arXiv subjects

Jonathan Braine

Publications and source records attributed to Jonathan Braine.

34 records · Page 2Linked to original sources

arXiv2015

Ram Pressure Stripping in the Virgo Cluster

Gas can be violently stripped from their galaxy disks in rich clusters, and be dispersed over 100kpc-scale tails or plumes. Young stars have been observed in these tails, suggesting they are formed in situ. This will contribute to the intracluster light, in addition to tidal stripping of old stars. We want to quantify the efficiency of intracluster star formation. We present CO(1--0) and CO(2--1) observations, made with the IRAM-30m telescope, towards the ram-pressure stripped tail northeast of NGC4388 in Virgo. HII regions found all along the tails, together with dust patches have been targeted. We detect molecular gas in 4 positions along the tail, with masses between 7x10$^5$ to 2x10$^6$ M$_\odot$. Given the large distance from the NGC 4388 galaxy, the molecular clouds must have formed in situ, from the HI gas plume. We compute the relation between surface densities of star formation and molecular gas in these regions, and find that the star formation has very low efficiency. The corresponding depletion time of the molecular gas can be up to 500 Gyr and more. Since this value exceeds a by far Hubble time, this gas will not be converted into stars, and will stay in a gaseous phase to join the intracluster medium.

astro-ph.GA↗

arXiv2014

The effect of sideband ratio on line intensity for Herschel/HIFI

The Heterodyne Instrument for the Far Infrared (HIFI) on board the Herschel Space Observatory is composed of a set of fourteen double sideband mixers. We discuss the general problem of the sideband ratio (SBR) determination and the impact of an imbalanced sideband ratio on the line calibration in double sideband heterodyne receivers. The HIFI SBR is determined from a combination of data taken during pre-launch gas cell tests and in-flight. The results and some of the calibration artefacts discovered in the gas cell test data are presented here along with some examples of how these effects appear in science data taken in orbit.

astro-ph.IM↗

arXiv2011

The Dependence of the Galactic Star Formation Laws on Metallicity

We describe results from semi-analytical modelling of star formation in protocluster clumps of different metallicities. In this model, gravitationally bound cores form uniformly in the clump following a prescribed core formation efficiency per unit time. After a contraction timescale which is equal to a few times their free-fall times, the cores collapse into stars and populate the IMF. Feedback from the newly formed OB stars is taken into account in the form of stellar winds. When the ratio of the effective energy of the winds to the gravitational energy of the system reaches unity, gas is removed from the clump and core and star formation are quenched. The power of the radiation driven winds has a strong dependence on metallicity and it increases with increasing metallicity. Thus, winds from stars in the high metallicity models lead to a rapid evacuation of the gas from the protocluster clump and to a reduced star formation efficiency, as compared to their low metallicity counterparts. We derive the metallicity dependent star formation efficiency per unit time in this model as a function of the gas surface density Sigma_g. This is combined with the molecular gas fraction in order to derive the dependence of the surface density of star formation Sigma_SFR on Sigma_g. This feedback regulated model of star formation reproduces very well the observed star formation laws in galaxies extending from low gas surface densities up to the starburst regime. Furthermore, the results show a dependence of Sigma_SFR on metallicity over the entire range of gas surface densities, and can also explain part of the scatter in the observations.

astro-ph.GA↗

arXiv2011

Studying the spatially resolved Schmidt-Kennicutt law in interacting galaxies: the case of Arp 158

Recent studies have shown that star formation in mergers does not seem to follow the same Schmidt-Kennicutt (KS) relation as in spiral disks, presenting a higher star formation rate (SFR) for a given gas column density. In this paper we study why and how different models of star formation arise. To do so we examine the process of star formation in the interacting system Arp 158 and its tidal debris. We perform an analysis of the properties of specific regions of interest in Arp 158 using observations tracing the atomic and the molecular gas, star formation, the stellar populations as well as optical spectroscopy to determine their exact nature. We also fit their spectral energy distribution with an evolutionary synthesis code. Finally, we compare star formation in these objects to star formation in the disks of spiral galaxies and mergers. Abundant molecular gas is found throughout the system and the tidal tails appear to have many young stars compared to their old stellar content. One of the nuclei is dominated by a starburst whereas the other is an active nucleus. We estimate the SFR throughout the system and find that most regions follow closely the KS relation seen in spiral galaxies with the exception of the nuclear starburst and the tip of one of the tails. We examine whether this diversity is due to uncertainties in the manner the SFR is determined or whether the conditions in the nuclear starburst region are such that it does not follow the same KS law as other regions. Observations of the interacting system Arp 158 provide the first evidence in a resolved fashion that different star-forming regions in a merger may be following different KS laws. This suggests that the physics of the interstellar medium at a scale no larger than 1 kpc, the size of the largest gravitational instabilities and the injection scale of turbulence, determines the origin of these laws.

astro-ph.CO↗

arXiv2011

Star formation efficiency as a function of metallicity: from star clusters to galaxies

We explore how the star formation efficiency in a protocluster clump is regulated by metallicity dependent stellar winds from the newly formed massive OB stars (Mstar >5 Msol). The model describes the co-evolution of the mass function of gravitationally bound cores and of the IMF in a protocluster clump. Dense cores are generated uniformly in time at different locations in the clump, and contract over lifetimes that are a few times their free fall times. The cores collapse to form stars that power strong stellar winds whose cumulative kinetic energy evacuates the gas from the clump and quenches further core and star formation. This sets the final star formation efficiency, SFEf. Models are run with various metallicities in the range Z/Zsol=[0.1,2]. We find that the SFEf decreases strongly with increasing metallicity.The SFEf-metallicity relation is well described by a decaying exponential whose exact parameters depend weakly on the value of the core formation efficiency. We find that there is almost no dependence of the SFEf-metallicity relation on the clump mass. This is due to the fact that an increase (decrease) in the clump mass leads to an increase (decrease) in the feedback from OB stars which is opposed by an increase (decrease) in the gravitational potential of the clump. The clump mass-cluster mass relations we find for all of the different metallicity cases imply a negligible difference between the exponent of the mass function of the protocluster clumps and that of the young clusters mass function. By normalizing the SFEs to their value for the solar metallicity case, we compare our results to SFE-metallicity relations derived on galactic scales and find a good agreement. As a by-product of this study, we also provide ready-to-use prescriptions for the power of stellar winds of main sequence OB stars in the mass range [5,80] Msol in the metallicity range we have considered

astro-ph.GA↗

arXiv2010

Molecular Cloud Formation and the Star Formation Efficiency in M~33

Does star formation proceed in the same way in large spirals such as the Milky Way and in smaller chemically younger galaxies? Earlier work suggests a more rapid transformation of H$_2$ into stars in these objects but (1) a doubt remains about the validity of the H$_2$ mass estimates and (2) there is currently no explanation for why star formation should be more efficient. M~33, a local group spiral with a mass $\sim 10$\% and a metallicity half that of the Galaxy, represents a first step towards the metal poor Dwarf Galaxies. We have searched for molecular clouds in the outer disk of M~33 and present here a set of detections of both $^{12}$CO and $^{13}$CO, including the only detections (for both lines) beyond the R$_{25}$ radius in a subsolar metallicity galaxy. The spatial resolution enables mass estimates for the clouds and thus a measure of the $N({\rm H}_2) / I_{\rm CO}$ ratio, which in turn enables a more reliable calculation of the H$_2$ mass. Our estimate for the outer disk of M~33 is $N({\rm H}_2) / I_{\rm CO(1-0)} \sim 5 \times 10^{20} \,{\rm cm^{-2}/(K{\rm \ km \ s^{-1}})}$ with an estimated uncertainty of a factor $\le 2$. While the $^{12/13}$CO line ratios do not provide a reliable measure of $N({\rm H}_2) / I_{\rm CO}$, the values we find are slightly greater than Galactic and corroborate a somewhat higher $N({\rm H}_2) / I_{\rm CO}$ value. Comparing the CO observations with other tracers of the interstellar medium, no reliable means of predicting where CO would be detected was identified. In particular, CO detections were often not directly on local HI or FIR or H$α$ peaks, although generally in regions with FIR emission and high HI column density. The results presented here provide support for the quicker transformation of H$_2$ into stars in M~33 than in large local universe spirals.

astro-ph.GA↗

arXiv2007

The Detection of Molecular Gas in the Outskirts of NGC 6946

We present the results of a search for molecular gas emission via the CO line in the far outer disk of the nearby spiral, NGC 6946. The positions targeted were chosen to lie on or near previously-identified outer disk HII regions. Molecular gas was clearly detected out to 1.3 R$_{25}$, with a further tentative detection at 1.4 R$_{25}$. The CO detections show excellent agreement with the HI velocities and imply beam-averaged column densities of $0.3-9\times 10^{20}$ cm$^{-2}$ and molecular gas masses of (2-70)$\times 10^{5}$ M$_{\sun}$ per 21$''$ beam (560pc). We find evidence for an abrupt decrease in the molecular fraction at the edge of the optical disk, similar to that seen previously in the azimuthally-averaged areal star formation rate. Our observations provide new constraints on the factors that determine the presence and detectability of molecular gas in the outskirts of galaxies, and suggest that neither the HI column, the metallicity or the local heating rate alone plays a dominant role.

astro-ph↗

arXiv2007

VCC 2062: an old Tidal Dwarf Galaxy in the Virgo Cluster?

Numerical simulations predict the existence of old Tidal Dwarf Galaxies (TDGs) that would have survived several Gyr after the collision lying at their origin. Such survivors, which would by now have become independent relaxed galaxies, would be ideal laboratories, if nearby enough, to tackle a number of topical issues, including the distribution of Dark Matter in and around galaxies. However finding old dwarf galaxies with a confirmed tidal origin is an observational challenge. A dwarf galaxy in the Virgo Cluster, VCC 2062, exhibits several unusual properties that are typical of a galaxy made out of recycled material. We discuss whether it may indeed be a TDG. We analysed multi-wavelength observations of VCC 2062, including an IRAM CO map, an optical spectrum of its HII regions, GALEX ultraviolet and archival broad-band and narrow-band optical images as well as a VLA HI datacube, originally obtained as part of the VIVA project. VCC 2062 appears to be the optical, low surface brightness counterpart of a kinematically detached, rotating condensation that formed within an HI tail apparently physically linked to the disturbed galaxy NGC 4694. In contrast to its faint optical luminosity, VCC 2062 is characterised by strong CO emission and a high oxygen abundance more typical of spiral disks. Its dynamical mass however, is that of a dwarf galaxy. VCC 2062 was most likely formed within a pre-enriched gaseous structure expelled from a larger galaxy as a result of a tidal interaction. The natural provider for the gaseous tail is NGC 4694 or rather a former companion which subsequently has been accreted by the massive galaxy. According to that scenario, VCC 2062 has been formed by a past tidal encounter. Since its parent galaxies have most probably already totally merged, it qualifies as an old TDG.

astro-ph↗

arXiv2006

Intergalactic Star Formation

Star formation in interacting systems may take place in various locations, from the dust--enshrouded core of Ultraluminous Infrared Galaxies to more unusual places such as the debris of colliding galaxies expelled into the intergalactic medium. Determining whether star-formation proceeds in the latter environment, far from the parent galaxies, in a similar way as in spiral disks has motivated the multi--wavelength study presented here. We collected VLA/HI, UV/GALEX, optical Halpha and MIR/Spitzer images of a few nearby interacting systems chosen for their prominent "intergalactic" star formation activity. Preliminary results on the spectacular collisional HI ring around NGC 5291 are presented.

astro-ph↗

arXiv2006

Infrared dust emission in the outer disk of M51

We examine faint infrared emission features detected in Spitzer Space Telescope images of M51, which are associated with atomic hydrogen in the outer disk and tidal tail at R greater than R_25 (4.9', ~14 kpc at d=9.6 Mpc). The infrared colors of these features are consistent with the colors of dust associated with star formation in the bright disk. However, the star formation efficiency (as a ratio of star formation rate to neutral gas mass) implied in the outer disk is lower than that in the bright disk of M51 by an order of magnitude, assuming a similar relationship between infrared emission and star formation rate in the inner and outer disks.

astro-ph↗

arXiv2004

Molecular hydrogen beyond the optical edge of an isolated spiral galaxy

We know little about the outermost portions of galaxies because there is little light coming from them. We do know that in many cases atomic hydrogen (HI) extends well beyond the optical radius \cite{Casertano91}. In the centers of galaxies, however, molecular hydrogen (H2) usually dominates by a large factor, raising the question of whether H2 is abundant also in the outer regions but hitherto unseen.Here we report the detection of emission from carbon monoxide (CO), the most abundant tracer of H2, beyond the optical radius of the nearby galaxy NGC 4414. The molecular clouds probably formed in the regions of relatively high HI column density and in the absence of spiral density waves. The relative strength of the lines from the two lowest rotational levels indicates that both the temperature and density of the H2 are quite low compared to conditions closer to the center. The inferred surface density of the molecular material continues the monotonic decrease from the inner regions. We conclude that while molecular clouds can form in the outer region of this galaxy, there is little mass associated with them.

astro-ph↗

arXiv2004

HCN and HCO+ emission in the disk of M31

We report observations made with the IRAM 30m radiotelescope in the HCN(1-0) and HCO+(1-0) lines towards a sample of molecular complexes (GMCs) in the disk of the Andromeda galaxy (M31). The targets were identified bright CO GMCs selected from the IRAM 30m CO survey with various morphologies and environments. The clouds vary in galactocentric distances from 2.4 to 15.5kpc. The HCN and HCO+ emission is easily detected in almost all observed positions, with line widths generally similar to the CO ones and there is a good correlation between the two dense gas tracers. The HCO+ emission is slightly stronger than the HCN, in particular towards GMCs with a strong star formation activity. However the HCO+ emission is weaker than the HCN towards a quiescent cloud in the inner part of M31, which could be due to a lower abundance of HCO+. We derive I_HCN/I_CO ratios between 0.008 and 0.03 and I_HCO+/I_CO ratios between less than 0.003 and 0.04. We study the radial distribution of the dense gas in the disk of M31. Unlike our Galaxy the HCO+/CO ratio is lower in the center of M31 than in the arms, which can be explained by both a lower abundance of HCO+ and different conditions of excitation. Furthermore the HCN/CO and HCO+/CO ratios appear to be higher in the inner spiral arm and weaker in the outer arm.

astro-ph↗

arXiv2002

Molecular gas in the intergalactic medium of Stephan's Quintet

Stephan's Quintet (SQ) is a Hickson Compact Group well known for its complex dynamical and star formation history and its rich intergalactic medium (IGM). In order to study the extent, origin and fate of the intergalactic molecular gas and its relation to the formation of stars outside galaxies and Tidal Dwarf Galaxies (TDGs), we mapped with the IRAM 30m antenna carbon monoxide (CO) towards several regions of the IGM in SQ. In two star forming regions (SQ A and B), situated in very different environments, we detected unusually large amounts of molecular gas ($3.1 \times 10^9$ \msun and $7 \times 10^8$ \msun, respectively), covering an extended area (between 15 and 25 kpc). In both regions the CO clouds have different properties and may be of a distinct nature. The integrated CO line of SQ A is in particular much wider than in SQ B. Its CO spectrum shows emission at two velocities (6000 and 6700 \kms), coincident with two HI lines, with the stronger emission at 6000 \kms being very smoothly distributed without a distinct peak in the starburst region. In SQ B the CO emission coincides with that of tracers of star formation (\halpha, near-infrared 15 $μ$m and radio continuum). The CO peak lies close to the HI peak towards a steep HI gradient. This is indicating that the molecular gas is forming in-situ, with subsequent star formation taking place. The star forming region at SQ B is the object in SQ that most resembles a TDG.

astro-ph↗

arXiv2002

Abundant molecular gas in the intergalactic medium of Stephan's Quintet

Stephan's Quintet (SQ) is a system consisting of at least four interacting galaxies which is well known for its complex dynamical and star formation history. It possesses a rich intergalactic medium (IGM), where hydrogen clouds, both atomic and molecular, associated with two starbursts (refered to as SQ A and B) have been found. In order to study the extent, origin and fate of the intergalactic molecular gas and its relation to the formation of stars outside galaxies and Tidal Dwarf Galaxies (TDGs), we mapped with the IRAM 30m antenna the carbon monoxide (CO) towards several regions of the IGM in SQ. In both SQ A and B, we detected unusually large amounts of molecular gas (3.1 times 10^9 msun and 7 times 10^8 msun, respectively). In contrast, no significant CO detection was achieved towards HII regions south of the pair NGC 7318a/b despite their high H alpha luminosities. The molecular gas is very extended in both SQ A and SQ B, over areas of between 15 and 25 kpc. The CO clouds seem to have otherwise different properties and may be of a different nature. The integrated CO line of SQ A is in particular much wider than in SQ B. Its CO spectrum shows emission at two velocities (6000 and 6700 km s^{-1}) that are coincident with two HI lines. The strongest emission at 6000 km s^{-1} is however spatially offset from the HI emission and situated on a ridge south-east of the starburst region. In SQ B the CO emission coincides with that of tracers of star formation (halpha, 15 mu m and radio continuum). The CO peak lies slightly offset from the HI peak towards a steep HI gradient. This is indicating that the molecular gas is forming in-situ, possibly in a region of compressed HI, with subsequent star formation. The star forming region at SQ B is the object in SQ that most resembles a TDG.

astro-ph↗

arXiv2001

Mass loss from galaxies: feeding the IGM, recycling in the IGM

As a result of internal processes or environmental effects like ram-pressure stripping or collisions, galaxies lose a significant part of their stellar and gaseous content. Whereas the impact of such stripping on galaxy evolution has been well studied, much less attention has been given to the fate of the expelled material in the intergalactic or intracluster medium (IGM/ICM). Observational evidence exists showing that a fraction of the injected matter is actually recycled to form a new generation of galaxies, such as the Tidal Dwarf Galaxies discovered near numerous interacting systems. Using a set of multiwavelength data, we are now able to roughly analyze the processes pertaining to their formation: from an instability in the HI clouds, through the formation of molecular gas, and to the onset of star formation.

astro-ph↗

arXiv2000

Formation of Molecular Gas in the debris of violent Galaxy Interactions

In many gravitational interactions between galaxies, gas and stars that have been torn from either or both of the precursor galaxies can collect in 'tidal tails'. Star formation begins anew in these regions to produce 'tidal dwarf galaxies', giving insight into the process of galaxy formation through the well-defined timescale of the interaction. But tracking the star formation process has proved to be difficult: the tidal dwarf galaxies with young stars showed no evidence of the molecular gas out of which new stars form. Here we report the discovery of molecular gas (carbon monoxide emission) in two tidal dwarf galaxies. In both cases, the molecular gas peaks at the same location as the maximum in atomic-hydrogen density, unlike most gas-rich galaxies. We infer from this that the molecular gas formed from the HI, rather than being torn in molecular form from the interacting galaxies. Star formation in the tidal dwarfs appears to mimic that process in normal spiral galaxies like our own.

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