Cold gas outflows from the Small Magellanic Cloud traced with ASKAP

Feedback from massive stars plays a critical role in the evolution of the Universe by driving powerful outflows from galaxies that enrich the intergalactic medium and regulate star formation. An important source of outflows may be the most numerous galaxies in the Universe: dwarf galaxies. With small gravitational potential wells, these galaxies easily lose their star-forming material in the presence of intense stellar feedback. Here, we show that the nearby dwarf galaxy, the Small Magellanic Cloud (SMC), has atomic hydrogen outflows extending at least 2 kiloparsecs (kpc) from the star-forming bar of the galaxy. The outflows are cold, $T<400~{\rm K}$, and may have formed during a period of active star formation $25 - 60$ million years (Myr) ago. The total mass of atomic gas in the outflow is $\sim 10^7$ solar masses, ${\rm M_{\odot}}$, or $\sim 3$% of the total atomic gas of the galaxy. The inferred mass flux in atomic gas alone, $\dot{M}_{HI}\sim 0.2 - 1.0~{\rm M_{\odot}~yr^{-1}}$, is up to an order of magnitude greater than the star formation rate. We suggest that most of the observed outflow will be stripped from the SMC through its interaction with its companion, the Large Magellanic Cloud (LMC), and the Milky Way, feeding the Magellanic Stream of hydrogen encircling the Milky Way.Comment: Published in Nature Astronomy, 29 October 2018, http://dx.doi.org/10.1038/s41550-018-0608-

The LOFAR Two-metre Sky Survey: the radio view of the cosmic star formation history

© 2023 Oxford University Press. This is the accepted manuscript version of an article which has been published in final form at https://doi.org/10.1093/mnras/stad1602We present a detailed study of the cosmic star formation history over $90$ per cent of cosmic time ($0\lesssim z\lesssim4$), using deep, radio continuum observations that probe star formation activity independent of dust. The Low Frequency Array Two Metre Sky Survey has imaged three well-studied extragalactic fields, Elais-N1, Bo\"otes and the Lockman Hole, reaching $\sim20\,\mu\rm{Jy/beam}$ rms sensitivity at $150\,\rm{MHz}$. The availability of high-quality ancillary data from ultraviolet to far-infrared wavelengths has enabled accurate photometric redshifts and the robust separation of radio-bright AGN from their star-forming counterparts. We capitalise on this unique combination of deep, wide fields and robustly-selected star-forming galaxies to construct radio luminosity functions and derive the cosmic star formation rate density. We carefully constrain and correct for scatter in the $L_{150\,\rm{MHz}}-\rm{SFR}$ relation, which we find to be $\sim0.3\,\rm{dex}$. Our derived star formation rate density lies between previous measurements at all redshifts studied. We derive higher star formation rate densities between $z\sim0$ and $z\sim3$ than are typically inferred from short wavelength emission; at earlier times, this discrepancy is reduced. Our measurements are generally in good agreement with far-infrared and radio-based studies, with small offsets resulting from differing star formation rate calibrations.Peer reviewe

Star formation in the central 0.5 pc of the Milky Way

The supermassive black hole candidate at the Galactic Center is surrounded by a parsec-scale star cluster, which contains a number of early type stars. The presence of such stars has been called a "paradox of youth" as star formation in the immediate vicinity of a supermassive black hole seemed difficult, as well as the transport of stars from far out in a massive-star lifetime. I will recall 30 years of technological developments which led to the current understanding of the nuclear cluster stellar population. The number of early type stars known at present is sufficient to access the 3D structure of this population and its dynamics, which in turn allows discriminating between the various possible origins proposed along the years.Comment: 8 pages, invited review for the conference "The Universe under the Microscope" (AHAR 2008), to be published in Journal of Physics: Conference Series by Institute of Physics Publishin

LOFAR MSSS: Flattening low-frequency radio continuum spectra of nearby galaxies

Accepted for publication in Astronomy and AstrophysicsAims. The shape of low-frequency radio continuum spectra of normal galaxies is not well understood, the key question being the role of physical processes such as thermal absorption in shaping them. In this work we take advantage of the LOFAR Multifrequency Snapshot Sky Survey (MSSS) to investigate such spectra for a large sample of nearby star-forming galaxies. Methods. Using the measured 150 MHz flux densities from the LOFAR MSSS survey and literature flux densities at various frequencies we have obtained integrated radio spectra for 106 galaxies characterised by different morphology and star formation rate. The spectra are explained through the use of a three-dimensional model of galaxy radio emission, and radiation transfer dependent on the galaxy viewing angle and absorption processes. Results. Our galaxies' spectra are generally flatter at lower compared to higher frequencies: the median spectral index α low measured between ≈ 50 MHz and 1.5 GHz is -0.57 ± 0.01 while the high-frequency one α high, calculated between 1.3 GHz and 5 GHz, is -0.77 ± 0.03. As there is no tendency for the highly inclined galaxies to have more flattened low-frequency spectra, we argue that the observed flattening is not due to thermal absorption, contradicting the suggestion of Israel & Mahoney (1990, ApJ, 352, 30). According to our modelled radio maps for M 51-like galaxies, the free-free absorption effects can be seen only below 30 MHz and in the global spectra just below 20 MHz, while in the spectra of starburst galaxies, like M 82, the flattening due to absorption is instead visible up to higher frequencies of about 150 MHz. Starbursts are however scarce in the local Universe, in accordance with the weak spectral curvature seen in the galaxies of our sample. Locally, within galactic disks, the absorption effects are distinctly visible in M 51-like galaxies as spectral flattening around 100-200 MHz in the face-on objects, and as turnovers in the edge-on ones, while in M 82-like galaxies there are strong turnovers at frequencies above 700 MHz, regardless of viewing angle. Conclusions. Our modelling of galaxy spectra suggests that the weak spectral flattening observed in the nearby galaxies studied here results principally from synchrotron spectral curvature due to cosmic ray energy losses and propagation effects. We predict much stronger effects of thermal absorption in more distant galaxies with high star formation rates. Some influence exerted by the Milky Way's foreground on the spectra of all external galaxies is also expected at very low frequencies.Peer reviewedFinal Accepted Versio

The LOFAR Two-metre Sky Survey Deep Fields: A new analysis of low-frequency radio luminosity as a star-formation tracer in the Lockman Hole region

We have exploited LOFAR deep observations of the Lockman Hole field at 150 MHz to investigate the relation between the radio luminosity of star-forming galaxies (SFGs) and their star-formation rates (SFRs), as well as its dependence on stellar mass and redshift. The adopted source classification, SFRs, and stellar masses are consensus estimates based on a combination of four different spectral energy distribution fitting methods. We note a flattening of the radio spectra of a substantial minority of sources below ∼1.4 GHz. Such sources have thus a lower `radio-loudness' level at 150 MHz than expected from extrapolations from 1.4 GHz using the average spectral index. We found a weak trend towards a lower SFR/L150 MHz ratio for higher stellar mass, M⋆. We argue that such a trend may account for most of the apparent redshift evolution of the L150 MHz/SFR ratio, in line with previous work. Our data indicate a weaker evolution than found by some previous analyses. We also find a weaker evolution with redshift of the specific SFR than found by several (but not all) previous studies. Our radio selection provides a view of the distribution of galaxies in the SFR-M⋆ plane complementary to that of optical and near-IR selection. It suggests a higher uniformity of the star-formation history of galaxies than implied by some analyses of optical and near-IR data. We have derived luminosity functions at 150 MHz of both SFGs and radio-quiet (RQ) AGN at various redshifts. Our results are in very good agreement with the T-RECS simulations and with literature estimates. We also present explicit estimates of SFR functions of SFGs and RQ AGN at several redshifts derived from our radio survey data

Exploring the radio-loudness of SDSS quasars with spectral stacking

© 2024 The Author(s). Published by Oxford University Press on behalf of Royal Astronomical Society. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY), https://creativecommons.org/licenses/by/4.0/We use new 144 MHz observations over 5634 deg 2 from the LOFAR (Low Frequency Array) Two-metre Sky Survey (LoTSS) to compile the largest sample of uniformly selected, spectroscopically confirmed quasars from the 14th data release of the Sloan Digital Sky Survey (SDSS-DR14). Using the classical definition of radio loudness, R = log (L 1.4GHz/L i), we identify 3697 radio-loud (RL) and 111 132 radio-quiet (RQ) sources at 0.6 < z < 3.4. To study their properties, we develop a new rest-frame spectral stacking algorithm, designed with forthcoming massively multiplexed spectroscopic surveys in mind, and use it to create high signal-to-noise composite spectra of each class, matched in redshift and absolute i-band magnitude. We show that RL quasars have redder continuum and enhanced [O II] emission than their RQ counterparts. These results persist when additionally matching in black hole mass, suggesting that this parameter is not the defining factor in making a quasi-stellar object (QSO) RL. We find that these features are not gradually varying as a function of radio loudness, but are maintained even when probing deeper into the RQ population, indicating that a clear-cut division in radio loudness is not apparent. Upon examining the star formation rates (SFRs) inferred from the [O II] emission line, with the contribution from active galactic nucleus removed using the [Ne V] line, we find that RL quasars have a significant excess of star formation relative to RQ quasars out to z = 1.9 at least. Given our findings, we suggest that RL sources either preferably reside in gas-rich systems with rapidly spinning black holes, or represent an earlier obscured phase of QSO evolution.Peer reviewe

Central kinematics of the globular cluster NGC 2808: Upper limit on the mass of an intermediate-mass black hole

Globular clusters are an excellent laboratory for stellar population and dynamical research. Recent studies have shown that these stellar systems are not as simple as previously assumed. With multiple stellar populations as well as outer rotation and mass segregation they turn out to exhibit high complexity. This includes intermediate-mass black holes which are proposed to sit at the centers of some massive globular clusters. Today's high angular resolution ground based spectrographs allow velocity-dispersion measurements at a spatial resolution comparable to the radius of influence for plausible IMBH masses, and to detect changes in the inner velocity-dispersion profile. Together with high quality photometric data from HST, it is possible to constrain black-hole masses by their kinematic signatures. We determine the central velocity-dispersion profile of the globular cluster NGC 2808 using VLT/FLAMES spectroscopy. In combination with HST/ACS data our goal is to probe whether this massive cluster hosts an intermediate-mass black hole at its center and constrain the cluster mass to light ratio as well as its total mass. We derive a velocity-dispersion profile from integral field spectroscopy in the center and Fabry Perot data for larger radii. High resolution HST data are used to obtain the surface brightness profile. Together, these data sets are compared to dynamical models with varying parameters such as mass to light ratio profiles and black-hole masses. Using analytical Jeans models in combination with variable M/L profiles from N-body simulations we find that the best fit model is a no black hole solution. After applying various Monte Carlo simulations to estimate the uncertainties, we derive an upper limit of the back hole mass of M_BH < 1 x 10^4 M_SUN (with 95 % confidence limits) and a global mass-to-light ratio of M/L_V = (2.1 +- 0.2) M_SUN/L_SUN.Comment: 12 pages, 9 figures, 2 tables, accepted for publication in A&

Evidence for Warped Disks of Young Stars in the Galactic Center

The central parsec around the super-massive black hole in the Galactic Center hosts more than 100 young and massive stars. Outside the central cusp (R~1") the majority of these O and Wolf-Rayet (WR) stars reside in a main clockwise system, plus a second, less prominent disk or streamer system at large angles with respect to the main system. Here we present the results from new observations of the Galactic Center with the AO-assisted near-infrared imager NACO and the integral field spectrograph SINFONI on the ESO/VLT. These include the detection of 27 new reliably measured WR/O stars in the central 12" and improved measurements of 63 previously detected stars, with proper motion uncertainties reduced by a factor of four compared to our earlier work. We develop a detailed statistical analysis of their orbital properties and orientations. Half of the WR/O stars are compatible with being members of a clockwise rotating system. The rotation axis of this system shows a strong transition as a function of the projected distance from SgrA*. The main clockwise system either is either a strongly warped single disk with a thickness of about 10 degrees, or consists of a series of streamers with significant radial variation in their orbital planes. 11 out of 61 clockwise moving stars have an angular separation of more than 30 degrees from the clockwise system. The mean eccentricity of the clockwise system is 0.36+/-0.06. The distribution of the counter-clockwise WR/O star is not isotropic at the 98% confidence level. It is compatible with a coherent structure such as stellar filaments, streams, small clusters or possibly a disk in a dissolving state. The observed disk warp and the steep surface density distribution favor in situ star formation in gaseous accretion disks as the origin of the young stars.Comment: ApJ in pres