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Register a new userThe distance to the giant elliptical galaxy M87 and the size of its stellar subsystem
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Olga Galazutdinova A.
Publication date
2020
fields
Physics
and research's language is
English
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Stellar photometry in nine fields around the giant elliptical galaxy M87 in the Virgo cluster is obtained from archival images of the Hubble Space Telescope. The resulting Hertzsprung--Russell diagrams show populated red-giant and AGB branches. The position of the tip of the red-giant branch (the TRGB discontinuity) is found to vary with galactocentric distance. This variation can be interpreted as the effect of metal-rich red giants on the procedure of the measurement of the TRGB discontinuity or as a consequence of the existence of a weak gas-and-dust cloud around M87 extending out to $10^prime$ along the galactocentric radius and causing $I$-band absorption of up to $0.^m2$ near the center of the galaxy. The TRGB stars located far from the M87 center yield an average distance modulus of $(m-M) = 30.91pm0.08$, which corresponds to the distance of $D=15.4pm0.6$ Mpc. It is shown that stars in the field located between M86 and M87 galaxies at angular separations of $37^prime$ and $40^prime$ are not intergalactic stars, but belong to the M87 galaxy, i.e., that the stellar halo of this galaxy can be clearly seen at a galactocentric distance of 190 kpc. The distances are measured to four dwarf galaxies P4anon, NGC4486A, VCCA039, and dSph-D07, whose images can be seen in the fields studied. The first three galaxies are M87 satellites, whereas dSph-D07 is located at a greater distance and is a member of the M86 group.
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The distance to NGC 5128, the central galaxy of the Centaurus group and the nearest giant elliptical to us, has been determined using two independent distance indicators: the Mira period-luminosity (PL) relation and the luminosity of the tip of the red giant branch (RGB). The data were taken at two different locations in the halo of NGC 5128 with the ISAAC near-IR array on ESO VLT. From more than 20 hours of observations with ISAAC a very deep Ks-band luminosity function was constructed. The tip of the RGB is detected at Ks=21.24 pm 0.05 mag. Using an empirical calibration of the K-band RGB tip magnitude, and assuming a mean metallicity of [M/H]=-0.4 dex and reddening of E(B-V)=0.11, a distance modulus of NGC 5128 of (m-M)_0=27.87 pm 0.16 was derived. The comparison of the H-band RGB tip magnitude in NGC 5128 and the Galactic Bulge implies a distance modulus of NGC 5128 of (m-M)_0=27.9 pm 0.2 in good agreement with the K-band RGB tip measurement. The population of stars above the tip of the RGB amounts to 2176 stars in the outer halo field and 6072 stars in the inner halo field. The large majority of these sources belong to the asymptotic giant branch (AGB) population in NGC 5128 with numerous long period variables. Mira variables were used to determine the distance of NGC 5128 from a period-luminosity relation calibrated using the Hipparcos parallaxes and LMC Mira period-luminosity relation in the K-band. This is the first Mira period-luminosity relation outside the Local Group. A distance modulus of 27.96 pm 0.11 was derived, adopting the LMC distance modulus of 18.50 pm 0.04. The mean of the two methods yields a distance modulus to NGC 5128 of 27.92 pm 0.19 corresponding to D=3.84 pm 0.35 Mpc.
There is a well-known discrepancy in the distance estimation for M60, a giant elliptical galaxy in Virgo: the planetary nebula luminosity function (PNLF) distance moduli for this galaxy are, on average, $~0.4$ mag smaller than the values based on the surface brightness fluctuation (SBF) in the literature. We present photometry of the resolved stars in an outer field of M60 based on deep F775W and F850LP images in the Hubble Space Telescope obtained as part of the Pure Parallel Program in the archive. Detected stars are mostly old red giants in the halo of M60. With this photometry we determine a distance to M60 using the tip of the red giant branch (TRGB). A TRGB is detected at $F850LP_{rm TRGB}=26.70pm0.06$ mag, in the luminosity function of the red giants. This value corresponds to $F814W_{0,rm TRGB}=27.13pm0.06$ mag and $QT_{rm TRGB}=27.04pm0.07$ mag, where $QT$ is a color-corrected F814W magnitude. From this we derive a distance modulus, $(m-M)_0=31.05pm0.07{rm(ran)}pm0.06{rm (sys)}$ ($d=16.23pm0.50{rm (ran)}pm0.42{rm (sys)}$ Mpc). This value is $0.3$ mag larger than the PNLF distances and $0.1$ mag smaller than the SBF distances in the previous studies, indicating that the PNLF distances to M60 in the literature have larger uncertainties than the suggested values.
We report on our search for very-long-term variability (weeks to years) in X-ray binaries (XRBs) in the giant elliptical galaxy M87. We have used archival Chandra imaging observations to characterise the long-term variability of 8 of the brightest members of the XRB population in M87. The peak brightness of some of the sources exceeded the ultra luminous X-ray source (ULX) threshold luminosity of ~ 10^{39} erg/s, and one source could exhibit dips or eclipses. We show that for one source, if it has similar modulation amplitude as in SS433, then period recoverability analysis on the current data would detect periodic modulations, but only for a narrow range of periods less than 120 days. We conclude that a dedicated monitoring campaign, with appropriately defined sampling, is essential if we are to investigate properly the nature of the long-term modulations such as those seen in Galactic sources.
We present Chandra observations of hot gas structures, characteristic of gas stripping during infall, in the Virgo cluster elliptical galaxy M60 (NGC4649) located $1$ Mpc east of M87. $0.5-2$ keV Chandra X-ray images show a sharp leading edge in the surface brightness $12.4 pm 0.1$ kpc north and west of the galaxy center in the direction of M87 characteristic of a merger cold front due to M60s motion through the Virgo ICM. We measured a temperature of $1.00 pm 0.02$ keV for abundance $0.5 Z_odot$ inside the edge and $1.37^{+0.35}_{-0.19}$ keV for abundance $0.1 Z_odot$ in the Virgo ICM free stream region. We find that the observed jump in surface brightness yields a density ratio of $6.44^{+1.04}_{-0.67}$ between gas inside the edge and in the cluster free stream region. If the edge is a cold front due solely to the infall of M60 in the direction of M87, we find a pressure ratio of $4.7^{+1.7}_{-1.4}$ and Mach number $1.7 pm 0.3$. For 1.37 keV Virgo gas we find a total infall velocity for M60 of $1030 pm 180$ kms$^{-1}$. We calculate the motion in the plane of the sky to be $1012^{+183}_{-192}$ km$^{-1}$ implying an inclination angle $xi = 11 pm 3$ degrees. Surface brightness profiles show the presence of a faint diffuse gaseous tail. We identify filamentary, gaseous wing structures caused by the galaxys motion through the ICM. The structure and dimensions of these wings are consistent with simulations of Kelvin-Helmholtz instabilities as expected if the gas stripping is close to inviscid.
The scatter in the galaxy size versus stellar mass (Mstar) relation gets largely reduced when, rather than the half-mass radius Re, the size at a fixed surface density is used. Here we address why this happens. We show how a reduction is to be expected because any two galaxies with the same Mstar have at least one radius with identical surface density, where the galaxies have identical size. However, the reason why the scatter is reduced to the observed level is not trivial, and we pin it down to the galaxy surface density profiles approximately following Sersic profiles with their Re and Sersic index (n) anti-correlated (i.e., given Mstar, n increases when Re decreases). Our analytical results describe very well the behavior of the observed galaxies as portrayed in the NASA Sloan Atlas (NSA), which contains more than half a million local objects with 7 < log(Mstar/Msun) < 11.5. The comparison with NSA galaxies also allows us to find the optimal values for the mass surface density (2.4m0.9p1.3 Msun/pc2) and surface brightness (r-band 24.7pm0.5 mag/arcsec2) that minimize the scatter, although the actual values depend somehow on the subset of NSA galaxies used for optimization. The physical reason for the existence of optimal values is unknown but, as Trujillo+20 point out, they are close to the gas surface density threshold to form stars and thus may trace the physical end of a galaxy. Our NSA-based size--mass relation agrees with theirs on the slope as well as on the magnitude of the scatter. As a by-product of the narrowness of the size--mass relation (only 0.06 dex), we propose to use the size of a galaxy to measure its stellar mass. In terms of observing time, it is not more demanding than the usual photometric techniques and may present practical advantages in particular cases.
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