Shardha Jogee

Multiepoch multiwavelength spectra and models for blazar 3C 279

Of the blazars detected by EGRET in GeV γ-rays, 3C 279 is not only the best observed by EGRET but also one of the best monitored at lower frequencies. We have assembled 11 spectra, from GHz radio through GeV γ-rays, from the time intervals of EGRET observations. Although some of the data have appeared in previous publications, most are new, including data taken during the high states in early 1999 and early 2000. All of the spectra show substantial γ-ray contribution to the total luminosity of the object; in a high state, the γ-ray luminosity dominates over that at all other frequencies by a factor of more than 10. There is no clear pattern of time correlation; different bands do not always rise and fall together, even in the optical, X-ray, and γ-ray bands. The spectra are modeled using a leptonic jet, with combined synchrotron self-Compton plus external Compton γ-ray production. Spectral variability of 3C 279 is consistent with variations of the bulk Lorentz factor of the jet, accompanied by changes in the spectral shape of the electron distribution. Our modeling results are consistent with the UV spectrum of 3C 279 being dominated by accretion disk radiation during times of low γ-ray intensity.

A Gas-rich Nuclear Bar Fueling a Powerful Central Starburst in NGC 2782

We present evidence that the peculiar interacting starburst galaxy NGC 2782 (Arp 215) harbors a gas-rich nuclear stellar bar feeding an M82-class powerful central starburst, from a study based on high-resolution interferometric CO (J = 1 → 0) data and optical BVR and Hα observations, along with available near-infrared (NIR) images, a 5 GHz radio continuum map, and archival Hubble Space Telescope (HST) images. Morphological and kinematic data show that NGC 2782 harbors a clumpy, barlike CO feature of radius ~75 (1.3 kpc) that leads a nuclear stellar bar of similar size. The nuclear barlike CO feature is massive: it contains ~2.5 × 109 M☉ of molecular gas, which makes up ~8% of the dynamical mass present within a 1.3 kpc radius. Within the CO bar, emission peaks in two extended clumpy lobes that lie on opposite sides of the nucleus, separated by ~6 (1 kpc). Between the CO lobes, in the inner 200 pc radius, resides a powerful central starburst that is forming stars at a rate of 3-6 M☉ yr-1. While circular motions dominate the CO velocity field, the CO lobes show weak barlike streaming motions on the leading side of the nuclear stellar bar, suggestive of gas inflow. We estimate semianalytically the gravitational torque from the nuclear stellar bar on the gas and suggest large gas inflow rates from the CO lobes into the central starburst. These observations, which are among the first ones showing a nuclear stellar bar fueling molecular gas into an intense central starburst, are consistent with simulations and theory, which suggest that nuclear bars provide an efficient way of transporting gas closer to the galactic center to fuel central activity. Furthermore, several massive (107-108 M☉) clumps are present at low radii, and dynamical friction might produce further gas inflow. We suggest that the nuclear barlike molecular gas feature and central activity will be very short lived, likely disappearing within 5 × 108 yr.

Gasdynamics in NGC 5248: Fueling a circumnuclear starburst ring of super-star clusters

Through observations and modeling, we demonstrate how the recently discovered large-scale bar in NGC 5248 generates spiral structure that extends from 10 kpc down to 100 pc, fuels star formation on progressively smaller scales, and drives disk evolution. Deep inside the bar, two massive molecular spirals cover nearly 180° in azimuth, show streaming motions of 20-40 km s-1, and feed a starburst ring of super-star clusters at 375 pc. They also connect to two narrow K-band spirals that delineate the UV bright star clusters in the ring. The data suggest that the K-band spirals are young, and the starburst has been triggered by a bar-driven spiral density wave (SDW). The latter may even have propagated closer to the center where a second Hα ring and a dust spiral are found. The molecular and Hubble Space Telescope data support a scenario where stellar winds and supernovae efficiently clear out gas from dense star-forming regions on timescales less than a few Myr. We have investigated the properties of massive CO spirals within the framework of bar-driven SDWs, incorporating the effect of gas self-gravity. We find good agreement between the model predictions and the observed morphology, kinematics, and pitch angle of the spirals. This combination of observations and modeling provides the best evidence to date for a strong dynamical coupling between the nuclear region and the surrounding disk. It also confirms that a low central mass concentration, which may be common in late-type galaxies, is particularly favorable to the propagation of a bar-driven gaseous SDW deep into the central region of the galaxy, whereas a large central mass concentration favors other processes, such as the formation and decoupling of nuclear bars.

Discovery and Implications of a new large-scale stellar bar in NGC 5248

For decades, the grand-design SAB spiral galaxy NGC 5248 has been postulated to host a short bar of semimajor axis 22 (1.6 kpc). From dynamical and morphological arguments, however, we argue that its spiral structure is being driven by a large-scale bar whose corotation radius lies at ~115 (8.6 kpc). Our estimate is based partially on a deep R-band image, which reveals that the feature previously thought to be an inclined disk is in fact an extended stellar bar. The bar is embedded within a fainter outer disk visible out to a radius of 230 (17.2 kpc). The bar has a deprojected ellipticity of 0.44 and a semimajor axis of 95 (7.1 kpc). The classical grand-design spirals of NGC 5248, prominent in B, R, and K light, lie on the leading edge of the large-scale stellar bar and are accompanied by concave dust lanes out to at least 70. The offset between the dust and young stars is consistent with our understanding of gas flows in barred galaxies, where shocks along the leading edges of a moderately strong bar compress the gas to form massive young stars. While in many strongly barred galaxies, optical spiral arms are prominent outside the bar but not within it, NGC 5248 illustrates how intense star formation along a moderately strong bar can lead to conspicuous open spiral arms within the bar itself. NGC 5248 also provides a clear example of how a large-scale stellar bar embedded within a faint outer optical disk can be misidentified as an inclined disk when imaging studies lack the sensitivity to detect the actual outer disk. We discuss the implications for the estimated bar fraction at higher redshifts.

The Remarkable Starburst-driven Outflow in NGC 2782

We show that the starburst-driven outflow in the peculiar galaxy NGC 2782 forms a well-defined collimated bubble that has an extent of ~1 kpc and a closed shell at its edge, as seen in Hα, [O III], and the 5 GHz radio continuum. The shell coincides with the maximum in intensity and line widths of [O III] lines in a blueshifted emission nebula that was previously detected via optical spectroscopy by Boer et al. in 1992. Such a remarkable outflow morphology has not been observed to date in any other starburst galaxy of comparable luminosity. The radio continuum map reveals a second bubble of similar size on the opposite side of the nucleus, forming a striking double-bubble outflow morphology. We argue from the morphology and short timescale (~4×106 yr) of the outflow that it is dynamically younger than freely expanding outflows seen in other galaxies that harbor circumnuclear starbursts of comparable luminosity, e.g., M82. We suggest that the outflow in NGC 2782 is in the early stage where thermal instabilities have not yet completely ruptured the outflow bubble. We present evidence that the outflow is driving warm and hot ionized gas, and possibly cold molecular gas, out of the central kiloparsec of the galaxy. We estimate the contribution of the hot, warm, and cold phases of the interstellar medium to the energetics of the outflow. This study is based on our optical BVR, Hα, and [O III] observations from the Wisconsin-Indiana-Yale-NRAO telescope and Owens Valley Radio Observatory CO interferometric data, along with available 5 GHz radio continuum and ROSAT X-ray maps.

The Molecule-rich Tail of the Peculiar Galaxy NGC 2782 (Arp 215)

We present the first detection of a large quantity of molecular gas in the extended tail of an interacting galaxy. Using the NRAO 12 m telescope, we have detected CO (1–0) at five locations in the eastern tail of the peculiar starburst galaxy NGC 2782. The CO velocities and narrow (FWHM ~ 50 km s-1) line widths in these positions agree with those seen in H I, which confirms that the molecular gas is indeed associated with the tail rather than the main disk. As noted previously, the gas in this tail has an apparent counterrotation compared to gas in the core of the galaxy, probably because the tails do not lie in the same plane as the disk. Assuming the standard Galactic conversion N/ICO factor, these observations indicate a total molecular gas mass of 6 × 108 M⊙ in this tail. This may be an underestimate of the total H2 mass if the gas is metal poor. This molecular gas mass, and the implied H2/H I mass ratio of 0.6, are higher than that found in many dwarf irregular galaxies. Comparison with an available Hα map of this galaxy, however, shows that the rate of star formation in this feature is extremely low relative to the available molecular gas, compared to LHα/M values for both spiral and irregular galaxies. Thus, the timescale for depletion of the gas in this feature is very long.

Dynamical influences on star formation in spiral galaxies

The principal dynamical influences on large-scale star formation in relatively undisturbed spiral galaxies are reviewed, using recent observations on flocculent galaxies, spiral arms, bars, resonance rings, and circumnuclear starbursts. Non-axisymmetric features in the gravitational potential like bars and spiral arms impact star formation in at least two ways. They cause radial flows of gas, influencing where gas concentrates and therefore where star formation is likely. They also affect the kinematics and density of gas, and therefore its susceptibility to instabilities. The local gravitational instability theory is relatively successful in predicting whether or not gas in a given location undergoes star formation, on scales ranging from outer disks to starbursting central regions. Much about galaxy evolution depends on the relative values of the radial flow rate and the star formation rate, both of which are strongly influenced by galaxy dynamics.

Starbursts: Triggers and Evolution

Why do the circumnuclear (inner 1–2 kpc) regions of spirals show vastly different star formation rates (SFR) even if they have a comparable molecular gas content? Why do some develop starbursts which are intense short-lived (t ≪ 1 Gyr) episodes of star formation characterized by a high star formation rate per unit mass of molecular gas (SFR/MH2 ), which I refer to as star formation efficiency (SFE). I address these questions using high resolution (2″ or 100–200 pc) CO (J=1→0) observations from the Owens Valley Radio Observatory, optical and NIR images, along with published radio continuum (RC) and Brγ data. The sample of eleven galaxies includes the brightest nearby starbursts comparable to M82 and control non-starbursts. More detailed results are in [8] and [10].

The Interplay Between The Nuclear Bars, Central Starburst, And Remarkable Outflow In Ngc 2782

We show that the nearby peculiar interacting galaxy NGC 2782 (Arp 215) harbors a clumpy molecular CO bar. The gas bar has a radius of ∼ 7.5″ (1.3 kpc), a mass of ∼ 2.4 x 109 M⨀, and leads a nuclear stellar bar of similar extent (Jogee et al. 1997b). We estimate the gravitational torque exerted by the nuclear stellar bar and find large gas inflow rates (≫ 1 M⨀ yr-1) into the central 200 pc where the starburst activity peaks. We suggest that the nuclear gas bar will disappear on timescales ranging from few x 108 years to a Gyr, under the effect of star formation, dynamical friction, and gravitational torque. We also show that NGC 2782 harbors a starburstdriven outflow with a remarkable bubble morphology that has not been observed to date in any other starburst galaxy of comparable luminosity (Jogee et al. 1997a). The outflow is driving hot, warm, and possibly cold phases of the ISM out of the central kpc. The estimated outflow rate is less than the inflow rate, and this suggests the dynamical mass in the inner 100 pc might increase by several folds within a Gyr. These results might be relevant to theory (Shlosman et al. 1989) and simulations (Friedli & Martinet 1993) which propose nuclear bars and ‘bars within bars’ as efficient mechanisms for fueling central starbursts/AGNs.