Central Starburst Research Articles

Little Massive Substructure in CMZ Molecular Clouds

The Central Molecular Zone (CMZ; inner ∼100pc) hosts some of the most dense and massive molecular clouds of the Milky Way. Studying these clouds can potentially lead to a better understanding of the dense clouds seen in the central starburst regions in nearby galaxies or in the early universe. The clouds share an unusual feature: they form stars at an unusually slow rate compared to other Milky Way clouds of similar mass and density. Here we use interferometer data from ALMA and the SMA to show that this reduced star formation rate is a consequence of the cloud density structure: CMZ clouds have unusually flat density slopes. The clouds do, for example, exceed the average density of the Orion A molecular cloud by an order of magnitude on spatial scales ∼5 pc, but the cores of CMZ clouds with ∼0.1 pc radius often have masses and densities lower than what is found in the Orion KL region. This relative absence of highest-density gas probably explains the suppression of star formation. The clouds are relatively turbulent, and ALMA observations of H 2 CO and SiO indicate that the turbulence is induced by high-velocity shocks. We speculate that these shocks might prevent the formation of high-mass cores.

Witness of gas infall and outflow in the young starburst dwarf galaxy NGC 5253

Abstract We present 12CO(2–1) observations towards the dwarf galaxy NGC 5253 using the Submillimeter Array. The data shows that a large amount of molecular gas is located in the central ∼ 200 pc starburst region, physically associated with two young super stellar clusters (SSCs). The molecular gas traced by 12CO(2–1) is elongated along the minor axis (dust lane) of the galaxy and its kinematics suggest that there is an inflow of molecular gas to the direction of the central SSCs, as is also observed in H i gas at a larger scale. Due to their correlation in spatial and velocity domains, the central SSCs were likely formed from molecular gas in the nucleus. We compare the 12CO(2–1) with available H2 1–0 S(1) data, and show that while the relatively cold gas traced by 12CO(2–1) is distributed around the central starburst region but also along the dust lane, the warm gas traced by H2 1–0 S(1) is associated with the central H ii region and other star-forming regions. Interestingly, a cavity in the H2 1–0 S(1) emission is found to be spatially correlated with a Hα shell. This Hα shell may trace a bipolar outflow from the central SSCs and the H2 1–0 S(1) gas, the gas shocked by the outflow encountering the surrounding quiescent gas. We calculate a molecular gas inflow rate of ∼ 2 M⊙ yr−1, a star formation rate of 0.3–0.5 M⊙ yr−1, and an ionized gas outflow being emitted from the SSCs with a rate of (5–25)× 10−3 f M⊙ yr−1, where f is a volume filling factor.

EXTENDED HCN AND HCO+EMISSION IN THE STARBURST GALAXY M82

We mapped 3 mm continuum and line emission from the starburst galaxy M82 using the Combined Array for Research in Millimeter-wave Astronomy. We targeted the HCN, HCO$^{+}$, HNC, CS and HC$_{3}$N lines, but here we focus on the HCN and HCO$^{+}$ emission. The map covers a field of 1.2' with a ~5" resolution. The HCN and HCO$^{+}$ observations are combined with single dish images. The molecular gas in M82 had been previously found to be distributed in a molecular disk, coincident with the central starburst, and a galactic scale outflow which originates in the central starburst. With the new short spacings-corrected maps we derive some of the properties of the dense molecular gas in the base of the outflow. From the HCN and HCO$^{+}$ J=(1-0) line emission, and under the assumptions of the gas being optically thin and in local thermodynamic equilibrium, we place lower limits to the amount of dense molecular gas in the base of the outflow. The lower limits are $7\times10^{6}$ $M_{\odot}$ and $21\times10^{6}$ $M_{\odot}$, or $\gtrsim2\%$ of the total molecular mass in the outflow. The kinematics and spatial distribution of the dense gas outside the central starburst suggests that it is being expelled through chimneys. Assuming a constant outflow velocity, the derived outflow rate of dense molecular gas is $\geq0.3$ $M_{\odot}$ yr$^{-1}$, which would lower the starburst lifetime by $\geq5\%$. The energy required to expel this mass of dense gas is $(1-10)\times10^{52}$ erg.

A FAR-INFRARED SPECTRAL SEQUENCE OF GALAXIES: TRENDS AND MODELS

We present a framework for the interpretation of the far-infrared spectra of galaxies in which we have expanded the model parameters compared with previous work by varying the ionization parameter $U$, column density $N$(H), and gas density at the cloud face $n$(H$^{+}$) for a central starburst or AGN. We compare these models carried out with the $Cloudy$ spectral synthesis code to trends in line-to-total far-infrared luminosity ratios, far-infrared fine-structure line ratios, IRAS colors, and OH and H$_{2}$O column densities with trends found in the well-studied sample of ten nearby galaxies from the IRAS Bright Galaxy Sample with infrared luminosities greater than 10$^{10}$ L$_{\odot}$ and IRAS 60 micron fluxes equal to or greater than that of the nearby ULIRG Arp 220. We find that the spectral sequence extending from normal starburst-type emission line spectra to ULIRG-type absorption-dominated spectra with significant absorption from excited levels, can be best explained by simultaneously increasing the hydrogen column density, from as low as 10$^{21}$ cm$^{-2}$ to as high as 10$^{24.8}$ cm$^{-2}$ or greater, and the ionization parameter, from as low as 10$^{-4}$ to as high as 1. The starburst models best reproduce most of the sequence, while AGN models are somewhat better able to produce the high OH and H$_{2}$O column densities in Arp 220. Our results suggest that the molecular ISM in ULIRG-like, absorption-dominated systems is located close to and at least partially obscures the source of power throughout much of the far-infrared, which must be taken into account in order to properly interpret diagnostics of both their sources of power and of feedback.

Probing the interstellar medium of NGC 1569 with Herschel★

NGC1569 has some of the most vigorous star formation among nearby galaxies. It hosts two super star clusters (SSCs) and has a higher star formation rate (SFR) per unit area than other starburst dwarf galaxies. Extended emission beyond the galaxy's optical body is observed in warm and hot ionised and atomic hydrogen gas; a cavity surrounds the SSCs. We aim to understand the impact of the massive star formation on the surrounding interstellar medium in NGC1569 through a study of its stellar and dust properties. We use Herschel and ancillary multiwavelength observations, from the ultraviolet to the submillimeter regime, to construct its spectral energy distribution, which we model with magphys on ~300pc scales at the SPIRE250 {\mu}m resolution. The multiwavelength morphology shows low levels of dust emission in the cavity, and a concentration of several dust knots in its periphery. The extended emission seen in the ionised and neutral hydrogen observations is also present in the far-infrared emission. The dust mass is higher in the periphery of the cavity, driven by ongoing star formation and dust emission knots. The SFR is highest in the central region, while the specific SFR is more sensitive to the ongoing star formation. The region encompassing the cavity and SSCs contains only 12 per cent of the dust mass of the central starburst, in accord with other tracers of the interstellar medium. The gas-to-dust mass ratio is lower in the cavity and fluctuates to higher values in its periphery.

EXTENDED STRUCTURE AND FATE OF THE NUCLEUS IN HENIZE 2-10

We investigate the structure and nuclear region of the black hole (BH) hosting galaxy Henize2-10. Surface brightness (SB) profiles are analyzed using Magellan/Megacam $g$- and $r$-band images. Excluding the central starburst, we find a best-fit two component S\'ersic profile with $n_{\rm in} \sim 0.6$, $r_{\text{eff,in}} \sim$ 260~pc, and $n_{\rm out}\sim 1.8$, $r_{\text{eff, out}}\sim$ 1 kpc. Integrating out to our outer most data point (100 arc sec $\sim$ 4.3 kpc), we calculate $M_g=-19.2$ and $M_r=-19.8$. The corresponding enclosed stellar mass is $M_{\star}\sim(10\pm3)\times10^9$ M$_\odot$, $\sim3\times$ larger than previous estimates. Apart from the central $\lesssim$500 pc, with blue colors and an irregular morphology, the galaxy appears to be an early-type system. The outer color is quite red, $(g-r)_0=0.75$, suggesting a dominant old population. We study the nuclear region of the galaxy using archival Gemini/NIFS $K$-band adaptive optics spectroscopy and {\it Hubble Space Telescope} imaging. We place an upper limit on the BH mass of $\sim10^7M_{\odot}$ from the NIFS data, consistent with that from the $M_{\rm BH}$-radio-X-ray fundamental plane. No coronal lines are seen, but a Br$\gamma$ source is located at the position of the BH with a luminosity consistent with the X-ray emission. The starburst at the center of Henize 2-10 has led to the formation of several super star clusters, which are within $\sim$100 pc of the BH. We examine the fate of the nucleus by estimating the dynamical masses and dynamical friction timescales of the clusters. The most massive clusters ($\sim 10^6 M_\odot$) have $\tau_{\rm dyn} \lesssim 200$ Myr, and thus Henize 2-10 may represent a rare snapshot of nuclear star cluster formation around a pre-existing massive BH.

Massive compact galaxies with high-velocity outflows: morphological analysis and constraints on AGN activity

We investigate the process of rapid star formation quenching in a sample of 12 massive galaxies at intermediate redshift (z~0.6) that host high-velocity ionized gas outflows (v>1000 km/s). We conclude that these fast outflows are most likely driven by feedback from star formation rather than active galactic nuclei (AGN). We use multiwavelength survey and targeted observations of the galaxies to assess their star formation, AGN activity, and morphology. Common attributes include diffuse tidal features indicative of recent mergers accompanied by bright, unresolved cores with effective radii less than a few hundred parsecs. The galaxies are extraordinarily compact for their stellar mass, even when compared with galaxies at z~2-3. For 9/12 galaxies, we rule out an AGN contribution to the nuclear light and hypothesize that the unresolved core comes from a compact central starburst triggered by the dissipative collapse of very gas-rich progenitor merging disks. We find evidence of AGN activity in half the sample but we argue that it accounts for only a small fraction (<10%) of the total bolometric luminosity. We find no correlation between AGN activity and outflow velocity and we conclude that the fast outflows in our galaxies are not powered by on-going AGN activity, but rather by recent, extremely compact starbursts.

Multicomponent simulation of emission of low-metallicity H II regions

Using multicomponent photoionization simulation, we investigated the impact of bubble-like structures around starbursts inside the low-metallicity H II regions on the ionization spectrum shape and emission line forming. Radial distribution of density values and other physical parameters of bubble-like structures were taken from Weaver et al. (Weaver et al., 1977, p. 377). The first and second inner components of such models describe the free expansion zone of superwind from the central starburst region and rarefied hot gas of the cavern thermalized by inverse shock wave, respectively. The gas density and electron temperature distributions into these components are obtained from the solution of the system of equations of continuity and energy transfer, including heat conductivity. The third component is a thin shell of high density gas formed from the gas surrounding a bubble and made by direct shock wind wave. The gas density in this component was obtained from isobaric condition at contact discontinuity between the second and third components. Input spectra of the ionizing radiation were obtained from the starburst evolutional models. The evolution grid of the multicomponent low-metallicity photoionization models with free parameters determining physical conditions inside the bubble-like structure was calculated. The impact of bubble-like structure on the change of ionization spectrum shape and the formation of fluxes of important emission lines in low-metallicity case was analyzed in detail.

DISCOVERY OF A SMALL CENTRAL DISK OF CO AND H I IN THE MERGER REMNANT NGC 34

We present CO(1-0) and HI(21-cm) observations of the central region of the wet merger remnant NGC 34. The Combined Array for Research in Millimeter-wave Astronomy (CARMA) observations detect a regularly rotating disk in CO with a diameter of 2.1 kpc and a total molecular hydrogen mass of ($2.1 \pm 0.2) \times10^9~M_\odot$. The rotation curve of this gas disk rises steeply, reaching maximum velocities at 1" (410 pc) from the center. Interestingly, HI observations done with the Karl G. Jansky Very Large Array show that the absorption against the central continuum has the exact same velocity range as the CO in emission. This strongly suggests that the absorbing HI also lies within 1" from the center, is mixed in and corotates with the molecular gas. A comparison of HI absorption profiles taken at different resolutions (5"-45") shows that the spectra at lower resolutions are less deep at the systemic velocity. This provides evidence for HI emission in the larger beams, covering the region from 1 kpc to 9 kpc from the center. The central rapidly rotating disk was likely formed either during the merger or from fall-back material. Lastly, the radio continuum flux of the central source at mm wavelengths ($5.4\pm1.8$ mJy) is significantly higher than expected from an extrapolation of the synchrotron spectrum, indicating the contribution of thermal free-free emission from the central starburst.

THE GREEN BANK TELESCOPE MAPS THE DENSE, STAR-FORMING GAS IN THE NEARBY STARBURST GALAXY M82

Observations of the Milky Way and nearby galaxies show that dense molecular gas correlates with recent star formation, suggesting that the formation of this gas phase may help regulate star formation. A key test of this idea requires wide-area, high-resolution maps of dense molecular gas in galaxies to explore how local physical conditions drive dense gas formation, but these observations have been limited because of the faintness of dense gas tracers like HCN and HCO+. Here we demonstrate the power of the Robert C. Byrd Green Bank Telescope -- the largest single-dish millimeter radio telescope -- for mapping dense gas in galaxies by presenting the most sensitive maps yet of HCN and HCO+ in the starburst galaxy M82. The HCN and HCO+ in the disk of this galaxy correlates with both recent star formation and more diffuse molecular gas and shows kinematics consistent with a rotating torus. The HCO+ emission extending to the north and south of the disk is coincident with the outflow previously identified in CO and traces the eastern edge of the hot outflowing gas. The central starburst region has a higher ratio of star formation to dense gas than the outer regions, pointing to the starburst as a key driver of this relationship. These results establish that the GBT can efficiently map the dense molecular gas at 90 GHz in nearby galaxies, a capability that will increase further with the 16 element feed array under construction.

Modeling the emission of an H II region containing a bubble-like structure

The influence of a bubble-like structure around a starburst within the H II regions on the shape of the ionizing spectrum and the generation of the observed emission lines was studied using multicomponent photoionization modeling. The distributions of density and other physical parameters in such bubble-like structures were determined by Weaver et al. in 1977. The first two components represent the region of the stellar wind from the central starburst region. The gas density and electron temperature distributions in these components were described by the solution of the system of equations of continuity and energy transfer with account for the heat conductivity. The third component represents a thin layer of gas with a high density that emerges due to the passage of a normal shock wave of the stellar wind. The fourth component represents a “typical” H II region. The ionizing radiation spectra were set from the calculated evolutionary models whose free parameters determine the physical conditions within the “bubble.” The influence of the stellar wind bubble on the shape of the ionizing radiation spectrum and the generation of fluxes in important emission lines was analyzed.

Constrained simulations of the Antennae galaxies: comparison with Herschel-PACS observations★

We present a set of hydro-dynamical numerical simulations of the Antennae galaxies in order to understand the origin of the central overlap starburst. Our dynamical model provides a good match to the observed nuclear and overlap star formation, especially when using a range of rather inefficient stellar feedback efficiencies (0.01 < q_EoS < 0.1). In this case a simple conversion of local star formation to molecular hydrogen surface density motivated by observations accounts well for the observed distribution of CO. Using radiative transfer post-processing we model synthetic far-infrared spectral energy distributions (SEDs) and two-dimensional emission maps for direct comparison with Herschel-PACS observations. For a gas-to-dust ratio of 62:1 and the best matching range of stellar feedback efficiencies the synthetic far-infrared SEDs of the central star forming region peak at values of ~65 - 81 Jy at 99 - 116 um, similar to a three-component modified black body fit to infrared observations. Also the spatial distribution of the far-infrared emission at 70 um, 100 um, and 160 um compares well with the observations: >50% (> 35%) of the emission in each band is concentrated in the overlap region while only < 30% (< 15%) is distributed to the combined emission from the two galactic nuclei in the simulations (observations). As a proof of principle we show that parameter variations in the feedback model result in unambiguous changes both in the global and in the spatially resolved observable far-infrared properties of Antennae galaxy models. Our results strengthen the importance of direct, spatially resolved comparative studies of matched galaxy merger simulations as a valuable tool to constrain the fundamental star formation and feedback physics.

Suppression of star formation in the galaxy NGC 253 by a starburst-driven molecular wind

The under-abundance of very massive galaxies in the Universe is frequently attributed to the effect of galactic winds. Although ionized galactic winds are readily observable, most of the expelled mass (that is, the total mass flowing out from the nuclear region) is likely to be in atomic and molecular phases that are cooler than the ionized phases. Expanding molecular shells observed in starburst systems such as NGC 253 (ref. 12) and M 82 (refs 13, 14) may facilitate the entrainment of molecular gas in the wind. Although shell properties are well constrained, determining the amount of outflowing gas emerging from such shells and the connection between this gas and the ionized wind requires spatial resolution better than 100 parsecs coupled with sensitivity to a wide range of spatial scales, a combination hitherto not available. Here we report observations of NGC 253, a nearby starburst galaxy (distance ∼ 3.4 megaparsecs) known to possess a wind, that trace the cool molecular wind at 50-parsec resolution. At this resolution, the extraplanar molecular gas closely tracks the Hα filaments, and it appears to be connected to expanding molecular shells located in the starburst region. These observations allow us to determine that the molecular outflow rate is greater than 3 solar masses per year and probably about 9 solar masses per year. This implies a ratio of mass-outflow rate to star-formation rate of at least 1, and probably ∼3, indicating that the starburst-driven wind limits the star-formation activity and the final stellar content.

THE FUELING DIAGRAM: LINKING GALAXY MOLECULAR-TO-ATOMIC GAS RATIOS TO INTERACTIONS AND ACCRETION

To assess how external factors such as local interactions and fresh gas accretion influence the global ISM of galaxies, we analyze the relationship between recent enhancements of central star formation and total molecular-to-atomic (H2/HI) gas ratios, using a broad sample of field galaxies spanning early-to-late type morphologies, stellar masses of 10^(7.2-11.2) Msun, and diverse stages of evolution. We find that galaxies occupy several loci in a "fueling diagram" that plots H2/HI vs. mass-corrected blue-centeredness, a metric tracing the degree to which galaxies have bluer centers than the average galaxy at their stellar mass. Spiral galaxies show a positive correlation between H2/HI and mass-corrected blue-centeredness. When combined with previous results linking mass-corrected blue-centeredness to external perturbations, this correlation suggests a link between local galaxy interactions and molecular gas inflow/replenishment. Intriguingly, E/S0 galaxies show a more complex picture: some follow the same correlation, some are quenched, and a distinct population of blue-sequence E/S0 galaxies (with masses below key transitions in gas richness) defines a separate loop in the fueling diagram. This population appears to be composed of low-mass merger remnants currently in late- or post-starburst states, in which the burst first consumes the H2 while the galaxy center keeps getting bluer, then exhausts the H2, at which point the burst population reddens as it ages. Multiple lines of evidence suggest connected evolutionary sequences in the fueling diagram. In particular, tracking total gas-to-stellar mass ratios within the diagram provides evidence of fresh gas accretion onto low-mass E/S0s emerging from central starbursts. Drawing on a comprehensive literature search, we suggest that virtually all galaxies follow the same evolutionary patterns found in our broad sample.

A REDLINE STARBURST: CO(2-1) OBSERVATIONS OF AN EDDINGTON-LIMITED GALAXY REVEAL STAR FORMATION AT ITS MOST EXTREME

We report observations of the CO(2-1) emission of SDSSJ1506+54, a compact (r_e~135pc) starburst galaxy at z=0.6. SDSSJ1506+54 appears to be forming stars close to the limit allowed by stellar radiation pressure feedback models: the measured L_IR/L'_CO 1500 is one of the highest measured for any galaxy. With its compact optical morphology but extended low surface brightness envelope, post-starburst spectral features, high infrared luminosity (L_IR>10^12.5 L_Sun), low gas fraction (M_H2/M_stars~15%), and short gas depletion time (tens of Myr), we speculate that this is a feedback- limited central starburst episode at the conclusion of a major merger. Taken as such, SDSSJ1504+54 epitomizes the brief closing stage of a classic model of galaxy growth: we are witnessing a key component of spheroid formation during what we term a 'redline' starburst.

Properties of free–free, dust and CO emissions in the starbursts of blue compact dwarf galaxies

The central star-forming regions in three blue compact dwarf galaxies (He 2-10, NGC 5253, and II Zw 40) were observed in the 340 GHz (880 micron) band at 5 arcsec resolution with the Submillimetre Array (SMA). Continuum emission associated with the central star-forming complex was detected in all these galaxies. The SMA 880 micron flux is decomposed into free-free emission and dust emission by using centimetre-wavelength data in the literature. We find that free-free emission contributes half or more of the SMA 880 micron flux in the central starbursts in those three galaxies. In spite of the dominance of free-free emission at 880 micron, the radio-to-far infrared (FIR) ratios in the central star-forming regions are not significantly higher than those of the entire systems, showing the robustness of radio-FIR relation. Based on the robustness of the radio-FIR relation, we argue that the free--free fraction in the 880 micron emission is regulated by the dust temperature. We also analyze the CO (J = 3--2) emission data. We find that CO is a good tracer of the total gas mass in solar-metallicity object He 2-10. Low-metallicity objects, NGC 5253 and II Zw 40, have apparently high star formation efficiencies; however, this may be an artifact of significant dissociation of CO in the low-metallicity environments. We also point out a potential underestimate of dust mass, since the dust traced by emission is biased to the most luminous high-temperature regions, particularly when a system hosts a compact star-forming region where the dust temperature is high.

IONIZATION SOURCE OF A MINOR-AXIS CLOUD IN THE OUTER HALO OF M82

The M82 `cap' is a gas cloud at a projected radius of 11.6 kpc along the minor axis of this well known superwind source. The cap has been detected in optical line emission and X-ray emission and therefore provides an important probe of the wind energetics. In order to investigate the ionization source of the cap, we observed it with the Kyoto3DII Fabry-Perot instrument mounted on the Subaru Telescope. Deep continuum, Ha, [NII]6583/Ha, and [SII]6716,6731/Ha maps were obtained with sub-arcsecond resolution. The superior spatial resolution compared to earlier studies reveals a number of bright Ha emitting clouds within the cap. The emission line widths (< 100 km s^-1 FWHM) and line ratios in the newly identified knots are most reasonably explained by slow to moderate shocks velocities (v_shock = 40--80 km s^-1) driven by a fast wind into dense clouds. The momentum input from the M82 nuclear starburst region is enough to produce the observed shock. Consequently, earlier claims of photoionization by the central starburst are ruled out because they cannot explain the observed fluxes of the densest knots unless the UV escape fraction is very high (f_esc > 60%), i.e., an order of magnitude higher than observed in dwarf galaxies to date. Using these results, we discuss the evolutionary history of the M82 superwind. Future UV/X-ray surveys are expected to confirm that the temperature of the gas is consistent with our moderate shock model.

Hot gas, cold gas and sub-haloes in a Lyman α blob at redshift 2.38

We present integral field spectroscopy of a Lyman-alpha blob at redshift 2.38, with a spectral resolution three times better than previous published work. As with previous observations, the blob has a chaotic velocity structure, much of which breaks up into multiple components. Our spectroscopy shows, however, that some of these multiple components are extremely narrow: they have velocity widths of less than 100 km/s. Combining these new data with previous observations, we argue that this Lyman-alpha blob resides in a dark-matter halo of around 10^13 solar masses. At the centre of this halo are two compact red massive galaxies. They are surrounded by hot gas, probably a super-wind from merger-induced nuclear starbursts. This hot gas has shut down star formation in the non-nuclear region of these galaxies, leading to their red-and-dead colours. A filament or lump of infalling cold gas is colliding with the hot gas phase and being shocked to high temperatures, while still around 30kpc from the red galaxies. The shock region is self-absorbed in Lyman-alpha but produces C IV emission. Further out still, the cold gas in a number of sub-halos is being lit up, most likely by a combination of tidally triggered star formation, bow-shocks as they plough through the hot halo medium, resonant scattering of Lyman-alpha from the filament collision, and tidal stripping of gas which enhances the Lyman-alpha escape fraction. The observed Lyman-alpha emission from the Blob is dominated by the sum of the emission from these sub-halos. On statistical grounds, we argue that Lyman-alpha blobs are not greatly elongated in shape, and that most are not powered by ionisation or scattering from a central active galactic nucleus or starburst.

THE STELLAR AND GAS KINEMATICS OF THE LITTLE THINGS DWARF IRREGULAR GALAXY NGC 1569

In order to understand the formation and evolution of dIm galaxies, one needs to understand their three-dimensional structure. We present measurements of the stellar velocity dispersion in NGC 1569, a nearby post-starburst dIm galaxy. The stellar vertical velocity dispersion, $\sigma_{\rm z}$, coupled with the maximum rotational velocity derived from \ion{H}{1} observations, $V_{\rm max}$, gives a measure of how kinematically hot the galaxy is, and, therefore, indicates its structure. We conclude that the stars in NGC 1569 are in a thick disk with a $V_{\rm max} / \sigma_{\rm z}$ = 2.4 $\pm$ 0.7. In addition to the structure, we analyze the ionized gas kinematics from \ion{O}{3} observations along the morphological major axis. These data show evidence for outflow from the inner starburst region and a potential expanding shell near supermassive star cluster (SSC) A. When compared to the stellar kinematics, the velocity dispersion of the stars increase in the region of SSC A supporting the hypothesis of an expanding shell. The stellar kinematics closely follow the motion of the gas. Analysis of high resolution \ion{H}{1} data clearly reveals the presence of an \ion{H}{1} cloud that appears to be impacting the eastern edge of NGC 1569. Also, an ultra-dense \ion{H}{1} cloud can be seen extending to the west of the impacting \ion{H}{1} cloud. This dense cloud is likely the remains of a dense \ion{H}{1} bridge that extended through what is now the central starburst area. The impacting \ion{H}{1} cloud was the catalyst for the starburst, thus turning the dense gas into stars over a short timescale, $\sim$ 1 Gyr. We performed a careful study of the spectral energy distribution using infrared, optical, and ultraviolet photometry producing a state-of-the-art mass model for the stellar disk. This mass modeling shows that stars dominate the gravitational potential in the inner 1 kpc.

Probing the Build-Up of Stellar Mass in the Center of IR Luminous Major Mergers with HST

AbstractInteractions and mergers are important drivers of galaxy evolution, transform spiral galaxies into massive ellipticals, and fuel both powerful starbursts and massive nuclear black holes. In particular one galaxy population, namely Luminous Infrared Galaxies (LIRGs), are believed to be responsible for most of the star formation that happened in the history of the universe (see e.g. Le Floch et al. 2005, Caputi et al. 2007, Magnelli et al. 2009), and hence represent a critical phase in the evolution of galaxies where most of the galaxies mass is building up. During a merger process, violent relaxation acts on stars present in gas-rich progenitor disks, while the centers are structured by the relics of dissipational, compact starbursts, imprinting a central “extra light” component or “cusp” into the surface brightness profiles of merger remnants. Our HST NICMOS/WFC3 imaging program of the 88 most luminous LIRGs in the Great Observatories Allsky LIRG Survey (GOALS, see Armus et al. 2009) shows that the central luminosity surface density in nearby LIRGs increases significantly along the merger sequence, indicating that the gas inflow fuels a central starburst and subsequently builds a compact stellar cusp (Haan et al. 2011). A large fraction of all galaxies in our sample possess double or multiple nuclei (~63%). Half of these double nuclei are not visible in the HST B-band images due to dust obscuration, which implies strong limitations on the ability to detect the true nuclear structures of luminous infrared galaxies at high-redshift (z &gt;2) and may explain some of the apparent discrepancy of the LIRG population and merger ratio between local and high-redshift galaxies. We find that ULIRGs (log[LIR/L⊙] &gt; 12.0) have significantly smaller nuclear separations than LIRGs (log[LIR/L⊙] = 11.4 — 12.0) with a median value of 1.2 kpc and 6.7 kpc, respectively. In our sample, merger (regardless of whether LIRG or ULIRG) seem to be prevalent at two time scales (based on the projected nuclear separation and mass ratio of the nuclei): First, at a remaining merger time scale of 0.3&lt;[t-tmerg]&lt;1.3 Gyr (53% of mergers in our sample), and second, at [t-tmerg]~ 0 (26%), likely representing the first passage of interacting galaxies and the final nuclear coalescence, respectively, with a post-merger time (starburst phase after the nuclei merged) of roughly 300 Myrs.