Field Of NGC Research Articles

Standardized Luminosity of the Tip of the Red Giant Branch Utilizing Multiple Fields in NGC 4258 and the CATs Algorithm

The tip of the red giant branch provides a luminous standard candle for calibrating distance ladders that reach Type Ia supernova (SN Ia) hosts. However, recent work reveals that tip measurements vary at the ∼0.1 mag level for different stellar populations and locations within a host, which may lead to inconsistencies along the distance ladder. We pursue a calibration of the tip using 11 Hubble Space Telescope fields around the maser host, NGC 4258, that is consistent with SN Ia hosts by standardizing tip measurements via their contrast ratios. We find F814W-band tips that exhibit a full 0.3 mag range and 0.1 mag dispersion. We do not find any correlation between H i column density and the apparent tip to 0.04 ± 0.03 mag/cm−2. We search for a tip–contrast relation (TCR) and measure the TCR within the fields of NGC 4258 of −0.015 ± 0.008 mag/R, where R is the contrast ratio. This value is consistent with the TCR originally discovered in the GHOSTS sample of −0.023 ± 0.005 mag/R. Combining these measurements, we find a global TCR of −0.021 ± 0.004 mag/R and a calibration of mag. We also use stellar models to simulate single age and metallicity stellar populations with [Fe/H] from −2.0 to −0.7 and ages from 3 to 12 Gyr and reconstruct the global TCR found here to a factor of ∼2. This work is combined in a companion analysis with tip measurements of nearby SN Ia hosts to measure H 0.

Deviation from a Continuous and Universal Turbulence Cascade in NGC 6334 due to Massive Star Formation Activity

We use molecular line data from the Atacama Large Millimeter/submillimeter Array, Submillimeter Array, James Clerk Maxwell Telescope, and NANTEN2 to study the multiscale (∼15–0.005 pc) velocity statistics in the massive star formation region NGC 6334. We find that the nonthermal motions revealed by the velocity dispersion function (VDF) stay supersonic over scales of several orders of magnitude. The multiscale nonthermal motions revealed by different instruments do not follow the same continuous power law, which is because the massive star formation activities near central young stellar objects have increased the nonthermal motions in small-scale and high-density regions. The magnitudes of VDFs vary in different gas materials at the same scale, where the infrared dark clump N6334S in an early evolutionary stage shows a lower level of nonthermal motions than other more evolved clumps due to its more quiescent star formation activity. We find possible signs of small-scale-driven (e.g., by gravitational accretion or outflows) supersonic turbulence in clump N6334IV with a three-point VDF analysis. Our results clearly show that the scaling relation of velocity fields in NGC 6334 deviates from a continuous and universal turbulence cascade due to massive star formation activities.

MUSE crowded field 3D spectroscopy in NGC 300

Context. There are known differences between the physical properties of H II and diffuse ionized gas (DIG). However, most of the studied regions in the literature are relatively bright, with log10 L(Hα)[erg s−1] ≳ 37. Aims. We compiled an extremely faint sample of 390 H II regions with a median Hα luminosity of 34.7 in the flocculent spiral galaxy NGC 300, derived their physical properties in terms of metallicity, density, extinction, and kinematics, and performed a comparative analysis of the properties of the DIG. Methods. We used MUSE data of nine fields in NGC 300, covering a galactocentric distance of zero to ~450 arcsec (~4 projected kpc), including spiral arm and inter-arm regions. We binned the data in dendrogram leaves and extracted all strong nebular emission lines. We identified H II and DIG regions and compared their electron densities, metallicity, extinction, and kinematic properties. We also tested the effectiveness of unsupervised machine-learning algorithms in distinguishing between the H II and DIG regions. Results. The gas density in the H II and DIG regions is close to the low-density limit in all fields. The average velocity dispersion in the DIG is higher than in the H II regions, which can be explained by the DIG being 1.8 kK hotter than H II gas. The DIG manifests a lower ionization parameter than H II gas, and the DIG fractions vary between 15–77%, with strong evidence of a contribution by hot low-mass evolved stars and shocks to the DIG ionization. Most of the DIG is consistent with no extinction and an oxygen metallicity that is indistinguishable from that of the H II gas. We observe a flat metallicity profile in the central region of NGC 300, without a sign of a gradient. Conclusions. The differences between extremely faint H II and DIG regions follow the same trends and correlations as their much brighter cousins. Both types of objects are so heterogeneous, however, that the differences within each class are larger than the differences between the two classes.

Revisiting FUSE O vi Emission in Galaxy Halos

Abstract A significant fraction of baryons in galaxies are in the form of diffuse gas of the circumgalactic medium (CGM). One critical component of the multiphases of CGM, the so-called “coronal” warm-hot phase gas (105–106 K) traced by O vi 1031.93, 1037.62 Å resonance lines, has rarely been detected in emission from galaxy halos other than that of the Milky Way. Here we report four additional detections of O vi emission gas in the halos of nearby edge-on galaxies, NGC 4631 and NGC 891, using archival Far Ultraviolet Spectroscopic Explorer data and an updated data pipeline. We find the most intense O vi emission to be from fields forming a vertical line near the center of NGC 4631, despite the close proximity to the disk of two other fields. The detected O vi emission surface brightness is about (1.1 ± 0.3) × 10−18 to (3.9 ± 0.8) × 10−18 erg s−1 cm−2 arcsec−2. The spatial distribution of the five 30″ × 30″ O vi detection fields in NGC 4631 can be interpreted as the existence of filamentary structures of more intense O vi emission superimposed within a diffuse and faint O vi halo in star-forming galaxies. Volume-filled O vi emission mapping is greatly needed to determine the structure and prevalence of warm-hot gas and the role it plays in the cycling of gas between the galaxy disk and the halo. Finally, we present the sensitivity of future funded and proposed UV missions (LUVOIR-A, LUVOIR-B, CETUS, and Aspera) to the detection of diffuse and faint O vi emission in nearby galaxy halos.

Large-scale magnetic field structure of NGC 3627 based on a magnetic vector map

Abstract We analyzed the data of Stokes I, Q, and U in the C and X bands and investigated the large-scale magnetic field structure of NGC 3627. The polarization intensity and angle in each band were derived using Stokes Q and U maps. The rotation measure was calculated using polarization angle maps. Moreover, the magnetic field strength was calculated by assuming energy equipartition with cosmic ray electrons. The structure of the magnetic field was well aligned with the spiral arms, which were consistent with those in the former studies. We applied the magnetic vector reconstruction method to NGC 3627 to derive a magnetic vector map, which showed that the northern and southern disks were dominant with inward and outward magnetic vectors, respectively. Furthermore, we considered the large-scale structure of the magnetic field in NGC 3627 and observed that the structure is bi-symmetric spiral in nature, and that the number of magnetic field modes is mB = 1 in the outer region of galaxy. In addition, NGC 3627 has a mode of two spiral arms that were clearly visible in an optical image. The ratio of the mode of the spiral arms to that of the magnetic field is 2 : 1. In terms of NGC 3627, the large-scale magnetic field may be generated via the parametric resonance induced by the gravitational potential of the spiral arms.

Using Gaia as an Astrometric Tool for Deep Ground-based Surveys

AbstractGaia DR1 positions are used to astrometrically calibrate three epochs’ worth of Subaru SuprimeCam images in the fields of globular cluster NGC 2419 and the Sextans dwarf spheroidal galaxy. Distortion-correction “maps” are constructed from a combination of offset dithers and reference to Gaia DR1. These are used to derive absolute proper motions in the field of NGC 2419. Notably, we identify the photometrically-detected Monoceros structure in the foreground of NGC 2419 as a kinematically-cold population of stars, distinct from Galactic-field stars. This project demonstrates the feasibility of combining Gaia with deep, ground-based surveys, thus extending high-quality astrometry to magnitudes beyond the limits of Gaia.

Kinematic Decoupled Core Paradigm: Counterrotation or Just Warping

Several mechanisms have been proposed in recent years to explain kinematic decoupled cores (KDCs) in early type galaxies as well as the large differences in angular momentum between KDCs and host galaxy. Most of the proposed scenarios involve large fractions of merging events, high speed interactions with dwarf spheroidal galaxies, cusp effect of the dark matter density profiles, etc. We here argue that counterrotation as well as fast and slow rotation of disks or spheroids at the center of galaxies can also be explained by a misalignment of the central spheroid equatorial plane with regard to that defined by the observed external stellar rotation. Contrary to what happens at the outer region of disk galaxies, once instability has led to the inner warped core, the perturbed orbits can maintain a common orientation due to the rigid body like rotation at the central region of the galaxy. The spatial configuration that furnishes the smallest angular momentum difference between the KDC and the host galaxy is completely defined by observed parameters in the plane of the sky, namely, the inclination of the inner and outer disks and the angle between the two lines of nodes. As an example we modeled the paradigmatic and extreme case of the 2D radial velocity field of NGC 4382 nucleus. Tilt angles of the KDC not larger than 30 degrees also allow explaining fast and low rotators of the called “Sauron paradigm” in a unified scenario. The maximum for the three parameters, namely, velocity of the inner rotator, difference of position angle and difference with the outer rotation velocity of the whole Sauron sample, are consistently correlated in agreement with the proposed scenario. These quantities do not correlate with the galaxies magnitude, mass (since large and dwarf spheroidals show apparent counterrotation as well) or environment, also suggesting that an internal phenomenon like the central spheroid warping, that we are here proposing, may be at work.

Red giants in the outer halo of the elliptical galaxy NGC 5128/Centaurus A

We used VIMOS on VLT to perform $V$ and $I$ band imaging of the outermost halo of NGC 5128 / Centaurus A ($(m-M)_0=27.91\pm0.08$), 65 kpc from the galaxy's center and along the major axis. The stellar population has been resolved to $I_0 \approx 27$ with a $50\%$ completeness limit of $I_0 = 24.7$, well below the tip of the red-giant branch (TRGB), which is seen at $I_0 \approx 23.9$. The surface density of NGC 5128 halo stars in our fields was sufficiently low that dim, unresolved background galaxies were a major contaminant in the source counts. We isolated a clean sample of red-giant-branch (RGB) stars extending to $\approx 0.8$ mag below the TRGB through conservative magnitude and color cuts, to remove the (predominantly blue) unresolved background galaxies. We derived stellar metallicities from colors of the stars via isochrones and measured the density falloff of the halo as a function of metallicity by combining our observations with HST imaging taken of NGC 5128 halo fields closer to the galaxy center. We found both metal-rich and metal-poor stellar populations and found that the falloff of the two follows the same de Vaucouleurs' law profiles from $\approx 8$ kpc out to $\approx$ 70 kpc. The metallicity distribution function (MDF) and the density falloff agree with the results of two recent studies of similar outermost halo fields in NGC 5128. We found no evidence of a "transition" in the radial profile of the halo, in which the metal-rich halo density would drop rapidly, leaving the underlying metal-poor halo to dominate by default out to greater radial extent, as has been seen in the outer halo of two other large galaxies. If NGC 5128 has such a transition, it must lie at larger galactocentric distances.

An imaging and spectroscopic study of the planetary nebulae in NGC 5128 (Centaurus A)

Imaging and spectroscopic observations of planetary nebulae (PNe) in the nearest large elliptical galaxy NGC 5128 (Centaurus A), were obtained to find more PNe and measure their radial velocities. NTT imaging was obtained in 15 fields in NGC 5128 over an area of about 1 square degree with EMMI using [O III] and off-band filters. Newly detected sources, combined with literature PNe, were used as input for VLT FLAMES multi-fibre spectroscopy in MEDUSA mode. Spectra of the 4600-5100A region were analysed and velocities measured based on emission lines of [O III]4959,5007A and often H-beta. The chief results are catalogues of 1118 PN candidates and 1267 spectroscopically confirmed PNe in NGC 5128. The catalogue of PN candidates contains 1060 PNe discovered with EMMI imaging and 58 from literature surveys. The spectroscopic PN catalogue has FLAMES radial velocity and emission line measurements for 1135 PNe, of which 486 are new. Another 132 PN radial velocities are available from the literature. For 629 PNe observed with FLAMES, H-beta was measured in addition to [O III]. Nine targets show double-lined or more complex profiles, and their possible origin is discussed. FLAMES spectra of 48 globular clusters were also targetted: 11 had emission lines detected (two with multiple components), but only 3 are PNe likely to belong to the host globular. The total of 1267 confirmed PNe in NGC 5128 with radial velocity measurements (1135 with small velocity errors) is the largest collection of individual kinematic probes in an early-type galaxy. This PN dataset, as well as the catalogue of PN candidates, are valuable resources for detailed investigation of the stellar population of NGC 5128. [Abridged]

3D Study Of Magnetic Fields In NGC 6946

AbstractRecent advancements in both radio observatories and computing have opened a new regime of 3D observations. Not only do these instruments measure emission lines and radio continuum over much larger bandpasses, but they also simultaneously observe the polarized emission over the same large bandpasses with increased sensitivity. This “polarization spectrum" can be used to recover information about the 3D structure of magnetic fields in the universe. Our combined 3-20~cm observations of NGC 6946 taken with the Westerbork Synthesis Radio Telescope provide highly sensitive diagnostics of the internal depolarization across the galaxy. We use model fitting to determine likely mechanisms for depolarization in different regions of the galaxy, and glean information about the coherent and turbulent magnetic fields in NGC 6946. We produce Faraday dispersion maps that illustrate how we can probe different depths into the galaxy at different wavelengths and display new features of the line of sight magnetic field. This work is just a sample of the new 3D studies that are possible with upgraded and new radio instruments like the VLA, ATCA, and SKA.

THE MAGNETIC FIELD OF THE IRREGULAR GALAXY NGC 4214

We examine the magnetic field in NGC 4214, a nearby irregular galaxy, using multi-wavelength radio continuum polarization data from the Very Large Array. We find that the global radio continuum spectrum shows signs that free-free absorption and/or synchrotron losses may be important. The 3cm radio continuum morphology is similar to that of the Halpha, while the 20cm emission is more diffuse. We estimate that 50% of the radio continuum emission in the center of the galaxy is thermal. Our estimate of the magnetic field strength is $30\pm 9.5$ \uG\ in the center and $10\pm3$ \uG\ at the edges. We find that the hot gas, magnetic, and the gravitational pressures are all the same order of magnitude. Inside the central star forming regions, we find that the thermal and turbulent pressures of the HII regions dominate the pressure balance. We do not detect any significant polarization on size scales greater than 200 pc. We place an upper limit of 8 \uG\ on the uniform field strength in this galaxy. We suggest that the diffuse synchrotron region, seen to the north of the main body of emission at 20cm, is elongated due to a uniform magnetic field with a maximum field strength of 7.6 \uG. We find that, while the shear in NGC 4214 is comparable to that of the Milky Way, the supernova rate is half that of the Milky Way and suggest that the star formation episode in NGC 4214 needs additional time to build up enough turbulence to drive an $\alpha-\omega$ dynamo.

REVISITING THE ABUNDANCE GRADIENT IN THE MASER HOST GALAXY NGC 4258

New spectroscopic observations of 36 HII regions in NGC 4258 obtained with the Gemini telescope are combined with existing data from the literature to measure the radial oxygen abundance gradient in this galaxy. The [OIII]4363 auroral line was detected in four of the outermost targets (17 to 22 kpc from the galaxy center), allowing a determination of the electron temperature Te of the ionized gas. From the use of different calibrations of the R23 abundance indicator an oxygen abundance gradient of approximately -0.012 +/- 0.002 dex/kpc is derived. Such a shallow gradient, combined with the difference in the distance moduli measured from the Cepheid Period-Luminosity relation by Macri et al. between two distinct fields in NGC 4258, would yield an unrealistically strong effect of metallicity on the Cepheid distances. This strengthens the suggestion that systematic biases might affect the Cepheid distance of the outer field. Evidence for a similar effect in the differential study of M33 by Scowcroft et al. is presented. A revision of the transformation between strong-line and Te-based abundances in Cepheid-host galaxies is discussed. In the Te abundance scale, the oxygen abundance of the inner field of NGC 4258 is found to be comparable with the LMC value.

THE ROLE OF THE MAGNETIC FIELD IN THE INTERSTELLAR MEDIUM OF THE POST-STARBURST DWARF IRREGULAR GALAXY NGC 1569

(abridged) NGC 1569 is a nearby dwarf irregular galaxy which underwent an intense burst of star formation 10 to 40 Myr ago. We present observations that reach surface brightnesses two to eighty times fainter than previous radio continuum observations and the first radio continuum polarization observations. These observations allow us to probe the relationship of the magnetic field of NGC 1569 to the rest of its interstellar medium. We confirm the presence of an extended radio continuum halo at 20 cm and see for the first time the radio continuum feature associated with the western Halpha arm at wavelengths shorter than 20cm. The spectral index trends in this galaxy support the theory that there is a convective wind at work in this galaxy. We derive a total magnetic field strength of 38 microG in the central regions and 10-15 microG in the halo. The magnetic field is largely random in the center of the galaxy; the uniform field is ~3-9 microG and is strongest in the halo. We find that the magnetic pressure is the same order of magnitude but, in general, a factor of a few less than the other components of the interstellar medium in this galaxy. The uniform magnetic field in NGC 1569 is closely associated with the Halpha bubbles and filaments. We suggest that a supernova-driven dynamo may be operating in this galaxy. The outflow of hot gas from NGC 1569 is clearly shaping the magnetic field, but the magnetic field in turn may be aiding the outflow by channeling gas out of the disk of the galaxy. Dwarf galaxies with extended radio continuum halos like that of NGC 1569 may play an important role in magnetizing the intergalactic medium.

Magnetic fields of AGNs and standard accretion disk model: testing by optical polarimetry

We have developed the method that allows us to estimate the magnetic field strength at the horizon of a supermassive black hole (SMBH) through the observed polarization of optical emission of the accreting disk surrounding SMBH. The known asymptotic formulae for the Stokes parameters of outgoing radiation are azimuthal averaged, which corresponds to an observation of the disk as a whole. We consider two models of the embedding 3D-magnetic field, the regular field, and the regular field with an additional chaotic (turbulent) component. It is shown that the second model is preferable for estimating the magnetic field in NGC 4258. For estimations we used the standard accretion disk model assuming that the same power-law dependence of the magnetic field follows from the range of the optical emission down to the horizon. The observed optical polarization from NGC 4258 allowed us to find the values 10^3 - 10^4 Gauss at the horizon, depending on the particular choice of the model parameters. We also discuss the wavelength dependencies of the light polarization, and possibly applying them for a more realistic choice of accretion disk parameters.

Are rotation curves in NGC 6946 and the Milky Way magnetically supported?

AbstractThe inner disk rotation of NGC 6946 and the Milky Way is dominated by gravity but magnetism is not negligible at radii where the rotation curve becomes flat, and indeed could become dominant at very large radii. Values of the order of 1 μG, or even less, produce a centripetal force when the absolute value of the slope of the curve [B φ , R ] (azimuthal field strength versus radius) is less than the slope of a B φ ‐profile proportional to R –1. The ∝ R –1‐profile is here called the critical profile. From the hypothesis of magnetically driven rotation curves, the following is to be expected: at large radii, a “subcritical” profile (slope flatter than R –1); at still larger radii a B φ ‐profile becoming asymptotically critical as the density becomes asymptotically vanishing. Recent observations of magnetic fields in NGC 6946 and the Milky Way are in very good agreement with these predictions. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A New Cepheid Distance to the Maser‐Host Galaxy NGC 4258 and Its Implications for the Hubble Constant

We present initial results from a time series BVI survey of two fields in NGC 4258 using the HST ACS. This galaxy was selected because of its accurate maser-based distance, which is anticipated to have a total uncertainty of ~3%. The goal of the HST observations is to provide an absolute calibration of the Cepheid distance scale and to measure its dependence on chemical abundance (the so-called metallicity effect). We carried out observations of two fields at different galactocentric distances with a mean abundance difference of 0.5 dex. We discovered a total of 281 Cepheids with periods ranging from 4 to 45 days (the duration of our observing window). We determine a Cepheid distance modulus for NGC 4258 (relative to the LMC) of Δμ0 = 10.88 ± 0.04 (random) ± 0.05 (systematic) mag. Given the published maser distance to the galaxy, this implies μ0(LMC) = 18.41 ± 0.10r ± 0.13s mag or D(LMC) = 48.1 ± 2.3r ± 2.9s kpc. We measure a metallicity effect of γ = -0.29 ± 0.09r ± 0.05s mag dex-1. We see no evidence for a variation in the slope of the period-luminosity relation as a function of abundance. We estimate a Hubble constant of H0 = 74 ± 3r ± 6s km s-1 Mpc-1 using a recent sample of four well-observed Type Ia SNe and our new calibration of the Cepheid distance scale. It may soon be possible to measure the value of H0 with a total uncertainty of 5%, with consequent improvement in the determination of the equation of state of dark energy.

Variable Stars in the Field of NGC 6882/6885: The Case of V381 Vulpeculae and V382 Vulpeculae

ABSTRACTWe present photometric and spectroscopic results for two reported δ Scuti stars in the field of NGC 6882/6885. We find that V381 Vul has a period of 0.1185 days and is a δ Scuti variable, as previously reported. The spectra of V382 Vul shows it to be a B3 star and therefore not a δ Scuti. All evidence points to V382 Vul being a β Cephei star with a period of 0.1808 days. Additionally, we report five new variables and eight suspected variable stars. Of the five new variables, two are pulsators and three are eclipsing binary systems. In our search for new variable stars, we use a “robust median statistic” that is proven to be better at finding low‐amplitude variables than the traditional error curve approach.

Analysis of USNO-B1 proper motions of stars in open-cluster fields

USNO-B1 proper motions for stars in the fields of NGC 457 and χ and h Per open clusters are analyzed and compared to the the proper motions based on plate material taken with normal astrographs. USNO-B1 proper motions in central regions of clusters are found to have large errors due mostly to numerous misidentifications in crowded fields. At the same time, USNO-B1 proper motions for the rest of cluster stars proved to compare fairly well to those inferred in classical astrometric studies. Standard deviations of the proper-motion differences between USNO-B1 and cluster proper-motion studies are found to be on the order of 3.4–4.2 mas yr−1.

Deep ACS Imaging of the Halo of NGC 5128: Reaching the Horizontal Branch

Using the Hubble Space Telescope (HST) Wide Field Camera (WFC) of the Advanced Camera for Surveys (ACS), we have obtained deep (V, I) photometry of an outer halo field in NGC 5128, to a limiting magnitude of I 29. Our photometry directly reveals the core helium burning stellar population (the red clump or horizontal branch) in a giant E/S0 galaxy for the first time. The color-magnitude diagram displays a very wide red giant branch (RGB), an asymptotic giant branch (AGB) bump, and the red clump; no noticeable population of blue horizontal branch stars is present, confirming previous suggestions that old, very metal-poor population is not ubiquitous in the halo of this galaxy. From the upper RGB we derive the metallicity distribution, which we find to be very broad and moderately metal-rich, with average [M/H] = -0.64 and dispersion 0.49 dex. The metallicity distribution function is virtually identical to that found in other halo fields observed previously with HST, but with an enhanced metal-rich population that was partially missed in the previous surveys due to V-band incompleteness for these very red stars. Combining the metallicity-sensitive colors of the RGB stars with the metallicity- and age-sensitive features of the AGB bump and the red clump, we infer the average age of the halo stars to be ~8 Gyr. As part of our study, we present an empirical calibration of the ACS F606W and F814W filters to the standard V and I bands, achieved with ground-based observations of the same field made from the EMMI camera of the New Technology Telescope of the ESO La Silla Observatory.

ChandraX-Ray Observations of Young Clusters. I. NGC 2264 Data

We present results of a Chandra observation of a field in NGC 2264. The observations were taken with the ACIS-I camera with an exposure time of 48.1 ks. We present a catalog of 263 sources, which includes X-ray luminosity, optical and infrared photometry, and X-ray variability information. We found 41 variable sources, 14 of which have a flarelike light curve, and two of which have a pattern of a steady increase or decrease over a 10 hr period. The optical and infrared photometry for the stars identified as X-ray sources are consistent with most of these objects being pre–main-sequence stars with ages younger than 3 Myr.