Nucleus Of M31 Research Articles

Spectral Variability of the Nucleus of M33 in aChandra/ACIS Observation

We have analyzed a 90 ksec long observation of the bright nucleus of M33 made with Chandra/ACIS. We detected low-amplitude (~sim10%) highly significant variability on timescales of ~5000 sec. We also find associated spectral variability. The two main spectral components (a power-law with \Gamma~2 and a multicolor disk -MCD- with kT~0.9-1.2 keV) vary in relative flux. The MCD temperature also increases with increasing MCD flux. The pattern of variability is reminiscent of (but not identical to) galactic black hole binaries. A ~5 M$_{\odot}$ accreting black hole may explain this source.

TeV γ-rays and cosmic rays from the nucleus of M87, a mis-aligned BL Lac object

The unresolved nuclear region of M87 emits strong non-thermal emission from radio to X-rays. Assuming this emission to originate in the pc scale jet aligned at θ∼30° to the line of sight, we interpret this emission in the context of the synchrotron proton blazar model. We find the observed nuclear jet emission to be consistent with M87 being a mis-aligned BL Lac object and predict γ-ray emission extending up to at least 100 GeV at a level easily detectable by GLAST and MAGIC, and possibly by VERITAS depending on whether it is high-frequency or low-frequency peaked. Predicted neutrino emission is below the sensitivity of existing and planned neutrino telescopes. Ultra-high-energy neutrons produced in pion photoproduction interactions decay into protons after escaping from the host galaxy. Because energetic protons are deflected by the intergalactic magnetic field, the protons from the decay of neutrons emitted in all directions, including along the jet axis where the Doppler factor and hence emitted neutron energies are higher, can contribute to the observed ultra-high-energy cosmic rays. We consider the propagation of these cosmic ray protons to Earth and conclude that M87 could account for the observed flux if the extragalactic magnetic field topology were favourable.

Accretion onto the Supermassive Black Hole in M87

Chandra X-ray observations of the giant elliptical galaxy M87 resolve the thermal state of the hot interstellar medium into the accretion (Bondi) radius of its central 3 10^9 Msun black hole. We measure the X-ray gas temperature and density profiles and calculate the Bondi accretion rate, Mdot_Bondi \sim 0.1 Msun/yr. The X-ray luminosity of the active nucleus of M87 observed with Chandra is L_{x, 0.5-7 \keV} \sim 7 \times 10^{40}erg/s. This value is much less than the predicted nuclear luminosity, L_{Bondi} \sim 5 \times 10^{44} erg/s, for accretion at the Bondi rate with a canonical accretion radiative efficiency of 10%. If the black hole in M87 accretes at this rate it must do so at a much lower radiative efficiency than the canonical value. The multiwavelength spectrum of the nucleus is consistent with that predicted by an advection-dominated flow. However, as is likely, the X-ray nucleus is dominated by jet emission then the properties of flow must be modified, possibly by outflows. We show that the overall energetics of the system are just consistent with the predicted Bondi nuclear power. This suggests that either most of the accretion energy is released in the relativistic jet or that the central engine of M87 undergoes on-off activity cycles. We show that, at present, the energy dumped into the ISM by the jet may reduce the accretion rate onto the black hole by a factor \propto (v_j/c_s)^{-2}, where v_j is the jet velocity and c_s the ISM sound speed, and that this is sufficient to account for the low nuclear luminosity.

Star cluster formation and disruption time-scales - I. An empirical determination of the disruption time of star clusters in four galaxies

We present a new method to derive the cluster disruption time in selected regions of galaxies from the mass or age distribution of magnitude-limited cluster samples. If the disruption time of clusters in a region of a galaxy depends on their initial mass as t_4 x (M_cluster/10^4 M_sun)^gamma and if the cluster formation rate is constant, then the mass and age distributions of the observed clusters will each show two powerlaw relations. The values of t_4 and gamma can be derived from these relations. We used this method to derive the cluster disruption time in specific regions in four galaxies: the inner region of M51, a region of M33, the SMC and the solar neighbourhood. The values of gamma are the same in the four galaxies within the uncertainty and the mean value is gamma= 0.62 +- 0.06. However the disruption time t_4 of a cluster of 10^4 M_sun is very different in the different galaxies. The clusters in the SMC have the longest disruption time, t_4 = 8 Gyr, and the clusters at 1 to 3 kpc from the nucleus of M51 have the shortest disruption time of t_4 = 0.04 Gyr. The disruption time of clusters 1 to 5 kpc from the nucleus of M33 is t_4 = 0.13 Gyr and for clusters within 1 kpc from the Sun we find t_4 = 1.0 Gyr.

Emission and Absorption in the M87 LINER

The nucleus of M87 displays a LINER spectrum at optical wavelengths, with a nuclear disk of nebulosity that is resolved by the Hubble Space Telescope (HST). We present new results from optical and ultraviolet spectra of the central ~40 pc as measured by HST. In contrast with previous results for the extended disk, the emission-line spectrum of the central region is best reproduced by a multicomponent photoionization scenario, rather than shock heating. The nebular properties as well as energetic considerations suggest a transition on scales of several tens of parsecs, from predominantly photoionization by a central accretion source to shock ionization within the disk. If this source is at all representative, it suggests that many LINERs may be composite in terms of the energetic processes that give rise to the emission spectrum. We also report measurements of resonance-line absorption for the nucleus. The absorption spectrum, like the emission lines, is characterized by low ionization. The absorption-line measurements coupled with independent constraints suggest a total hydrogen column density of 1019-1020 cm-2, outflowing from the galaxy center with a velocity of ~126 km s-1. The kinematic signature of an outflow, along with evidence that the absorber covers the power-law continuum source but not the emission-line clouds, suggests that the absorbing matter is related to accretion phenomena in the nucleus. The presence of such an outflow resembles similar behavior in luminous active galactic nuclei, although the low ionization that characterizes LINERs is probably indicative of a different mode of accretion in these sources.

AChandraHigh Resolution Camera Observation of X‐Ray Point Sources in M31

We present results from a 47 ks observation of the Andromeda galaxy, M31, using the High Resolution Camera of the Chandra X-Ray Observatory. We detect 142 point sources spanning 3 orders of magnitude in luminosity, from LX = 2 × 1035 to 2 × 1038 ergs s-1 in the 0.1-10 keV band. The X-ray source location accuracy is better than 1'' in the central regions of the galaxy. One source lies within 13 of SN 1885 but does not coincide with the UV absorption feature identified as the supernova remnant. However, there is an optical transient, which is likely an optical nova, at the location of the X-ray source. There is a weak source, LX ~ 4 × 1036 ergs cm2 s-1, coincident with the nucleus of M31 and 14 sources coincident with globular clusters. Our observation has very high efficiency down to luminosities of 1.5 × 1036 ergs s-1 for sources within 5' of the nucleus. Comparing with a ROSAT observation made 11 yr earlier, we find that 0.46 ± 0.26 of the sources with LX > 5 × 1036 ergs s-1 are variable. We find no evidence for X-ray pulsars in this region, indicating that the population is likely dominated by low-mass X-ray binaries. The source density radial profile follows a power-law distribution with an exponent of 1.25 ± 0.10 and is inconsistent with the optical surface brightness profile. The X-ray point-source luminosity function is well fitted by a differential broken power law with a break at a luminosity of (4.5) × 1037 ergs s-1. The luminosity function is consistent with a model of an aging population of X-ray binaries.

Effects of wind on radiation spectra from magnetized accretion discs

The effects of a wind on the emerging spectrum from an inefficiently radiating accretion flow in a global magnetic field are examined, based on the analytic solution obtained recently by one of the present authors. The results exhibit the steepening of the negative slope appearing in the intermediate frequency range of bremsstrahlung spectrum and the decrease in the luminosity ratio of thermal synchrotron to bremsstrahlung radiation, in accordance with the increasing wind strength. Both effects are caused by a suppressed mass accretion rate in the inner disc, caused by a mass loss in terms of wind. In order to demonstrate the reliability of this model, Sagittarius A* (Sgr A*) and the nucleus of M31, both of which have been resolved in the X-ray band by Chandra, are taken up as the best candidates for the broad-band spectral fittings. Although the observed X-ray data are reproduced for these objects by both of the inverse-Compton and the bremsstrahlung fittings, some evidence of a preference for the latter is recognized. The wind effects are clearly seen in the latter fitting case, in which we can conclude that a widely extending accretion disc is present in each nucleus, with no or only weak wind in Sgr A* and with a considerably strong wind in the nuclear region of M31. Especially in Sgr A*, the inferred mass accretion rates are much smaller than the Bondi rate, the estimate of which has become reliable because of Chandra. This fact strongly suggests that the accretion in this object does not proceed in accordance with Bondi's prediction, though its extent almost reaches the Bondi radius.

Widespread HCO Emission in the Nuclear Starburst of M82

We present a high-resolution (~5'') image of the nucleus of M82 showing the presence of widespread emission of the formyl radical (HCO). The HCO map, the first obtained in an external galaxy, reveals the existence of a structured disk of ~650 pc full diameter. The HCO distribution in the plane mimics the ring morphology displayed by other molecular/ionized gas tracers in M82. More precisely, rings traced by HCO, CO, and H II regions are nested, with the HCO ring lying in the outer edge of the molecular torus. Observations of HCO in Galactic clouds indicate that the abundance of HCO is strongly enhanced in the interfaces between the ionized and molecular gas. The surprisingly high overall abundance of HCO measured in M82 [X(HCO) ~ 4 × 10-10] indicates that its nuclear disk can be viewed as a giant photon-dominated region (PDR) of ~650 pc size. The existence of various nested gas rings, with the highest HCO abundance occurring at the outer ring [X(HCO) ~ 0.8 × 10-9], suggests that PDR chemistry is propagating in the disk. We discuss the inferred large abundances of HCO in M82 in the context of a starburst evolutionary scenario, picturing the M82 nucleus as an evolved starburst.

Chandra observations of the nucleus of M33

The nearby galaxy M33 hosts the most luminous steady X-ray source in the Local Group. The high spatial resolution of Chandra allows us to confirm that this ultra-luminous X-ray source is within the nucleus and rule out at the 4.6 sigma level a previously proposed possible counterpart located away. The X-ray spectrum is well fitted by a disc blackbody with kT=1.18+-0.02 keV and R cos(theta)^(0.5}=57+-3 km, consistent with earlier results from ASCA and BeppoSAX. The source flux is steady between 1 and 1e4 s (<3% rms variability, 0.5-10 keV) with a 0.5-10 keV luminosity of 1.5e39 erg/s. The X-ray properties and an association with a radio source are reminiscent of the galactic microquasar GRS 1915+105.

Hubble Space TelescopeSpectroscopy of the Nucleus of M331

We have used the Hubble Space Telescope to obtain moderate resolution 1150-5700 A spectroscopy of the nucleus of M33 and a blue star ~1'' north-northwest of the nucleus in an attempt to find the optical counterpart of the nuclear X-ray source and to characterize stellar populations in the nuclear region. The Space Telescope Imaging Spectrograph spectra of the nucleus can be modeled in terms of two starbursts: one with a mass of about 9000 M☉ at 40 Myr and the other with a mass of about 76,000 M☉ at 1 Gyr. The blue star is a late-type O giant with no obvious spectral anomalies to indicate that it is associated with the luminous X-ray source. The nuclear region is not heavily reddened; 2200 A absorption features in the spectra of both the nucleus and the star are weak. The data and the star formation history support the hypothesis that the M33 nuclear source, the brightest persistent source in the Local Group, is an ~10 M☉ black hole binary.

Line Emission from Cooling Accretion Flows in the Nucleus of M31

The recent Chandra X-ray observations of the nucleus of M31, combined with earlier VLA radio and HST UV spectral measurements, provide the strictest constraints on the nature of accretion onto the supermassive black hole (called M31* hereafter) in this region. One of the two newly-detected sources within roughly an arcsec of M31* may be its X-ray counterpart. If not, the X-ray flux from the nucleus must be even lower than inferred previously. Some uncertainty remains regarding the origin of the UV excess from the compact component known as P2. In our earlier analysis, we developed a unified picture for the broadband spectrum of this source. Contrary to the `standard' picture in which the infalling plasma attains temperatures in excess of 10^{10} K near the event horizon, the best fit model for M31*, under the assumption that the UV radiation is in fact produced by this source, appears to correspond to a cool branch solution, arising from strong line cooling inside the capture radius. Starting its infall with a temperature of about 10^6 K in the post-shock region, the plasma cools down efficiently to about 10^4 K toward smaller radii. An important prediction of this model is the appearance of a prominent UV spike from hydrogen line emission. In this paper we model this line emission with significantly greater accuracy, using the algorithm CLOUDY, and to correctly take into account the attenuation along the line of sight. We predict a spectrum with several additional prominent emission lines that can be used to verify the model with future high-resolution observations.

No Supermassive Black Hole in M33?

We observed the nucleus of M33, the third-brightest galaxy in the Local Group, with the Space Telescope Imaging Spectrograph at a resolution at least a factor of 10 higher than previously obtained. Rather than the steep rise expected within the radius of gravitational influence of a supermassive black hole, the random stellar velocities showed a decrease within a parsec of the center of the galaxy. The implied upper limit on the mass of the central black hole is only 3000 solar masses, about three orders of magnitude lower than the dynamically inferred mass of any other supermassive black hole. Detecting black holes of only a few thousand solar masses is observationally challenging, but it is critical to establish how supermassive black holes relate to their host galaxies, and which mechanisms influence the formation and evolution of both.

Long-lived Lopsided Modes of Annular Disks Orbiting a Central Mass

We demonstrate the existence of long-lived, large-amplitude eccentric modes in massive, annular particle disks in orbit about a central mass. The lopsided modes we have found precess slowly in the prograde direction at a rate that increases with disk mass and decreases with the amplitude of the distortion. The lopsidedness generally survives for as long as we run the calculations and may last indefinitely; we found no significant decay in over 700 disk-particle orbits in one case. These strongly nonlinear modes are readily created using a number of different perturbing rules, but we find no evidence for linear instabilities in unperturbed disks. Our results suggest that Tremaine's eccentric disk model for the nucleus of M31 may be viable, but none of our models matches all aspects of the observational data.

First Detection of Ammonia in M82

We report the detection of the (J, K) = (1, 1), (2, 2), and (3, 3) inversion lines of ammonia (NH3) toward the southwestern molecular lobe in M82. The relative intensities of the ammonia lines are characterized by a rotational temperature of Trot = 29 ± 5 K, which implies an average kinetic temperature of Tkin ≈ 60 K. A Gaussian decomposition of the observed spectra indicates increasing kinetic temperatures toward the nucleus of M82, consistent with recent findings based on CO observations. The observations imply a very low NH3 abundance relative to H2, X(NH3) ≈ 5 × 10-10. We present evidence for a decreasing NH3 abundance toward the central active regions in M82 and interpret this abundance gradient in terms of photodissociation of NH3 in photodissociation regions. The low temperature derived here from NH3 also explains the apparent underabundance of complex molecules like CH3OH and HNCO, which has previously been reported.

Eccentric Nuclear Disks with Self-Gravity: Predictions for the Double Nucleus of M31

We present a method for constructing models of weakly self-gravitating, finite dispersion eccentric stellar disks around central black holes. The disk is stationary in a frame rotating at a constant precession speed. The stars populate quasi-periodic orbits whose parents are numerically integrated periodic orbits in the total potential. We approximate the quasi-periodic orbits by distributions of Kepler orbits dispersed in eccentricity and orientation, using an approximate phase-space distribution function written in terms of the Kepler integrals of motion. We show an example of a model with properties similar to those of the double nucleus of M31. The properties of our models are primarily determined by the behavior of the periodic orbits. Self-gravity in the disk causes these orbits to assume a characteristic radial eccentricity profile, which gives rise to distinctive multipeaked line-of-sight velocity distributions along lines of sight near the black hole. The multipeaked features should be observable in M31 at the resolution of the Space Telecope Imaging Spectrograph. These features provide the best means of identifying an eccentric nuclear disk in M31 and can be used to constrain the disk properties and black hole mass.

Accretion Processes in the Nucleus of M31

The hypothesized supermassive black hole in the nucleus of M31 (which we shall hereafter call M31*) has many features in common with Sgr A* at the Galactic Center, yet they differ in several significant and important ways. Though M31* is probably ten times heavier, its radio luminosity at 3.6 cm is only one third that of Sgr A*. At the same time, M31* is apparently thousands of times more luminous in X-rays than its Galactic Center counterpart. Thus, a comparative study of these objects can be valuable in helping us to understand the underlying physical basis for their activity. We show here that the accretion model being developed for Sgr A* comprises two branches of solutions, distinguished by the relative importance of cooling compared to compressional heating at the radius $r_C$ where the ambient gas is captured by the black hole. For typical conditions in the ISM, the initial temperature ($T[r_C]\sim 10^6-10^7$ K) sits on the unstable branch of the cooling function. Depending on the actual value of $T(r_C)$ and the accretion rate, the plasma settles either onto a hot branch (attaining a temperature as high as $10^{10} K$ or so at small radii) or a cold branch, in which $T$ drops to $\sim 10^4$ K. Sgr A* is presumably a `hot' black hole. We show here that the VLA, UV and {\it Chandra} observations of M31* reveal it to be a member of the `cold' black hole family. We discuss several predicted features in the spectrum of M31* that may be testable by future multi-wavelength observations, including the presence of a prominent UV spike (from hydrogen line emission) that would be absent on the hot branch.

The Nuclear Dynamics of M32. I. Data and Stellar Kinematics

We have obtained optical long-slit spectroscopy of the nucleus of M32 using the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope. The stellar rotation velocity and velocity dispersion, as well as the full line-of-sight velocity distribution (LOSVD), were determined as a function of position along the slit using two independent spectral deconvolution algorithms. We see three clear kinematical signatures of the nuclear black hole: a sudden upturn, at ~03 from the center, in the stellar velocity dispersions; a flat or rising rotation curve into the center; and strong, non-Gaussian wings on the central LOSVD. The central velocity dispersion is ~130 km s-1 (Gaussian fit) or 175 km s-1 (corrected for the wings). The central kinematics are consistent with the presence of a supermassive compact object in M32 with a mass in the range × 106 M☉.

Integrable models of galactic discs with double nuclei

We introduce a new class of 2-D mass models, whose potentials are of Stackel form in elliptic coordinates. Our model galaxies have two separate strong cusps that form double nuclei. The potential and surface density distributions are locally axisymmetric near the nuclei and become highly non-axisymmetric outside the nucleus. The surface density diverges toward the cuspy nuclei with the law �/ r 2 . Our model is sustained by four general types of regular orbits: butterfly, nucleuphilic banana, horseshoe and aligned loop orbits. Horseshoes and nucleuphilic bananas support the existence of cuspy regions. Butterflies and aligned loops control the non-axisymmetric shape of outer regions. Without any need for central black holes, our distributed mass models resemble the nuclei of M31 and NGC4486B. It is also shown that the self-gravity of the stellar disc can prevent the double nucleus to collapse.

Discovery of a Luminous, Variable, Off-Center Source in the Nucleus of M82 with the [ITAL]Chandra[/ITAL] High-Resolution Camera

We present results from observations of the most famous starburst galaxy M82 with the High-Resolution Camera onboard the Chandra X-Ray Observatory. We found nine sources in the central 1' x 1' region, but no source was detected at the galactic center. Comparing the observations on 1999 October 28 and on 2000 January 20, we found four of the nine sources showed significant time variability. In particular, CXOM82J095550.2+694047, which is 9 away from the galactic center, showed extremely large time variability. We conclude that this source is the origin of the hard X-ray time variability of M82 detected with ASCA. Assuming a spectral shape obtained by the ASCA observation, its luminosity in the 0.5 -- 10 keV band changed from 1.2E40 erg/s on 1999 October 28 to 8.7E40 erg/s on 2000 January 20.

The Peak Brightness and Spatial Distribution of Asymptotic Giant Branch Stars Near the Nucleus of M32

The bright stellar content near the center of the Local Group elliptical galaxy M32 is investigated with 012 FWHM H and K images obtained with the Gemini Mauna Kea telescope. Stars with K = 15.5, which are likely evolving near the tip of the asymptotic giant branch (AGB), are resolved to within 2'' of the nucleus, and it is concluded that the peak stellar brightness near the center of M32 is similar to that in the outer regions of the galaxy. Moreover, the projected density of bright AGB stars follows the visible light profile to within 2'' of the nucleus, indicating that the brightest stars are well mixed throughout the galaxy. Thus, there is no evidence for an age gradient, and the radial variations in spectroscopic indices and ultraviolet colors that have been detected previously must be due to metallicity and/or some other parameter. We suggest that either the bright AGB stars formed as part of a highly uniform and coherent galaxy-wide episode of star formation or they originated in a separate system that merged with M32.