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Register a new userMinkowskis Object: A Starburst Triggered by a Radio Jet, Revisited
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We present neutral hydrogen, ultraviolet, optical and near-infrared imaging, and optical spectroscopy, of Minkowskis Object (MO), a star forming peculiar galaxy near NGC 541. The observations strengthen evidence that star formation in MO was triggered by the radio jet from NGC 541. Key new results are the discovery of a 4.9E8 solar mass double HI cloud straddling the radio jet downstream from MO, where the jet changes direction and decollimates; strong detections of MO, also showing double structure, in UV and H-alpha; and numerous HII regions and associated clusters in MO. In UV, MO resembles the radio-aligned, rest-frame UV morphologies in many high redshift radio galaxies (HzRGs), also thought to be caused by jet-induced star formation. MOs stellar population is dominated by a 7.5 Myr-old, 1.9E7 solar mass instantaneous burst, with current star formation rate 0.52 solar masses per year (concentrated upstream from where the HI column density is high). This is unlike the jet-induced star formation in Centaurus A, where the jet interacts with pre-existing cold gas; in MO the HI may have cooled out of a warmer, clumpy intergalactic or interstellar medium as a result of jet interaction, followed by collapse of the cooling clouds and subsequent star formation (consistent with numerical simulations). Since the radio source that triggered star formation in MO is much less luminous, and therefore more common, than powerful HzRGs, and because the environment around MO is not particularly special in terms of abundant dense, cold gas, jet-induced star formation in the early universe might be even more prevalent than previously thought.
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We use ALMA to detect and image CO (1-0) emission from Minkowskis Object, a dwarf galaxy that is interacting with a radio jet from a nearby elliptical galaxy. These observations are the first to detect molecular gas in Minkowskis Object. We estimate the range in the mass of molecular gas in Minkowskis Object assuming two different values of the ratio of the molecular gas mass to the CO luminosity, $alpha_{rm CO}$. For the Milky Way value of $alpha_{rm CO}=4.6~M_{odot}{rm (K~km~s^{-1}~pc^2)^{-1}}$ we obtain a molecular gas mass of $M_{rm H_2} =3.0 times 10^7~M_{odot}$, 6% of the HI gas mass. We also use the prescription of Narayanan et al. (2012) to estimate an $alpha_{rm CO}=27~M_{odot}{rm (K~km~s^{-1}~pc^2)^{-1}}$, in which case we obtain $M_{rm H_2} =1.8 times 10^8~M_{odot}$, 36% of the HI mass. The observations are consistent with previous claims of star formation being induced in Minkowskis Object via the passage of the radio jet, and it therefore being a rare local example of positive feedback from an AGN. In particular, we find highly efficient star formation, with gas depletion timescales $sim 5times 10^7 - 3times 10^8$yr (for assumed values of $alpha_{rm CO}=4.6$ and $27~M_{odot}{rm (K~km~s^{-1}~pc^2)^{-1}}$, respectively) in the upstream regions of Minkowskis Object that were struck first by the jet, and less efficient star formation downstream. We discuss the implications of this observation for models of jet induced star formation and radio mode feedback in massive galaxies.
W49A is a giant molecular cloud which harbors some of the most luminous embedded clusters in the Galaxy. However, the explanation for this starburst-like phenomenon is still under debate. Methods. We investigated large-scale Spitzer mid-infrared images together with a Galatic Ring Survey 13CO J = 1-0 image, complemented with higher resolution (~ 11 arcsec) 13CO J = 2-1 and C18O J = 2-1 images over a ~ 15 x 13 pc^2 field obtained with the IRAM 30m telescope. Two expanding shells have been identified in the mid-infrared images, and confirmed in the position-velocity diagrams made from the 13CO J = 2-1 and C18O J = 2-1 data. The mass of the averaged expanding shell, which has an inner radius of ~ 3.3 pc and a thickness of ~ 0.41 pc, is about 1.9 x 10^4 M*. The total kinetic energy of the expanding shells is estimated to be ~ 10^49 erg which is probably provided by a few massive stars, whose radiation pressure and/or strong stellar winds drive the shells. The expanding shells are likely to have a common origin close to the two ultracompact Hii regions (source O and source N), and their expansion speed is estimated to be ~ 5 km/s, resulting in an age of ~ 3-7 x 10^5 years. In addition, on larger (~ 35 x 50 pc^2) scales, remnants of two gas ejections have been identified in the 13CO J = 1 - 0 data. Both ejections seem to have the same center as the expanding shells with a total energy of a few times 10^50 erg. The main driving mechanism for the gas ejections is unclear, but likely related to the mechanism which triggers the starburst in W49A.
The nature of the far-IR source HOPS 108 has been a matter of debate in the last years. Previous radio observations detected a 3.6 cm source (VLA 12), coincident with HOPS 108, that was interpreted as a radio jet from this protostar. We present new multi-wavelength (0.7-5 cm), multi-configuration VLA observations as well as archive data (3.6 cm) that reveal VLA 12 as three knots of non-thermal emission, with HOPS 108 close to the central knot. We show that these knots have not been ejected by HOPS 108. We propose that the VLA 12 knots are actually part of a radio jet driven by VLA 11 (HOPS 370), a strong nearby source clearly elongated in the direction of the knots. The position of HOPS 108 in the path of the VLA 11-VLA 12 jet suggests an appealing new scenario: the triggered formation of HOPS 108 by the interaction of the jet with the surrounding medium.
M1-92 can be considered an archetype of bipolar pre-planetary nebulae. It shows a clear axial symmetry, along with the kinematics and momentum excess characteristic of this class of envelopes around post-AGB stars. By taking advantage of the new extended configuration of the IRAM Plateau de Bure interferometer, we wanted to study the morphology and velocity field of the molecular gas better in this nebula, particularly in its central part. We performed sub-arcsecond resolution interferometric observations of the J=2-1 rotational line 13CO M1-92. We found that the equatorial component is a thin flat disk, which expands radially with a velocity proportional to the distance to the center. The kinetic age of this equatorial flow is very similar to that of the two lobes. The small widths and velocity dispersion in the gas forming the lobe walls confirm that the acceleration responsible for the nebular shape could not last more than 100-120 yr. The present kinematics of the molecular gas can be explained as the result of a single brief acceleration event, after which the nebula reached an expansion velocity field with axial symmetry. In view of the similarity to other objects, we speculate on the possibility that the whole nebula was formed as a result of a magneto-rotational explosion in a common-envelope system.
Local examples of jet-induced star formation lend valuable insight into its significance in galaxy evolution and can provide important observational constraints for theoretical models of positive feedback. Using optical integral field spectroscopy, we present an analysis of the ISM conditions in Minkowskis Object ($z = 0.0189$), a peculiar star-forming dwarf galaxy located in the path of a radio jet from the galaxy NGC 541. Full spectral fitting with PPXF indicates that Minkowskis Object primarily consists of a young stellar population $sim 10$ Myr old, confirming that the bulk of the objects stellar mass formed during a recent jet interaction. Minkowskis Object exhibits line ratios largely consistent with star formation, although there is evidence for a low level ($lesssim 15$ per cent) of contamination from a non-stellar ionising source. Strong-line diagnostics reveal a significant variation in the gas-phase metallicity within the object, with $logleft( rm O / H right) + 12$ varying by $sim 0.5$ dex, which cannot be explained by in-situ star formation, an enriched outflow from the jet, or enrichment of gas in the stellar bridge between NGC 541 and NGC 545/547. We hypothesise that Minkowskis Object either (a) was formed as a result of jet-induced star formation in pre-existing gas clumps in the stellar bridge, or (b) is a gas-rich dwarf galaxy that is experiencing an elevation in its star formation rate due to a jet interaction, and will eventually redden and fade, becoming an ultra-diffuse galaxy as it is processed by the cluster.
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