Keywords

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.

We present a photometric and spectroscopic analysis of the Hubble–Sandage variable star AF And in M31. The data have been taken under the Nainital Microlensing Survey during 1998–2002, and follow-up observations were carried out until 2011. During this period, photometric observations in Cousins R and I bands were obtained for 169 nights spanning about 5000 days. AF And showed a prominent outburst around 1999 mid-January, followed by a gradual decrease in brightness of about 1.5 mag in the next 3 yr with a declining rate of ∼0.0015 mag day⁻¹, leading to a quiescent phase at the end of 2001. After lying low for about 9 yr, AF And again went through a secondary outburst phase in late 2010 with an amplitude of 0.44 mag, where it lasted for one year before fading back to its quiescent phase. Spectroscopic observations of AF And show prominent Balmer and He i emission lines along with the comparatively weaker Fe ii and [Fe ii] emissions. Asymmetric emission line profiles in its spectrum imply a mass-loss rate of about 2.2 × 10⁻⁴ M_⊙ yr⁻¹ through the stellar winds in the photosphere. Using spectral energy distribution fitting, we find a photospheric temperature of 33,000 ± 3000 K during the visual minimum. Using a weak P Cygni profile of the He i emission line, the wind terminal velocity for AF And is found to be around 280–300 km s⁻¹.

We present the first measurements of [Fe/H] and $[\alpha /\mathrm{Fe}]$ abundances, obtained using spectral synthesis modeling, for red giant branch stars in M31's giant stellar stream (GSS). The spectroscopic observations, obtained at a projected distance of 17 kpc from M31's center, yielded 61 stars with [Fe/H] measurements, including 21 stars with $[\alpha /\mathrm{Fe}]$ measurements, from 112 targets identified as M31 stars. The [Fe/H] measurements confirm the expectation from photometric metallicity estimates that stars in this region of M31's halo are relatively metal rich compared to stars in the Milky Way's inner halo: more than half the stars in the field, including those not associated with kinematically identified substructure, have [Fe/H] abundances $\gt -1.0$. The stars in this field are α-enhanced at lower metallicities, while $[\alpha /\mathrm{Fe}]$ decreases with increasing [Fe/H] above metallicities of [Fe/H] ≳ −0.9. Three kinematical components have been previously identified in this field: the GSS, a second kinematically cold feature of unknown origin, and M31's kinematically hot halo. We compare probabilistic [Fe/H] and $[\alpha /\mathrm{Fe}]$ distribution functions for each of the components. The GSS and the second kinematically cold feature have very similar abundance distributions, while the halo component is more metal poor. Although the current sample sizes are small, a comparison of the abundances of stars in the GSS field with abundances of M31 halo and dSph stars from the literature indicate that the progenitor of the stream was likely more massive, and experienced a higher efficiency of star formation, than M31's existing dSphs or the dEs NGC 147 and NGC 185.

The Andromeda galaxy is the closest spiral galaxy to us and has been the subject of numerous studies. It harbors a massive dark matter halo, which may span up to ∼600 kpc across and comprises ∼90% of the galaxy's total mass. This halo size translates into a large diameter of 42° on the sky, for an M31–Milky Way (MW) distance of 785 kpc, but its presumably low surface brightness makes it challenging to detect with γ-ray telescopes. Using 7.6 yr of Fermi Large Area Telescope (Fermi–LAT) observations, we make a detailed study of the γ-ray emission between 1–100 GeV toward M31's outer halo, with a total field radius of 60° centered at M31, and perform an in-depth analysis of the systematic uncertainties related to the observations. We use the cosmic-ray propagation code GALPROP to construct specialized interstellar emission models to characterize the foreground γ-ray emission from the MW, including a self-consistent determination of the isotropic component. We find evidence for an extended excess that appears to be distinct from the conventional MW foreground, having a total radial extension upward of ∼120–200 kpc from the center of M31. We discuss plausible interpretations of the excess emission, but emphasize that uncertainties in the MW foreground—and in particular, modeling of the H i-related components—have not been fully explored and may impact the results.

We analyze resolved stellar near-infrared photometry of 21 Hubble Space Telescope (HST) fields in M31 to constrain the impact of metallicity on the formation of carbon stars. Observations of nearby galaxies show that carbon stars are increasingly rare at higher metallicity. Models indicate that carbon star formation efficiency drops due to the decrease in dredge-up efficiency in metal-rich thermally pulsing Asymptotic Giant Branch (TP-AGB) stars, coupled to a higher initial abundance of oxygen. However, while models predict a metallicity ceiling above which carbon stars cannot form, previous observations have not yet pinpointed this limit. Our new observations reliably separate carbon stars from M-type TP-AGB stars across 2.6–13.7 kpc of M31's metal-rich disk using HST WFC3/IR medium-band filters. We find that the ratio of C to M stars (C/M) decreases more rapidly than extrapolations of observations in more metal-poor galaxies, resulting in a C/M that is too low by more than a factor of 10 in the innermost fields and indicating a dramatic decline in C star formation efficiency at metallicities higher than [M/H] ≈ −0.1 dex. The metallicity ceiling remains undetected, but must occur at metallicities higher than what is measured in M31's inner disk ([M/H] ≳ +0.06 dex).

As galaxies evolve, they must enrich and exchange gas with the surrounding medium, but the timing of these processes and how much gas is involved remain poorly understood. In this work, we leverage metals as tracers of past gas flows to constrain the history of metal ejection and redistribution in M31. This roughly L* galaxy is a unique case where spatially resolved measurements of the gas-phase and stellar metallicity, dust extinction, and neutral interstellar gas content are all available, enabling a census of the current metal mass. We combine spatially resolved star formation histories from the Panchromatic Hubble Andromeda Treasury survey with a metal production model to calculate the history of metal production in M31. We find that 1.8 × 10⁹ M_⊙ of metals, or 62% of the metal mass formed within r < 19 kpc, is missing from the disk in our fiducial model, implying that the M31 disk has experienced significant gaseous outflows over its lifetime. Under a conservative range of model assumptions, we find that between 3% and 88% of metals have been lost (1.9 × 10⁷–6.4 × 10⁹ M_⊙), which means that metals are missing even when all model parameters are chosen to favor metal retention. We show that the missing metal mass could be harbored in the circumgalactic medium of M31 if the majority of the metals reside in a hot gas phase. Finally, we find that some metal mass produced in the past 1.5 Gyr in the central ∼5 kpc has likely been redistributed to larger radii within the disk.

We have observed the Andromeda galaxy, Messier 31 (M31), at 6.7 GHz with the Sardinia Radio Telescope. We mapped the radio emission in the C-band, re-analyzed WMAP and Planck maps, as well as other ancillary data, and we have derived an overall integrated flux density spectrum from the radio to the infrared. This allowed us to estimate the emission budget from M31. Integrating over the whole galaxy, we found strong and highly significant evidence for anomalous microwave emission (AME), at the level of ${1.45}_{-0.19}^{+0.17}$ Jy at the peaking frequency of ≃25 GHz. Decomposing the spectrum into known emission mechanisms such as free–free, synchrotron, thermal dust, and AME arising from electric dipole emission from rapidly rotating dust grains, we found that the overall emission from M31 is dominated, at frequencies below 10 GHz, by synchrotron emission with a spectral index of $-{1.10}_{-0.08}^{+0.10}$, with subdominant free–free emission. At frequencies ≳10 GHz, AME has a similar intensity to that of synchrotron and free–free emission, overtaking them between 20 and 50 GHz, whereas thermal dust emission dominates the emission budget at frequencies above 60 GHz, as expected.

We calibrate commonly used star formation rate (SFR) prescriptions using observations in five kiloparsec-sized fields in the nearby galaxy Andromeda (M31) at 10 pc spatial resolution. Our observations at different scales enable us to resolve the star-forming regions and to distinguish them from non-star-forming components. We use extinction-corrected Hα from optical integral field spectroscopy as our reference tracer and have verified its reliability via tests. It is used to calibrate monochromatic and hybrid (Hα+a×IR and far-UV+b×IR) SFR prescriptions, which use far-UV (GALEX), 22 μm (Wide-field Infrared Survey Explorer), and 24 μm (MIPS). Additionally, we evaluate other multiwavelength infrared tracers. Our results indicate that the SFR prescriptions do not change (in M31) with spatial scales or with subtraction of the diffuse component. For the calibration factors in the hybrid SFR prescriptions, we find a ≈ 0.2 and b ≈ 22 in M31, which are a factor of 5 higher than in the literature. As the fields in M31 exhibit high attenuation and low dust temperatures, lie at large galactocentric distances, and suffer from high galactic inclination compared to measurements in other galaxies, we propose that the fields probe a dust layer extended along the line of sight that is not directly spatially associated with star-forming regions. This (vertically) extended dust component increases the attenuation and alters the SFR prescriptions in M31 compared to literature measurements. We recommend that SFR prescriptions should be applied with caution at large galactocentric distances and in highly inclined galaxies, due to variations in the relative (vertical) distribution of dust and gas.

We analyze the kinematics of Andromeda's disk as a function of stellar age by using photometry from the Panchromatic Hubble Andromeda Treasury survey and spectroscopy from the Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo survey. We use H i 21 cm and CO ($J=1\to 0$) data to examine the difference between the deprojected rotation velocity of the gas and that of the stars. We divide the stars into four stellar age bins, from shortest lived to longest lived: massive main-sequence stars (0.03 Gyr), more luminous intermediate-mass asymptotic giant branch (AGB) stars (0.4 Gyr), less luminous intermediate-mass AGB stars (2 Gyr), and low-mass red giant branch stars (4 Gyr). There is a clear correlation between the offset of the stellar and the gas rotation velocity, or the asymmetric drift: the longer-lived populations lag farther behind the gas than short-lived populations. We also examine possible causes of the substructure in the rotation curves (RCs) and find that the most significant cause of scatter in the RCs comes from the tilted ring model being an imperfect way to account for the multiple warps in Andromeda's disk.

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).