Searching for X-ray luminous 'normal' galaxies in 2dfGRS

A. Georgakakis; I. Georgantopoulos; P. Tzanavaris

REVIEW

Searching for X-ray luminous 'normal' galaxies in 2dfGRS

P. Tzanavaris, I. Georgantopoulos, A. Georgakakis

Astrophysics · Submitted 27 Feb 2006 · 26 references

Pith's verdict: A careful, modest paper that makes a specific new point—the log(fX/fO)<-2 cut misses the most luminous normal ellipticals—but the bright-end extreme rests on only three Fabbiano galaxies whose 'normal' status is not independently verified.

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arXiv IDastro-ph/0602585 (version 1)

Pith ID7E3LARFR, Pith's permanent record of this paper and its review

CitationA. Georgakakis, I. Georgantopoulos, P. Tzanavaris. "Searching for X-ray luminous 'normal' galaxies in 2dfGRS." arXiv:astro-ph/0602585 (2006). https://pith.science/paper/7E3LARFR

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Editor's note

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The paper's real contribution is narrow but solid: it shows, with an independent optical classification, that the empirical log(fX/fO) < -2 cut used to separate normal galaxies from AGN tends to exclude the most X-ray-luminous ellipticals. That is a useful caution for anyone building X-ray-selected samples of normal galaxies, and I don't think the point was made this explicitly before. The authors also deserve credit for doing the classification before applying the flux-ratio criterion, so the test is not circular, and for being upfront that their own 2dFGRS sample is incomplete in exactly the region where the effect matters.

Where I would push back is on the strength of the bright-end claim. The three Fabbiano et al. galaxies with LX > 10^42 are doing a lot of work, and their normality depends on 1992-era Einstein flags plus an unspecified literature search. If even one of them hosts a weak AGN, the 100% fraction above 10^42 is meaningless. That is not a fatal flaw—the paper itself uses cautious language and the pattern below 10^42 (13/26 ellipticals above the cut in the 41

Other soft spots are minor: Table 3 fractions have no Poisson errors, the fixed B-V conversions are crude (but probably adequate for a flux ratio good to a few tenths of a dex), and many 2dFGRS spectral classifications are tentative. None of these change the main message. The cross-correlation samples are small, but the authors use them appropriately, mostly as a check on the literature samples.

This paper is for people who build X-ray luminosity functions of normal galaxies or use X-ray-to-optical flux ratios as a selection tool. It is not a breakthrough, but it is a legitimate, honest piece of work that identifies a real selection bias. I would send it to a referee, with the main request being a more transparent treatment of the Fabbiano AGN exclusion and some error bars on the fractions. I would not cite the 100% LX>42 number without checking the three galaxies myself.

Referee report

Significance

The paper addresses a question of genuine relevance to X-ray survey design: whether the log(fX/fO) < -2 empirical boundary, calibrated on deep Chandra fields, misses the most luminous normal galaxies. Its main strength is that the optical spectral classification is performed before the flux-ratio test is applied, so the test is not circular for the star-forming galaxies; the inclusion of the Zezas (2001) and Fabbiano et al. (1992) comparison samples provides external anchoring; and the final claim is appropriately hedged in wording ('seems to select'). The complementary finding that no confirmed AGN in the 2dFGRS sample falls below log(fX/fO) = -2 is a useful validation of the AGN side of the criterion. If the bright-end incompleteness is real, it bears directly on the luminosity-function discrepancy that motivates the paper. The main limitations are statistical (small numbers, unpropagated uncertainties) and evidentiary (the normality of the three high-LX Fabbiano galaxies is asserted rather than documented in the manuscript).

Major objections (3)

[§4; Eqs. (2)-(4); Fig. 1; Table 3] The bright-end result in Table 3 (3/3 galaxies with log LX >= 42 above the log(fX/fO) > -2 line) and the Section 5 summary claim that the criterion selects against 'the brightest, massive ellipticals' rest on the premise that the three Fabbiano et al. (1992) objects (two massive BGGs and one cD BCG) are genuinely normal. The manuscript states only that galaxies 'flagged as AGN hosts in the original sample have been excluded' and that 'a further literature search' was carried out, with no details of the search (databases, criteria, scope) and no list of the 16 galaxies with log(fX/fO) > -2 (identities, B magnitudes, fX, LX, and computed log(fX/fO)). This matters because brightest group/cluster galaxies are exactly the class in which low-luminosity AGN and LINERs are frequently found, and the Einstein-era compilation predates sensitive nuclear X-ray studies that reveal such sources. If even one of the three is an AGN, the 100% bright-end fraction becomes a statement about AGN contamination rather than about normal galaxies. I ask the authors to (i) tabulate the 16 galaxies and their plotted quantities, (ii) specify what the literature search covered and how AGN status was adjudicated, and (iii) either quote high-quality nuclear X-ray or radio data for the three LX >= 42 objects or soften the '3/3' and 'brightest' wording accordingly.
No uncertainties are propagated into log(fX/fO) or into the fractions in Table 3. Three systematics are relevant near the sharp boundary at -2: (i) the fixed colour conversions (B-V = 0.53 in Eq. 4 for the 2dFGRS sample; B-V = 0.655 for the Fabbiano et al. sample) differ from the true colours of the massive ellipticals in question (B-V ~ 0.9-1.0), giving a systematic offset of order 0.1-0.25 mag in V and hence ~0.05-0.1 dex in log(fX/fO); (ii) the bandpass conversion from the 0.2-4.0 keV Fabbiano fluxes to the 0.5-2.0 keV band used in Fig. 1, and the assumed photon index Gamma = 1.8, are not stated for the literature points; and (iii) the Table 3 fractions carry large binomial uncertainties (for example, 4/18 has a 95% interval of roughly 0.06-0.51, and 3/3 has a lower limit near 0.29). Since several objects lie within ~0.3 dex of the -2 line, the placement of individual objects is sensitive to these systematics. I ask for typical error boxes in Fig. 1, confidence intervals on the Table 3 entries, and an explicit statement of how many of the 16 (or 8) galaxies sit within 0.3 dex of the cut.
The stress test in Section 3, that LX and log(fX/fO) were not used to classify sources, is only partially accurate. For the 11 absorption-line galaxies, Section 4 states that 'the LX, log(fX/fO) and HR values, considered together, suggest that these are also not AGN-dominated.' The normal-galaxy status of the A-type galaxies therefore incorporates the very quantities whose discriminating power is being tested, and these galaxies provide most of the 2dFGRS bright-end entries in Table 3 (2/2 and 1/1), as well as the single absorption-line galaxy at log LX = 42 used in the expected-count comparison. The test is clean for the H II nuclei (5 of 8 above the cut), but for the A galaxies it is partially circular. I ask the authors to state explicitly which classifications rely on X-ray properties and to recompute the '~40%' fraction and the Table 3 A-rows with and without this subset so the reader can gauge the sensitivity of the conclusion.

Minor objections (5)

[Table 3] The caption says the denominator is the total number of NGs in the same sample and LX range, but the entries (e.g., 'A 2/18') invite a per-type reading, and the middle column overlaps the first. Please make the caption match the entries and give per-type percentages with binomial errors.
[Fig. 1] The figure has no error bars. A representative error box (or per-point errors) is needed, and the 16 Fabbiano et al. galaxies with log(fX/fO) > -2, especially the three with LX >= 10^42, should be marked so the central claim can be verified visually.
[§4 (expected-count comparison)] The 'expected <~1 galaxy per telescope' comparison has negligible constraining power, since Poisson fluctuations are of order unity; a Poisson interval should be stated, and if feasible the expectation should also be quoted from the Norman et al. (2004) luminosity function, because reconciling the two luminosity functions is the stated motivation.
[§4 (random-selection assumption)] The statement that galaxies from different populations are 'randomly selected' needs justification, given the very different selection functions of the three samples; at minimum, the interpretation of the Table 3 fractions should be flagged as sample-dependent.
[Abstract and Table 2 footnotes] The abstract's 'no luminous normal galaxies have been found' refers to the 2dFGRS sample alone; it would be useful to state that the three Fabbiano et al. galaxies with LX >= 10^42 lie above the -2 cut, which is the point of the paper. In addition, Table 2 footnotes 2 and 3 read 'Suggested by a an H beta FWHM' and should be corrected.

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The paper

The paper claims

Brightest normal galaxies break the standard X-ray cutoff

The reading

Galaxies whose X-rays come from stars and hot gas, not from an active nucleus, are usually picked out by demanding that their X-ray-to-optical flux ratio lie below a fixed cutoff, $\log(f_X/f_O) < -2$. The paper tests this rule against galaxies bright enough that a violation would matter, cross-correlating Chandra and XMM-Newton catalogs with the 2dFGRS spectroscopic survey and reexamining two nearby-galaxy samples from the literature. It finds no truly X-ray-luminous star-forming galaxies, but it does find that roughly 40 percent of its spectroscopically classified normal galaxies and, in the comparison sample, every galaxy with $L_X > 10^{42}\,\mathrm{erg\,s^{-1}}$ falls above the cutoff. The conclusion is that the standard cutoff systematically selects against the brightest, most massive ellipticals, so surveys that impose it are incomplete for luminous normal galaxies and will undercount the bright end of the local X-ray luminosity function.

What carries the argument

The central device is the empirical X-ray-to-optical flux ratio $\log(f_X/f_O)$ and its conventional boundary at $-2$, together with the luminosity threshold $L_X \sim 10^{42}\,\mathrm{erg\,s^{-1}}$ that is said to cap stellar-process X-ray emission. The paper's argument is carried by the $L_X$–$L_B^{1.8}$ correlation (Fabbiano et al., 1992), which predicts $\log(f_X/f_O) \sim 0.5 \log L_X$; that is why, as luminosity climbs, a fixed flux-ratio cutoff must cut into the normal-galaxy population. The authors use optical emission-line diagnostics (the Ho et al. 1997 adaptation of the BPT diagrams) to classify galaxies without looking at their X-ray properties, then compare that physical classification with the flux-ratio boundary.

Extended reading notes

Core claim

The paper's central claim is that the empirical AGN/normal-galaxy divide at $\log(f_X/f_O) < -2$ is not universal. Using emission-line ratios to classify galaxies independently of their X-ray properties, the authors show that a substantial minority of normal galaxies—about 40 percent in their own 2dFGRS sample—lands on the AGN side of the flux-ratio boundary, and that the violators become the majority at the highest luminosities. In the Fabbiano et al. (1992) sample, every normal galaxy with $L_X \ge 10^{42}\,\mathrm{erg\,s^{-1}}$, and most of those with $L_X > 10^{41}$, have $\log(f_X/f_O) > -2$; almost all of these are brightest group or cluster ellipticals (BGGs/BCGs). Surveys that impose the flux-ratio cut therefore do not simply miss rare outliers—they preferentially exclude the very class of luminous, massive elliptical galaxies that dominate counts at the bright end, biasing the local X-ray luminosity function.

Load-bearing premise

That every galaxy counted as 'normal' above the flux-ratio cut really is AGN-free—in particular the three Fabbiano et al. (1992) galaxies with $L_X > 10^{42}\,\mathrm{erg\,s^{-1}}$, whose classification rests on the original sample's flags plus the authors' own literature search rather than on new high-quality X-ray spectra.

Editorial extensions

If this is right

Any survey that selects normal galaxies using $\log(f_X/f_O) < -2$ will undercount the brightest members of the population, especially brightest cluster and group ellipticals.
The local X-ray luminosity function of normal galaxies built from such cuts (e.g., Georgantopoulos et al. 2005) will be suppressed at the bright end relative to samples that do not impose the cut (Norman et al. 2004), resolving the reported discrepancy in favor of the latter being more complete.
The cutoff remains useful in the opposite direction: the paper finds no confirmed AGN below $-2$, so AGN selected this way are not significantly contaminated.
Fully flux-calibrated, higher signal-to-noise optical spectra covering all four diagnostic lines would sharpen the classification of the currently unclassifiable 2dFGRS sources and would test the estimated violation rate.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

If the three $L_X > 10^{42}$ galaxies are confirmed AGN-free, the standard picture of 'normal' galaxy X-ray emission will need to include cool-core group and cluster ellipticals whose halos are not simply stellar-wind products, blurring the boundary between galaxy and intra-group medium in luminosity-function work.
A natural extension is to measure the same flux-ratio distribution for a volume-limited sample of brightest group galaxies; the trend reported here predicts that the median $\log(f_X/f_O)$ rises with local galaxy density.
A luminosity-dependent selection boundary—one that lets $\log(f_X/f_O)$ rise with $L_X$—would remove the incompleteness while still excluding AGN, and the data in this paper provide calibration points for such a revised criterion.

Original abstract

We cross-correlated the Chandra XASSIST and XMM-Newton Serendipitous Source Catalogues with the 2 degree Field Galaxy Redshift Survey (2dfGRS) database. Our aim was to identify the most X-ray luminous (L_X > 10^42 erg s^-1) examples of galaxies in the local Universe whose X-ray emission is dominated by stellar processes rather than AGN activity ('normal' galaxies) as well as to test the empirical criterion log(f_X/f_O) < -2 for separating AGN from NGs. With XMM-Newton (Chandra) we covered an area of ~8.2 (~5.8)deg^2 down to a flux limit of ~10^-15 (~1.6 x 10^-15) erg cm^-2 s^-1 and found 18 (20) 2dfGRS galaxies. Using emission-line intensity ratios, we classified 6 2dfGRS spectra as star-forming, H II nuclei, and 2 spectra as possible H II nuclei. The rest of the objects are absorption-line galaxies and AGN, including 3 possible LINERs. No luminous 'normal' galaxies have been found but out of 19 'normal' galaxies in this sample 5 H II and 3 absorption-line galaxies have log(f_X/f_O) > -2. We performed a similar search in two nearby-galaxy samples from the literature. All 44 galaxies in the Zezas (2001) sample have log(f_X/f_O) < -2 and L_X < 10^42 erg s^-1. In the Fabbiano et al. (1992) sample, out of a total of 170 'normal' galaxies, we found 16 galaxies with log(f_X/f_O) >-2, the majority of which are massive ellipticals. Three of these have L_X > 10^42 erg s^-1 .

The compiled record

The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.

Compiled by classifier-1.13.0 via TypeSafe API, question bank abstract-p1-20260923, 2026-09-23: 1 request, 7,963 input tokens.

What the abstract itself states, as probabilities: claims first 0.04 · states limitation 0.79 · human participants 0.01 · controlled comparison 0.42 · randomized assignment 0.01 · states quantitative result 0.99 · compares with prior results 0.97 · mentions code or data availability 0.02.

The abstract, sentence by sentence

arXiv listing abstract v1. Results and the main claim are underlined; show every role.

We cross-correlated the Chandra XASSIST and XMM-Newton Serendipitous Source Catalogues with the 2 degree Field Galaxy Redshift Survey (2dfGRS) database. Our aim was to identify the most X-ray luminous (L_X > 10^42 erg s^-1) examples of galaxies in the local Universe whose X-ray emission is dominated by stellar processes rather than AGN activity ('normal' galaxies) as well as to test the empirical criterion log(f_X/f_O) < -2 for separating AGN from NGs. With XMM-Newton (Chandra) we covered an area of ~8.2 (~5.8)deg^2 down to a flux limit of ~10^-15 (~1.6 x 10^-15) erg cm^-2 s^-1 and found 18 (20) 2dfGRS galaxies. Using emission-line intensity ratios, we classified 6 2dfGRS spectra as star-forming, H II nuclei, and 2 spectra as possible H II nuclei. The rest of the objects are absorption-line galaxies and AGN, including 3 possible LINERs. No luminous 'normal' galaxies have been found but out of 19 'normal' galaxies in this sample 5 H II and 3 absorption-line galaxies have log(f_X/f_O) > -2. We performed a similar search in two nearby-galaxy samples from the literature. All 44 galaxies in the Zezas (2001) sample have log(f_X/f_O) < -2 and L_X < 10^42 erg s^-1. In the Fabbiano et al. (1992) sample, out of a total of 170 'normal' galaxies, we found 16 galaxies with log(f_X/f_O) >-2, the majority of which are massive ellipticals. Three of these have L_X > 10^42 erg s^-1 .

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#	sentence	role	the authors' own work	states the main conclusion	strength
s0	We cross-correlated the Chandra XASSIST and XMM-Newton Serendipitous Source Catalogues with the 2 degree Field Galaxy Redshift Survey (2dfGRS) database.	method 0.99	0.94	0.01	asserted 1.00
s1	Our aim was to identify the most X-ray luminous (L_X > 10^42 erg s^-1) examples of galaxies in the local Universe whose X-ray emission is dominated by stellar processes rather than AGN activity ('normal' galaxies) as well as to test the empirical criterion log(f_X/f_O) < -2 for separating AGN from NGs.	contribution 0.69	0.28	0.05	asserted 0.75
s2	With XMM-Newton (Chandra) we covered an area of ~8.2 (~5.8)deg^2 down to a flux limit of ~10^-15 (~1.6 x 10^-15) erg cm^-2 s^-1 and found 18 (20) 2dfGRS galaxies.	method 0.60	0.97	0.04	asserted 1.00
s3	Using emission-line intensity ratios, we classified 6 2dfGRS spectra as star-forming, H II nuclei, and 2 spectra as possible H II nuclei.	result 0.55	0.95	0.03	asserted 0.97
s4	The rest of the objects are absorption-line galaxies and AGN, including 3 possible LINERs.	result 1.00	0.88	0.04	asserted 0.95
s5	No luminous 'normal' galaxies have been found but out of 19 'normal' galaxies in this sample 5 H II and 3 absorption-line galaxies have log(f_X/f_O) > -2.	result 1.00	0.94	0.66	asserted 1.00
s6	We performed a similar search in two nearby-galaxy samples from the literature.	contribution 0.53	0.92	0.02	asserted 0.97
s7	All 44 galaxies in the Zezas (2001) sample have log(f_X/f_O) < -2 and L_X < 10^42 erg s^-1.	result 0.99	0.24	0.06	asserted 1.00
s8	In the Fabbiano et al. (1992) sample, out of a total of 170 'normal' galaxies, we found 16 galaxies with log(f_X/f_O) >-2, the majority of which are massive ellipticals.	result 0.98	0.71	0.08	asserted 1.00
s9	Three of these have L_X > 10^42 erg s^-1 .	result 1.00	0.67	0.07	asserted 1.00

What kind of paper the classifier reports this is: empirical study 1.00 · evidence from analysis of existing data 0.95.

What was examined, and what was not

sources read

arXiv listing abstract v1

not examined

full text
figures and tables
references and citing sentences
relations to other papers

levels run

P abstract

not run

S sections
T body sentences
C citations
R cross-paper relations

Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

Figures

**Figure 1.** Plot of log(fX/fO) versus log LX. Plotted here are values calculated from our XMM-Newton and Chandra data, as well as from the literature (see legend in the plot). For all samples only galaxies which are classified as H ii or absorption-line are plotted. All luminosities are for the same energy band (0.5 − 2.0 keV). The two dashed lines demarcate the regions of log(fX/fO)−LX space which may be inhabited mainly either by NGs (log LX < 42, log(fX/fO) < −2) or by AGN. The dash-dotted line indicates an estimate for log(fX/fO) from low mass X-ray binaries in a 1011M⊙ galaxy (see text).…

**Figure 2.** Plot of bJ magnitude versus logarithmic X-ray flux for all galaxies in Tables 1 and 2. The region to the right of the line log(fX/fO) = −1 is expected to be occupied mainly by AGN. NGs are expected to be found mainly in the region to the left of the line log(fX/fO) = −2. It is also useful to compare observed values of log(fX/fO) to the numbers expected due to low mass X-ray binaries (LXRBs), whose combined luminosity has been shown to scale with the stellar mass of the host galaxy (Gilfanov, 2004). We estimated the value of log(fX/fO) expected for a 1011M⊙ galaxy, using the…

Figures reproduced from arXiv:astro-ph/0602585 by the authors.

References

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Discussion

Disagree with the review, or know something it missed? Leave a comment, highlight any passage above to comment on it, or challenge this review formally.

[1] [1]

Alexander, D. M., Bauer, F. E., Brandt, W. N., et al. 2003, AJ, 126, 539

View on Pith 2003

[2] [2]

Baldwin, J. A., Phillips, M. M., & Terlevich, R. 1981, PASP, 93, 5

View on Pith 1981

[3] [3]

Colless, M., Dalton, G., Maddox, S., et al. 2001, MNRAS, 328, 1039

View on Pith 2001

[4] [4]

Colless, M., Peterson, B. A., Jackson, C., et al. 2003, astro-ph/0306581

View on Pith Pith review arXiv 2003

[5] [5]

Fabbiano, G. 1989, ARA&A, 27, 87

View on Pith 1989

[6] [6]

Fabbiano, G., Kim, D.-W., & Trinchieri, G. 1992, ApJS, 80, 531

View on Pith 1992

[7] [7]

Fukugita, M., Shimasaku, K., & Ichikawa, T. 1995, PASP, 107, 945

View on Pith 1995

[8] [8]

Shanks, T., & Boyle, B. J. 2003, MNRAS, 344, 161 8 P. Tzanavaris et al.: X-ray luminous ‘normal’ galaxies in 2dFGRS

View on Pith 2003

[9] [9]

Georgantopoulos, I., Georgakakis, A., & Koulouridis, E. 2005, MNRAS, 360, 782

View on Pith 2005

[10] [10]

Giacconi, R., Zirm, A., Wang, J., et al. 2002, ApJS, 139, 369

View on Pith 2002

[11] [11]

Gilfanov, M. 2004, MNRAS, 349, 146

View on Pith 2004

[12] [12]

Gilfanov, M., Grimm, H.-J., & Sunyaev, R. 2004, MNRAS, 347, L57

View on Pith 2004

[13] [13]

Helsdon, S. F. & Ponman, T. J. 2000, MNRAS, 315, 356

View on Pith 2000

[14] [14]

Ho, L. C., Filippenko, A. V., & Sargent, W. L. W. 1997, ApJS, 112, 315

View on Pith 1997

[15] [15]

Hornschemeier, A. E., Bauer, F. E., Alexander, D. M., et al. 2003, AJ, 126, 575

View on Pith 2003

[16] [16]

Levenson, N. A., Weaver, K. A., & Heckman, T. M. 2001, ApJ, 550, 230

View on Pith 2001

[17] [17]

Mannucci, F., Basile, F., Poggianti, B. M., et al. 2001, MNRAS, 326, 745

View on Pith 2001

[18] [18]

Moran, E. C., Lehnert, M. D., & Helfand, D. J. 1999, ApJ, 526, 649

View on Pith 1999

[19] [19]

Norberg, P., Cole, S., Baugh, C. M., et al. 2002, MNRAS, 336, 907

View on Pith 2002

[20] [20]

Norman, C., Ptak, A., Hornschemeier, A., et al. 2004, ApJ, 607, 721

View on Pith 2004

[21] [21]

Ptak, A. & Griﬃths, R. 2003, in ASP Conf. Ser. 295: Astronomical Data Analysis Software and Systems XII, 465

View on Pith 2003

[22] [22]

Stevens, I. R., Read, A. M., & Bravo-Guerrero, J. 2003, MNRAS, 343, L47

View on Pith 2003

[23] [23]

Stocke, J. T., Morris, S. L., Gioia, I. M., et al. 1991, ApJS, 76, 813

View on Pith 1991

[24] [24]

Veilleux, S. & Osterbrock, D. E. 1987, ApJS, 63, 295

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