arXiv subjects

Y. Krongold

Publications and source records attributed to Y. Krongold.

At least 37 records · Page 2Linked to original sources

arXiv2018

The Changing-Look Quasar Mrk 590 is Awakening

Mrk 590 was originally classified as a Seyfert 1 galaxy, but then it underwent dramatic changes: the nuclear luminosity dropped by over two orders of magnitude and the broad emission lines all but disappeared from the optical spectrum. Here we present followup observations to the original discovery and characterization of this "changing look" active galactic nucleus (AGN). The new Chandra and HST observations from 2014 show that Mrk 590 is awakening, changing its appearance again. While the source continues to be in a low state, its soft excess has re-emerged, though not to the previous level. The UV continuum is brighter by more than a factor of two and the broad MgII emission line is present, indicating that the ionizing continuum is also brightening. These observations suggest that the soft excess is not due to reprocessed hard X-ray emission. Instead, it is connected to the UV continuum through warm Comptonization. Variability of the Fe K-alpha emission lines suggests that the reprocessing region is within about 10 light years or 3 pc of the central source. The AGN type change is neither due to obscuration, nor due to one-way evolution from type-1 to type-2, as suggested in literature, but may be related to episodic accretion events.

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arXiv2018

Early Science with the Large Millimeter Telescope: an energy-driven wind revealed by massive molecular and fast X-ray outflows in the Seyfert Galaxy IRAS 17020+4544

We report on the coexistence of powerful gas outflows observed in millimeter and X-ray data of the Radio-Loud Narrow Line Seyfert 1 Galaxy IRAS 17020+4544. Thanks to the large collecting power of the Large Millimeter Telescope, a prominent line arising from the 12CO(1-0) transition was revealed in recent observations of this source. The complex profile is composed by a narrow double-peak line and a broad wing. While the double-peak structure may be arising in a disk of molecular material, the broad wing is interpreted as the signature of a massive outflow of molecular gas with an approximate bulk velocity of -660 km/s. This molecular wind is likely associated to a multi-component X-ray Ultra-Fast Outflow with velocities reaching up to ~0.1c and column densities in the range 10^{21-23.9} cm^-2 that was reported in the source prior to the LMT observations. The momentum load estimated in the two gas phases indicates that within the observational uncertainties the outflow is consistent with being propagating through the galaxy and sweeping up the gas while conserving its energy. This scenario, which has been often postulated as a viable mechanism of how AGN feedback takes place, has so far been observed only in ULIRGs sources. IRAS 17020+4544 with bolometric and infrared luminosity respectively of 5X10^{44} erg/s and 1.05X10^{11} L_sun appears to be an example of AGN feedback in a NLSy1 Galaxy (a low power AGN). New proprietary multi-wavelength data recently obtained on this source will allow us to corroborate the proposed hypothesis.

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arXiv2018

Observations of the MIssing Baryons in the warm-hot intergalactic medium

It has been known for decades that the observed number of baryons in the local universe falls about 30-40% short of the total number of baryons predicted by Big-Bang Nucleosynthesis, as inferred from density fluctuations of the Cosmic Microwave Background and seen during the first 2-3 billion years of the universe in the so called Lyman-alpha Forest. A theoretical solution to this paradox locates the missing baryons in the hot and tenuous filamentary gas between galaxies, known as the warm-hot intergalactic medium. However, it is difficult to detect them there because the largest by far constituent of this gas - hydrogen - is mostly ionized and therefore almost invisible in far-ultraviolet spectra with typical signal-to-noise ratios. Indeed, despite the large observational efforts, only a few marginal claims of detection have been made so far. Here we report observations of two absorbers of highly ionized oxygen (OVII) in the high signal-to-noise-ratio X-ray spectrum of a quasar at redshift >0.4. These absorbers show no variability over a 2-year timescale and have no associated cold absorption, making the assumption that they originate from the quasar's intrinsic outflow or the host galaxy's interstellar medium implausible. The OVII systems lie in regions characterized by large (x4 compared to average) galaxy over-densities and their number (down to the sensitivity threshold of our data), agrees well with numerical simulation predictions for the long-sought warm-hot intergalactic medium (WHIM). We conclude that the missing baryons have been found.

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arXiv2017

Space Telescope and Optical Reverberation Mapping Project. VII. Understanding the UV anomaly in NGC 5548 with X-Ray Spectroscopy

During the Space Telescope and Optical Reverberation Mapping Project (STORM) observations of NGC 5548, the continuum and emission-line variability became de-correlated during the second half of the 6-month long observing campaign. Here we present Swift and Chandra X-ray spectra of NGC 5548 obtained as a part of the campaign. The Swift spectra show that excess flux (relative to a power-law continuum) in the soft X-ray band appears before the start of the anomalous emission-line behavior, peaks during the period of the anomaly, and then declines. This is a model-independent result suggesting that the soft excess is related to the anomaly. We divide the Swift data into on- and off-anomaly spectra to characterize the soft excess via spectral fitting. The cause of the spectral differences is likely due to a change in the intrinsic spectrum rather than being due to variable obscuration or partial covering. The Chandra spectra have lower signal-to-noise ratios, but are consistent with Swift data. Our preferred model of the soft excess is emission from an optically thick, warm Comptonizing corona, the effective optical depth of which increases during the anomaly. This model simultaneously explains all the three observations: the UV emission line flux decrease, the soft-excess increase, and the emission line anomaly.

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arXiv2017

Hubble Space Telescope Observations of BALQSO Ton 34 Reveal a Connection between the Broad Line Region and the BAL Outflow

Ton 34 recently transitioned from non-absorbing quasar into a BALQSO.Here, we report new HST-STIS observations of this quasar. Along with CIV absorption, we also detect absorption by NV+Ly alpha and possibly OVI+Ly beta. We follow the evolution of the CIV BAL, and find that, for the slower outflowing material, the absorption trough varies little (if at all) on a rest-frame timescale of 2 yr. However, we detect a strong deepening of the absorption in the gas moving at larger velocities (-20,000 - -23,000 km s-1). The data is consistent with a multistreaming flow crossing our line of sight to the source. The transverse velocity of the flow should be few thousand km s-1, similar to the rotation velocity of the BLR gas (2,600 km s-1). By simply assuming Keplerian motion, these two components must have similar locations, pointing to a common outflow forming the BLR and the BAL. We speculate that BALs, mini-BALs, and NALs, are part of a common, ubiquitous, accretion-disk outflow in AGN, but become observable depending on the viewing angle towards the flow. The absorption troughs suggest a wind covering only 20% of the emitting source, implying a maximum size of 10^-3 pc for the clouds forming the BAL/BLR medium. This is consistent with constraints of the BLR clouds from X-ray occultations. Finally, we suggest that the low excitation broad emission lines detected in the spectra of this source lie beyond the wind, and this gas is probably excited by the shock of the BAL wind with the surrounding medium.

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arXiv2017

An ultra-dense fast outflow in a quasar at z=2.4

We present Adaptive Optics assisted near-IR integral field spectroscopic observations of a luminous quasar at $z = 2.4$, previously observed as the first known example at high redshift of large scale quasar-driven outflow quenching star formation in its host galaxy. The nuclear spectrum shows broad and blueshifted H$β$ in absorption, which is tracing outflowing gas with high densities ($>10^8$ - $10^9$ cm$^{-3}$) and velocities in excess of 10,000 km s$^{-1}$. The properties of the outflowing clouds (covering factor, density, column density and inferred location) indicate that they likely originate from the Broad Line Region. The energetics of such nuclear regions are consistent with that observed in the large scale outflow, supporting models in which quasar driven outflows originate from the nuclear region and are energy conserving. We note that the asymmetric profile of both the H$β$ and H$α$ emission lines is likely due to absorption by the dense outflowing gas along the line of sight. This outflow-induced asymmetry has implications on the estimation of the black hole mass using virial estimators, and warns about such effects for several other quasars characterized by similar line asymmetries. More generally, our findings may suggest a broader revision of the decomposition and interpretation of quasar spectral features, in order to take into account the presence of potential broad blueshifted Balmer absorption lines. Our high spatial resolution data also reveals redshifted, dynamically colder nebular emission lines, likely tracing an inflowing stream.

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arXiv2017

Coexistence of a non thermal jet and a complex ultra fast X-ray outflow in a moderately luminous AGN

Context: Recent XMM-Newton observations have revealed that IRAS 17020+4544 is a very unusual example of black hole wind-produced feedback by a moderately luminous AGN in a spiral galaxy. Aims: Since the source is known for being a radio emitter, we investigated about the presence and the properties of a non-thermal component. Methods: We observed IRAS 17020+4544 with the Very Long Baseline Array at 5, 8, 15, and 24 GHz within a month of the 2014 XMM-Newton observations. We further analysed archival data taken in 2000 and 2012. Results: We detect the source at 5 GHz and on short baselines at 8 GHz. At 15 and 24 GHz, the source is below our baseline sensitivity for fringe fitting, indicating the lack of prominent compact features. The morphology is that of an asymmetric double, with significant diffuse emission. The spectrum between 5 and 8 GHz is rather steep ($S(ν)\simν^{-(1.0\pm0.2)}$). Our re-analysis of the archival data at 5 and 8 GHz provides results consistent with the new observations, suggesting that flux density and structural variability are not important in this source. We put a limit on the separation speed between the main components of $<0.06c$. Conclusions: IRAS 17020+4544 shows interesting features of several classes of objects: its properties are typical of compact steep spectrum sources, low power compact sources, radio-emitting narrow line Seyfert 1 galaxies. However, it can not be classified in any of these categories, remaining so far a one-of-a-kind object.

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arXiv2016

A Decade of WHIM Searches: Where do we Stand and Where do we Go?

In this article we first review the past decade of efforts in detecting the missing baryons in the Warm Hot Intergalactic Medium (WHIM) and summarize the current state of the art by updating the baryon census and physical state of the detected baryons in the local Universe. We then describe observational strategies that should enable a significant step forward in the next decade, while waiting for the step-up in quality offered by future missions. In particular we design a multi-mega-second and multiple cycle XMM-Newton legacy program (which we name the Ultimate Roaming Baryon Exploration, or URBE) aimed to secure detections of the peaks in the density distribution of the Universe missing baryons over their entire predicted range of temperatures.

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arXiv2016

The Milky Way Hot Baryons and their Peculiar Density Distribution: a Relic of Nuclear Activity

We know that our Galaxy is permeated by tenuous, hot, metal-rich gas. However much remains unknown about its origin, the portion of the Galaxy that it permeates, its total mass, as any role it may play in regulating activity in the Galaxy. In a Letter currently in the press with the ApJ, we show that this hot gas permeates both the disk of the Galaxy and a large spherical volume, centered on the Galactic nucleus, and extending out to distances of at least 60-200 kpc from the center. This gas displays a peculiar density distribution that peaks about 6 kpc from the Galaxy's center, likely witnessing a period of strong activity of the central super-massive black hole of the Milky Way that occurred 6 Myrs ago. With our study we are also able to update the total baryonic mass of the Galaxy to Mb = (0.8-4)x1e11 Solar Masses, sufficient to close the Galaxy's baryon census.

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arXiv2016

A Distant Echo of Milky Way Central Activity closes the Galaxy's Baryon Census

We report on the presence of large amounts of million-degree gas in the Milky Way's interstellar and circum-galactic medium. This gas (1) permeates both the Galactic plane and the halo, (2) extends to distances larger than 60-200 kpc from the center, and (3) its mass is sufficient to close the Galaxy's baryon census. Moreover, we show that a vast, $\sim 6$ kpc radius, spherically-symmetric central region of the Milky Way above and below the 0.16 kpc thick plane, has either been emptied of hot gas or the density of this gas within the cavity has a peculiar profile, increasing from the center up to a radius of $\sim 6$ kpc, and then decreasing with a typical halo density profile. This, and several other converging pieces of evidence, suggest that the current surface of the cavity, at 6 kpc from the Galaxy's center, traces the distant echo of a period of strong nuclear activity of our super-massive black-hole, occurred about 6 Myrs ago.

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arXiv2016

Studying the evolution of galaxies in compact groups over the past 3 Gyr - II. The importance of environment in the suppression of star formation

We present an in depth study on the evolution of galaxy properties in compact groups over the past 3 Gyr. We are using the largest multi-wavelength sample to-date, comprised 1770 groups (containing 7417 galaxies), in the redshift range of 0.01<z<0.23. To derive the physical properties of the galaxies we rely on ultraviolet (UV)-to-infrared spectral energy distribution modeling, using CIGALE. Our results suggest that during the 3 Gyr period covered by our sample, the star formation activity of galaxies in our groups has been substantially reduced (3-10 times). Moreover, their star formation histories as well as their UV-optical and mid-infrared colors are significantly different from those of field and cluster galaxies, indicating that compact group galaxies spend more time transitioning through the green valley. The morphological transformation from late-type spirals into early-type galaxies occurs in the mid-infrared transition zone rather than in the UV-optical green valley. We find evidence of shocks in the emission line ratios and gas velocity dispersions of the late-type galaxies located below the star forming main sequence. Our results suggest that in addition to gas stripping, turbulence and shocks might play an important role in suppressing the star formation in compact group galaxies.

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arXiv2016

Diffuse Low-Ionization Gas in the Galactic Halo Casts Doubts on $z\simeq 0.03$ WHIM Detections

In this Letter we demonstrate that the two claims of $z\simeq 0.03$ OVII K$α$ absorption lines from Warm Hot Intergalactic Medium (WHIM) along the lines of sight to the blazars H~2356-309 (Buote et al., 2009; Fang et al., 2010) and Mkn~501 (Ren, Fang \& Buote, 2014) are likely misidentifications of the $z=0$ OII K$β$ line produced by a diffuse Low-Ionization Metal Medium in the Galaxy's Interstellar and Circum-Galactic mediums. We perform detailed modeling of all the available high signal-to-noise Chandra LETG and XMM-Newton RGS spectra of H 2356-309 and Mkn 501 and demonstrate that the $z\simeq 0.03$ WHIM absorption along these two sightlines is statistically not required. Our results, however, do not rule out a small contribution from the $z\simeq 0.03$ OVII K$α$ absorber along the line of sight to H~2356-309. In our model the temperature of the putative $z = 0.031$ WHIM filament is T$= 3\times 10^5$ K and the OVII column density is N$_{OV II} \le 4\times 10^{15}$ cm$^{-2}$, twenty times smaller than the OVII column density previously reported, and now more consistent with the expectations from cosmological hydrodynamical simulations.

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arXiv2015

X-Ray Detection of Warm Ionized Matter in the Galactic Halo

We report on a systematic investigation of the cold and mildly ionized gaseous baryonic metal components of our Galaxy, through the analysis of high resolution Chandra and XMM-Newton spectra of two samples of Galactic and extragalactic sources. The comparison between lines of sight towards sources located in the disk of our Galaxy and extragalactic sources, allows us for the first time to clearly distinguish between gaseous metal components in the disk and halo of our Galaxy. We find that a Warm Ionized Metal Medium (WIMM) permeates a large volume above and below the Galaxy's disk, perhaps up to the Circum-Galactic space (CGM). This halo-WIMM imprints virtually the totality of the OI and OII absorption seen in the spectra of our extragalactic targets, has a temperature of T(Halo-WIMM)=2900 +/- 900 K, a density (Halo-WIMM) = 0.023 +/- 0.009 cm-3 and a metallicity Z(Halo-WIMM) = (0.4 +/- 0.1) Z_Solar. Consistently with previous works, we also confirm that the disk of the Galaxy contains at least two distinct gaseous metal components, one cold and neutral (the CNMM: Cold Neutral Metal Medium) and one warm and mildly ionized, with the same temperature of the Halo-WIMM, but higher density ( (Disk-WIMM) = 0.09 +/- 0.03 cm-3) and metallicity (Z(Disk-WIMM) = 0.8 +/- 0.1$ Z_Solar). By adopting a simple disk+sphere geometry for the Galaxy, we estimates masses of the CNMM and the total (disk + halo) WIMM of M(CNMM) <~ 8e8 Solar masses and M(WIMM) ~ 8.2e9 Solar masses.

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arXiv2015

Studying the evolution of galaxies in compact groups over the past 3 Gyr. I. The nuclear activity

We present the first -- of a series -- study of the evolution of galaxies in compact groups over the past 3 Gyr. This paper focuses on the evolution of the nuclear activity and how it has been affected by the dense environment of the groups. Our analysis is based on the largest multiwavelength compact group sample to-date, containing complete ultraviolet-to-infrared (UV-to-IR) photometry for 1,770 isolated groups (7,417 galaxies). We classified the nuclear activity of the galaxies based on optical emission line and mid-infrared diagnostic methods, as well as using spectral energy distribution fitting. We observe a 15% increase on the number of the AGN-hosting late-type galaxies found in dynamically old groups, over the past 3 Gyr, accompanied by the corresponding decrease of their circumnuclear star formation. Comparing our compact group results with those of local isolated field and interacting pair galaxies, we find no differences in the AGN at the same redshift range. Based on both optical and mid-IR colour classifications, we report the absence of Seyfert 1 nuclei and we attribute this to the low accretion rates, caused by the depletion of gas. We propose that the observed increase of LINER and Seyfert 2 nuclei (at low-z's), in the early-type galaxies of the dynamically young groups, is due to the morphological transformation of lenticular into elliptical galaxies. Finally, we show that at any given stellar mass, galaxies found in dynamically old groups are more likely to host an AGN. Our findings suggest that the depletion of gas, due to past star formation and tidal stripping, is the major mechanism driving the evolution of the nuclear activity in compact groups of galaxies.

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arXiv2013

Probing the anisotropy of the Milky Way gaseous halo: Sight-lines toward Mrk 421 and PKS2155-304

(Abridged) We recently found that the halo of the Milky Way contains a large reservoir of warm-hot gas that contains a large fraction of the missing baryons from the Galaxy. The average physical properties of this circumgalactic medium (CGM) are determined by combining average absorption and emission measurements along several extragalactic sightlines. However, there is a wide distribution of both, the halo emission measure and the \ovii column density, suggesting that the Galactic warm-hot gaseous halo is anisotropic. We present {\it Suzaku} observations of fields close to two sightlines along which we have precise \ovii absorption measurements with \chandran. The column densities along these two sightlines are similar within errors, but we find that the emission measures are different. Therefore the densities and pathlengths in the two directions must be different, providing a suggestive evidence that the warm-hot gas in the CGM of the Milky Way is not distributed uniformly. However, the formal errors on derived parameters are too large to make such a claim. The average density and pathlength of the two sightlines are similar to the global averages, so the halo mass is still huge, over 10 billion solar masses. With more such studies, we will be able to better characterize the CGM anisotropy and measure its mass more accurately. We also show that the Galactic disk makes insignificant contribution to the observed \ovii absorption; a similar conclusion was also reached independently about the emission measure. We further argue that any density inhomogeneity in the warm-hot gas, be it from clumping, from the disk, or from a non-constant density gradient, would strengthen our result in that the Galactic halo path-length and the mass would become larger than what we estimate here. As such, our results are conservative and robust.

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arXiv2013

The properties of the clumpy torus and BLR in the polar-scattered Seyfert 1 galaxy ESO 323-G77 through X-ray absorption variability

We report results from multi-epoch X-ray observations of the polar-scattered Seyfert 1 galaxy ESO 323-G77. The source exhibits remarkable spectral variability from months to years timescales. The observed spectral variability is entirely due to variations of the column density of a neutral absorber towards the intrinsic nuclear continuum. The column density is generally Compton-thin ranging from a few times 10$^{22}$ cm$^{-2}$ to a few times 10$^{23}$ cm$^{-2}$. However, one observation reveals a Compton-thick state with column density of the order of 1.5 $\times$ 10$^{24}$ cm$^{-2}$. The observed variability offers a rare opportunity to study the properties of the X-ray absorber(s) in an active galaxy. We identify variable X-ray absorption from two different components, namely (i) a clumpy torus whose individual clumps have a density of $\leq$ 1.7 $\times$ 10$^8$ cm$^{-3}$ and an average column density of $\sim$ 4 $\times$ 10$^{22}$ cm$^{-2}$, and (ii) the broad line region (BLR), comprising individual clouds with density of 0.1-8 $\times$ 10$^9$ cm$^{-3}$ and column density of 10$^{23}$-10$^{24}$ cm$^{-2}$. The derived properties of the clumpy torus can also be used to estimate the torus half-opening angle, which is of the order of 47 $^\circ$. We also confirm the previously reported detection of two highly ionized warm absorbers with outflow velocities of 1000-4000 km s$^{-1}$. The observed outflow velocities are consistent with the Keplerian/escape velocity at the BLR. Hence, the warm absorbers may be tentatively identified with the warm/hot inter-cloud medium which ensures that the BLR clouds are in pressure equilibrium with their surroundings. The BLR line-emitting clouds may well be the cold, dense clumps of this outflow, whose warm/hot phase is likely more homogeneous, as suggested by the lack of strong variability of the warm absorber(s) properties during our monitoring.

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arXiv2013

Discovery of Relativistic Outflow in the Seyfert Galaxy Ark 564

We present \chandra high energy transmission grating spectra of the narrow-line Seyfert-1 galaxy Ark 564. The spectrum shows numerous absorption lines which are well modeled with low velocity outflow components usually observed in Seyfert galaxies \citep{Gupta2013}. There are, however, some residual absorption lines which are not accounted for by low-velocity outflows. Here we present identifications of the strongest lines as $Kα$ transitions of \oviin (two lines) and \ovin at outflow velocities of $\sim 0.1c$. These lines are detected at $6.9σ$, $6.2σ$, and $4.7σ$ respectively and cannot be due to chance statistical fluctuations. Photoionization models with ultra-high velocity components improves the spectral fit significantly, providing further support for the presence of relativistic outflow in this source. Without knowing the location of the absorber, its mass and energy outflow rates cannot be well constrained; we find $\dot{E}(outflow)/L_{bol}$ lower limit of $\geq 0.006$% assuming a bi-conical wind geometry. This is the first time that absorption lines with ultra-high velocities are unambiguously detected in the soft X-ray band. The presence of outflows with relativistic velocities in AGNs with Seyfert-type luminosities is hard to understand and provides valuable constraints to models of AGN outflows. Radiation pressure is unlikely to be the driving mechanism for such outflows and magneto-hydrodynamic may be involved.

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arXiv2013

A Two-Phase Low-velocity Outflow in the Seyfert 1 Galaxy Ark~564

The Seyfert 1 galaxy Ark 564 was observed with Chandra high energy transmission gratings for 250 ks. We present the high resolution X-ray spectrum that shows several associated absorption lines. The photoionization model requires two warm absorbers with two different ionization states (logU=0.39\pm0.03 and logU=-0.99\pm0.13), both with moderate outflow velocities (~100 km s^-1) and relatively low line of sight column densities (logNH=20.94 and 20.11 cm^-2). The high ionization phase produces absorption lines of OVII, OVIII, NeIX, NeX, MgXI, FeXVII and FeXVIII while the low ionization phase produces lines at lower energies (OVI & OVII). The pressure--temperature equilibrium curve for the Ark 564 absorber does not have the typical "S" shape, even if the metallicity is super-solar; as a result the two warm-absorber phases do not appear to be in pressure balance. This suggests that the continuum incident on the absorbing gas is perhaps different from the observed continuum. We also estimated the mass outflow rate and the associated kinetic energy and find it to be at most 0.006% of the bolometric luminosity of Ark~564. Thus it is highly unlikely that these outflows provide significant feedback required by the galaxy formation models.

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