Deep Hubble Space Telescope Wide Field Camera (WFC-1) imaging of a region 3.7' east of the V=13mag Seyfert galaxy NGC 5548 in the F555W and F785LP bands, and deep ground-based V and I imaging, show a new extended, blue, low surface brightness structure, apparently a remnant of a tidal interaction associated with NGC 5548. If this straight, low surface brightness (V~27-28mag/arcsec^2^) tidal tail is associated with NGC 5548, it extends at least 80kpc from it and has an absolute magnitude of M_V~-16.4 (H_0_=65km/s/Mpc). Previous imaging surveys of Seyfert galaxies would have missed such low surface brightness tails. Morphologically similar to some brighter tails seen in other interacting systems and in simulations of merging galaxies, this faint tail is a useful diagnostic of an earlier interaction and of the progenitors' halo-to-disk mass ratios. Luminous ripples and a brighter tail wrapped around the galaxy are seen in the inner 1-10kpc region. Unresolved blue objects in the long tail have the colors and absolute magnitudes of young globular clusters. Taken together, the two tails and other features suggest that two spirals merged less than ~1Gyr ago. The inner luminosity profile of NGC 5548 is a good fit to a de Vaucouleurs profile with r_eff_=5.8kpc. Recent simulations of merged galaxies with high-mass halos fail to form lasting tidal tails, suggesting that the NGC 5548 progenitor spirals had modest halo masses.
The distance of NGC1316, the brightest galaxy in the Fornax cluster, provides an interesting test for the cosmological distance scale. First, because Fornax is the second largest cluster of galaxies within <~25Mpc after Virgo and, in contrast to Virgo, has a small line-of-sight depth; and second, because NGC1316 is the single galaxy with the largest number of detected Type Ia supernovae (SNe Ia), giving the opportunity to test the consistency of SNe Ia distances both internally and against other distance indicators. We measure surface brightness fluctuations (SBF) in NGC1316 from ground- and space-based imaging data. The sample provides a homogeneous set of measurements over a wide wavelength interval. The SBF magnitudes, coupled with empirical and theoretical absolute SBF calibrations, are used to estimate the distance to the galaxy. We also present the first B-band SBF measurements of NGC1316 and use them together with the optical and near-IR SBF data to analyze the properties of field stars in the galaxy. We obtain m-M=31.59+/-0.05(stat)+/-0.14(sys.)mag, or d=20.8+/-0.5(stat)+/-1.5(sys.)Mpc. When placed in a consistent Cepheid distance scale, our result agrees with the distances from other indicators. On the other hand, our distance is ~17% larger than the most recent estimate based on SNe Ia. Possible explanations for this disagreement are the uncertain level of internal extinction, and/or calibration issues. Concerning the stellar population analysis, we confirm the results from other spectro-photometric indicators: the field stars in NGC1316 are dominated by a component with roughly solar metallicity and intermediate age. A non-negligible mismatch exists between B-band SBF models and data. We confirm that such behavior can be accounted for by an enhanced percentage of hot horizontal branch stars. Our study of the SBF distance to NGC1316, and the comparison with distances from other indicators, raises some concern about the homogeneity between the calibrations of different indicators. If not properly placed in the same reference scale, significant differences can occur, with dramatic impact on the cosmological distance ladder. Our results on the stellar populations properties show that SBF data over a broad wavelength interval are an efficient means of studying the properties of unresolved systems in peculiar cases like NGC1316.
We present BVRI CCD photometry of ~600 stars in the central region of a resolved spiral galaxy NGC 1560 which is seen almost edge-on, apparently located in the IC 342 group. The V-(B-V) diagram shows a dominant group of significantly reddened blue plume stars consisting of young massive main-sequence and evolved blue supergiant stars. The tip of the blue plume is found to be at V~21 mag and (B-V)~0.4mag. The I-(R-I) diagram shows a large group of red supergiants the brightest of which are extending to I~19.5mag at (R-I)~1.1mag. The logarithmic slope of the V luminosity function of the blue plume stars is 0.69+/-0.10, similar to that for other nearby galaxies. The mean color of the blue plume stars is <(B-V)>=0.25+/-0.03mag. The total reddening, including both foreground and internal reddening, is estimated from the mean color of the blue plume stars, to be E(B-V)=0.40+/-0.10mag, whereas the foreground reddening based on the H I column density and faint galaxy counts is E(B-V)=0.16mag. Combining our results with those in the literature, we have obtained distance estimates using four methods: brightest blue stars [(m-M)_0_=27.11mag], brightest red stars [(m-M)_0_=27.02mag], B-band Tully-Fisher relation [(m-M)_0_=26.96mag], and H-band Tully-Fisher relation [(m-M)_0_=26.86mag]. These four estimates agree very well. Taking the average of these four estimates, we obtain a value for the true distance modulus to NGC 1560, (m-M)_0_=27.0+/-0.1mag, corresponding to a distance of 2.5+/-0.1Mpc.
BVRI/CCD photometry of ~2,300 stars in the central 2.2'x3.5' area of the dwarf elliptical galaxy NGC205 in the Local Group is presented. The color-magnitude diagrams of NGC 205 show: (1) a blue plume around (B-V)~0.0mag, extending up to V~19.3mag (M_V_~-5.4mag); and (2) a strong asymptotic giant branch (AGB) population, the brightest of which reaches I~19.0mag, 1.5 mag brighter than the tip of the red giant branch. BVRI photometry of three globular clusters (Hubble IV. V, and VI) is also presented. Hubble V is much bluer [(B-V)=0.37] than other globular clusters. This blue color of Hubble V leads to an estimate of the age, ~300Myrs. A V luminosity function for the blue stars has been derived: the logarithmic slope of the luminosity function for -5.7<M_V_<-3.2 is 0.64+/-0.12, which is similar to those of other nearby galaxies. The AGB stars extend to Mbol~-5.7mag, which is ~0.7mag brighter than those in NGC 185. The bolometric luminosity function of the AGB stars in NGC 205 is found to be flatter than that of the similar dwarf elliptical galaxy NGC 185, but similar to that of a bulge field 4' of the nucleus of M31. Surface photometry for the central area within R=50" is presented, showing that the colors get bluer as the galactocentric radius decreases except for the nucleus region within R=1.6" where the colors get redder inward. Star formation history in NGC 205 is briefly discussed.
Recent estimates of the Cepheid distance modulus of NGC 6822 differ by 0.18mag. To investigate this we present new multi-epoch JHKs photometry of classical Cepheids in the central region of NGC 6822 and show that there is a zero-point difference from earlier work. These data together with optical and mid-infrared observations from the literature are used to derive estimates of the distance modulus of NGC 6822. A best value of 23.40mag is adopted, based on a Large Magellanic Cloud (LMC) distance modulus of 18.50mag. The standard error of this quantity is ~0.05mag. We show that to derive consistent moduli from Cepheid observations at different wavelengths, it is necessary that the fiducial LMC period-luminosity relations at these wavelengths should refer to the same subsample of stars. Such a set is provided. A distance modulus based on RR Lyrae variables agrees with the Cepheid result.
We present the results of a Chandra/Hubble Space Telescope (HST) study of the point sources of the Virgo Cluster giant elliptical galaxy NGC 4472. We identify 144 X-ray point sources outside the nuclear region, 72 of which are located within the HST fields. The optical data show 1102 sources, of which 829 have colors consistent with being globular clusters (with only four in the restricted central 10" region). Thirty matches are found between the two lists - these are likely to be low-mass X-ray binaries (LMXBs) associated with globular clusters, while ~42 of the X-ray sources have no optical counterparts to V<~25 and I<~24, indicating that they are likely to be predominantly LMXBs in the field star population with a small amount of possible contamination from background active galactic nuclei. Thus approximately 40% of the X-ray sources are in globular clusters and 4% of the globular clusters contain X-ray sources.
We used SOFIA/GREAT [CII] 158um observations as well as HI data from THINGS and CO(2-1) data from HERACLES to decompose the spectrally resolved [CII] line profiles in NGC4124 into components associated with neutral atomic and molecular gas. We infer gas masses traced by [CII] under different ISM conditions. We find that the molecular gas mass is dominated by CO-dark gas and that we can only assign 9 percent of [CII] intensity to the cold neutral medium.
We investigate the fueling and the feedback of nuclear activity in the Seyfert 2 galaxy NGC1068, by studying the distribution and kinematics of molecular gas in the torus and its connections. We use ALMA to image the emission of a set of molecular gas tracers in the circumnuclear disk (CND) and the torus of the galaxy using the CO(2-1), CO(3-2) and HCO^+^(4-3) lines with spatial resolutions ~0.03"-0.09"(2-6pc). ALMA resolves the CND as an asymmetric ringed disk of D~400pc-size and mass of ~1.4x10^8^M_{sun}_. The inner edge of the ring is associated with edge-brightened arcs of NIR polarized emission identified with the working surface of the AGN ionized wind. ALMA proves the existence of a molecular torus of M_torus ~3x10^5^M_{sun}_, which extends over a large range of spatial scales D=10-30pc around the central engine. The new observations evidence the density radial stratification of the torus: the HCO^+^(4-3) torus, with a full size D=11pc, is a factor of 2-3 smaller than its CO(2-1) and CO(3-2) counterparts, which have full-sizes D=26pc and D=28pc, respectively. The torus is connected to the CND through a network of gas streamers. The kinematics of molecular gas show strong departures from circular motions in the torus, the gas streamers, and the CND. These velocity distortions are interconnected and are part of a 3D outflow that reflects the effects of AGN feedback on the kinematics of molecular gas across a wide range of spatial scales. We conclude that a wide-angle AGN wind launched from the accretion disk is impacting a sizeable fraction of the gas inside the torus (~0.4-0.6xM_torus_). However, a large gas reservoir (~1.2-1.8x10^5^M_{sun}_) close to the equatorial plane of the torus remains unaffected by the AGN wind and can continue fueling the AGN for ~1-4Myr. AGN fueling seems nevertheless thwarted on intermediate scales (15pc<r<50pc).
NGC 4654 is a Virgo galaxy seen almost face-on, which undergoes nearly edge-on gas ram pressure stripping and a fly-by gravitational interaction with another massive galaxy, NGC 4639. NGC 4654 shows a strongly compressed gas region near the outer edge of the optical disk, with HI surface densities (HSDR), exceeding the canonical value of 10-15M_{sun}_/pc^2^. New IRAM 30m HERA CO(2-1) data of NGC 4654 are used to study the physical conditions of the ISM. The CO-to-H_2_ conversion factor was estimated and found to be one to two times the Galactic value with significant decrease in the ratio between the molecular fraction and the total ISM pressure in the HSDR, self-gravitating gas, a Toomre parameter below Q=1 and star-formation efficiency 1.5-2 times higher. Analytical models were used to reproduce radial profiles of the SFR and the atomic and molecular surface densities. A Toomre parameter of Q~0.8 combined with an increase in the velocity dispersion of {DELTA}_vdisp_~5km/s are necessary conditions to simultaneously reproduce the gas surface densities and the SFR. A dynamical model was used to reproduce the gas distribution of NGC 4654. The comparison between the velocity dispersion given by the moment 2 map and the intrinsic 3D velocity dispersion from the model were used to discriminate between regions of broader linewidths caused by a real increase in the velocity dispersion and those caused by an unresolved velocity gradient only. We found that the 5km/s increase in the intrinsic velocity dispersion is compatible with observations. During a period of gas compression through external interactions, the gas surface density is enhanced, leading to an increased SFR and stellar feedback. Under the influence of stellar feedback, the gas density increases only moderately. The stellar feedback acts as a regulator of star-formation, increasing the turbulent velocity within the region.
The variations of the [OIII]{lambda}5007 line profile in NGC 5548 are examine d from CCD spectra obtained at the Ohio State University and Crimean Astrophysic al Observatory in December 1988 - October 1994.