arXiv subjects

N. A. Hatch

Publications and source records attributed to N. A. Hatch.

At least 55 records · Page 3Linked to original sources

arXiv2011

Riding the wake of a merging galaxy cluster

Using WHT OASIS integral field unit observations, we report the discovery of a thin plume of ionised gas extending from the brightest cluster galaxy in Abell 2146 to the sub-cluster X-ray cool core which is offset from the BCG by ~37 kpc. The plume is greater than 15 kpc long and less than 3 kpc wide. This plume is unique in that the cluster it is situated in is currently undergoing a major galaxy cluster merger. The brightest cluster galaxy is unusually located behind the X-ray shock front and in the wake of the ram pressure stripped X-ray cool core and evidence for recent disruption to the BCG is observed. We examine the gas and stellar morphology, the gas kinematics of the BCG and their relation to the X-ray gas. We propose that a causal link between the ionised gas plume and the offset X-ray cool core provides the simplest explanation for the formation of the plume. An interaction or merger between the BCG and another cluster galaxy is probably the cause of the offset.

astro-ph.CO↗

arXiv2011

A deep spectroscopic study of the filamentary nebulosity in NGC4696, the brightest cluster galaxy in the Centaurus cluster

We present results of deep integral field spectroscopy observations using high resolution optical (4150-7200 A) VIMOS VLT spectra, of NGC 4696, the dominant galaxy in the Centaurus cluster (Abell 3526). After the Virgo cluster, this is the second nearest (z=0.0104) example of a cool core cluster. NGC 4696 is surrounded by a vast, luminous H alpha emission line nebula (L = 2.2 \times 10^40 ergs per second). We explore the origin and excitation of the emission-line filaments and find their origin consistent with being drawn out, under rising radio bubbles, into the intracluster medium as in other similar systems. Contrary to previous observations we do not observe evidence for shock excitation of the outer filaments. Our optical spectra are consistent with the recent particle heating excitation mechanism of Ferland et al.

astro-ph.CO↗

arXiv2011

Discovery of a high-z protocluster with tunable filters: the case of 6C0140+326 at z=4.4

We present the first results obtained using a tunable narrowband filter in the search for high-z protoclusters. Using the recently commissioned red tunable filter on the Gran Telescopio Canarias we have searched for Lya emitters in a 75 arcmin^2 field centered on the z=4.413 radio galaxy 6C0140+326. With three different wavelength tunings we find a total of 27 unique candidate Lya emitters. The availability of three different wavelength tunings allows us to make estimates of the redshifts for each of the objects. It also allows us to separate a possible protocluster from structure in the immediate foreground. This division shows that the foreground region contains significantly fewer Lya emitters. Also, the spatial distribution of the objects in the protocluster field deviates from a random distribution at the 2.5 sigma level. The observed redshift distribution of the emitters is different from the expected distribution of a blank field at the ~3 sigma level, with the Lya emitters concentrated near the radio galaxy at z>4.38. The 6C0140+326 field is denser by a factor of 9+/-5 than a blank field, and the number density of Lya emitters close to the radio galaxy is similar to that of the z~4.1 protocluster around TNJ1338-1942. We thus conclude that there is an overdensity of Lya emitters around the radio galaxy 6C0140+326. This is one of few known overdensities at such a high redshift.

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arXiv2011

A SINFONI view of flies in the Spiderweb: a galaxy cluster in the making

The environment of the high-z radio galaxy PKS 1138-262 at z~2.2 is a prime example of a forming galaxy cluster. We use deep SINFONI data to perform a detailed study of the kinematics of the galaxies within 60 kpc of the radio core and we link this to the kinematics of the protocluster on the megaparsec scale. Identification of optical emission lines shows that 11 galaxies are at the redshift of the protocluster. The density of line emitters is more than an order of magnitude higher in the core of the protocluster than the larger scale environment. This implies a matter overdensity in the core of delta_m~70 which is similar to the outskirts of local galaxy clusters. The velocity distribution of the confirmed satellite galaxies shows a broad, double-peaked velocity structure with sigma=1360+/-206 km/s. A similar broad, double-peaked distribution was found in a previous study targeting the large scale protocluster structure, indicating that a common process is acting on both small and large scales. Including all spectroscopically confirmed protocluster galaxies, a velocity dispersion of 1013+/-87 km/s is found. We show that the protocluster has likely decoupled from the Hubble flow and is a dynamically evolved structure. Comparison to the Millenium simulation indicates that the protocluster velocity distribution is consistent with that of the most massive haloes at z~2, but we rule out that the protocluster is a fully virialized structure based on dynamical arguments and its X-ray luminosity. Comparison to merging haloes in the Millennium simulation shows that the structure as observed in and around the Spiderweb galaxy is best interpreted as being the result of a merger between two massive haloes. We propose that this merger can result in an increase in star formation and AGN activity in the protocluster core and is possibly an important stage in the evolution of massive cD galaxies.

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arXiv2011

H-alpha emitters in z~2 proto-clusters: evidence for faster evolution in dense environments

This is a study of H-alpha emitters in two dense galaxy proto-clusters surrounding radio galaxies at z~2. We show that the proto-cluster surrounding MRC 1138-262 contains 14+/-2 times more H-alpha candidates than the average field (9 sigma significance), and the z=2.35 radio galaxy 4C+10.48 is surrounded by 12+/-2 times more emitters than the field (5 sigma), so it is also likely to reside in a dense proto-cluster environment. We compared these H-alpha emitters, situated in dense environments, to a control field sample selected from 3 separate fields forming a total area of 172 arcmin^2. We constructed and compared H-alpha and rest-frame R continuum luminosity functions of the emitters in both environments. The star formation density is on average 13 times greater in the proto-clusters than the field at z~2, so the total star formation rate within the central 1.5Mpc of the proto-clusters exceeds 3000Msun/yr. However, we found no significant difference in the shape of the H-alpha luminosity functions, implying that environment does not substantially affect the strength of the H-alpha line from strongly star forming galaxies. The proto-cluster emitters are typically 0.8mag brighter in rest-frame R continuum than field emitters, implying they are twice as massive as their field counterparts at the same redshift. We also show the proto-cluster galaxies have lower specific star formation rates than field galaxies, meaning the emitters in the dense environments formed more of their stars earlier than the field galaxies. We conclude that galaxy growth in the early Universe was accelerated in dense environments, and that cluster galaxies differed from field galaxies even before the cluster had fully formed.

astro-ph.CO↗

arXiv2011

Far Ultraviolet Emission in the A2597 and A2204 Brightest Cluster Galaxies

We use the Hubble Space Telescope ACS/SBC and Very Large Telescope FORS cameras to observe the Brightest Cluster Galaxies in Abell 2597 and Abell 2204 in the far-ultraviolet (FUV) F150LP and optical U, B, V, R, I Bessel filters. The FUV and U band emission is enhanced in bright, filamentary structures surrounding the BCG nuclei. These filaments can be traced out to 20 kpc from the nuclei in the FUV. Excess FUV and U band light is determined by removing emission due to the underlying old stellar population and mapped with 1 arcsec spatial resolution over the central 20 kpc regions of both galaxies. We find the FUV and U excess emission to be spatially coincident and a stellar interpretation requires the existence of a significant amount of 10000-50000 K stars. Correcting for nebular continuum emission and dust intrinsic to the BCG further increases the FUV to U band emission ratio and implies that stars alone may not suffice to explain the observations. However, lack of detailed information on the gas and dust distribution and extinction law in these systems prevents us from ruling out a purely stellar origin. Non-stellar processes, such as the central AGN, Scattering, Synchrotron and Bremsstrahlung emission are investigated and found to not be able to explain the FUV and U band measurements in A2597. Contributions from non-thermal processes not treated here should be investigated. Comparing the FUV emission to the optical H-alpha line emitting nebula shows good agreement on kpc-scales in both A2597 and A2204. In concordance with an earlier investigation by O'Dea et al. (2004) we find that O-stars can account for the ionising photons necessary to explain the observed H-alpha line emission.

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arXiv2010

Detection of optical coronal emission from 10^6 K gas in the core of the Centaurus cluster

We report a detection (3.5x10^37 \pm 5.6x10^36 ergps) of the optical coronal emission line [Fe X]6374 and upper limits of four other coronal lines using high resolution VIMOS spectra centred on NGC 4696, the brightest cluster galaxy in the Centaurus cluster. Emission from these lines is indicative of gas at temperatures between 1 and 5 million K so traces the interstellar gas in NGC 4696. The rate of cooling derived from the upper limits is consistent with the cooling rate from X-ray observations (~10 solar masses per year) however we detect twice the luminosity expected for [Fe X]6374 emission, at 1 million K, our lowest temperature probe. We suggest this emission is due to the gas being heated rather than cooling out of the intracluster medium. We detect no coronal lines from [Ca XV], which are expected from the 5 million K gas seen near the centre in X-rays with Chandra. Calcium is however likely to be depleted from the gas phase onto dust grains in the central regions of NGC 4696.

astro-ph.GA↗

arXiv2010

Galaxy protocluster candidates around z ~ 2.4 radio galaxies

We study the environments of 6 radio galaxies at 2.2 < z < 2.6 using wide-field near-infrared images. We use colour cuts to identify galaxies in this redshift range, and find that three of the radio galaxies are surrounded by significant surface overdensities of such galaxies. The excess galaxies that comprise these overdensities are strongly clustered, suggesting they are physically associated. The colour distribution of the galaxies responsible for the overdensity are consistent with those of galaxies that lie within a narrow redshift range at z ~ 2.4. Thus the excess galaxies are consistent with being companions of the radio galaxies. The overdensities have estimated masses in excess of 10^14 solar masses, and are dense enough to collapse into virizalised structures by the present day: these structures may evolve into groups or clusters of galaxies. A flux-limited sample of protocluster galaxies with K < 20.6 mag is derived by statistically subtracting the fore- and background galaxies. The colour distribution of the protocluster galaxies is bimodal, consisting of a dominant blue sequence, comprising 77 +/- 10% of the galaxies, and a poorly populated red sequence. The blue protocluster galaxies have similar colours to local star-forming irregular galaxies (U -V ~ 0.6), suggesting most protocluster galaxies are still forming stars at the observed epoch. The blue colours and lack of a dominant protocluster red sequence implies that these cluster galaxies form the bulk of their stars at z < 3.

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arXiv2010

Herschel observations of FIR emission lines in brightest cluster galaxies

The question of how much gas cools in the cores of clusters of galaxies has been the focus of many, multiwavelength studies in the past 30 years. In this letter we present the first detections of the strongest atomic cooling lines, [C II], [O I] and [N I] in two strong cooling flow clusters, A1068 and A2597, using Herschel PACS. These spectra indicate that the substantial mass of cold molecular gas (>10^9 Mo) known to be present in these systems is being irradiated by intense UV radiation, most probably from young stars. The line widths of these FIR lines indicate that they share dynamics similar but not identical to other ionised and molecular gas traced by optical, near-infrared and CO lines. The relative brightness of the FIR lines compared to CO and FIR luminosity is consistent with other star-forming galaxies indicating that the properties of the molecular gas clouds in cluster cores and the stars they form are not unusual. These results provide additional evidence for a reservoir of cold gas that is fed by the cooling of gas in the cores of the most compact clusters and provide important diagnostics of the temperature and density of the dense clouds this gas resides in.

astro-ph.CO↗

arXiv2010

Herschel photometry of brightest cluster galaxies in cooling flow clusters

The dust destruction timescales in the cores of clusters of galaxies are relatively short given their high central gas densities. However, substantial mid-infrared and sub-mm emission has been detected in many brightest cluster galaxies. In this letter we present Herschel PACS and SPIRE photometry of the brightest cluster galaxy in three strong cooling flow clusters, A1068, A2597 and Zw3146. This photometry indicates that a substantial mass of cold dust is present (>3 x 10^7 Mo) at temperatures significantly lower (20-28K) than previously thought based on limited MIR and/or sub-mm results. The mass and temperature of the dust appear to match those of the cold gas traced by CO with a gas-to-dust ratio of 80-120.

astro-ph.CO↗

arXiv2010

Central galaxy growth and feedback in the most massive nearby cool core cluster

We present multi-wavelength observations of the centre of RXCJ1504.1-0248 - the galaxy cluster with the most luminous and relatively nearby cool core at z~0.2. Although there are several galaxies within 100 kpc of the cluster core, only the brightest cluster galaxy (BCG), which lies at the peak of the X-ray emission, has blue colours and strong line-emission. Approximately 80 Msun/yr of intracluster gas is cooling below X-ray emitting temperatures, similar to the observed UV star formation rate of ~140 Msun/yr. Most star formation occurs in the core of the BCG and in a 42 kpc long filament of blue continuum, line emission, and X-ray emission, that extends southwest of the galaxy. The surrounding filamentary nebula is the most luminous around any observed BCG. The number of ionizing stars in the BCG is barely sufficient to ionize and heat the nebula, and the line ratios indicate an additional heat source is needed. This heat source can contribute to the Hα-deduced star formation rates (SFRs) in BCGs and therefore the derived SFRs should only be considered upper limits. AGN feedback can slow down the cooling flow to the observed mass deposition rate if the black hole accretion rate is of the order of 0.5 Msun/yr at 10% energy output efficiency. The average turbulent velocity of the nebula is vturb ~325 km/s which, if shared by the hot gas, limits the ratio of turbulent to thermal energy of the intracluster medium to less than 6%.

astro-ph.CO↗

arXiv2010

A galaxy populations study of a radio-selected protocluster at z~3.1

We present a population study of several types of galaxies within the protocluster surrounding the radio galaxy MRC0316-257 at z~3.1. In addition to the known population of Ly_alpha emitters (LAEs) and [OIII] emitters, we use colour selection techniques to identify protocluster candidates that are Lyman break galaxies (LBG) and Balmer break galaxies (BBGs). The radio galaxy field contains an excess of LBG candidates, with a surface density 1.6\pm0.3 times larger than found for comparable blank fields. This surface overdensity corresponds to an LBG volume overdensity of ~8\pm4. The BBG photometric redshift distribution peaks at the protocluster's redshift, but we detect no significant surface overdensity of BBG. This is not surprising because a volume overdensity similar to the LBGs would have resulted in a surface density of ~1.2 that found in the blank field. This could not have been detected in our sample. Masses and star formation rates of the candidate protocluster galaxies are determined using SED fitting. These properties are not significantly different from those of field galaxies. The galaxies with the highest masses and star formation rates are located near the radio galaxy, indicating that the protocluster environment influences galaxy evolution at z~3. We conclude that the protocluster around MRC0316-257 is still in the early stages of formation.

astro-ph.CO↗

arXiv2009

The growth and assembly of a massive galaxy at z ~ 2

We study the stellar mass assembly of the Spiderweb Galaxy (MRC 1138-262), a massive z = 2.2 radio galaxy in a protocluster and the probable progenitor of a brightest cluster galaxy. Nearby protocluster galaxies are identified and their properties are determined by fitting stellar population models to their rest-frame ultraviolet to optical spectral energy distributions. We find that within 150 kpc of the radio galaxy the stellar mass is centrally concentrated in the radio galaxy, yet most of the dust-uncorrected, instantaneous star formation occurs in the surrounding low-mass satellite galaxies. We predict that most of the galaxies within 150 kpc of the radio galaxy will merge with the central radio galaxy by z = 0, increasing its stellar mass by up to a factor of ~ 2. However, it will take several hundred Myr for the first mergers to occur, by which time the large star formation rates are likely to have exhausted the gas reservoirs in the satellite galaxies. The tidal radii of the satellite galaxies are small, suggesting that stars and gas are being stripped and deposited at distances of tens of kpc from the central radio galaxy. These stripped stars may become intracluster stars or form an extended stellar halo around the radio galaxy, such as those observed around cD galaxies in cluster cores.

astro-ph.CO↗

arXiv2008

Collisional heating as the origin of filament emission in galaxy clusters

[Abridged] Photoionization, whether by starlight or other sources, has difficulty in accounting for the observed spectra of the optical filaments that often surround central galaxies in large clusters. Our first paper examined whether heating by energetic particles or dissipative MHD wave can account for the observations. Here we include atomic and low-ionization regions. The model of the hydrogen atom, along with all elements of the H-like iso-electronic sequence, is now fully nl-resolved. We show how the predicted HI spectrum differs from the pure recombination case. The second update is to the rates for H^0 - H2 inelastic collisions. We now use the values computed by Wrathmall et al. The rates are often much larger. We calculate the chemistry, ionization, temperature, gas pressure, and emission-line spectrum for a wide range of gas densities and collisional heating rates. We assume that the filaments are magnetically confined and free to move along field lines so that the gas pressure is equal to that of the surrounding hot gas. A mix of clouds, some being dense and cold and others hot and tenuous, can exist. The observed spectrum will be the integrated emission from clouds with different densities and temperatures but the same pressure. We assume that the gas filling factor is given by a power law in density. The power-law index is set by matching the observed intensities of IR H2 lines relative to optical HI lines. We conclude that the filaments are heated by ionizing particles, either conducted in from surrounding regions or produced in situ by processes related to MHD waves.

astro-ph↗

arXiv2008

[OIII] emitters in the field of the MRC 0316-257 protocluster

Venemans et al. (2005) found evidence for an overdensity of Ly-alpha emission line galaxies associated with the radio galaxy MRC 0316-257 at z=3.13 indicating the presence of a massive protocluster. Here, we present the results of a search for additional star-forming galaxies and AGN within the protocluster. Narrow-band infrared imaging was used to select candidate [O III] emitters in a 1.1 x 1.1 Mpc^2 region around the radio galaxy. Thirteen candidates have been detected. Four of these are among the previously confirmed sample of Ly-alpha galaxies, and an additional three have been confirmed through follow-up infrared spectroscopy. The three newly confirmed objects lie within a few hundred km/s of each other, but are blueshifted with respect to the radio galaxy and Ly-alpha emitters by ~2100 km/s. Although the sample is currently small, our results indicate that the radio-selected protocluster is forming at the centre of a larger, ~60 co-moving Mpc super-structure. On the basis of an HST/ACS imaging study we calculate dust-corrected star-formation rates and investigate morphologies and sizes of the [O III] candidate emitters. From a comparison of the star formation rate derived from UV-continuum and [O III] emission, we conclude that at least two of the [O III] galaxies harbour an AGN which ionized the O+ gas.

astro-ph↗

arXiv2008

Cold gas in the Perseus cluster core: Excitation of molecular gas in filaments

We have recently detected CO lines in the well-known filaments around NGC 1275, the galaxy at the centre of the Perseus cluster of galaxies. These previous observations, with the HERA multi-beam array at the IRAM 30m telescope enabled us to make a large map of the CO(2-1) line and to see hints of molecular gas far away from the cluster centre. To confirm the presence of CO emission lines in the outer filaments and to study the CO(2-1)/CO(1-0) line ratio, we observed seven regions of interest again with the 30m telescope in both CO(1-0) and CO(2-1). The regions we observed were: the eastern filament, the horseshoe, the northern filament and a southern extension, all selected from Halpha emission line mapping. Molecular gas is detected in all the observed regions. This result confirms the large extent of the cold molecular gas filaments. We discuss the CO(2-1)/CO(1-0) ratios in the filaments. The eastern filament has optically thick gas, whereas further away, the line ratio increases close to values expected for a warmer optically thin medium. We also show CO(1-0) and CO(2-1) lines in 9 regions closer to the centre. The kinematics of the CO is studied here in more detail and confirms that it follows the motions of the warm H_2 gas found in the near-infrared. Finally, we searched for dense gas tracers around 3C84 and claim here the first detection of HCN(3-2).

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arXiv2008

The origin of molecular hydrogen emission in cooling-flow filaments

The optical filaments found in many cooling flows in galaxy clusters consist of low density ($\sim 10^3 \pcc$) cool ($\sim 10^3$ K) gas surrounded by significant amounts of cosmic-ray and magnetic-field energy. Their spectra show anomalously strong low-ionization and molecular emission lines when compared with galactic molecular clouds exposed to ionizing radiation such as the Orion complex. Previous studies have shown that the spectra cannot be produced by O-star photoionization. Here we calculate the physical conditions in dusty gas that is well shielded from external sources of ionizing photons and is energized either by cosmic rays or dissipative MHD waves. Strong molecular hydrogen lines, with relative intensities similar to those observed, are produced. Selection effects introduced by the microphysics produce a correlation between the \htwo line upper level energy and the population temperature. These selection effects allow a purely collisional gas to produce \htwo emission that masquerades as starlight-pumped \htwo but with intensities that are far stronger. This physics may find application to any environment where a broad range of gas densities or heating rates occur.

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arXiv2007

Diffuse UV light associated with the Spiderweb Galaxy: evidence for in-situ star formation outside galaxies

We present detailed images of diffuse UV intergalactic light (IGL), situated in a 60kpc halo that surrounds the radio galaxy MRC 1138-262 at z=2. We discuss the nature of the IGL and rule out faint cluster galaxies, nebular continuum emission, synchrotron, inverse Compton emission and scattering of galactic stellar light as possible sources of the IGL. Dust scattered quasar light is an unlikely possibility that cannot be ruled out entirely. We conclude that the source of the IGL is most likely to be a young stellar population distributed in a halo encompassing the radio and satellite galaxies, undergoing star formation at a rate greater than 57 Msun/yr. Within 70kpc of the radio core, approximately 44% of the star formation that is traced by UV light occurs in this diffuse mode. The average UV colour of the IGL is bluer than the average galaxy colour, and there is a trend for the IGL to become bluer with increasing radius from the radio galaxy. Both the galaxies and the IGL show a UV colour--surface brightness relation which can be obtained by variations in either stellar population age or extinction. These observations show a different, but potentially important mode of star formation, that is diffuse in nature. Star formation, as traced by UV light, occurs in two modes in the high redshift universe: one in the usual Lyman break galaxy clump-like mode on kpc scales, and the other in a diffuse mode over a large region surrounding massive growing galaxies. Such a mode of star formation can easily be missed by high angular resolution observations that are well suited for detecting high surface brightness compact galaxies. Extrapolating from these results, it is possible that a significant amount of star formation occurs in large extended regions within the halos of the most massive galaxies forming at high redshift.

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