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Register a new userALMA Observations toward the starburst dwarf galaxy NGC 5253: I. Molecular cloud properties and scaling relations
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We present high-spatial-resolution ($sim 0farcs2$, or $sim$3,pc) CO(2--1) observations of the nearest young starburst dwarf galaxy, NGC,5253, taken with the Atacama Large Millimeter/submillimeter Array. We have identified 118 molecular clouds with average values of 4.3,pc in radius and 2.2,kms, in velocity dispersion, which comprise the molecular cloud complexes observed previously with $sim$100,pc resolution. We derive for the first time in this galaxy the $I{rm (CO)}$--$N$(H$_2$) conversion factor, $X$ = $4.1^{+5.9}_{-2.4}times10^{20}$,cm$^{-2}$(K,kms)$^{-1}$, based on the virial method. The line-width and mass-to-size relations of the resolved molecular clouds present an offset on average toward higher line-widths and masses with respect to quiescent regions in other nearby spiral galaxies and our Galaxy. The offset in the scaling relation reaches its maximum in regions close to the central starburst, where velocity dispersions are $sim$ 0.5 dex higher and gas mass surface densities are as high as $Sigma_{rm H_2}$ = 10$^3$,Msol,pc $^{-2}$. These central clouds are gravitationally bound despite the high internal pressure. A spatial comparison with star clusters found in the literature enables us to identify six clouds that are associated with young star clusters. Furthermore, the star formation efficiencies (SFEs) of some of these clouds exceed those found in star-cluster-forming clouds within our Galaxy. We conclude that once a super star cluster is formed, the parent molecular clouds are rapidly dispersed by the destructive stellar feedback, which results in such a high SFE in the central starburst of NGC,5253.
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There are few observational constraints on how the escape of ionizing photons from starburst galaxies depends on galactic parameters. Here, we report on the first major detection of an ionization cone in NGC 5253, a nearby starburst galaxy. This high-excitation feature is identified by mapping the emission-line ratios in the galaxy using [S III] lambda 9069, [S II] lambda 6716, and H_alpha narrow-band images from the Maryland-Magellan Tunable Filter at Las Campanas Observatory. The ionization cone appears optically thin, which is suggestive of the escape of ionizing photons. The cone morphology is narrow with an estimated solid angle covering just 3% of 4pi steradians, and the young, massive clusters of the nuclear starburst can easily generate the radiation required to ionize the cone. Although less likely, we cannot rule out the possibility of an obscured AGN source. An echelle spectrum along the minor axis shows complex kinematics that are consistent with outflow activity. The narrow morphology of the ionization cone supports the scenario that an orientation bias contributes to the difficulty in detecting Lyman continuum emission from starbursts and Lyman break galaxies.
VLA and Parkes 64 m radiotelescope 21-cm observations of the starburst dwarf galaxy NGC 5253 reveal a multi-component non-axisymmetric HI distribution. The component associated with the stellar body shows evidence for a small amount of rotational support aligned with the major axis, in agreement with optically measured kinematics and consistent with the small galaxian mass. Approximately 20-30% of the HI emission is associated with a second component, an HI plume extending along the optical minor axis to the southeast. We consider outflow, inflow, and tidal origins for this feature. Outflow appears improbable, inflow is a possibility, and tidal debris is most consistent with the observations. These observations also reveal a filamentary third component that includes an 800 pc diameter HI shell or bubble to the west of the nucleus, coinciding with an Halpha shell. The mass of HI in the shell may be as large as ~4x10^6 Msun. This large mass, coupled with the lack of expansion signatures in the neutral and ionized gas (v<30 km/s), suggests that this feature may be an example of a starburst-blown bubble stalled by interaction with a massive neutral envelope. Many other HI kinematic features closely resemble those seen in Halpha emission from the ionized gas, supporting the interpretation of neutral and ionized gas outflow at velocities of ~30 km/s. Comparison between extinction estimates from the Balmer emission-line decrement and the HI column densities suggest a gas-to-dust ratio 2-3 times the Galactic value in this low-metallicity (Z=1/4 Zsun) galaxy.
We report the detection of CO(2-1) and 3.1 mm and 1.3 mm continuum emission towards the extremely young starburst in NGC 5253, with data taken from the Owens Valley Millimeter Array. Faint CO emission originates in five molecular clouds distributed along the prominent dust lane seen in visual images. With the gas, the morphology of NGC 5253 looks much like a dwarf elliptical version of the ``dust-lane ellipticals or ``polar-ring class of galaxies. The molecular gas resides in GMCs well away from the radio-IR super-star cluster/supernebula seen in the radio and infrared. The millimeter continuum data confirm that the 2 cm flux from the supernebula is optically thick; the Lyman continuum rate derived from the 1.3 mm continuum is N$_{Lyc} sim 6x10^{52} s^{-1}$ for the central ~20. CO may underestimate the true molecular column density, as expected for a low metallicity system, although there are regions along the dust lane that appear to have near-Galactic conversion factors. We estimate a total molecular gas mass of $M_{H_{2}}lsim 10^{7} M_{odot}$. The molecular gas in the dust lane is falling into the galaxy, supporting an accretion hypothesis. The dust lane gas cannot therefore be causally associated with the current burst of star formation. A relatively small amount, $M_{H_{2}}lsim 5x10^{5} M_{odot}$, of molecular gas is associated with the current starburst. We estimate a star formation efficiency of at least 25 % and more likely ~75 %, consistent with the formation of a bound cluster. Despite the extreme youth of the starburst, the specific trigger of the starburst remains elusive, although the infall of gas in the dust lane suggests that there is more star formation to come in NGC 5253.
We present the results of CO observations toward 14 host galaxies of long-duration gamma-ray bursts (GRBs) at z = 0.1-2.5 by using the Atacama Large Millimeter/submillimeter Array. We successfully detected CO(3-2) or CO(4-3) emission in eight hosts (z = 0.3-2), which more than doubles the sample size of GRB hosts with CO detection. The derived molecular gas mass is $M_{rm gas} = (0.2-6) times 10^{10}$ $M_{odot}$ assuming metallicity-dependent CO-to-H$_2$ conversion factors. By using the largest sample of GRB hosts with molecular gas estimates (25 in total, of which 14 are CO-detected) including results from the literature, we compared molecular gas properties with those of other star-forming galaxies (SFGs). The GRB hosts tend to have a higher molecular gas mass fraction ($mu_{rm gas}$) and a shorter gas depletion timescale ($t_{rm depl}$) as compared with other SFGs at similar redshifts especially at $z lesssim 1$. This could be a common property of GRB hosts or an effect introduced by the selection of targets which are typically above the main-sequence line. To eliminate the effect of selection bias, we analyzed $mu_{rm gas}$ and $t_{rm depl}$ as a function of the distance from the main-sequence line ($delta$MS). We find that the GRB hosts follow the same scaling relations as other SFGs, where $mu_{rm gas}$ increases and $t_{rm depl}$ decreases with increasing $delta {rm MS}$. No molecular gas deficit is observed when compared to other SFGs of similar SFR and stellar mass. These findings suggest that the same star-formation mechanism is expected to be happening in GRB hosts as in other SFGs.
Galactic winds are essential to regulation of star formation in galaxies. To study the distribution and dynamics of molecular gas in a wind, we imaged the nearby starburst galaxy NGC 1482 in CO ($J=1rightarrow0$) at a resolution of 1 ($approx100$ pc) using the Atacama Large Millimeter/submillimeter Array. Molecular gas is detected in a nearly edge-on disk with a radius of 3 kpc and a biconical outflow emerging from the central 1 kpc starburst and extending to at least 1.5 kpc perpendicular to the disk. In the outflow, CO gas is distributed approximately as a cylindrically symmetrical envelope surrounding the warm and hot ionized gas traced by H$alpha$ and soft X-rays. The velocity, mass outflow rate, and kinetic energy of the molecular outflow are $v_mathrm{w}sim100~mathrm{km~s^{-1}}$, $dot{M}_mathrm{w}sim7~M_odot~mathrm{yr}^{-1}$, and $E_mathrm{w}sim7times10^{54}~mathrm{erg}$, respectively. $dot{M}_mathrm{w}$ is comparable to the star formation rate ($dot{M}_mathrm{w}/mathrm{SFR}sim2$) and $E_mathrm{w}$ is $sim1%$ of the total energy released by stellar feedback in the past $1times10^7~mathrm{yr}$, which is the dynamical timescale of the outflow. The results indicate that the wind is starburst driven.
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