arXiv subjects

Kartik Sheth

Publications and source records attributed to Kartik Sheth.

108 records · Page 6Linked to original sources

arXiv2005

Spitzer Observations Of The Supergiant Shell Region In IC 2574

We present spatially resolved Spitzer imaging of the supergiant shell region of the M81 group dwarf galaxy IC 2574 obtained as part of the Spitzer Infrared Nearby Galaxies Survey. This region harbors one of the best nearby examples of a kinematically distinct HI shell, with an associated remnant stellar cluster; the shell is initiating sequential star formation as it interacts with the surrounding interstellar medium. This region dominates the infrared luminosity of IC 2574 and is spatially resolved in all Spitzer imaging bands. We study the differences in dust temperature as a function of local environment and compare local star formation rates as inferred from H Alpha and total infrared luminosities. We find that the strong H Alpha sources are associated with regions of warm dust; however, the most luminous infrared and H Alpha sources are not necessarilyco-spatial. The coolest dust is found in the regions farthest from the rim of the shell; these regions show the best agreement between star formation rates derived from H Alpha and from total infrared luminosities (although discrepancies at the factor of 3-4 level still exist). There is considerable variation in the radio-far infrared correlation in different regions surrounding the shell. The low dust content of the region may influence the scatter seen in these relations; these data demonstrate that the expanding shell is dramatically affecting its surroundings by triggering star formation and altering the dust temperature.

astro-ph↗

arXiv2005

Secular Evolution Via Bar-Driven Gas Inflow: Results from BIMA SONG

We present an analysis of the molecular gas distributions in the 29 barred and 15 unbarred spirals in BIMA SONG. For CO-bright galaxies, we confirm the conclusion by Sakamoto et al. (1999b) that barred spirals have higher molecular gas concentrations in the central kiloparsec. The SONG sample also includes 27 galaxies below the CO brightness limit used by Sakamoto et al. Even in these CO-faint galaxies we show that high central gas concentrations are more common in barred galaxies, consistent with radial inflow driven by the bar. However, there is a significant population of early-type (Sa--Sbc) barred spirals (6 of 19) that have little or no molecular gas detected in the nuclear region and out to the bar co-rotation radius. In these galaxies, the bar has already driven most of the gas within the bar to the nuclear region, where it has been consumed by star formation. The median nuclear gas mass is over four times higher in early type bars; since the gas consumption rate is an order of magnitude higher in early type bars, early types must have significantly higher bar-driven inflows. The lower inflow rates in late type bars can be attributed to differences in bar structure between early and late types. Despite bar-driven inflows, the data indicate that it is highly unlikely for a late type galaxy to evolve into an early type via bar-induced gas inflow. Nonetheless, secular evolutionary processes are undoubtedly present, and pseudo-bulges are inevitable; evidence for pseudo-bulges is likely to be clearest in early-type galaxies because of their high gas inflow rates and higher star formation activity (abridged).

astro-ph↗

arXiv2005

Molecular Gas in a z~2.5 Triply-Imaged, sub-mJy Submillimetre Galaxy Typical of the Cosmic Far-Infrared Background

We present the results of observations from the IRAM array of the submm galaxy SMMJ16359+6612 lying at z=2.516 behind the massive cluster A2218. The foreground gravitational lens produces 3 images with a total mag. of 45 of this faint submm galaxy, which has an intrinsic submm flux of f_850mic=0.8mJy placing it below the confusion limit of blank-field surveys. The substantial magnification provides a rare opportunity to probe the nature of a distant sub-mJy submm-selected galaxy, part of the population which produces the bulk of the submm cosmic far-infrared background. Our observations detect the CO(3-2) line in all 3 images, as well as the CO(7-6) line and the dust continuum at 1.3mm for the brightest image. The CO(3-2) velocity profile displays a double-peak profile which is well fit by two Gaussians with FWHM of 220km/s and separated by 280km/s. We estimate the dynamical mass of the system to be ~1.5 10^10 M_sun and an H2 gas mass of 2.6 10^9 M_sun. We identify a spatial offset of ~1'' between the two CO(3-2) velocity components, modeling of which indicates that the offset corresponds to just ~3kpc in projection at z=2.5. The spatial and velocity properties of these two components are closely related to features detected in previously published Halpha spectroscopy. We conclude that this source is likely to be a compact merger of 2 fairly typical Ly-break galaxies with a maximal separation between the two nuclei of ~3kpc. This system is much less luminous and massive than other high-z submm galaxies studied to date, but it bears a close similarity to similarly luminous, dusty starburst resulting from lower-mass mergers in the local Universe.

astro-ph↗

arXiv2005

Warm and Cold Molecular Gas in Galaxies

New and archival interferometric 12CO(1->0) datasets from six nearby galaxies are combined with H_2 2.122um and H-alpha maps to explore in detail the interstellar medium in different star-forming galaxies. We investigate the relation between warm (H_2 at T~2000 K) and cold (CO at T~50 K) molecular gas from 100 pc to 2 kpc scales. On these scales, the ratio of warm-to-cold molecular hydrogen correlates with the fnu(60um)/fnu(100um) ratio, a ratio that tracks the star formation activity level. This result also holds for the global properties of galaxies from a much larger sample drawn from the literature. The trend persists for over three orders of magnitude in the mass ratio, regardless of source nuclear activity.

astro-ph↗

arXiv2004

Detection of CO from SMM J16359+6612, The Multiply Imaged Submillimeter Galaxy Behind A2218

We report the detection of CO ($J$=3$\to$2) line emission from all three multiple images (A,B and C) of the intrinsically faint ($\simeq$ 0.8 mJy) submillimeter-selected galaxy SMM J16359+6612. The brightest source of the submm continuum emission (B) also corresponds to the brightest CO emission, which is centered at $z$=2.5168, consistent with the pre-existing redshift derived from \Ha. The observed CO flux in the A, B and C images is 1.2, 3.5 and 1.6 Jy \kms respectively, with a linewidth of $500\pm 100$ \kms. After correcting for the lensing amplification, the CO flux corresponds to a molecular gas mass of $\sim 2\times 10 ^{10} h_{71}^{-2}$ \Msun, while the extent of the CO emission indicates that the dynamical mass of the system $\sim9\times10^{10}$ \Msun. Two velocity components are seen in the CO spectra; these could arise from either a rotating compact ring or disk of gas, or merging substructure. The star formation rate in this galaxy was previously derived to be $\sim$100--500 \Msun \yr. If all the CO emission arises from the inner few kpc of the galaxy and the galactic CO-to-H$_2$ conversion factor holds, then the gas consumption timescale is a relatively short 40 Myr, and so the submm emission from SMM J16359+6612 may be produced by a powerful, but short-lived circumnuclear starburst event in an otherwise normal and representative high-redshift galaxy.

astro-ph↗

arXiv2004

CO in the Bipolar-Radio-Continuum Galaxy NGC 3367

CO emission has been imaged at 2" resolution in the central 10 kpc of the barred spiral galaxy NGC 3367. This galaxy has bipolar synchrotron lobes out to a radii of 6 kpc, straddling the compact nucleus. The peak molecular emission is in a source of radius 2" (425 pc) centered on the galaxy nucleus. The molecular mass is about 3 times 10**8 Msun in this peak an about 5.9 times 10**8 Msun within a radius of 4".5 (950 pc). The very large gas masses in the central source imply extinctions sufficiently high to completely obscure optical emission lines (e.g.BLR) associated with the nuclear radio source. The observed Balmer lines probably originate in the narrow line region few hundred pc from the nucleus. The CO emission in the central region is elongated NE-SW, very similar to the PA of the large scale synchrotron lobes. This elongation is likely due to non-axisymmetric gravitational potential of the stellar bar. We infer that the NE radio continuum lobe is on the far side of the galaxy and the SW lobe is on the near side. The central mass of molecular gas is sufficient mass to power the AGN accretion luminosity for over 10**8 yrs at 3 Msun per yr.

astro-ph↗

arXiv2003

Barred Galaxies at z > 0.7: NICMOS HDFN Observations

Previous optical studies found an unexpected deficit of bars at z > 0.7. To investigate the effects of bandshifting, we have studied the fraction of barred spirals in the NICMOS Deep Field North. At z > 0.7 we find at least four barred spirals, doubling the number previously detected. The number of barred galaxies is small because these (and previous) data lack adequate spatial resolution. A typical 5 kpc bar at z > 0.7 is only marginally detectable for WFPC2 at 0.8microns; the NICMOS data have even lower resolution and can only find the largest bars. The average size of the four bars seen at z > 0.7 is 12 kpc. The fraction of such large bars (4/95) is higher than that seen in nearby spirals (1/44); all known selection effects suggest that the observed fraction is a lower limit. However, important caveats such as small numbers and difficulties in defining comparable samples at high and low redshifts should be noted. We conclude that there is no significant evidence for a decrease in the fraction of barred spirals beyond z ~ 0.7.

astro-ph↗

arXiv2003

The BIMA Survey of Nearby Galaxies (BIMA SONG). II. The CO Data

The BIMA Survey of Nearby Galaxies is a systematic imaging study of the 3 mm CO J = 1--0 molecular emission within the centers and disks of 44 nearby spiral galaxies. The typical spatial resolution of the survey is 6", or 360 pc at the average distance (12 Mpc) of the sample, over a field of view of 10kpc. The velocity resolution of the CO observations is 4 km/s. The sample was not chosen based on CO or infrared brightness; instead, all spirals were included that met the selection criteria of vsun <= 2000 km/s, dec >= -20deg, inc <= 70deg, D25 < 70', and BT < 11.0. The detection rate was 41/44 sources or 93%. Fully-sampled single-dish CO data were incorporated into the maps for 24 galaxies; these single-dish data comprise the most extensive collection of fully-sampled, two-dimensional single-dish CO maps of external galaxies to date. We also tabulate direct measurements of the global CO flux densities for these 24 sources. We demonstrate that the measured ratios of flux density recovered are a function of the signal-to-noise of the interferometric data. We examine the degree of central peakedness of the molecular surface density distributions and show that the distributions exhibit their brightest CO emission within the central 6" in only 20/44 or 45% of the sample. We show that all three Local Group spiral galaxies have CO morphologies that are represented in SONG, though the Milky Way CO luminosity is somewhat below the SONG average, and M31 and M33 are well below average. This survey provides a unique public database of integrated intensity maps, channel maps, spectra, and velocity fields of molecular emission in nearby galaxies.

astro-ph↗

arXiv2002

Central Mass Concentration and Bar Dissolution in Nearby Spiral Galaxies

We use data from the BIMA Survey of Nearby Galaxies (SONG) to investigate the relationship between ellipticity and central mass concentration in barred spirals. Existing simulations predict that bar ellipticity decreases as inflowing mass driven by the bar accumulates in the central regions, ultimately destroying the bar. Using the ratio of the bulge mass to the mass within the bar radius as an estimate of the central mass concentration, we obtain dynamical mass estimates from SONG CO 1-0 rotation curve data. We find an inverse correlation between bar ellipticity and central mass concentration, consistent with simulations of bar dissolution.

astro-ph↗

arXiv2002

Molecular Gas and Star Formation in Bars of Nearby Spiral Galaxies

We compare the distribution of molecular gas and star formation activity in the bar region of six spirals (NGC 2903, NGC 3627, NGC 4321, NGC 5457, NGC 6946, & IC 342) from the BIMA Survey of Nearby Galaxies (SONG). The molecular gas, traced using the CO (J=1--0) emission line, is brightest along the leading edge of the stellar bar in the bar dust lanes. The star formation activity, traced using the Halpha emission line, is offset towards the leading side of the CO emission. A cross-correlation analysis shows that a) the HII regions are offset 0--800 pc on the leading side of the CO emission, b) the largest offsets are found in the strongest bars, and c) there is a wide range in offsets in a single bar with no systematic pattern as a function of the galacto-centric radius. The CO-Halpha offset constrains how stars may form depending on the gas flow. We examine possible star formation scenarios in context of the two main classes of bar gas flow simulations, the N-body/sticky particle and hydrodynamic models. Though both model gas flows are generally consistent with the observed offsets, we suggest the inclusion of a two- or multi-phase medium to improve the agreement between models and observation.

astro-ph↗

arXiv2002

Inner Molecular Rings in Barred Galaxies: BIMA SONG CO Observations

Although inner star-forming rings are common in optical images of barred spiral galaxies, observational evidence for the accompanying molecular gas has been scarce. In this paper we present images of molecular inner rings, traced using the CO (1-0) emission line, from the Berkeley-Illinois-Maryland-Association Survey of Nearby Galaxies (BIMA SONG). We detect inner ring CO emission from all five SONG barred galaxies classified as inner ring (type (r)). We also examine the seven SONG barred galaxies classified as inner spiral (type (s)); in one of these, NGC 3627, we find morphological and kinematic evidence for a molecular inner ring. Inner ring galaxies have been classified as such based on optical images, which emphasize recent star formation. We consider the possibility that there may exist inner rings in which star formation efficiency is not enhanced. However, we find that in NGC 3627 the inner ring star formation efficiency is enhanced relative to most other regions in that galaxy. We note that the SONG (r) galaxies have a paucity of CO and H alpha emission interior to the inner ring (except near the nucleus), while NGC 3627 has relatively bright bar CO and H alpha emission; we suggest that galaxies with inner rings such as NGC 3627 may be misclassified if there are significant amounts of gas and star formation in the bar.

astro-ph↗

arXiv2001

CO Observations of the Interacting Galaxy Pair NGC 5394/95

BIMA CO 1-0 observations are presented of the spiral galaxies NGC 5394 and NGC 5395 that have undergone a recent, grazing encounter. In NGC 5394, 80% of the CO emission detected by BIMA is concentrated in the central 800 pc (FWHM) starburst region.In an encounter simulation that reproduces some of the main features of this galaxy pair, a considerable amount of gas in NGC 5394 falls into the central region early in the collision. The observed total gas distribution in the disk of NGC 5394 is lopsided, with more HI, CO, and H-alpha emission coming from the western or southwestern side. The innermost western arm of NGC 5394 is seen in CO and H-alpha emission, but the eastern inner-disk arm, which is very bright in the optical continuum, is not detected in CO or H-alpha emission. From a comparison of the radio continuum, H-alpha, 60 micron, and CO luminosities, we estimate that the average visual extinction of the starburst is 3 - 4 mag and the conversion factor N(H2)/I(CO) in the starburst is a factor of 3 - 4 below the standard value. Comparison of NGC 5394 with two other systems previously studied suggests that in prograde grazing encounters a central starburst may not develop until near the end of the ocular phase. Very little of the CO emission from NGC 5395 found in previous single-dish observations is detected by BIMA.

astro-ph↗

arXiv2001

Molecular Gas Properties in the Central Kiloparsec of Barred and Unbarred Spirals

We study the molecular gas properties in the central kiloparsec of 29 barred and 15 unbarred spirals from the BIMA Survey of Nearby Galaxies (SONG). We find that the mean nuclear molecular gas surface density of barred spirals ( =309 +/- 71 Msun pc^{-2}) is three times higher than that of unbarred spirals ( =107 +/- 29 Msun pc^{-2}). Nine out of the eleven bars with Sigma_{nuc} > 300 Msun pc^{-2} are early types. Comparison with estimates of the star formation threshold density (Sigma_{crit}) indicates that enhanced star formation in bars may be due to a larger fraction having Sigma_{nuc} > typical Sigma_{crit}. We also find that barred spirals are more centrally concentrated than unbarred spirals. The median value of the concentration parameter for barred spirals (f_{con}=Sigma_{nuc}/Sigma_{disk}=27.7) is a factor of four higher than that for the unbarred spirals (f_{con}=6.2). Finally we investigate the dependence of the central concentration on bar properties and find it may be weakly correlated with the bulge size, but is not correlated with the bar length. This suggests that inner Lindblad resonances, bar ellipticity and circumnuclear star formation play important roles in determining the gas accretion in the central kiloparsec. See http://www.astro.caltech.edu/~kartik/thesis.html for more detailed description of these thesis results. This research was made possible with funding from NSF grants AST-9981308, AST-9981289 and AST-9981546.

astro-ph↗

arXiv2001

The BIMA Survey of Nearby Galaxies (BIMA SONG): I. The Radial Distribution of CO Emission in Spiral Galaxies

We present the first results of the BIMA Survey of Nearby Galaxies (BIMA SONG), an imaging survey of the CO J=(1-0) emission in 44 nearby spiral galaxies at a typical resolution of 6". BIMA SONG differs from previous high-resolution CO surveys in that: (1) CO brightness was not an explicit selection criterion; (2) a larger area (200" diameter for most galaxies) of each galaxy was imaged; and (3) fully-sampled single-dish CO data (55" resolution) were obtained for over half of the sample galaxies, so all of the CO flux is imaged in these galaxies. The radial distribution of stellar light in galactic disks is generally characterized as an exponential. It is, therefore, of interest to investigate whether the molecular gas, which is the star-forming medium, has a similar distribution. Though our low-resolution single-dish radial profiles of CO emission can be described by simple exponentials, this is not true for the emission at our full 6" resolution. The scale lengths of the CO disks are correlated with the scale lengths of the stellar disks with a mean ratio of the scale lengths of about one. We also find that:(1) there is also a weak correlation between the ratio of K-band to CO luminosity and Hubble type; (2) in half of the galaxies presented here, CO emission does not peak at the location of the stellar nucleus; (3) averaged over the inner kiloparsec, the CO emission in one-half of the galaxies exhibits an excess over that expected from an exponential disk which is similar to the excess in stellar light caused by the bulge stars; and (4) this excess CO emission may be due to an increase in the total molecular gas content in the bulge region, or alternatively, to an increase in the CO emissivity caused by the increased pressure of the bulge region.

astro-ph↗

arXiv2000

Comparing Giant Molecular Clouds in M31, M33 & the Milky Way

We present new observations of a 2' field in the north-eastern spiral arm of M31. In the 0.8 x 3.6 kpc mosaicked region, we have detected six distinct, large complexes of molecular gas, most of which lie along the spiral arm dust lane or in the vicinity of HII regions. The mean properties of these complexes are as follows: diameter ~ 57+/-13 pc, fwhm velocity ~ 6.5+/-1.2 km/s, M(CO) ~ 3.0+/-1.6 x 10^5 solar masses, peak brightness temperatures ~ 1.6--4.2 K. We investigate the effects of spatial filtering on the quantitative comparison of Local Group and Milky Way giant molecular clouds properties and distributions. We also discuss different cloud identification techniques and their impact on derived cloud properties. When we employ the same cloud identification method and account for differences in data acquisition for M31, Milky Way, and M33, we find that the molecular cloud complexes in all three galaxies are similar. While the global distribution of molecular gas may vary from galaxy to galaxy, cloud complexes are similar, suggesting that cloud formation and destruction is determined by local physics. This work is supported by grants AST-9613716 & AST-9981289 from the National Science Foundation.

astro-ph↗

arXiv2000

Molecular gas and star formation in BIMA SONG bars

Using a sample of 7 barred spirals from the BIMA Survey of Nearby Galaxies (SONG), we compare the molecular gas distribution in the bar, to recent massive star formation activity. In all 7 galaxies, Halpha is offset azimuthally from the CO on the downstream side. The maximum offset, at the bar ends, ranges from 170-570 pc, with an average of 320 +/- 120 pc. We discuss whether the observed offsets are consistent with the description of gas flows in bars provided by the two main classes of models: n-body models and hydrodynamic models. This work is supported by NSF grants AST 9981289, AST 9981308.

astro-ph↗

arXiv1999

Molecular Gas, Dust and Star Formation in the Barred Spiral NGC 5383

We present multi-wavelength (interferometer and single-dish CO J=1-0, Halpha, broadband optical and near-infrared) observations of the classic barred spiral NGC 5383. We compare the observed central gas and dust morphology to the predictions of recent hydrodynamic simulations. In the nuclear region, our observations reveal three peaks lying along a S-shaped gas and dust distribution. In contrast, the model predicts a circumnuclear ring, not the observed S-shaped distribution; moreover, the predicted surface density contrast between the central gas accumulation and the bar dust lanes is an order of magnitude larger than observed. The discrepancies are not due to unexplored model parameter space or a nuclear bar but are probably due to the vigorous (7 solar masses per year) star formation activity in the center. As is common in similar bars, the star formation rate in the bar between the bar ends and the central region is low (~0.5 solar masses per yr), despite the high gas column density in the bar dust lanes; this is generally attributed to shear and shocks. We note a tendency for the HII regions to be associated with the spurs feeding the main bar dust lanes, but these are located on the leading side of the bar. We propose that stars form in the spurs, which provide a high column density but low shear environment. HII regions can therefore be found even on the leading side of the bar because the ionizing stars pass ballistically through the dust lane

astro-ph↗

arXiv1999

Molecular Gas Kinematics in Barred Spiral Galaxies

To quantify the effect that bar driven mass inflow can have on the evolution of a galaxy requires an understanding of the dynamics of the inflowing gas. In this paper we study the kinematics of the dense molecular gas in a set of seven barred spiral galaxies to determine which dynamical effects dominate. The kinematics are derived from observations of the CO J=(1-0) line made with the Berkeley-Illinois-Maryland Association (BIMA) millimeter array. We compare the observed kinematics to those predicted by ideal gas hydrodynamic and ballistic cloud-based models of gas flow in a barred potential. The hydrodynamic model is in good qualitative agreement with both the current observations of the dense gas and previous observations of the kinematics of the ionized gas. The observed kinematics indicate that the gas abruptly changes direction upon entering the dust lanes to flow directly down the dust lanes along the leading edge of the bar until the dust lanes approach the nuclear ring. Near the location where the dust lanes intersect the nuclear ring, we see two velocity components: a low velocity component, corresponding to gas on circular orbits, and a higher velocity component, which can be attributed to the fraction of gas flowing down the bar dust lane which sprays past the contact point toward the other half of the bar. The ballistic cloud-based model of the ISM is not consistent with the observed kinematics. The kinematics in the dust lanes require large velocity gradients which cannot be reproduced by an ISM composed of ballistic clouds with long mean-free-paths. Therefore, even the dense ISM responds to hydrodynamic forces.

astro-ph↗