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At least 631 records · Page 35
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NASA NTRS2010

Deep Galex Observations of the Coma Cluster: Source Catalog and Galaxy Counts

We present a source catalog from deep 26 ks GALEX observations of the Coma cluster in the far-UV (FUV; 1530 Angstroms) and near-UV (NUV; 2310 Angstroms) wavebands. The observed field is centered 0.9 deg. (1.6 Mpc) south-west of the Coma core, and has full optical photometric coverage by SDSS and spectroscopic coverage to r-21. The catalog consists of 9700 galaxies with GALEX and SDSS photometry, including 242 spectroscopically-confirmed Coma member galaxies that range from giant spirals and elliptical galaxies to dwarf irregular and early-type galaxies. The full multi-wavelength catalog (cluster plus background galaxies) is 80% complete to NUV=23 and FUV=23.5, and has a limiting depth at NUV=24.5 and FUV=25.0 which corresponds to a star formation rate of 10(exp -3) solar mass yr(sup -1) at the distance of Coma. The GALEX images presented here are very deep and include detections of many resolved cluster members superposed on a dense field of unresolved background galaxies. This required a two-fold approach to generating a source catalog: we used a Bayesian deblending algorithm to measure faint and compact sources (using SDSS coordinates as a position prior), and used the GALEX pipeline catalog for bright and/or extended objects. We performed simulations to assess the importance of systematic effects (e.g. object blends, source confusion, Eddington Bias) that influence source detection and photometry when using both methods. The Bayesian deblending method roughly doubles the number of source detections and provides reliable photometry to a few magnitudes deeper than the GALEX pipeline catalog. This method is also free from source confusion over the UV magnitude range studied here: conversely, we estimate that the GALEX pipeline catalogs are confusion limited at NUV approximately 23 and FUV approximately 24. We have measured the total UV galaxy counts using our catalog and report a 50% excess of counts across FUV=22-23.5 and NUV=21.5-23 relative to previous GALEX measurements, which is not attributed to cluster member galaxies. Our galaxy counts are a better match to deeper UV counts measured with HST.

Hammer, D.↗
DOE OSTI2023

Searching for Milky Way twins: Radial abundance distribution as a strict criterion

We search for Milky Way-like galaxies among a sample of approximately 500 galaxies. The characteristics we considered of the candidate galaxies are the following: stellar mass M $\star$ , optical radius R 25 , rotation velocity V rot , central oxygen abundance (O/H) 0 , and abundance at the optical radius (O/H) R 25 . If the values of R 25 and M $\star$ of the galaxy were close to that of the Milky Way, then the galaxy was referred to as a structural Milky Way analogue (sMWA). The oxygen abundance at a given radius of a galaxy is defined by the evolution of that region (astration level, that is, the fraction of gas converted into stars, as well as gas exchange with the surroundings), and we then assumed that the similarity of (O/H) 0 and (O/H) R 25 in two galaxies suggests a similarity in their (chemical) evolution. If the values of (O/H) 0 and (O/H) R 25 in the galaxy were close to that of the Milky Way, then the galaxy was referred to as an evolutionary Milky Way analogue (eMWA). If the galaxy was simultaneously an eMWA and sMWA, then the galaxy was considered a Milky Way twin. We find that the position of the Milky Way on the (O/H) 0 –(O/H) R 25 diagram shows a large deviation from the general trend in the sense that the (O/H) R 25 in the Milky Way is appreciably lower than in other galaxies of similar (O/H) 0 . This feature of the Milky Way evidences that its (chemical) evolution is not typical. We identify four galaxies (NGC 3521, NGC 4651, NGC 2903, and MaNGA galaxy M-8341-09101) that are simultaneously sMWA and eMWA and can therefore be considered as Milky Way twins. In previous studies, Milky Way-like galaxies were selected using structural and morphological characteristics, that is, sMWAs were selected. We find that the abundances at the centre and at the optical radius (evolutionary characteristics) provide a stricter criterion for selecting real Milky Way twins.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

Photometric Signature of Ultraharmonic Resonances in Barred Galaxies

Bars may induce morphological features, such as rings, through their resonances. Previous studies suggested that the presence of "dark gaps," or regions of a galaxy where the difference between the surface brightness along the bar major axis and that along the bar minor axis is maximal, can be attributed to the location of bar corotation. Here, using GALAKOS, a high-resolution N-body simulation of a barred galaxy, we test this photometric method's ability to identify the bar corotation resonance. Contrary to previous work, our results indicate that "dark gaps" are a clear sign of the location of the 4:1 ultraharmonic resonance instead of bar corotation. Measurements of the bar corotation can indirectly be inferred using kinematic information, e.g., by measuring the shape of the rotation curve. We demonstrate our concept on a sample of 578 face-on barred galaxies with both imaging and integral field observations and find that the sample likely consists primarily of fast bars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

The Effect of Bars on the Ionized ISM: Optical Emission Lines from Milky Way Analogs

Gas interior to the bar of the Milky Way has recently been shown to be the closest example of a low-ionization (nuclear) emission region (LI(N)ER) in the universe. To better understand the nature of this gas, a sample of face-on galaxies with integral field spectroscopy is used to study the ionized gas conditions of 240 barred and 250 nonbarred galaxies, focusing on those that are most similar to the Milky Way. Strong optical line emission of [N ii] λ6584, Hα, [O iii] λ5007, and Hβ are used to diagnose the dominant ionization mechanisms of gas across galaxies and the Galaxy via Baldwin-Phillips-Terlevich diagrams. Barred galaxies show a strong suppression of star formation and an increase in composite and LI(N)ER-like spectra in their inner regions when compared with similar nonbarred counterparts. This effect is lessened in galaxies of very low (log 10 (M * /M ⊙ ) ≲ 10.4) or very high (log 10 (M * /M ⊙ ) ≳ 11.1) total stellar mass. Bar masks from Galaxy Zoo:3D show the bar's nonaxisymmetric effect on the ionized gas and help predict the face-on distribution of ionized gas conditions near the bar of the Milky Way.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Effects of Magnetic Fields on Gas Dynamics and Star Formation in Nuclear Rings

Nuclear rings at the centers of barred galaxies are known to be strongly magnetized. To explore the effects of magnetic fields on star formation in these rings and nuclear gas flows, we run magnetohydrodynamic simulations in which there is a temporally constant magnetized inflow to the ring, representing a bar-driven inflow. The mass inflow rate is 1M ⊙ yr –1 , and we explore models with a range of field strength in the inflow. We adopt the TIGRESS framework developed by Kim & Ostriker to handle radiative heating and cooling, star formation, and resulting supernova (SN) feedback. We find that magnetic fields are efficiently amplified in the ring due to rotational shear and SN feedback. Within a few 100 Myr, the turbulent component Btrb in the ring saturates at ~35 μG (in rough equipartition with the turbulent kinetic energy density), while the regular component Breg exceeds 50 μG. Expanding superbubbles created by clustered SN explosions vertically drag predominantly toroidal fields from near the midplane to produce poloidal fields in high-altitude regions. The growth of magnetic fields greatly suppresses star formation at late times. Simultaneously, strong magnetic tension in the ring drives radially inward accretion flows from the ring to form a circumnuclear disk in the central region; this feature is absent in the unmagnetized model.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

DIISC-II: Unveiling the Connections between Star Formation and Interstellar Medium in the Extended Ultraviolet Disk of NGC 3344

We present our investigation of the extended ultraviolet (XUV) disk galaxy, NGC 3344, conducted as part of Deciphering the Interplay between the Interstellar medium, Stars, and the Circumgalactic medium survey. We use surface and aperture photometry of individual young stellar complexes to study star formation and its effect on the physical properties of the interstellar medium. We measure the specific star formation rate (sSFR) and find it to increase from 10 –10 yr –1 in the inner disk to >10 –8 yr –1 in the extended disk. This provides evidence for inside-out disk growth. If these sSFRs are maintained, the XUV disk stellar mass can double in ~0.5 Gyr, suggesting a burst of star formation. The XUV disk will continue forming stars for a long time due to the high gas depletion times (τ dep ). The stellar complexes in the XUV disk have high-Σ H I and low-Σ SFR with τ dep ~ 10 Gyr, marking the onset of a deviation from the traditional Kennicutt–Schmidt law. We find that both far-ultraviolet (FUV) and a combination of FUV and 24 μm effectively trace star formation in the XUV disk. Hα is weaker in general and prone to stochasticities in the formation of massive stars. Investigation of the circumgalactic medium at 29.5 kpc resulted in the detection of two absorbing systems with metal-line species: the stronger absorption component is consistent with gas flows around the disk, most likely tracing inflow, while the weaker component is likely tracing corotating circumgalactic gas.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

SDSS-IV MaNGA: Stellar M/L gradients and the M/L-colour relation in galaxies

The stellar mass-to-light ratio gradient in SDSS r-band ∇(M * /L r ) of a galaxy depends on its mass assembly history, which is imprinted in its morphology and gradients of age, metallicity, and stellar initial mass function (IMF). Taking a MaNGA sample of 2051 galaxies with stellar masses ranging from 10 9 to 10 12 M ⊙ released in SDSS DR15, we focus on face-on galaxies, without merger and bar signatures, and investigate the dependence of the 2D ∇(M * /L r ) on other galaxy properties, including M * /L r -colour relationships by assuming a fixed Salpeter IMF as the mass normalization reference. The median gradient is ∇M * /L r ~ –0.1 (i.e. the M * /L r is larger at the centre) for massive galaxies, becomes flat around M * ~ 10 10 M ⊙ and change sign to ∇M * /L r ~ 0.1 at the lowest masses. The M * /L r inside a half-light radius increases with increasing galaxy stellar mass; in each mass bin, early-type galaxies have the highest value, while pure-disc late-type galaxies have the smallest. Correlation analyses suggest that the mass-weighted stellar age is the dominant parameter influencing the M * /L r profile, since a luminosity-weighted age is easily affected by star formation when the specific star formation rate (sSFR) inside the half-light radius is higher than 10 –3 Gyr –1 . With increased sSFR gradient, one can obtain a steeper negative ∇(M * /L r ). The scatter in the slopes of M * /L-colour relations increases with increasing sSFR, for example, the slope for post-starburst galaxies can be flattened to 0.45 from the global value 0.87 in the M * /L versus g – r diagram. Hence converting galaxy colours to M * /L should be done carefully, especially for those galaxies with young luminosity-weighted stellar ages, which can have quite different star formation histories.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

Hubble Space Telescope Captures UGC 12591: bulge/disc properties, star formation and ‘missing baryons’ census in a very massive and fast-spinning hybrid galaxy

We present Hubble Space Telescope (HST) observations of the nearby, massive, highly rotating hybrid galaxy UGC 12591, along with observations in the UV to FIRbands. HST data in V, I, and H bands is used to disentangle the structural components. Surface photometry shows a dominance of the bulge over the disc with an H-band B/D ratio of $69{{\,\rm per\,cent}}$. The spectral energy distribution (SED) fitting reveals an extremely low global star formation rate (SFR) of $\rm \sim 0.1\!-\!0.2\, M_\odot \, yr^{-1}$, exceptionally low for the galaxy’s huge stellar mass of $\rm 1.6\times 10^{11}M_\odot$, implying a strong quenching of its SFR with a star formation efficiency of 3–5 per cent. For at least the past $\rm 10^{8}$ yr, the galaxy has remained in a quiescent state as a sterile, ‘red and dead’ galaxy. UGC 12591 hosts a supermassive black hole (SMBH) of $\rm 6.18\times 10^{8} \, M_\odot$, which is possibly quiescent at present, i.e. we neither see large ($\rm \gt 1 kpc$) radio jets nor the SMBH contributing significantly to the mid-IR SED, ruling out strong radiative feedback of AGN. We obtained a detailed census of all observable baryons with a total mass of $\rm 6.46\times 10^{11} \, M_\odot$ within the virial radius, amounting to a baryonic deficiency of ${\sim }85{{\,\rm per\,cent}}$ relative to the cosmological mean. Only a small fraction of these baryons reside in a warm/hot circumgalactic X-ray halo, while the majority are still unobservable. We discussed various astrophysical scenarios to explain its unusual properties. Our work is a major step forward in understanding the assembly history of such extremely massive, isolated galaxies.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Discovery of compact disc galaxies with high-surface brightness in the Sloan Digital Sky Survey

Compact disc galaxies (CDGs) with high-surface brightness were identified in the Sloan Digital Sky Survey data. We determined the surface profiles of the CDGs and compared them to those of normal-sized disc galaxies (NDGs). The CDGs have higher central brightness and older stellar age than the NDGs. Furthermore, the brightness profiles of the CDGs fit a Sérsic model with n ≈ 2.11 and have a zero g′ − r′ colour gradient on average. By contrast, the NDGs fit an exponential profile and have a negative colour gradient on average. These results indicate that the structure and stellar population of the CDGs and NDGs differ. We suggest that the CDGs are ancient galaxies in the quenching phase following the initial central starburst.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1988

A new technique for measuring extragalactic distances

A relatively direct technique of determining extragalactic distances is described. The method relies on measuring the luminosity fluctuations that arise from the counting statistics of the stars contributing the flux in each pixel of a high signal-to-noise CCD image of a galaxy. The amplitude of these fluctuations is inversely proportional to the distance of the galaxy. This approach bypasses most of the successive stages of calibration required in the traditional extragalactic distance ladder; the only serious drawback to this method is that it requires an accurate knowledge of the bright end of the luminosity function. Potentially, this method can produce accurate distances of elliptical galaxies and spiral bulges at distances out to about 20 Mpc. In this paper, it is explained how to calculate the value of the fluctuations, taking into account various sources of contamination and the effects of finite spatial resolution, and the feasibility and limitations of this technique are demonstrated via simulations and CCD images of M 32 and N 3379.

Tonry, John↗
NASA NTRS1994

Powerful model for the point source sky: Far-ultraviolet and enhanced midinfrared performance

I report further developments of the Wainscoat et al. (1992) model originally created for the point source infrared sky. The already detailed and realistic representation of the Galaxy (disk, spiral arms and local spur, molecular ring, bulge, spheroid) has been improved, guided by CO surveys of local molecular clouds, and by the inclusion of a component to represent Gould's Belt. The newest version of the model is very well validated by Infrared Astronomy Satellite (IRAS) source counts. A major new aspect is the extension of the same model down to the far ultraviolet. I compare predicted and observed far-utraviolet source counts from the Apollo 16 'S201' experiment (1400 A) and the TD1 satellite (for the 1565 A band).

Cohen, Martin↗
NASA NTRS2014

Galaxy Evolution Spectroscopic Explorer (GESE): Science Rationale, Optical Design, and Telescope Architecture

One of the key goals of NASA’s astrophysics program is to answer the question: How did galaxies evolve into the spiral, elliptical, and irregular galaxies that we see today? We describe a space mission concept called Galaxy Evolution Spectroscopic Explorer (GESE) to help address this question by making a large ultraviolet spectroscopic survey of galaxies at a redshift, z approximately 1 (look-back time of approximately 8 billion years). GESE is a 1.5-m space telescope with an near-ultraviolet (NUV) multi-object slit spectrograph covering the spectral range, 0.2-0.4 micrometers (0.1-0.2 micrometers as emitted by galaxies at a redshift, z approximately 1) at a spectral resolution of delta lambda=6 A.

elliptical↗