M31 from APOGEE Integrated-light Spectroscopy

We present an analysis of nearly 1000 near-infrared, integrated-light spectra from APOGEE in the inner ∼7 kpc of M31. We utilize full-spectrum fitting with A-LIST simple stellar population spectral templates that represent a population of stars with the same age, [M/H], and [α/M]. With this, we determine the mean kinematics, metallicities, α abundances, and ages of the stellar populations of M31's bar, bulge, and inner disk (∼4–7 kpc). We find a nonaxisymmetric velocity field in M31 resulting from the presence of a bar. The bulge of M31 is less metal-rich (mean [M/H] = dex) than the disk, features minima in metallicity on either side of the bar ([M/H] ∼ −0.2), and is enhanced in α abundance (mean [α/M] = ). The disk of M31 within ∼7 kpc is enhanced in both metallicity ([M/H] = ) and α abundance ([α/M] = ). Both of these structural components are uniformly old at ≃12 Gyr. We find the mean metallicity increases with distance from the center of M31, with the steepest gradient along the disk major axis (0.043 ± 0.021 dex kpc−1). This gradient is the result of changing light contributions from the bulge and disk. The chemodynamics of stellar populations encodes information about a galaxy’s chemical enrichment, star formation history, and merger history, allowing us to discuss new constraints on M31's formation. Our results provide a stepping stone between our understanding of the Milky Way and other external galaxies.

Observational signatures of carbon–oxygen white dwarf merger remnants

ABSTRACT Many double white dwarf (WD) mergers likely do not lead to a prompt thermonuclear explosion. We investigate the prospects for observationally detecting the surviving remnants of such mergers, focusing on the case of mergers of double Carbon–Oxygen WDs. For ∼104 yr, the merger remnant is observationally similar to an extreme AGB star evolving to become a massive WD. Identifying merger remnants is thus easiest in galaxies with high-stellar masses (high WD merger rate) and low star formation rates (low birth rate of ∼6–10 M⊙ stars). Photometrically identifying merger remnants is challenging even in these cases because the merger remnants appear similar to He stars and post-outburst classical novae. We propose that the most promising technique for discovering WD merger remnants is through their unusual surrounding photoionized nebulae. We use CLOUDY photoionization calculations to investigate their unique spectral features. Merger remnants should produce weak hydrogen lines, strong carbon and oxygen recombination, and fine-structure lines in the UV, optical and IR. With narrow-band imaging or integral field spectrographs, we predict that multiple candidates are detectable in the bulge of M31, the outskirts of M87 and other nearby massive galaxies, and the Milky Way. Our models roughly reproduce the WISE nebula surrounding the Galactic WD merger candidate IRAS 00500+6713; we predict detectable [Ne vi] and [Mg vii] lines with JWST but that the mid-IR WISE emission is dominated by dust not fine-structure lines.

The Complex Structure of the Bulge of M31

The bulge of M31 is of interest regarding the nature of galactic bulges and how their structure relates to bulge formation mechanisms and their subsequent evolution. With the UVIT instrument on AstroSat, we have observed the bulge of M31 in five far-ultraviolet (FUV) and near-ultraviolet (NUV) filters at 1″ spatial resolution. Models for the luminosity distribution of the bulge are constructed using the UVIT data and the galaxy image fitting algorithm GALFIT. We fit the bulge without the nuclear region with a Sérsic function for the five images and find Sérsic indices (≃2.1–2.5) similar to previous studies but smaller R e values (≃0.5–0.6 kpc). When fitting the images including the nuclear region, a multicomponent model is used. We use an eight-component model for the FUV 148 nm image, which has the highest sensitivity. The other images (169–279 nm) are fit with four-component models. The dust lanes in the bulge region are recovered in the residual images, which have subtraction of the bright bulge light using the multicomponent models. The dust lanes show that M31's nuclear spiral is visible in absorption at NUV and FUV wavelengths. The bulge images show boxy contours in all five UVIT wave bands, which is confirmed by fitting using GALFIT. The Sérsic indices of ∼2.1–2.5 are intermediate between the expected values for a classical bulge and for a pseudobulge. The boxiness of the bulge provides further evidence that M31's bulge has contributions from a classical bulge and a pseudobulge.

AstroSat/UVIT Cluster Photometry in the Northern Disk of M31

The Andromeda galaxy (M31) is an object of ongoing study with the Ultraviolet Imaging Telescope (UVIT) on AstroSat. UVIT far-UV (FUV) and near-UV (NUV) photometry is carried out here for a set of 239 clusters in the NE disk and bulge of M31 that overlap with the HST/PHAT survey. Padova stellar models were applied to derive ages, masses, metallicities, and extinctions for 170 clusters. The ages show a narrow peak at ∼4 Myr and a broad peak around 100 Myr. log(Z/Z ⊙) values are mostly between −0.3 and +0.3. The 7 clusters in the bulge have a low metallicity and high mass. Most clusters are in the spiral arms and have metallicities in the range noted above. The youngest clusters mostly have high metallicity and are concentrated along the brightest parts of the spiral arms. The UVIT FUV and NUV data are sensitive to young stars and detect a new metal-rich peak in star formation in the disk at age ∼4 Myr.

The Andromeda gamma-ray excess: background systematics of the millisecond pulsars and dark matter interpretations

ABSTRACT Since the discovery of an excess in gamma rays in the direction of M31, its cause has been unclear. Published interpretations focus on dark matter or stellar related origins. Studies of a similar excess in the Milky Way centre motivate a correlation of the spatial morphology of the signal with the distribution of stellar mass in M31. However, a robust determination of the best theory for the observed excess emission is challenging due to uncertainties in the astrophysical gamma-ray foreground model. We perform a spectro-morphological analysis of the M31 gamma-ray excess using state-of-the-art templates for the distribution of stellar mass in M31 and novel astrophysical foreground models for its sky region. We construct maps for the old stellar populations of M31 based on data from the PAndAS survey and carefully remove the foreground stars. We also produce improved astrophysical foreground models via novel image inpainting techniques based on machine learning methods. Our stellar maps, mimicking the location of a population of millisecond pulsars in the bulge of M31, reach a 5.4σ significance, making them as strongly favoured as the simple phenomenological models usually considered in the literature, e.g. disc-like templates. This detection is robust to generous variations of the astrophysical foreground model. Once the stellar templates are included in the astrophysical model, we show that the dark matter annihilation interpretation of the signal is unwarranted. We demonstrate that about one million unresolved millisecond pulsars naturally explain the observed gamma-ray luminosity per stellar mass, energy spectrum, and stellar bulge-to-disc flux ratio.

Far-ultraviolet to FIR Spectral-energy Distribution Modeling of the Stellar Formation History of the M31 Bulge

Abstract M31 is being surveyed at far- and near-ultraviolet with the UVIT telescope on AstroSat. The central bulge of M31 was observed in the N279N (275–280 nm), N219M (200–240 nm), F172M (160–185 nm), F169M (145–175 nm), and F148W (120–180 nm) filters. These images are made publicly available here. The UVIT data are supplemented with Sloan Digital Sky Survey data in optical, Spitzer data in near-infrared and Herschel data in mid- and far-infrared. The resulting far-ultraviolet to IR spectral-energy distributions for the bulge and for 10 subregions, are modeled using combinations of simple stellar populations and with CIGALE models. We find a dominant old (10–12 Gyr) metal-rich ([Z/H] ∼ 0.3) population and a younger (600 Myr) solar abundance ([Z/H] ∼ 0) population throughout the bulge. For the innermost 120″ we find an additional very young (25 Myr) metal-poor ([Z/H] ∼ −0.7) population. The results are consistent with the most recent stellar population studies of the bulge, which find the two populations for the whole bulge and a third young population in the innermost bulge.

Gamma-ray image reconstruction of the Andromeda galaxy

We analyze about 12 years of Fermi-LAT data in the direction of the Andromeda galaxy (M31). We robustly characterize its spectral and morphological properties against systematic uncertainties related to the modeling of the Galactic diffuse emission. We perform this work by adapting and exploiting the potential of the SkyFACT adaptive template fitting algorithm. We reconstruct the γ-ray image of M31 in a template-independent way, and we show that flat spatial models are preferred by data, indicating an extension of the γ-ray emission of about 0.3 — 0.4° for the bulge of M31. This study also suggests that a second component, extending to at least 1°, contributes to the observed total emission. We quantify systematic uncertainties related to mis-modeling of Galactic foreground emission at the level of 2.9%.

Excitation of Polycyclic Aromatic Hydrocarbon Emission: Dependence on Size Distribution, Ionization, and Starlight Spectrum and Intensity

Abstract Using physical models, we study the sensitivity of polycyclic aromatic hydrocarbon (PAH) emission spectra to the character of the illuminating starlight, to the PAH size distribution, and to the PAH charge distribution. The starlight models considered range from the emission from a 3 Myr old starburst, rich in far-ultraviolet (FUV) radiation, to the FUV-poor spectrum of the very old population of the M31 bulge. A wide range of starlight intensities is considered. The effects of reddening in dusty clouds are investigated for different starlight spectra. For a fixed PAH abundance parameter q PAH (the fraction of the total dust mass in PAHs with <103 C atoms), the fraction of the infrared power appearing in the PAH emission features can vary by a factor of two as the starlight spectrum varies from FUV-poor (M31 bulge) to FUV-rich (young starburst). We show how q PAH can be measured from the strength of the 7.7 μm emission. The fractional power in the 17 μm feature can be suppressed by high starlight intensities.

Mild radial variations of the stellar IMF in the bulge of M31

ABSTRACT Using new, homogeneous, long-slit spectroscopy in the wavelength range from ∼0.35 to $\sim 1 \, \mu$m, we study radial gradients of optical and near-infrared (NIR) initial mass function (IMF)-sensitive features along the major axis of the bulge of M31, out to a galactocentric distance of ∼200 arcsec (∼800 pc). Based on state-of-the-art stellar population synthesis models with varying Na abundance ratio, we fit a number of spectral indices, from different chemical species (including TiO’s, Ca, and Na indices), to constrain the low-mass (≲0.5 M⊙) end slope (i.e. the fraction of low-mass stars) of the stellar IMF, as a function of galactocentric distance. Outside a radial distance of ∼10 arcsec, we infer an IMF similar to a Milky Way-like distribution, while at small galactocentric distances, an IMF radial gradient is detected, with a mildly bottom-heavy IMF in the few inner arcsec. We are able to fit Na features (both NaD and $\rm Na\,{\small I}8190$), without requiring extremely high Na abundance ratios. $\rm [Na/Fe]$ is ∼0.4 dex for most of the bulge, rising up to ∼0.6 dex in the innermost radial bins. Our results imply an overall, luminosity-weighted, IMF and mass-to-light ratio for the M31 bulge, consistent with those for a Milky Way-like distribution, in contrast to results obtained, in general, for most massive early-type galaxies.

Gamma-ray image reconstruction of the Andromeda galaxy

We analyze about 12 years of Fermi-LAT data in the direction of the Andromeda galaxy (M31). We robustly characterize its spectral and morphological properties against systematic uncertainties related to the modeling of the Galactic diffuse emission. We perform this work by adapting and exploiting the potential of the skyFACT adaptive template fitting algorithm. We reconstruct the gamma-ray image of M31 in a template-independent way, and we show that flat spatial models are preferred by data, indicating an extension of the $\gamma$-ray emission of about 0.3-0.4 degree for the bulge of M31. This study also suggests that a second component, extending to at least 1 degree, contributes to the observed total emission. We quantify systematic uncertainties related to mis-modeling of Galactic foreground emission at the level of 2.9%.

ASTROSAT/UVIT Near and Far Ultraviolet Properties of the M31 Bulge

AstroSat has surveyed M31 with the UVIT telescope from 2017 to 2019. The central bulge of M31 was observed in 2750 - 2850 A, 2000 - 2400 A, 1600 - 1850 A, 1450 - 1750 A, and 1200 - 1800 A filters. A radial profile analysis, averaged along elliptical contours which approximate the bulge shape, was carried out in each filter. The profiles follow a Sersic function with an excess for the inner ~8" in all filters, or can be fitted with two Sersic functions (including the excess). The ultraviolet colours of the bulge are found to change systematically with radius, with the center of the bulge bluer (hotter). We fit the UVIT spectral energy distributions (SEDs) for the whole bulge and for 10 elliptical annuli with single stellar population (SSP) models. A combination of two SSPs fits the UVIT SEDs much better than one SSP, and three SSPs fit the data best. The properties of the three SSPs are age, metallicity (Z) and mass of each SSP. The best fit model is a dominant old, metal-poor (1010 yr, , with the solar metallicity) population plus a 15% contribution from an intermediate (109.5 yr, ) population plus a small contribution (~2%) from a young high-metallicity (108.5 yr, ) population. The results are consistent with previous studies of M31 in optical: both reveal an active merger history for M31.

Measuring Star Formation Histories, Distances, and Metallicities with Pixel Color–Magnitude Diagrams. II. Applications to Nearby Elliptical Galaxies

Abstract We present spatially resolved measurements of star formation histories (SFHs), metallicities, and distances in three nearby elliptical galaxies and the bulge of M31 derived using the pixel color–magnitude diagram (pCMD) technique. We compute pCMDs from archival Hubble Space Telescope photometry of M87, M49, NGC 3377, and M31, and we fit the data using the new code PCMDPy. We measure distances to each system that are accurate to ∼10%. The recovered nonparametric SFHs place reasonable (±1 dex) constraints on the recent (<2 Gyr) star formation in M31 and NGC 3377, both of which show evidence of inside-out growth. The SFHs in M87 and M49 are constrained only at the oldest ages. The pCMD technique is a promising new avenue for studying the evolutionary history of the nearby universe and is highly complementary to existing stellar population modeling techniques.

XMM-Newton RGS Spectroscopy of the M31 Bulge. I. Evidence for a Past AGN Half a Million Years Ago

Abstract Existing analysis based on spectra from the Reflection Grating Spectrometer (RGS) on board XMM-Newton already shows that the G-ratio of the O vii Heα triplet in the inner bulge of M31 is too high to be consistent with a pure optically thin thermal plasma in collisional ionization equilibrium (CIE). Different processes that may affect properties of diffuse hot plasma were proposed, such as resonance scattering (RS) and charge exchange (CX) with cold gas. To determine which physical process(es) may be responsible for this inconsistency, we present a systematic spectroscopic analysis based on 0.8 Ms XMM-Newton/RGS data, together with complementary Chandra/ACIS-S images. The combination of these data enables us to reveal multiple non-CIE spectroscopic diagnostics, including but not limited to the large G-ratios of Heα triplets (O vii, N vi, and Ne ix) and the high Lyman series line ratios (O viii Lyβ/Lyα and Lyγ/Lyα, and N vii Lyβ/Lyα), which are not expected for a CIE plasma, and the high iron line ratios (Fe xviii 14.2 Å/Fe xvii 17 Å and Fe xvii 15 Å/17 Å), which suggest much higher temperatures than other line ratios, as well as their spatial variations. Neither CX nor RS explains all these spectroscopic diagnostics satisfactorily. Alternatively, we find that an active galactic nucleus (AGN) relic scenario provides a plausible explanation for virtually all the signatures. We estimate that an AGN was present at the center of M31 about half a million years ago and that the initial ionization parameter ξ of the relic plasma is in the range of 3–4.

Searching for GeV gamma-ray emission from the bulge of M31

The three major large-scale, diffuse γ-ray structures of the Milky Way are the Galactic disk, a bulge-like GeV excess towards the Galactic center, and the Fermi bubble. Whether such structures can also be present in other normal galaxies remains an open question. M31, as the nearest massive normal galaxy, holds promise for spatially-resolving the γ-ray emission. Based on more than 8 years of Fermi-LAT observations, we use (1) disk, (2) bulge, and (3) disk-plus-bulge templates to model the spatial distribution of the γ-ray emission from M31. Among these, the disk-plus-bulge template delivers the best-fit, in which the bulge component has a TS value 25.7 and a photon-index of 2.57 ± 0.17, providing strong evidence for a centrally-concentrated γ-ray emission from M31, that is analogous to the Galactic center excess. The total 0.2–300 GeV γ-ray luminosity from this bulge component is (1.16 ± 0.14) × 1038 erg s−1, which would require ∼ 1.5 × 105 millisecond pulsars, if they were the dominant source. We also search for a Fermi bubble-like structure in M31 using the full dataset (pass8), but no significant evidence has been found. In addition, a likelihood analysis using only photons with the most accurate reconstructed direction (i.e., PSF3-only data) reveals a 4.8 σ point-like source located at ∼10 kpc to the northwest of the M31 disk, with a luminosity of (0.97 ± 0.27) × 1038 erg s−1 and a photon-index of 2.31 ± 0.18. Lacking a counterpart on the southeast side of the disk, the relation between this point-like source and a bubble-like structure remains elusive.

ASTROSAT/UVIT Survey of M31, First Results: UV-bright Stars in the Bulge

Abstract A survey of M31 is being carried out with the ASTROSAT Observatory in ultraviolet (UV), soft X-rays, and hard X-rays using the UltraViolet Imaging Telescope (UVIT), soft X-ray telescope, LAXPC, and CZT instruments. Here we analyze UVIT observations of the first field covering the central (7′) part of the bulge of M31. The UV bands observed cover the range 120–280 nm with four filters with a spatial resolution of 1″. They are sensitive to the UV emission from hot stars (T eff > 10000 K), so can be used to detect the hottest stars in the bulge of M31. We cross-match the UVIT sources with sources from Panchromatic Hubble Andromeda Treasury to obtain multi-band spectral energy distributions (SEDs) for 26 sources. Fits of the SEDs with stellar models show that these are likely main-sequence stars with masses in the range 5–20 M ⊙. This provides evidence for a low, but significant (∼10−5 M ⊙ yr−1), rate of star formation in the bulge of M31, which has occurred in the very recent past (∼10–100 million years).